STELLER SEA LION RESEARCH :
Why Did They Decline? Related Publications

PredationOverfishing  | Ocean Climate Change | Other Factors

PREDATION


2012
 
Predation on an upper trophic marine predator, the Steller sea lion: evaluating high juvenile mortality in a density dependent conceptual framework.
Horning, M. and J.-A.E. Mellish. 2012.
PLoS ONE. Vol 7(1):e30173
abstract
The endangered western stock of the Steller sea lion (Eumetopias jubatus) ˆ the largest of the eared seals ˆ has declined by 80% from population levels encountered four decades ago. Current overall trends from the Gulf of Alaska to the Aleutian Islands appear neutral with strong regional heterogeneities. A published inferential model has been used to hypothesize a continuous decline in natality and depressed juvenile survival during the height of the decline in the mid-late 1980‚s,followed by the recent recovery of juvenile survival to pre-decline rates. However, these hypotheses have not been tested by direct means, and causes underlying past and present population trajectories remain unresolved and controversial. We determined post-weaning juvenile survival and causes of mortality using data received post-mortem via satellite from telemetry transmitters implanted into 36 juvenile Steller sea lions from 2005 through 2011. Data show high post-weaning mortality by predation in the eastern Gulf of Alaska region. To evaluate the impact of such high levels of predation, we developed a conceptual framework to integrate density dependent with density independent effects on vital rates and population trajectories. Our data and model do not support the hypothesized recent recovery of juvenile survival rates and reduced natality. Instead, our data demonstrate continued low juvenile survival in the Prince William Sound and Kenai Fjords region of the Gulf of Alaska. Our results on contemporary predation rates combined with the density dependent conceptual framework suggest predation on juvenile sea lions as the largest impediment to recovery of the species in the eastern Gulf of Alaska region. The framework also highlights the necessity for demographic models based on age-structured census data to incorporate the differential impact of predation on multiple vital rates.
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2009
 
Predator-prey relationships.
Trites, A.W. 2009.
In B. Wursig W.F. Perrin (ed.), Encyclopedia of Marine Mammals. Academic Press, San Diego. pp. 933-936.
abstract
Marine mammal predator – prey interactions occur over different spatial and temporal scales, making it difficult to empirically decipher the influences they have on one another and on their ecosystems. However, their coexistence suggests that marine mammal predators and their prey have had profound influences on each other’s behaviors, physiologies, morphologies, and life-history strategies. The diversity of niches filled by marine mammals makes it difficult to generalize about the evolutionary consequences of their interactions with prey, beyond stating the obvious: marine mammals have adapted to catch food, while their prey have adapted to avoid being caught. On the shorter ecological time scale, marine mammals can affect the abundance of other species by consuming or outcompeting them. They can also indirectly affect the abundance of nontargeted species by consuming one of their predators, and can have strong impacts on the overall dynamics and structure of their ecosystems. One of the best tools for understanding marine mammal predator – prey interactions is the ecosystem model. However, more work is required through experimental manipulations and observational studies to evaluate the choices made by marine mammals and the costs of obtaining different species of prey.
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2007
 
Ecosystem models of the Aleutian Islands and Southeast Alaska show that Steller sea lions are impacted by killer whale predation when sea lion numbers are low.
Guenette S., S.J.J. Heymans, V. Christensen, A.W. Trites. 2007.
In J.F. Piatt and S.M. Gende (eds), Proceedings of the Fourth Glacier Bay Science Symposium, U.S. Geological Survey, Juneau , Alaska. pp. 150-154.
abstract
We constructed ecosystem models using the Ecopath with Ecosim software to evaluate whether predation by killer whales might explain the decline of Steller sea lions since the late 1970s in the western Aleutian Islands. We also sought to understand why sea lions increased in the presence of killer whales in Southeast Alaska. Modeling results reproduced the time series of abundances for exploited species and sea lions in both ecosystems. Simulation results suggest that killer whale predation contributed to the decline of sea lions in the western Aleutians, but that predation was not the primary cause of the population decline. Predation could however have become a significant source of mortality during the 1990s when sea lions numbers were much lower. In Southeast Alaska, predation was also found to be a significant source of mortality in the 1960s when sea lions were low, but ceased to control population growth through the 1980s and 1990s. Overall, the ecosystem models suggest that large populations of Steller sea lions can withstand predation, but that small populations are vulnerable to killer whales.
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Killer whales, whaling and sequential megafaunal collapse in the North Pacific: a comparative analysis of the dynamics of marine mammals in Alaska and British Columbia following commercial whaling.
Trites, A. W., V. B. Deecke, E. J. Gregr, J. K. B. Ford, and P. F. Olesiuk. 2007.
Marine Mammal Science 23:751-765.
abstract
The hypothesis that commercial whaling caused a sequential megafaunal collapse in the North Pacific Ocean by forcing killer whales to eat progressively smaller species of marine mammals is not supported by what is known about the biology of large whales, the ecology of killer whales and the patterns of ecosystem change that took place in Alaska, British Columbia, and elsewhere in the world following whaling. A comparative analysis shows that populations of seals, sea lions and sea otters increased in British Columbia following commercial whaling, unlike the declines noted in the Gulf of Alaska and Aleutian Islands. The declines of seals and sea lions that began in western Alaska around 1977 were mirrored by increases in numbers of these species in British Columbia. A more likely explanation is the seal and sea lion declines and other ecosystem changes in Alaska stems from a major oceanic regime shift that occurred in 1977. Killer whales are unquestionably a significant predator of seals, sea lions and sea otters but not because of commercial whaling.
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2006
 
