Steller sea lion survival

Another hypothesis under consideration is that ecosystem change resulted primarily from variability in patterns of weather and climate, particularly as related to the Aleutian Low pressure system. Biological correlates of atmospheric regime shifts, known as the "Pacific Decadal Oscillation," continue to be discovered over a wide part of the North Pacific and Bering Sea, and evidence of several regime shifts have been detected in historical meteorological data. These include broad scale changes in primary and secondary ocean production, numbers of salmon and forage fishes, and the abundance of certain species of seabirds and marine mammals. A significant shift occurred in 1977 and another may have taken place in 1998. Researchers suspect the structure and functioning of the ecosystem, including abundance and distribution of both fish and marine mammals, may respond to changes in atmospheric forcing.

A team of researchers headed by Dr. Trites (UBC) constructed a mathematical model of the Eastern Bering Sea ecosystem. With financial support from the David and Lucille Packard Foundation, the model was used to test the hypothesized cascading effects on Steller sea lions of commercial whaling, over fishing, and climate change.

  

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A novel presence-only validation technique leads to improved habitat descriptions for a wide-ranging marine predator, the Steller sea lion (Eumetopias jubatus). Gregr, E.J. and A.W. Trites. (in press). Marine Ecology Progress Series abstract We used published information about foraging behaviour, terrestrial resting sites, bathymetry, and seasonal ocean climate to develop hypotheses relating life history traits and physical variables to the at-sea habitat of a wide-ranging marine predator, the Steller sea lion (Eumetopias jubatus). We used the hypotheses to develop a series of habitat models that predicted the probability of sea lions occurring within 3 x 3 km2 grids overlaid on the Gulf of Alaska and Bering Sea; and compared these deductive model predictions with opportunistic at-sea observations of sea lions (presence-only data) using 1) a likelihood approach in a small area where effort was assumed to be uniformly distributed, and 2) an adjusted skewness (Skadj) test that evaluated the distribution of the predicted values associated with true presence observations. We found the Skadj statistic was comparable to the likelihood test when using pseudo-absence data, but it was more powerful for assessing the relative performance of the different predictive spatial models. We also found that the habitat maps we produced for adult female sea lions using the deductive modelling approach captured a higher proportion of presence observations than the current habitat model (Critical Habitat) used by fisheries managers since 1993 to manage Steller sea lions. Such improved predictions of habitat are necessary to effectively design, implement, and evaluate fishery mitigation measures. The deductive approach we propose is suitable for modelling the habitat use of other age- and sex- classes, and for integrating these age/sex class specific models into a revised definition of Critical Habitat for Steller sea lions. It can also be readily used to identify the at-sea habitat of other central place foragers.
Evaluating network analysis indicators of ecosystem status in the Gulf of Alaska. Heymans, S.J.J., S. Guénette and V. Christensen. 2008. Ecosystems 10:488-502. abstract This is the first study on the emergent properties for empirical ecosystem models that have been validated by time series information. Ecosystem models of the western and central Aleutian Islands and Southeast Alaska were used to examine indices of ecosystem status generated from network analysis and incorporated into Ecopath with Ecosim. Dynamic simulations of the two ecosystems over the past 40 years were employed to examine if these indices reflect the dissimilar changes that occurred in the ecosystems. The results showed that the total systems throughput (TST) and ascendency (A) followed the climate change signature (Pacific decadal oscillation, PDO) in both ecosystems, while the redundancy (R) followed the inverse trend. The different trajectories for important species such as Steller sea lions (Eumetopias jubatus), Atka mackerel (Pleurogrammus monopterygius), pollock (Theragra chalcograma), herring (Clupea pallasii), Pacific cod (Gadus macrocephalus) and hali! but (Hippoglossus stenolepis) were noticeable in the Finn cycling index (FCI), entropy (H) and average mutual information (AMI): not showing large change during the time that the Stellers sea lions, herring, Pacific cod, halibut and arrowtooth flounder (Atheresthes stomias) increased in Southeast Alaska, but showing large declines during the decline of Steller sea lions, sharks, Atka mackerel and arrowtooth flounder in the Aleutians. On the whole, there was a change in the emergent properties of the Aleutians around 1976 that was not seen in Southeast Alaska. Conversely, the emergent properties of both systems showed a change around 1988, which indicated that both systems were unstable after 1988.
