There are two ways the catch statistics might be interpreted. One is that they reflect a depletion of prey available to Steller sea lions. The other is that they are an index of the relative abundance of prey available to Stellers. High catches of pollock, for example, might mean that pollock is extremely abundant or it could be interpreted to mean that Steller sea lions are being out-competed. The key to sorting out these interpretations is to understand what sea lions eat, and what proportion of the various stocks are caught each year.

There is evidence that major changes have occurred over the past two decades in the relative and total abundance of various species of fish in the Gulf of Alaska and the Bering Sea. These changes may reflect long term periodic oceanographic changes and perhaps long term climatic change. Species in relatively high abundance during the 1950s included shrimp, crab and small pelagic fishes. Those in highest abundance in the 1980s included gadids (mostly pollock) and large flatfish (mostly halibut and arrowtooth flounders). Both gadids and flatfish share considerable dietary overlap with marine mammals and may be significant competitors. Furthermore, pollock is the dominant prey being consumed in the areas of sharpest population decline, and are generally poor in energy or nutritional content. They have about half the energy content of herring, and have less usable energy due to various costs of digestion.

NEXT: Human Disturbance and the decline of Steller sea lions

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.
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.
Sizes of walleye pollock (Theragra chalcogramma) consumed by the eastern stock of Steller sea lions (Eumetopias jubatus) in Southeast Alaska from 1994-1999. Tollit, D.J., Heaslip, S.G. and Trites, A.W. 2004. Fishery Bulletin 102(3):522-532. abstract Lengths of walleye pollock (Theragra chalcogramma) consumed by Steller sea lions (Eumetopias jubatus) were estimated using allometric regressions applied to seven diagnostic cranial structures recovered from 531 scats collected in Southeast Alaska between 1994-1999. Selected structural measurements were corrected for loss of size due to erosion using experimentally derived condition-specific digestion correction factors. Correcting for digestion increased the estimated length of fish consumed by 23%, and the average mass of fish consumed by 88%. Mean corrected fork length (FL) of pollock consumed was 42.4 11.6 cm (range=10.0-78.1 cm, n=909). Adult pollock (>45.0 cm FL) occurred more frequently in scats collected from rookeries along the open ocean coastline of Southeast Alaska during June and July (74% adults, mean FL=48.4 cm) than they did in scats from haulouts located in inside waters between October and May (51% adults, mean FL=38.4 cm). Overall, the contribution of juvenile pollock (20 cm) to the sea lion diet was insignificant, while adults contributed 44% to the diet by number and 74% by mass. On average, larger pollock were eaten in summer at rookeries throughout Southeast Alaska than at rookeries in the Gulf of Alaska or the Bering Sea. Overall it appears that Steller sea lions are capable of consuming a wide size range of pollock, with the bulk of fish falling between 20-60 cm. The use of cranial hard parts other than otoliths and the application of digestion correction factors are fundamental to correctly estimating the sizes of prey consumed by sea lions and for determining their overlap with commercial fisheries.
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.
Sizes of walleye pollock and Atka mackerel consumed by the Western stock of Steller sea lions (Eumetopias jubatus) in Alaska from 1998-2000. Zeppelin, T. K., Tollit, D.J., Call, K.A., Orchard, T. J. and Gudmundson, C. J. 2004. Fishery Bulletin 102(3):509-521. abstract Prey size selectivity by Steller sea lions (Eumetopias jubatus) is relevant for understanding the foraging ecology of this declining predator, but studies have been problematic due to the erosion or absence of prey skeletal structures and otoliths usually used to estimate fish length. We developed regression formulae to estimate fish length from seven diagnostic cranial structures of walleye pollock (Theragra chalcogramma) and Atka mackerel (Pleurogrammus monopterygius). For both species, all structure measurements were related with fork length of prey (r squared range: 0.78 - 0.99). Fork length of walleye pollock and Atka mackerel consumed by Steller sea lions was estimated by applying these regression models to cranial structures recovered from scats (feces) collected between 1998 and 2000 across the range of the Alaskan western stock of Steller sea lions. Experimentally derived digestion correction factors were applied to take into account loss of size due to digestion. Fork lengths (FL) of walleye pollock consumed by Steller sea lions ranged from 3.7 to 70.8 cm FL (mean = 1 39.3 cm, SD = 14.3 cm, n = 1 666) and Atka mackerel ranged from 15.3 to 49.6 cm FL (mean = 1 32.3 cm, SD = 5.9 cm, n = 1,685). Although sample sizes were limited, a greater proportion of juvenile (less than to 20 cm) walleye pollock were found in samples collected on summer (June - September) haul-out sites (64% juveniles, n = 1 11 scats) than on summer rookeries (9% juveniles, n = 1 132 scats) or winter (February - March) haul-out sites (3% juveniles, n = 1 69 scats). Annual changes in the size of Atka mackerel consumed by Steller sea lions corresponded to changes in the length distribution of Atka mackerel resulting from exceptionally strong year classes. Considerable overlap (> 51%) in the size composition of walleye pollock and Atka mackerel taken by Steller sea lions and the commercial trawl fishery was demonstrated.
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.
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.
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.