The Sequential Megafaunal Collapse Hypothesis: Testing with Existing Data.
DeMaster, D.P., A.W. Trites, P. Clapham, S. Mizroch, P. Wade, R.J. Small, and J. Ver Hoef. 2006.
Progress in Oceanography 68:329-342.
abstract
Springer et al. [Springer, A.M., Estes, J.A., van Vliet, G.B., Williams, T.M., Doak, D.F., Danner, E.M., Forney, K.A., Pfister, B., 2003. Sequential megafaunal collapse in the North Pacific Ocean: an ongoing legacy of industrial whaling? Proceedings of the National Academy of Sciences 100 (21), 12,223–12,228] hypothesized that great whales were an important prey resource for killer whales, and that the removal of fin and sperm whales by commercial whaling in the region of the Bering Sea/Aleutian Islands (BSAI) in the late 1960s and 1970s led to cascading trophic interactions that caused the sequential decline of populations of harbor seal, northern fur seal, Steller sea lion and northern sea otter. This hypothesis, referred to as the Sequential Megafaunal Collapse (SMC), has stirred considerable interest because of its implication for ecosystem-based management. The SMC has the following assumptions: (1) fin whales and sperm whales were important as prey species in the Bering Sea; (2) the biomass of all large whale species (i.e., North Pacific right, fin, humpback, gray, sperm, minke and bowhead whales) was in decline in the Bering Sea in the 1960s and early 1970s; and (3) pinniped declines in the 1970s and 1980s were sequential. We concluded that the available data are not consistent with the first two assumptions of the SMC. Statistical tests of the timing of the declines do not support the assumption that pinniped declines were sequential. We propose two alternative hypotheses for the declines that are more consistent with the available data. While it is plausible, from energetic arguments, for predation by killer whales to have been an important factor in the declines of one or more of the three populations of pinnipeds and the sea otter population in the BSAI region over the last 30 years, we hypothesize that the declines in pinniped populations in the BSAI can best be understood by invoking a multiple factor hypothesis that includes both bottom–up forcing (as indicated by evidence of nutritional stress in the western Steller sea lion population) and top–down forcing (e.g., predation by killer whales, mortality incidental to commercial fishing, directed harvests). Our second hypothesis is a modification of the top–down forcing mechanism (i.e., killer whale predation on one or more of the pinniped populations and the sea otter population is mediated via the recovery of the eastern North Pacific population of the gray whale). We remain skeptical about the proposed link between commercial whaling on fin and sperm whales, which ended in the mid-1960s, and the observed decline of populations of northern fur seal, harbor seal, and Steller sea lion some 15 years later.
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Ecosystem models show combined effects of fishing, predation, competition, and ocean productivity on Steller sea lions (Eumetopias jubatus) in Alaska.
Guénette, S., S.J.J. Heymans, V. Christensen, and A.W. Trites. 2006.
Canadian Journal of Fisheries and Aquatic Sciences 63:2495-2517.
abstract
Steller sea lions (Eumetopias jubatus) increased in the eastern portion of their range while declining in the Gulf of Alaska and Aleutian Islands from the late 1970s to late 1990s. We constructed ecosystem models of the central and western Aleutians and of Southeast Alaska to simultaneously evaluate four hypotheses explaining sea lion dynamics: killer whale (Orcinus orca) predation, ocean productivity, fisheries, and competition with other species. Comparisons of model predictions to historical time series data indicate that all four factors likely contributed to the trends observed in sea lion numbers in both ecosystems. Changes in ocean productivity conveyed by the Pacific Decadal Oscillation influenced the abundance trajectory of several species. Fishing could have affected the ecosystem structure by influencing the abundance of Atka mackerel (Pleurogrammus monopterygius) in the Aleutians, and herring (Clupea pallasii) in Southeast Alaska. Halibut (Hypoglossus stenolepis) in the Aleutians and arrowtooth flounder (Reinhardtius stomias) in Southeast Alaska appear to impede sea lion population growth through competitive interactions. Predation by killer whales was important when sea lions were less abundant in the 1990s in the Aleutians and in the 1960s in Southeast Alaska, but appear to have little effect when sea lion numbers were high.
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2004
 
Whales, whaling and ecosystem change in the Antarctic and Eastern Bering Sea: insights from ecosystem models.
Trites, A. W.,Bredesen, E.L. and Coombs,A.P. 2004.
In Investigating the roles of cetaceans in marine ecosystems. Monaco: CIESM Workshop Monographs pp. 85-92.
abstract
Ecosystem models were constructed for the Antarctic and the Bering Sea that incorporate current understanding of biological interactions of species within the ecosystem (i.e., who eats whom and how much). Within the limitations that are inherent to simulations, both models suggest that removal of large whales had little measurable effect on lower trophic levels or on the dynamics of other species in their polar ecosystems. Trophic interactions failed to explain the magnitude of changes in the biomass of the major species groups in the Antarctic and Bering Sea. Nor did fin-fisheries appear to have had a significant effect on the abundance of non-targeted species. This may mean that environmental effects (which were not modeled) play an important role in influencing the dynamics of marine ecosystems. Oceanographic factors such as changes in water temperature or ocean currents likely result in variations in ecosystem production and species recruitment patterns which are not captured by our Ecopath models. The Ecopath modeling approach is a powerful means of synthesizing knowledge about ecosystems and the factors that influence ecosystem dynamics. They provide a straightforward means for estimating trophic levels and niche overlaps with other species to assess the potential for resource competition. While the models failed to support the hypotheses that large whales play a significant structural role in the Antarctic and Bering Sea ecosystems, they do support what most already know ?- i.e., that populations of large whales are easily reduced to low numbers, but take a long, long time to recover. They also help in recognizing the need to consider factors other than food web interactions when assessing the status of cetaceans, as well as highlighting the potential tradeoffs that can result when other species are removed from ecosystems.
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2003
 
Examining the evidence for killer whale predation on Steller sea lions in British Columbia and Alaska.
Heise, K,. L.G. Barrett-Lennard, E. Saulitis, C. O. Matkin, D. Bain. 2003.
Aquatic Mammals 29:325-334.
abstract
The discovery of flipper tags from 14 Steller sea lions (Eumetopias jubatus) in the stomach of a dead killer whale (Orcinus orca) in 1992 focused attention on the possible role of killer whale predation in the decline of Steller sea lions in western Alaska. In this study, mariners in British Columbia and Alaska were surveyed to determine the frequency and out-come of observed attacks on sea lions, the age classes of sea lions taken, and the areas where predatory attacks occurred. The 126 survey respondents described 492 killer whale/sea lion interactions, of which at least 32 were fatal attacks on the sea lion. The greatest rate of observed predation occurred in the Aleutian Islands. The stomach contents of dead and stranded whales also were examined. Stomachs that were not empty contained only fish or marine mammal remains, but not both. This supports earlier evidence of dietary segregation between fish-eating resident and marine mammal-eating transient killer whales in Alaska. Steller sea lion remains were found in two of 12 killer whale stomachs examined from Alaska between 1990 and 2001. Stomach contents fromtwo oVshore killer whales provided the first direct evidence that this third formof killer whale feeds on fish.
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Sequential megafaunal collapse in the North Pacific Ocean: An ongoing legacy of industrial whaling?
Springer, A.M. , J. A. Estes , G. B. van Vliet , T. M. Williams, D. F. Doak, E. M. Danner, K. A. Forney, and B. Pfister. 2003.
Proceedings of the National Academy of Sciences of the United States of America 100:12223-12228.
abstract
Populations of seals, sea lions, and sea otters have sequentially collapsed over large areas of the northern North Pacific Ocean and southern Bering Sea during the last several decades. A bottom-up nutritional limitation mechanism induced by physical oceano-graphic change or competition with fisheries was long thought to be largely responsible for these declines. The current weight of evidence is more consistent with top-down forcing. Increased predation by killer whales probably drove the sea otter collapse and may have been responsible for the earlier pinniped declines as well. We propose that decimation of the great whales by post-World War II industrial whaling caused the great whales’ foremost natural predators, killer whales, to begin feeding more intensively on the smaller marine mammals, thus ‘‘fishing-down’’ this element of the marine food web. The timing of these events, information on the abundance, diet, and foraging behavior of both predators and prey, and feasibility analyses based on demographic and energetic modeling are all consistent with this hypothesis. food web dynamics brought about by human overharvesting initiated the change.
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2002
 
Killer whales and predation on Steller sea lions.
Matkin, C.G., L. Barrett-Lennard and G. Ellis. 2002.
In D. DeMaster and S. Atkinson (eds), Steller sea lion decline: Is it food II. University of Alaska Sea Grant, AK-SG-02-02, Fairbanks.
abstract
The western stock of Steller sea lions has declined from over 140,000 individuals in the 1960s to possibly fewer than 40,000 individuals in 2000. The primary hypotheses put forth by the National Marine Fisheries Service (NMFS) explaining this decline centers around food limitation.One alternative hypothesis that has recently received attention is that the decline or lack of recovery is due to the effects of predation by killer whales or sharks.Reports of large numbers of killer whales surrounding longline and trawl fishing vessels in western Alaska suggest that there are many killer whales in the region. In order to assess the impact of killer whale predation on this popula- tion decline,we need the following information:

1.Number of Steller sea lions.