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. Guénette, 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.
Bottom-up forcing and the decline of Steller sea lions (Eumetopias jubatus) in Alaska: assessing the ocean climate hypothesis. Trites, A. W., A. J. Miller, H. D. G. Maschner, M. A. Alexander, S. J. Bograd, J. A. Calder, A. Capotondi, K. O. Coyle, E. D. Lorenzo, B. P. Finney, E. J. Gregr, C. E. Grosch, S. R. Hare, G. L. Hunt, J. Jahncke, N. B. Kachel, H.-J. Kim, C. Ladd, N. J. Mantua, C. Marzban, W. Maslowski, R. Mendelssohn, D. J. Neilson, S. R. Okkonen, J. E. Overland, K. L. Reedy-Maschner, T. C. Royer, F. B. Schwing, J. X. L. Wang and A. J. Winship. 2007. Fisheries Oceanography 16:46-67. abstract Declines of Steller sea lion (Eumetopias jubatus) populations in the Aleutian Islands and Gulf of Alaska could be a consequence of physical oceanographic changes associated with the 1976-77 climate regime shift. Changes in ocean climate are hypothesized to have affected the quantity, quality and accessibility of prey, which in turn may have affected the rates of birth and death of sea lions. Recent studies of the spatial and temporal variations in the ocean climate system of the North Pacific support this hypothesis. Ocean climate changes appear to have created adaptive opportunities for various species that are preyed upon by Steller sea lions at mid-trophic levels. The east-west asymmetry of the oceanic response to climate forcing after 1976-77 is consistent with both the temporal aspect (populations decreased after the late 1970's) and the spatial aspect of the decline (western, but not eastern, sea lion populations decreased). These broad-scale climate variations appear to be modulated by regionally sensitive biogeographic structures along the Aleutian Islands and Gulf of Alaska, which include a transition point from coastal to open-ocean conditions at Samalga Pass westward along the Aleutian Islands. These transition points delineate distinct clusterings of different combinations of prey species, which are in turn correlated with differential population sizes and trajectories of Steller sea lions. Archaeological records spanning 4000 years further indicate that sea lion populations have experienced major shifts in abundance in the past. Shifts in ocean climate are the most parsimonious underlying explanation for the broad suite of ecosystem changes that have been observed in the North Pacific Ocean in recent decades.
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.
Diets of Steller sea lions (Eumetopias jubatus) in Southeast Alaska from 1993-1999. Trites, A.W., D.G Calkins and A.J. Winship. 2007. Fishery Bulletin 105:234-248. abstract Diet of Steller sea lions (Eumetopias jubatus) was determined from 1494 scats (feces) collected at breeding (rookeries) and non-breeding (haulout) sites in Southeast Alaska from 1993 to 1999. The most common prey of 61 species identified were walleye pollock (Theragra chalcogramma), Pacific herring (Clupea pallasii), Pacific sand lance (Ammodytes hexapterus), Pacific salmon (Salmonidae), arrowtooth flounder (Atheresthes stomias), rockfish (Sebastes spp.), skates (Rajidae), and cephalopods (squid and octopus). Sea lion diets at the three Southeast Alaska rookeries differed significantly from one another. Steller sea lions consumed the most diverse range of prey categories during summer, and the least diverse during fall. Diet was more diverse in Southeast Alaska during the 1990s than in any other region of Alaska (Gulf of Alaska and Aleutian Islands). Dietary differences between increasing and declining populations of sea lions in Alaska correlate with rates of population change, and add credence to the view that diet may have played a role in the decline of sea lions in the Gulf of Alaska and Aleutian Islands.