The role of pinnipeds in the ecosystem. Trites, A.W. 1997. In G. Stone, J. Goebel and S. Webster (eds), Symposium of the 127th Annual Meeting of the American Fisheries Society. New England Aquarium, Conservation Department, Boston. pp. 31-38. abstract The proximate role played by seals and sea lions is obvious: they are predators and consumers of fish and invertebrates. Less intuitive is their ultimate role (dynamic and structural) within the ecosystem. The limited information available suggests that some pinnipeds perform a dynamic role by transferring nutrients and energy, or by regulating the abundance of other species. Others may play a structural role by influencing the physical complexity of their environment; or they may synthesize the marine environment and serve as indicators of ecosystem change. Field observations suggest the ultimate role that pinnipeds fill is species specific and a function of the type of habitat and ecosystem they occupy. Their functional and structural roles appear to be most evident in simple short-chained food webs, and are least obvious and tractable in complex long-chained food webs due perhaps to high variability in the recruitment of fish or nonlinear interactions and responses of predators and prey. The impact of historic removals of whales, sea otters and seals are consistent with these observations. Many of these removals produced unexpected changes in other components of the ecosystem. Better insights into the role that pinnipeds play and the effect of removing them will come as better data on diets and predator-prey functional responses are included in ecosystem models. keywords     pinnipeds, ecosystems, predators, interactions, models, #4
Concepts and issues in marine ecosystem management. Larkin, P.A. 1996. Reviews in Fish Biology and Fisheries 6:139-164. abstract Ecosystem management means different things to different people, but the underlying concept is similar to that of the long-standing ethic of conservation. Current interest in marine ecosystem management stems from concerns about overexploitation of world fisheries and the perceived need for broader perspectives in fisheries management. A central scientific question is whether the effects of harvesting (top down) or changes in the physical environment (bottom up) are responsible for major changes in abundance. Historically, ecology, fisheries biology, oceanography, fisheries management and the fishing industry have gone somewhat separate ways. Since the 198Os, increasing attention has been given to multispecies aspects of fisheries, the linkages between oceanography and fish abundance and more holistic approaches to fisheries management. Sorting out the causes and effects of fluctuations in fish abundance is complicated by the lack of reliability of fisheries statistics. Discards, dishonesty and the inherent logistic difficulties of collecting statistics all combine to confuse interpretation. The overcapacity of fishing fleets and their unrestricted use are widely recognized as a contributing cause to overfishing and declines in fish stocks in many parts of the world. Ecosystem management, as shorthand for more holistic approaches to resource management, is, from a fisheries management perspective, centred on multispecies interactions in the context of a variable physical and chemical environment. Broader perspectives include social, economic and political elements which are best considered pragmaticaily as a part of the context of fisheries management. Objectives in marine ecosystem management are varied. From a biological perspective, an underlying principle of management is commonly assumed to be a sustained yield of products for human consumption. Whether that should be taken to mean that the yield . should always be of the same products is less certain. Fishing commonly changes the relative abundance of species of fishes. Thus, a biological objective should specify the species mix that is desired. Concern for the maintenance of global diversity has generated a substantial literature on threatened and endangered species. In general, it has not been considered likely that marine fish species could be rendered extinct and greatest attention has been given to marine mammals, sea birds and sea turtles. The provision of marine parks and sanctuary areas are obvious first steps in providing a measure of protection, at least for the less widely ranging species. Related to the current concepts of ecosystem management are expressions such as ecosystem health and ecosystem integrity which are given a wide range of different meanings, none of which are readily translated into operationa language for resource management. These and similar expressions are best assessed as rhetorical devices. The essential components of ecosystem management are sustainable yield, maintenance of biodiversity and protection from the effects of pollution and habitat degradation. Theory for marine ecosystem management has a long history in fisheries and ecological literature. Ecological models such as Lotka-Volterra equations, ECOPATH, trophic cascades and chaos theory do not give practical guidance for managcmcnt. Fleet interaction and multispecies virtual population analysis models hold more promise for fisheries managers. Alaska provides particular opportunities for developing new concepts in fisheries management. Statistics of catch are good, stock assessments are at the state-of-the-art level and management has been prudent. Debate is active on the causes of substantial changes in abundance of many species including marine mammals, because substantial changes in the fisheries have been accompanied by major changes in oceanographic conditions. As elsewhere, the resultant changes may be a consequence of top-down and bottom-up effects. The bottom part is beyond human control, and ecosystem management is centred on managing the top-down or fisheries component in the context of special measures of protection for particuiar species. Whether that is a realistic goal depends in part on how much special protection is to be afforded to which species. Marine mammals, for example, are given high priority for special protection, but like fisheries they too may have significant roles in shaping the structure of marine ecosystems. Eventually, ecosystem management must come to grips with the question of how much protection of particular species is desirable in achieving optimal use of living marine resources.
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.
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.