2.Intrinsic growth rate of Steller sea lion population.

3.Number of killer whales that prey on Steller sea lions.

4.Percentage of the killer whale diet that consists of Steller sea lions and age class of sea lion that is consumed.



keywords     Predation
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1999
 
Ecosystem change and the decline of marine mammals in the Eastern Bering Sea: testing the ecosystem shift and commercial whaling hypotheses.
Trites, A.W., P.A. Livingston, M.C. Vasconcellos, S. Mackinson, A.M. Springer and D. Pauly. 1999.
Fisheries Centre, University of British Columbia, Vancouver, Canada. pp. 106
abstract
Over the past 10 years there has been increasing criticism of management decisions that are based on single species approaches and a call for the implementation of ecosystem approaches. The major criticism of single species models is that they cannot predict changes in community struc ture. Unfortunately, our experience in modeling the Bering Sea shows that these same criticisms can also be leveled against ecosystem models. We employed trophic mass balance models (Ecopath and Ecosim) to examine some possible explanations for the changes that occurred in the Bering Sea between the 1950s and 1980s. We removed fish and mammals from the modeled system and tracked how other components of the eco system responded. Our mass balance models indicate that neither whal ing nor commercial fisheries were sufficient to explain the 400% increase in pollock biomass and other changes that may have occurred between the two time periods. The simulations further suggest that environmental factors, affecting recruitment or primary production, may be more impor tant in determining the dynamics of the Bering Sea ecosystem than preda tor prey interactions alone. These findings illustrate that mass balance models that do not account for the impact of climate variability on year class strength cannot provide reliable estimates of trends in marine fish production. However, our models can show how predation and fishing can affect trophic interactions among species. As such, ecosystem models are a useful scientific tool to identify gaps in understanding and data needs, but are unlikely to ever replace single species models. They may instead complement and provide parameters to single species models. Ecosystem models such as ours are still in the early stages of develop ment and will become increasingly more important as a management tool as they begin to incorporate spatial and oceanographic/climatic information.
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1995
 
The impact of killer whale predation on Steller sea lion populations in British Columbia and Alaska.
Barrett-Lennard, L.G., K.E.Saulitis Heise, G. Ellis and C. Matkin. 1995.
University of British Columbia, Fisheries Centre.
abstract
Steller sea lion populations in Alaska have declined precipitously over the last 25 years. Much research has been conducted on the role of anthropogenic factors in this decline. The retrieval of 14 sea lion flipper tags from a dead killer whale in 1992 underscored the need for a similar appraisal of predation. We used simulation models to examine (1) the extent to which killer whales contributed to the sea lion decline, and (2) the present effect of killer whale predation on depleted sea lion populations. We estimated the model parameters using three sources: a survey of researchers and mariners, the stomach contents of stranded killer whales, and killer whale identification photographs from several collections. The 126 survey respondents described 52 attacks including 32 reported kills. Eight out of 15 killer whale stomachs with identifiable contents contained marine mammals, and two contained Steller sea lion remains. The survey and stomach content data were consistent with earlier findings that only members of the transient killer whale population commonly prey on marine mammals. Based on identification photographs, we estimated that at least 250 transient killer whales feed in Alaskan waters. We ran Leslie matrix simulations under various assumptions concerning the functional responses of killer whales to changes in sea lion density. Our models suggest that killer whale predation did not cause the sea lion decline, but may now be a contributing factor. At present, approximately 18% of sea lions that die annually in Western Alaska may be taken by killer whales.
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OVERFISHING


2015
 
Assessment of competition between fisheries and Steller sea lions in Alaska based on estimated prey biomass, fisheries removals and predator foraging behaviour.
Hui, T.C.Y., R. Gryba, E.J. Gregr and A.W. Trites. 2015.
PLoS ONE Vol 10(5): e0123786
abstract
A leading hypothesis to explain the dramatic decline of Steller sea lions (Eumetopias jubatus) in western Alaska during the latter part of the 20th century is a change in prey availability due to commercial fisheries. We tested this hypothesis by exploring the relationships between sea lion population trends, fishery catches, and the prey biomass accessible to sea lions around 33 rookeries between 2000 and 2008. We focused on three commercially important species that have dominated the sea lion diet during the population decline: walleye pollock, Pacific cod and Atka mackerel. We estimated available prey biomass by removing fishery catches from predicted prey biomass distributions in the Aleutian Islands, Bering Sea and Gulf of Alaska; and modelled the likelihood of sea lions foraging at different distances from rookeries (accessibility) using satellite telemetry locations of tracked animals. We combined this accessibility model with the prey distributions to estima te the prey biomass accessible to sea lions by rookery. For each rookery, we compared sea lion population change to accessible prey biomass. Of 304 comparisons, we found 3 statistically significant relationships, all suggesting that sea lion populations increased with increasing prey accessibility. Given that the majority of comparisons showed no significant effect, it seems unlikely that the availability of pollock, cod or Atka mackerel was limiting sea lion populations in the 2000s.

keywords     Eumetopias jubatus, walleye pollock, Pacific cod, Atka mackerel, accessibility, prey distribution, CPUE, linear mixed-effects models
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2011
 
Steller sea lions and fisheries: competition at sea?
Hui, T.C.Y. 2011.
MSc, University of British Columbia, Vancouver. 114 pages
abstract
A leading hypothesis to explain the decline of Steller sea lions (Eumetopias jubatus) in western Alaska is the reduction of prey abundance or change in prey distributions caused by commercial fisheries. We sought to improve on past studies that attempted to assess competition between sea lions and fisheries by estimating the local amounts of prey accessible to sea lions. We explored the relationships between sea lion population trends, fishery catches and the prey biomass accessible to sea lions around 33 rookeries from 2000-2008. We focused on three commercially important species that dominate the sea lion diet: walleye pollock, Pacific cod and Atka mackerel. We estimated available prey biomass by removing fishery catches from predicted prey biomass distributions in the Aleutian Islands, Bering Sea and Gulf of Alaska; and modelled the likelihood of sea lions foraging at different distances from rookeries (accessibility) using satellite telemetry locations of tracked animals. We combined this accessibility model with the prey distributions to estimate the prey biomass accessible to sea lions by rookery. For each rookery, we compared sea lion population change to accessible prey biomass (estimated using our accessibility model and also within 10, 20 and 50 km of each rookery). Of the 304 statistical models we constructed to compare accessible prey biomass and catch to sea lion population trends, only three relationships were significant. These three suggest that sea lion population change rates increased (became less negative) with increasing accessible pollock biomass in the Aleutian Islands and with cod biomass in the Gulf of Alaska. No relationships were found between sea lion population trends and Atka mackerel biomass. Given that the majority of the relationships we explored were insignificant, it seems unlikely that the availability of pollock, cod or Atka mackerel was limiting sea lion populations in the 2000s. Sea lion population trends appeared to be affected by some unknown factor associated with regional differences. Removing fish catches or adding catch to our predicted distributions of groundfish abundances had no measurable effect on sea lion population trends. These observations suggest that sea lion populations were largely unaffected by fishery removals during this period.
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2008
 