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.
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.
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.
Food webs in the ocean: who eats whom, and how much? Trites, A.W. 2003. In M. Sinclair and G. Valdimarsson (eds), Responsible Fisheries in the Marine Ecosystem. FAO, Rome and CABI Publishing, Wallingford. pp. 125-143. abstract Over 100 food webs have been published for marine cosystems to describe the transfer of food energy from its source in plants,through herbivores,to carnivores and higher order predators.The webs suggest that the lengths of the chains that form food webs are typically short (3 –4 links),and that ecosystems with long food chains may be less stable than those with shorter food chains. Stomach contents have been the primary means for determining what marine organisms eat.More recently developed techniques include faecal analysis and fatty acid signatures from blood or fat samples. Consumption has been estimated from the volume of food found in stomachs,from the feeding rates of captive individuals and from bioenergetic modelling.Consumption of marine organisms,expressed as a percentage of an individual ’s body weight per day,ranges from about 4 –15% or zooplankton,to 1 –4% for cephalopods,1 –2%for fish,3 –5% or marine mammals and 15 –20%for sea birds.Immature age classes consume about twice as much (per unit of body weight)as do mature individuals. Furthermore,consumption is not constant throughout the year,but varies with seasonal periods of growth and reproduction.Most groups of species consume 3 –10 times more than they produce,and export or pass up the food web about 70 –95%of their production. Marine organisms tend to be larger at successive trophic levels and are limited in the sizes of food they can consume. Humans are one of the few species that can prey uponalmost any level of the food chain and any size of prey. Food web analysis and estimates of consumption are essential for understanding which ecosystems can support additional species,and which may be less stable and susceptible to species loss through the synergistic effects of fishing or culling.They are also critical tools for understanding changes in ecosystem dynamics as highlighted by a case study from the eastern Bering Sea.
Ecological effects of regime shifts in the Bering Sea and eastern North Pacific Ocean. Benson, A.J. and A.W. Trites. 2002. Fish and Fisheries 3:95-113. abstract Large-scale shifts occurred in climatic and oceanic conditions in 1925, 1947, 1977, 1989 and possibly 1998. These shifts affected the mix and abundance of suites of coexisting species during each period of relative environmental stability- from primary producers to apex predators. However, the 1989 regime shift was not a simple reversal of the 1977 shift. The regime shifts occurred abruptly and were neither random variations nor simple reversals to the previous conditions. Timing of these anomalous environmental events in the North Pacific Ocean appears to be linked to physical and biological responses in other oceanic regions of the world. Changes in the atmospheric pressure can alter wind patterns that affect oceanic circulation and physical properties such as salinity and depth of the thermocline. This, in turn, affects primary and secondary production. Data from the North Pacific indicate that regime shifts can have opposite effects on species living in different domains, or can affect similar species living within a single domain in opposite ways. Climatic forcing appears to indirectly affect fish and marine mammal populations through changes in the distribution and abundance of their predators and prey. Effects of regime shifts on marine ecosystems are also manifested faster at lower trophic levels. Natural variability in the productivity of fish stocks in association with regime shifts indicates that new approaches to managing fisheries should incorporate climatic as well as fisheries effects.