Economic valuation of critical habitat closures.
Berman, M., E.J. Gregr, G. Ishimura, R. Coatta, R. Flinn, U.R. Sumaila and A.W. Trites. 2008.
In Fisheries Centre Research Reports. Vol 16(8) pp. 102
abstract
We developed methods to estimate the spatial variation in economic values of ocean fisheries, and applied the methods to estimate the cost of closing groundfish fisheries in Steller sea lion Critical Habitat in the Bering Sea and Gulf of Alaska. The research addressed two related goals: (1) explicitly linking spatial variability of fisheries biomass and profitability over time to environmental variables; and (2) developing estimates of opportunity costs of time and area closures to the fishing industry at scales relevant to management. The approach involved two stages of statistical analyses. First, environmental conditions measured at 3 km and 9 km spatial scales and two-week and one-month intervals were used to predict fish biomass and fisheries catch per unit of effort (CPUE). Environmental variables included bathymetry, remotely sensed physical and biological observations, and output from a physical oceanographic circulation model. Second, we used predicted CPUE and spatial regulatory and cost factors to explain the spatial distribution of fishing effort over time. Our results suggested that 2001 Critical Habitat closures cost the North Pacific groundfish trawl fisheries 5-40 percent of their total potential net earnings. The improved methods for estimating opportunity costs of fisheries closures we present have direct applications to evaluating boundary changes to marine protected areas and other spatial management decisions. Limitations include the extensive data requirements and the need to bootstrap confidence intervals. If further research demonstrates the robustness and stability of the estimated relationships over time, the methods could project spatial fishery effects of climate variability and change, leading to dynamic spatial models linking fisheries with ecosystems.
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2006
 
Relationship between Steller sea lion diets and fish distributions in the eastern North Pacific.
Bredesen, E.L., A.P. Coombs, and A.W. Trites. 2006.
In A.W. Trites, S. Atkinson, D.P. DeMaster, L.W. Fritz, T.S. Gelatt, L.D. Rea and K. Wynne (eds), Sea Lions of the World. Alaska Sea Grant College Program, University of Alaska, Fairbanks. pp. 131-139.
abstract
Distributions of fish species were compared with diet information for Steller sea lions (Eumetopias jubatus) to assess the level of correspondence between potential prey availability and sea lion feeding habits. Fish distributions were compiled as part of the Sea Around Us Project at the UBC Fisheries Centre, and were based on published distributions and habitat preferences (e.g., latitude, depth). Sea lion scat samples were collected during the 1990s from seven geographic regions from Oregon to the western and central Aleutian Islands. The frequencies of occurrence of four prevalent species (walleye pollock, Theragra chalcogramma ; Pacific herring, Clupea pallasii ; Pacific cod, Gadus macrocephalus ; and North Pacific hake, Merluccius productus ) in the Steller sea lion diet were compared to their distributions in the North Pacific Ocean. The data suggest that Steller sea lion diets broadly reflect the distributions of these major prey species. However, some of the fish species that were regionally predicted to be present in high abundance were not proportionally reflected in the Steller sea lion diet, suggesting that other factors in addition to fish abundance influence their diets.
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Modeling spatial dynamics of Steller sea lions (Eumetopias jubatus) using maximum likelihood and Bayesian methods: evaluating causes for population decline.
Fay, G. and A.E. Punt. 2006.
In A.W. Trites, S. Atkinson, D.P. DeMaster, L.W. Fritz, T.S. Gelatt, L.D. Rea and K. Wynne (eds), Sea Lions of the World. Alaska Sea Grant College Program, University of Alaska, Fairbanks. pp. 405-433.
abstract
The timing and extent of the negative population trend in the abundance of the western stock of Steller sea lions has not been geographically uniform. A stochastic metapopulation dynamics model is developed for Steller sea lions. This model allows for geographical differences in factors affecting population processes, and can be parameterized to represent a wide range of hypotheses for the decline in Steller sea lion abundance. Bayesian and maximum likelihood methods are used to fit this model to pup and non-pup count data, age structure samples, and survival estimates. Inferences from model selection criteria highlight the spatial variability in the types of impact deemed to provide most parsimonious representation of the data. Bayesian posteriors for the estimated model parameters show that many combinations of parameter values are able to provide similar fits to the data, even given a specific hypothesis for the decline. This highlights the uncertainty in the precise nature of the impact of these hypotheses. Indeed, while pup production is generally estimated consistently among models, estimates of the size of other components of the Steller sea lion population (such as total population size) depend greatly on the assumptions regarding the cause of the decline. The results demonstrate that future simulation modeling approaches will require more formal, spatial, and mechanistic descriptions of the manner in which specific hypotheses for the decline affect the population.
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Can experimental manipulation be used to determine the cause of the decline of western stock of Steller sea lions (Eumetopias jubatus)?
Punt, A.E. and G. Fay. 2006.
In A.W. Trites, S. Atkinson, D.P. DeMaster, L.W. Fritz, T.S.ReaL.D. Gelatt and K. Wynne (eds), Sea Lions of the World. Alaska Sea Grant College Program, University of Alaska, Fairbanks. pp. 435-454.
abstract
A variety of reasons have been postulated for the decline of Steller sea lions (Otariidae: Eumetopias jubatus) in the Northeast Pacific. To date, however, it has proved impossible to distinguish among these reasons given the available data. In principle, experimental management based on spatial replication of treatments could be used to discriminate among some of these hypotheses. A simulation protocol was developed and applied to evaluate the power of a set of potential experiments to distinguish between whether the cause of the decline was fishing-induced or due to other factors. The simulations are based on an operating model that is individual-based and spatially explicit, and can be parameterized to represent the implications of a range of possible causes for the decline. This model can be used to generate the types of data typically available for the western stock of Steller sea lions. Experiments based on splitting four of the regions identified for past ana! lyses of population dynamics information into sectors that are either open to some fishing or completely closed are considered. The performance of these experiments is, however, poor, owing to the impact of movement, different historical trends in different areas, demographic stochasticity, and the likely size of the effect that the experiments are attempting to detect. These results suggest that the currently available information imply that large-scale experimental manipulation by means of additional spatial closures, where the results are analyzed by examining trends in pup counts, is unlikely to provide an effective means of discriminating among alternative hypotheses for the declining Steller sea lions in Alaska.
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Effects of fisheries on ecosystems: just another top predator?
Trites, A.W., V. Christensen and D. Pauly. 2006.
In I.L. Boyd, K. Camphuysen and S. Wanless (eds), Top predators in marine ecosystems: their role in monitoring and management. Cambridge University Press, Cambridge. pp. 11-27.
abstract
Apex predators such as pinnipeds, cetaceans, seabirds and sharks, are constrained by the sizes of prey they can consume and thus typically feed within a narrow range of trophic levels. Having co-evolved with their prey, they have influenced the behaviors, physiologies, morphologies and life history strategies of the species they target. In contrast, humans can consume prey of any size from all trophic levels using methods that can rapidly deplete populations. On an ecological time scale, fisheries, like apex predators, can directly affect the abundance of other species by consuming, or out-competing them; or they can indirectly affect the abundance of non-targeted species by removing other predators. However, there is growing evidence that the effects of fisheries go well beyond those imposed by apex predators. Theory and recent observations confirm that the continued development and expansion of fisheries over the past half century has led to a decrease in the! size and life spans of targeted species, with reproduction of fish occurring at earlier ages and at smaller sizes. Also, high levels of fishing have altered the makeup of many ecosystems, depressing the average trophic level of heavily fished ecosystems and speeding up the rate of nutrient turnover within them. An inevitable consequence of fishing down the food web is increased ecosystem instability, unsustainable fisheries and an inability for the ecosystem to support healthy abundant populations of apex predators.
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2005
 
Food web models and data for studying fisheries and environmental impacts on Eastern Pacific ecosystems.
Guénette, S., and V. Christensen. 2005.
Fisheries Centre Research Reports Vol 13(1) pp. 237
abstract
The North Pacific is a hot-bed for understanding how marine populations are impacted by humans as well as by environmental conditions. The 'Thompson-Burkenroad debate' has been ongoing since the late-1940s: what drives the marked fluctuations in Pacific halibut that has been observed over the past century' Dr William Thompson, who started up the work of the International Pacific Halibut Commission, IPHC, argued that the changes in halibut abundance could be fully explained by changes in fishing pressure, i.e. that they were the result of successful management on the part of IPHC, while his adversary, Dr Martin Burkenroad questioned if the populations trends could be accounted for by fishing pressure on its own, or if wasn't rather a question of environmental factors impacting halibut recruitment. While Thompson and Burkenroad actually never debated the relative role of fisheries and the environment ' indeed it may well be that they would actually agree that one factor in itself would not suffice to give us the full explanation their debate has lived on, and both sides still have proponents arguing for one over the other. Examining the Pacific halibut trends now, nearly 60 years after the debate started, still yields inconclusive answers only. We cannot name the culprit. The debate has widened since Thompson and Burkenroad's days, and we regularly hear about regime shifts in connection with the North Pacific. A notable debate in this context has emerged, seeking explanations for why the Steller sea lions have declined to become threatened in major parts of their North Pacific distribution area, while increasing in others. A multitude of explanations have been brought forward, and considerable research has been aimed at understanding the importance of nutritional conditions, of predators and of prey, of competition with commercial fisheries, of parasites and diseases, of the Pacific Decadal Oscillation Index, and of the potential impact of incidental culls, to mention some. As for the halibut, no conclusive explanation has emerged. Asking then, if the non-emergence of a single clear explanation may be due to the Steller sea lion being impacted by a combination of factors the North Pacific Universities Marine Mammal Research Consortium and the North Pacific Marine Science Foundation initiated a project 'Ecosystem analysis of Steller sea lion dynamics and their prey' through NOAA funding. The project, which was the brain child of Andrew Trites (Director of the Marine Mammal Research Unit, Fisheries Centre), employs ecosystem modelling of North Pacific ecosystems (Southeast Alaska, the Central Gulf of Alaska and the Western Aleutian Islands) in an attempt to evaluate (quantify) the relative role the various factors may have played in determining population trends. The methodologies applied for the modelling along with some of the preliminary findings from the study are described in this report. Notably, the models indicate that no single factor by itself can explain the population trends of Steller sea lion, several have to be invoked. In parallel to the work centered on Steller sea lion, the UBC 'Sea Around Us' project (www.seaaroundus.org) through funding from the Pew Charitable Trusts initiated a series of workshops aimed at evaluating the relative role of fisheries and environmental factors for North Pacific ecosystems. Bringing together researchers from the Department of Fisheries and Oceans, Pacific Biological Station, Nanaimo; the NOAA Alaska Fisheries Science Center, Seattle; the University of Washington, School of Fisheries, Seattle; and the University of British Columbia, Fisheries Centre, Vancouver, to analyze a series of ecosystems ranging from the Bering Sea to the Northern California Current, and coordinate the methodologies. We present descriptions of some studies in this report, while most of the findings are published separately. The present report also includes a reconstruction of North Pacific whale catches for the 20th century, which served to estimate the whale population at different periods in Southeast Alaska and the Western Aleutians. Finally, in the interest of preparing future work related to evaluating nutritional aspects of North Pacific ecosystems we have included a compilation of the energy content of invertebrates, fish and mammals in the Gulf of Alaska.
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2000
 
Forage fish abundance and distribution at Forrester Island, Alaska.
Norcross, B.L., B.A. Holladay and F. Mueter. 2000.
Institute of Marine Science, University of Alaska, Fairbanks, Alaska. pp. 75 + appendices
abstract
This study examined the abundance and species composition of forage fishes near sea lion and seabird rookeries in Southeast Alaska, East Aleutians, Pribilofs, Central Aleutians, and West Aleutians (Figure 1). Bottom trawls, midwater trawls, surveys of large predatory fish stomach contents, and measurements of physical parameters were conducted at some or all of these five regions during summer 1997. Sixty-one bottom trawl tows from among all regions collected 4539 fishes of 62 taxa. There were significant differences among regional values of depth, % mud, bottom temperature, bottom salinity, towing speed, cumulative fish abundance, and size of fish. Differences were not detected among regional mean values of %gravel, %sand, and species diversity. Among the tows on sand substrate, there were significant regional differences in species diversity. Regional species abundance and species distribution relative to physical parameters are reported and contrasted; parameters important to distribution are identified. Fourteen midwater trawl tows from Southeast and the Pribilof Islands collected 23,345 fishes of 10 taxa. Salinity, number of taxa caught, and species diversity were all significantly greater in Southeast Alaska than in the Pribilofs. Regional differences were not detected between depths, temperatures, towing speed, or cumulative fish abundance. Regional values of species abundance are reported. The stomach contents of 126 Pacific halibut and Pacific cod captured in the Pribilofs and East, Central, and West Aleutians were examined. Frequency of occurrence and numerical composition of prey taxa are reported for each predator species and region. Fish species diversity, composition, abundance, and size differed between Southeast Alaska and western locations. There were higher species diversity, different species, and fewer individuals in Southeast Alaska than in the Aleutians and Pribilofs. There were also considerable differences between sampling sites in Southeast Alaska and the Aleutian and Pribilof Islands. Trawlable areas within Southeast Alaska were significantly deeper than in the other regions, and were all of sand substrate. Bottom temperatures at bottom trawl tow sites in Southeast Alaska and in the East Aleutian Islands were warmer than in the Pribilofs, Central Aleutians and West Aleutian Islands. Samples were collected from a different vessel in Southeast Alaska than in other regions, in part resulting in a higher tow speed in Southeast Alaska. While these physical differences between regions compound the regional differences detected in fish distribution and abundance, the differences in fish are real, just their magnitude is uncertain.
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1999
 