Predator-prey relationships. Trites, A.W. 2002. In W.F. Perrin, B. Wursig and H.G.M. Thewissen (eds), Encyclopedia of Marine Mammals. Academic Press, San Diego. pp. 994-997. 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 if 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 out-competing 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. keywords     predation
Marine mammal trophic levels and interactions. Trites, Andrew W. 2001. In J. Steele, S. Thorpe and K. Turekian (eds), Encyclopedia of Ocean Sciences. Academic Press, London, UK. pp. 1628-1633. abstract Calculating trophic levels is necessary first step to quantifying and understanding trophic interactions between marine mammals and other species in marine ecosystems. This can be achieved using dietary information collected from stomachs and scats, or by measuring isotopic ratios contained in marine mammal tissues. These data indicate that marine mammals occupy a wide range of trophic levels beginning with dugong and manatees (trophic level 2.0), and followed by baleen whales (3.35), sea otters (3.45), seals (3.95), sea lions and fur seals (4.03), toothed whales (4.23), and polar bears (4.08). With the aid of ecosystem models and other quantitative analyses, the degree of competition can be quantified, and the consequences of changing predator-prey numbers can be predicted. These analyses show that many species of fish are major competitors of marine mammals. A number of field studies have also shown negative effects of reduced prey abundance on body size and survival of marine mammals. However, there are fewer examples of marine mammal populations affecting their prey due perhaps to the difficulty of monitoring such interactions, or to the complexity of most marine mammal food webs. keywords     PhdTLmarine mammalsdietbackground
Summary, conclusions, and recommendations. Springer, A.M. 1999. In T. Loughlin and T. Ohtani (eds), The Bering Sea: physical, chemical, and biological dynamics. Sea Grant, University of Alaska Fairbanks. 43:777-779. abstract Much of the interest in dynamics in the Bering Sea,now and in the past, has been spurred by concerns over the stability and sustainability of its vast living resources.Particularly prominent today are depressed popula tions of several species of marine mammals,notably great whales,Steller sea lions,fur seals,harbor seals,and sea otters,and of additional species of considerable economic importance,such as king crabs,shrimp,and Pacific Ocean perch.The reason for the collapse of whales,shrimp,and Pacific Ocean perch is known —they were killed by commercial fisheries. The recent decline of sea otters in the Aleutian Islands is thought to have been caused by increased predation.The reasons for diminished popula tions of other species are not known,or at least not agreed upon,and have been the stuff of extensive,often rancorous debate.No less dramatic, however,have been spectacular increases of certain fishes,such as flat fishes,walleye pollock that grew in abundance by nearly an order of mag nitude between the 1960s and 1980s,and Pacific salmon that provided record harvests across the northeastern North Pacific for many years in the 1980s and 1990s before collapsing in some regions,notably the Ber ing Sea,in 1997 and 1998. This volume is the most recent in a growing series of publications devoted to the Bering Sea and is basically a report on certain advances that have been made in our understanding of the ecosystem —what it is and why it behaves as it does —primarily in regard to issues of interest to PICES.Thus,many of the papers presented here,as the general title indi cates,address dynamics —dynamics of physical processes like meteor ology and ocean circulation,and of ecosystem processes that are important to biomass yield at higher trophic levels in pelagic food webs.Others provide more descriptive information that will be useful in future dynam ic contexts.My brief summary touches on some of the highlights of the foregoing chapters,but each must be read to fully appreciate the state of knowledge revealed in their pages.
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.