The decline of Steller sea lions (Eumetopias jubatus) and the development of commercial fisheries in the Gulf of Alaska and Aleutian Islands from 1950-1990.
Trites, A.W., J. Money, and P.A. Larkin. 1999.
Unpublished Report. Marine Mammal Research Unit, Fisheries Centre, University of British Columbia pp. 29
abstract
Biomass removed from the Gulf of Alaska and Aleutian Islands rose from 100,000 tons in the early 1950s to a high of 700,000 tons in 1985. Average landings through the 1980s were 550,000 tons. Major growth in domestic fisheries in the Gulf of Alaska and Aleutian Islands occurred after the declaration of 200-mile zones in 1976. The traditional fisheries for salmon, herring, halibut and shellfish were augmented by major groundfish fisheries in the late 1960s. Total numbers of vessels involved in each of the major fisheries also rose with time, from an average of 6,500 vessels in the 1950s and 1960s, to about 12,000 through the late 1970s and 1980s. Steller sea lions first began to decline in the eastern Aleutians in the late 1960s. Overall, however, the total population in the Gulf and Aleutians did not begin to decline until 1979 when it fell from a peak of 270,000 animals to 90,000 in 1990. A decline in the overall numbers of sea lions in Alaska has been coincidental with the growth in the numbers of vessels and the increase in catch. Human activities, such as the shooting and entangling of sea lions, undoubtedly contributed to the population decline, but there does not appear to be any direct link between the timing of different declines in different regions of Alaska and the amount of human activity (numbers of vessels and amount of fish caught). Positive correlations between catches of crabs and shrimp with numbers of sea lions, and negative correlations between sea lions and landings of halibut and gadoids may reflect changes in the structure of the ecosystem that underline the population declines, and may be independent of human activities. Finer scale analysis of seasonal and localized removals of fish may yet reveal a possible link between commercial fisheries and sea lion abundance. Similarly, consideration should be given to whether commercial removal of some fish species may have indirectly affected the quality and quantity of food sources by restructuring the complex interrelationships among! species in the marine ecosystem.
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Ecosystem Considerations and the limitations of ecosystem models in fisheries management: insights from the Bering Sea.
Trites, A.W., P.A. Livingston, M.C. Vasconcellos, S. Mackinson, A.M. Springer and D. Pauly. 1999.
In Ecosystem Approaches for Fisheries Management. Alaska Sea Grant College Program, Alaska. pp. 609-619.
abstract
Over the past 10 years there has been increasing criticism of management decisions that are based on singlespecies approaches and a call for the implementation of ecosystem approaches. The major criticism of singlespecies models is that they cannot predict changes in community structure. Unfortunately, our experience in modeling the Bering Sea shows that these same criticisms can also be leveled against ecosystem models. We employed trophic massbalance models (Ecopath and Ecosim) to examine some possible explanations for the changes that occurred in the Bering Sea between the 1950s and 1980s. We removed fish and mammalsfrom the modeled system and tracked how other components of the ecosystem responded. Our massbalance models indicate that neither whaling nor commercial fisheries were sufficient to explain the 400% increase in pollock biomass and other changes that may have occurred between the two time periods. The simulations further suggest that environmental factors, affecting recruitment or primary production, may be more important in determining the dynamics of the Bering Sea ecosystem than predator prey interactions alone. These findings illustrate that mass balance models that do not account for the impact of climate variability on yearclass strength cannot provide reliable estimates of trends in marine fish production. However, our models can show how predation and fishing can affect trophic interactions among species. As such, ecosystem models are a useful scientific tool to identify gaps in understanding and data needs, but are unlikely to ever replace singlespecies models. They may instead complement and provide parameters to singlespecies models. Ecosystem models such as ours are still in the early stages of development and will become increasingly more important as a management tool as they begin to incorporate spatial and oceanographic/climatic information.

keywords     PhD MMecosystem modelmodeling limitations Bering Sea fisheries management
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1997
 
Competition between fisheries and marine mammals for prey and primary production in the Pacific Ocean.
Trites, A.W., V. Christensen and D. Pauly. 1997.
Journal of Northwest Atlantic Fishery Science. 22:173-187.
abstract
The degree of competition between fisheries and marine mammals in the Pacific Ocean was estimated for 7 statistical areas defined by the Food and Agriculture Organization of the United Nations (FAO). Catch statistics compiled from FAO sources show that the amount of fish caught in the Pacific Ocean rose from 2 million tons in the late-1940s to over 50 million tons in the early-1990s. Recent stagnation and declines occurring in some areas of the Pacific suggest that Pacific fisheries cannot continue to expand as they had previously. Based on estimates of population size, total biomass and daily consumption rates, it was estimated that the 84 species of marine mammals inhabiting the Pacific Ocean con-sume about three times as much food as humans harvest. A large fraction (>60%) of the food caught by marine mammals consisted of deep sea squids and very small deep sea fishes not harvestable by humans, thus limiting the extent of direct competition between fisheries and marine mammals. Moreover, the most important consumers of commercially exploited fish are other predatory fish, not marine mammals. Although direct competition between fisheries and marine mammals for prey appears rather limited, there may be considerable indirect competition for primary production. The primary production required to sustain marine mammals in each of the 7 FAO areas varies within a narrow range, suggesting that the diversity and abundance of marine mam-mals may have slowly evolved to fully exploit their niche and maximize their use of avail-able primary production. This contrasts with the rapid expansion of fisheries and their relatively recent dependence on primary production, which may have led to what we call ‘ food web competition’.

keywords     competition, fisheries, food, feeding, marine mammals, Pacific Ocean, #3
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1995
 
An analysis of groundfish fishing activities near Steller sea lion rookeries in Alaska.
Sampson, D. 1995.
Oregon State University, Hatfield Marine Science Centre, Newport, Oregon, OR 97365. pp. 40
abstract
During the past few decades large commercial fisheries for groundfish developed in the Gulf of Alaska and Bering Sea. There has been speculation that these fishing operations may have reduced the available fish stocks and thereby contributed to the dramatic declines in the Alaskan populations of Steller sea lion (Eumetopias jubatus) that occurred during the same period. Previous studies that attempted to relate estimates of sea lion abundance with annual catches of walleye pollock (Theragra chalcogramma) produced inconclusive results. In this investigation principal component analysis was applied to data from 1979-90 on sea lion counts for 25 sea lion rookeries in the Gulf of Alaska and Aleutian Islands, and independently to fishery observer data from 1980-89 for the commercial groundfish fishing operations that occurred within a distance of about 37 kilometers of these rookeries. The component scores from the two data sets were then correlated to explore for similarities between the pattern of sea lion decline and the pattern of fishing operations.

There was an unusually large correlation between the second principal component for the adult sea lion declines and the second component for the winter pollock catches. Rookeries that suffered relatively large declines in sea lion counts early in the study period generally experienced large winter pollock catches, but rookeries that suffered declines late in the study period experienced either no winter pollock catches or ones that occurred late in the study period. There were no strong correlations between the components for the adult sea lion declines and any other fishery components (quarterly fishing effort and total catches of groundfish, catches of Pacific cod, Gadus macrocephalus, and of Atka mackerel, Pleurogrammus monopterygius). Also, there were no strong correlations between the components for the sea lion pups and any fishery components.


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1992
 
The status of Steller sea lion populations and the development of fisheries in the Gulf of Alaska and Aleutian Islands.
Trites, A.W. and P.A. Larkin. 1992.
In A report of the Pacific States Marine Fisheries Commission pursuant to National Oceanic and Atmospheric award no. NA17FD0177. University of British Columbia, Fisheries Centre, 2204 Main Mall, Vancouver, B.C. V6T 1Z4. pp. 134
abstract
The goal of our study was to assess the status of Steller sea lions in Alaska, review their population biology, and develop a simulation model to explore the role that harvesting and incidental kills by fisheries may have played in the sea lion decline. We also attempted to relate the population declines to the amount of fish caught in the Gulf of Alaska and Aleutian Islands, and to the number of vessels fishing from 1950 to 1990. Using life tables to estimate population size, the numbers of Steller sea lions were estimated for all rookeries for which information was available in each of six areas in the Gulf of Alaska and Aleutian Islands. The total population appears to have risen from 150,000 to 210,000 from the mid 1950s to 1967. The population was then stable for roughly ten years, then increased to 225,000 by 1979. Since then it has decreased to about 85,000. Most of the decline took place in Area 3 (Kodiak region) but there were also significant declines in Areas 4 to 6 (westward of Kodiak). Increases have occurred in the smaller populations of Areas 1 and 2 (southeast Alaska and Prince William Sound). A major growth in domestic fisheries occurred after the declaration of 200 mile zones. The traditional fisheries for salmon, herring and halibut were augmented by major groundfish fisheries. The decline in the numbers of Steller sea lions has been coincidental with the growth in the numbers and size of vessels and the increase in catch. The stabilization in the numbers of Steller sea lions in the Gulf of Alaska from 1956 to 1980 can be attributed to the direct effect of incidental capture in fishing gear, the shooting of sea lions and the harvesting of adults and pups. However, these factors explain but a small portion of the recent population decline, from 1980 to the present. Some sea lions are missing in the arithmetic of population dynamics which cannot be accounted for by movements of animals from one area to another. Whether these losses are caused by the removal of food resources is a circumstantial possibility, but evidence of local abundance of food resources at particular times of the year for particular segments of the population is needed to build a convincing case. Other causes, such as diseases and parasites must also be kept in mind as possible contributing factors. Research on Steller sea lions should focus on the decline in abundance since 1980, changes in body size, the diet at various seasons of the year, bioenergetics and nutritional requirements, and assessment of local abundance of various food items. Long term research on the ecosystem dynamics of the region will be necessary for long term management of all living resources but how best to focus that research is a matter of current scientific debate that will not be resolved quickly.
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OCEAN CLIMATE CHANGE

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OTHER FACTORS


2012
 
PBDE flame retardants and PCBs in migrating Steller sea lions (Eumetopias jubatus) in the Strait of Georgia, British Columbia, Canada.
Alava, J. J., D. Lambourn, P. Olesiuk, M. Lance, S. J. Jeffries, F. A. P. C. Gobas and P. S Ross. 2012.
Chemosphere 88:855-864.
abstract
Polybrominated diphenyl ethers (PBDEs) and polychlorinated biphenyls (PCBs) were measured in blubber biopsy samples from 22 live-captured Steller sea lions (Eumetopias jubatus) that had just entered the Strait of Georgia, British Columbia, Canada, for their overwintering feeding season. PPBDE ranged from 50 lg kg1 (lipid weight) in adult females to 3780 lg kg1 in subadult individuals. PPCBs ranged from 272 lg kg1 in adult females to 14280 lg kg1 in subadult individuals. While most PBDE and PCB congeners were transferred through milk to pups, PCBs with logKOW > 7.0 (PCBs 206, 207, 208 and 209) appeared constrained, resulting in a lighter mixture in pups compared to adult females. The ratio of individual PCB congeners by metabolic group (Groups I, II, III, IV and V) to PCB-153 regressed against length of males suggested poor biotransformation of these compounds (slopes did not differ from zero, p > 0.05). PBDE congeners 49, 99, 153 and 183 appeared bioaccumulative (slopes of ratio BDE/PCB 153 versus length were higher than zero, p < 0.05), but the dominance of the single congener, BDE-47 (64% of total PBDEs), likely due in part to debromination pathways, reduced our ability to explore congenerspecific dynamics of PBDEs in these pinnipeds. With 80% of our Steller sea lions exceeding a recent toxicity reference value for PCBs, the fasting-associated mobilization of these contaminants raises concerns about a heightened vulnerability to adverse effects during annual migrations.
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2006
 
A review of the potential effects of disturbance on sea lions: assessing response and recovery.
Kucey, L., and A.W. Trites. 2006.
In A.W. Trites, S. Atkinson, D.P. DeMaster, L.W. Fritz, T.S. Gelatt, L.D. Rea and K. Wynne (eds), Sea Lions of the World. Alaska Sea Grant College Program, University of Alaska, Fairbanks. pp. 581-589.
abstract
Human intrusion within areas of sea lion habitat is increasing worldwide, leading to concerns about disruption of distribution and daily activities of sea lions. Sea lion responses to disturbance can be quantified by recording changes in behavioural patterns, documenting numbers of animals on shore before, during and after the disturbance, or by measuring physiological stress of individual animals. However, assessing recovery is not so straightforward, as highlighted by an example from a study of the short-term effects of disturbance on Steller sea lions. Recovery is generally recognized as a return to an original state or normal condition, but is often operationally defined as a percent-return to pre-disturbance numbers or behaviours. Simple interpretation of disturbance effects can be easily confounded by concurrent natural seasonal changes in behaviours or haulout patterns, or by daily variability in numbers that can be attributed to weather, tidal cycle stage and other factors. Overall, a range of recovery criteria needs to be simultaneously applied when assessing the effects of human disturbance on sea lion populations. Insights gained from research on the effects of disturbance on Steller sea lions may help guide the development of studies undertaken on other species of sea lions.
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2005
 