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
Diet composition and trophic levels of marine mammals. Pauly, D., A.W. Trites, E. Capuli and V. Christensen. 1998. ICES Journal of Marine Science 55:467-481. abstract Standardized diet compositions were derived for 97 species of marine mammals from published accounts of stomach contents as well as from morphological, behavioural and other information. Diet was apportioned among eight categories of prey types (benthic invertebrates, large zooplankton, small squids, large squids, small pelagic fishes, mesopelagic fishes, miscellaneous fishes and higher invertebrates). Trophic levels were estimated for each species of marine mammals and compared with published estimates derived using stable isotope ratios. Trophic levels ranged from 3.2–3.4 in baleen whales and sea otters, to 3.8–4.4 in most pinnipeds and odontocete whales, to 4.5–4.6 in killer whales. Such information can be used for ecosystem modelling and related studies. keywords     marine mammals; diets; trophic levels; food organisms; stomach content; Cetacea; Balaenoptera; Odontocetes; Orcinus orca; Pinnipedia; Enhydra lutris; cetaceans; whales; Finback whales; Rorquals; Sea otter; Killer whale; Bering Sea species;
Is it all climate change? Why marine bird and mammal populations fluctuate in the North Pacific. Springer, A.M. 1998. In G. Holloway, P. Muller and D. Henderson (eds), Biotic impacts of extra tropical climate change in the Pacific. 'Aha Huliko'a proceedings. University of Hawaii. pp. 109-119. abstract Abstract The abundance and productivity of several species of marine birds and mammals have undergone extreme fluctuations during recent decades across a broad region of the North Pacific and western Arctic. The biological changes have had multiple temporal scales that correspond to oscillations in the strength and position of the Aleutian Low pressure system and in water temperature, and to longer-term trends in climate warming. Most variability at higher trophic levels apparently has been mediated by food web dynamics, although in at least one case the interaction between seabirds and the physical environment has been direct. Seabirds and marine mammals are sensitive indicators of change in the ocean environment and provide compelling evidence of fundamental ecosystem response to physical forcing.
A forage fish is what? Summary of the symposium. Springer, A.M. and S.G. Speckman. 1997. In Forage Fishes in marine ecosystems. Univ. of Alaska Sea Grant Program. Report 97-01:773-806. abstract The conference was organized around a number of themes that emerged as papers concerning one or more of the interrelated issues of forage fish basic biology, their role as predators and prey, causes of population fluctuations, assessment methodologies, and management considerations. The papers in this volume are grouped according to subject, but many of them contain information on a variety of aspects of forage fish biology and ecology that can only be discovered by examining them all.
Changes in the distribution and size of juvenile walleye pollock as indicated by seabird diets at the Pribilof Islands and by bottom trawl surveys in the eastern Bering Sea. Hunt Jr, G.L., A.S. Kitaysky and M.B. Decker. 1996. In R.D. Brodeur, P.A. Livingston and T.R.A.B.Hollowed Loughlin, Ecology of juvenile pollock. NOAA Tech. Rep. 126:125-139. abstract We tested whether the proportion of age-l walleye pollock, Theragra chalcogramma, in the diets of four species of seabirds-black-legged kittiwake, Rissa tridactyla; red-legged kittiwake, R brevirosttis; common murre, Uris aalge; and thick-billed murre, U. lomvia-decreased between the 1970’s and the 1980’s by examining otoliths present in food samples obtained from birds breeding on the Pribilof Islands. We examined the distribution of age-1 walleye pollock on the Bering Sea shelf over the same time period to determine if the changes in age classes of pollock taken by birds were reflected in data from National Marine Fisheries Service bottom trawl surveys. We examined the growth rates and sizes of age-0 pollock taken by the birds, and we sought evidence for mechanisms that might have influenced the distribution and abundance ofjuvenile pollock in the vicinity of the Pribilof Islands. We found that the proportion of age-l walleye pollock in seabird diets decreased significantly from the 1970’s to the 1980’s. Over the same period, age-l walleye pollock declined in trawl survey catches in the vicinity of the Pribilof Islands and also in the southern portion of the shelf edge. Although age-0 pollock taken near the end of August were longer in the 1970’s than in the 1980’s, growth rates of age-0 pollock in August were similar in the two periods. We found no significant correlation between the abundance of age-l walleye pollock near the Pribilof Islands and in the southern outer domain (strata 32, 42, and 50) and the extent of ice cover along the 17O”W meridian. Likewise, there were no significant correlations between the number of age-l pollock in strata 32,42, and 50 and the number of adult pollock present in these strata in the same or preceding year. We discuss additional evidence for both interannual and interdecadal changes in the marine environ-ment in the vicinity of the Pribilof Islands. Juvenile walleye pollock were an important component of the diets of breeding seabirds, but-contrary to our expectations-seabird reproductive success was not sensitive to the ratio of age-l to age-0 pollock in the seabirds’ diets.