Infectious disease and the decline of Steller sea lions (Eumetopias jubatus) in Alaska: insights from serology data.
Burek, K.A., F.M.D. Gulland, G. Sheffield, K.B. Beckman, E. Keyes, T.R. Spraker, A.W. Smith, D.E. Skilling, J.E. Evermann, J.L. Stott, J.T. Saliki and A.W. Trites. 2005.
Journal of Wildlife Diseases 41(3):512-524.
abstract
Serology data were examined to determine whether infectious disease may have played a role in the decline of Steller sea lions (Eumetopias jubatus) in the Gulf of Alaska and Aleutian Islands. Available published data, historical unpublished data, and recent collections (1997-2000) were compared and reviewed. Data was stratified by geography in order to compare the declining western Alaska population in the Aleutian Islands regions through eastern Prince William Sound to the increasing population in Southeast Alaska. Prevalences of antibodies from the 1970s to early 1990s were noted for Leptospira interrogans, Chlamydophila psittaci, Brucella spp., phocid herpesvirus 1, and canine parvovirus. Serum samples collected and analyzed from 1997?2000 were tested for antibodies to these agents as well as to caliciviruses, marine mammal morbilliviruses, and canine adenoviruses 1 and 2. Conclusions could not be drawn about changes in the prevalence of exposure to disease agents during the decline of Steller sea lions because data were not comparable either because of inconsistencies in test techniques, or because the samples were either not collected in all decades from all regions or were not tested for antibodies to the same disease agents in different decades. Despite these shortcomings, the available data contained no convincing evidence of significant exposure of Steller sea lions to morbilliviruses, B. spp., canine parvovirus or L. interrogans. Steller sea lions have been exposed to a phocid herpesvirus, caliciviruses, canine adenovirus, and C. psittaci or to cross reactive organisms in regions of both increasing and decreasing sea lion abundance. These disease agents are not likely to have been the primary cause of the decline because they are found at comparable levels in both the increasing and the decreasing populations. However they may have contributed to the decline or impeded recovery of the Steller sea lion population due to undetected mortality and morbidity, or reduction of fecundity and body condition in animals under other stresses. Systematic monitoring for disease agents and their effects is needed to determine whether infectious disease is currently playing a role in the decline and lack of recovery of Steller sea lions.
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Human disturbance and the haulout behaviour of Steller sea lions (Eumetopias jubatus).
Kucey, L. 2005.
Graduate Thesis, University of British Columbia, Vancouver. 75 pages
abstract
There is considerable interest in assessing and mitigating disruptive effects of humans on the behaviour of marine mammals, especially for species with uncertain or decreasing population trends. Steller sea lions (Eumetopias jubatus) have been under intensive study throughout their range over the past few decades in an attempt to identify the causes of a large population decline in the Gulf of Alaska and Aleutian Islands. Consequently, disturbance due to scientific research has also increased at rookeries and haulouts. The purpose of my study was to determine if there were measurable short-term effects of human disturbance on the numbers of Steller sea lions using terrestrial sites. Numbers and composition of sea lions were documented for 2 – 3 week periods in southeast Alaska and British Columbia during summer (n = 8 sites) and winter / spring (n = 6 sites). They revealed considerable daily variation in numbers of sea lions hauled out within and among study sites that was related in part to prevailing environmental conditions. However, counts could not be corrected to account for environmental influences on the total numbers of sea lions using haulouts. Hauling out trends were examined for pre- and post-disturbance periods across multiple sites over two seasons. Predetermined research disturbances occurred to collect scats at the haulouts, and to brand pups at the rookery. Three methods were explored to assess local population recovery that addressed both quantitative and temporal aspects of sea lions returning to the study locations. Disturbances resulted in significantly fewer sea lions using haulouts during the post-disturbance period. Variation in the numbers of animals using the haulouts increased following the disturbance, but rates of change in daily numbers did not differ significantly between periods. Six of ten disturbed sites reached full recovery (100% of the pre-disturbance mean) on average 4.3 days after the research disturbance. To determine if individual behaviour was affected by disturbance, sea lions arriving on shore were followed to determine normal patterns of interactions and behaviour. Significant differences were noted in hauling out behaviour between animals that remained on land and those that returned to the water. Sea lions that returned to the water exhibited higher rates of behaviour and interactions with other animals during the week that followed the disturbance. Seasonal differences were also noted in the rates of behaviour and interactions that may be indicative of certain times of the year when sea lions are more sensitive to human presence and disturbance. Increasing levels of human–sea lion contact are expected as more and more people visit the remote coastal habitat of Steller sea lions. Future studies are needed to assess the influence of disturbance on sea lion redistribution within a critical recovery period, as well as to determine the physiological effects that sea lions experience with repeated human disturbance. Disturbance studies are an important aspect of conservation initiatives because they can help guide policies and establish restrictions to protect wild populations from human intrusion.
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2003
 
Disease agents in Steller sea lions in Alaska: A review and analysis of serology data from 1975-2000.
Burek, K.A., F.M.D. Gulland, G. Sheffield, D. Calkins, E. Keyes, T.R. Spraker, A.W. Smith, D.E. Skilling, J. Evermann, J.L. Stott and A.W. Trites. 2003.
Fisheries Centre Reports Vol 11(4) pp. 26
abstract
Results of serology studies conducted from 1975-1996 on Alaskan populations of Steller sea lions(Eumetopias jubatus) were synthesized and supplemented with analyses of archived sera to assess the chronological and spatial patterns of exposure to disease agents and the role that infectious disease may have played in the decline of Steller sea lions in the Gulf of Alaska and Aleutian Islands. Serum samples were obtained during three periods (1970s, 1980s and 1990s) and were tested for exposure to Leptospira interrogans, caliciviruses, Chlamydophila sittaci, Brucella sp, morbilliviruses, influenza A, oxoplasma gondii, phocid herpesviruses and canine parvovirus. Testing for these agents and canine adenoviruses 1 and 2 continued through 2000. In most cases, conclusions cannot be drawn about chronological changes in the prevalence of disease agents during the decline of Steller sea lions because the samples were not collected from all regions in each time period, nor from sufficient numbers of animals in each age class. In addition, samples were not all analyzed by the same laboratories, were not stored under controlled conditions, were not tested for the same disease agents, and assays were not validated for Steller sea lions.

There is no convincing evidence of significant exposure to influenza A, morbilliviruses, Brucella abortus, canine parvovirus and Leptospira sp. However, there is evidence of exposure to a herpesvirus, C. psittaci, caliciviruses, T. gondii and canine adenovirus in regions of both increasing and decreasing sea lion abundance. As these agents are either present throughout the areas examined, or were not evident in all of the animals examined, it is unlikely that these disease agents caused the population decline of sea lions by epidemic mortality. However, as the number of samples tested for morbillivirus is low, and the assays used have not been validated for Steller sea lions, exposure to a morbillivirus during the peak of the decline cannot be completely ruled out from the data available.

Some pathogens become endemic and interact with malnutrition or predation to decrease survival or reproduction—therefore preventing recovery of depleted populations. In other species, C. psittaci, herpesviruses, adenoviruses, and T. gondii are more readily expressed as clinical diseases when individuals are stressed. It is possible that these agents could be contributing to the lack of recovery by causing undetected mortality and morbidity, or by reducing fecundity and juvenile survival rates. A systematic disease agent monitoring protocol should therefore be initiated to adequately test for disease agents in different time periods and regions.

Serological studies are limited in that they only assess immunological response following exposure to infectious agents. They do not give information on the prevalence of disease agents, or on presence of clinical disease. Further sstudies should be aimed at detecting infectious agents directly, and determining their association with morbidity and mortality, as well as changes in host population dynamics.

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