Decline of the Steller Sea Lion in Alaskan Waters Untangling Food Webs and Fishing Nets (2003) / Chapter Skim
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6. Steller Sea Lion Decline: Environmental Context and Compendium of Evidence
6. Steller Sea Lion Decline: Environmental Context and Compendium of Evidence
Pages 112-151
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From page 112... ...
This chapter describes four concepts that provide a context for analyzing the role of food web interactions in Steller sea lion population dynamics and then applies these concepts to evaluate the many hypotheses proposed to explain the Steller sea lion population decline.
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From page 113... ...
Although hypotheses based on direct effects could be sufficient to explain the decline of Steller sea lion populations, indirect effects may influence the pattern of decline and thereby complicate analysis of direct impacts (see Box 6.1~. How the number of intermediate species affects the strength of the interaction is generally unknown, but the number of potential indirect linkages is far greater than the number of direct linkages.
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From page 115... ...
Hence, even if the causes of the Steller sea lion decline are identified and addressed, the western sea lion population still may fail to reach its former abundance. MULTIPLE WORKING HYPOTHESES At least eight hypotheses have been proposed to explain the rapid decline of the western stock of Steller sea lions.
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From page 116... ...
This deficiency could be manifested as depletion of prey, reduced abundance of preferred prey species, or reduced accessibility to prey due to local depletion or disturbance of fish stocks. Nutritional limitation caused by either a climate regime shift and/or a fisheries effect requires that either the quantity or quality of food is insufficient for the recovery or maintenance of the Steller sea lion population.
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From page 117... ...
Although the cause of the sea lion decline likely falls within this breadth of hypotheses, more than one of the listed hypotheses may have contributed to the decline, additively, interactively, or in various degrees of relative importance in different places or at different times. Hypothesis Mechanism of Population Limitation Starvation and/or reproductive failure because of nutritional limitation Forcing Direction Acronym 1.
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From page 118... ...
These two mechanisms, climate regime shifts and fishery removals, may have had a combined effect that limited the availability of common Steller sea lion prey items during the earlier phases of the decline. Climate Regime Shift The regime shift hypothesis links climate-forced environmental changes to changes in the welfare of Steller sea lions through indirect trophic interactions in the marine food web.
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From page 119... ...
state similar to what existed before the steep decline in Steller sea lion populations (Hare and Mantua, 2000; Schwing and Moore, 2000; Peterson and Mackas,2001~. If PDO regime shifts exert a significant and reversible forcing on sea lion abundance, the western population should begin to recover in response to the 1998-2002 climate trends.
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From page 120... ...
In the 2000 Biological Opinion (BiOp #3) , the National Marine Fisheries Service attempted to determine whether there was sufficient groundfish prey for Steller sea lions by calculating the amount of food consumed by sea lions relative to the biomass of the groundfish in the Gulf of Alaska, Aleutian Islands, and Bering Sea in 1999.
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From page 121... ...
. Given these caveats, this analysis does not provide support for the hypothesis that the recent decline in the Steller sea lion population is due to depletion of sea lion prey by the groundfish fisheries.
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From page 122... ...
I T I 80 85 90 95 1 o 00 Year in_ 40 s b. 1 - 60 · -50 Cod · A\ 30 ° SSLs ~ ~ - 20 ~ - ~ 10 1 1 1 1 1 1 1 1 1 - O r r T T T 1 1 r r 55 60 65 70 75 80 85 90 95 00 Year FIGURE 6.1 Trends of Steller sea lion (SSL)
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From page 123... ...
ENVIRONMENTAL CONTEXT AND COMPENDIUM OF EVIDENCE Bering Sea/Aleutian Islands (BS/AI)
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From page 124... ...
per Steller sea lion (SSL) index count for (a)
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From page 125... ...
Data derived from fish stock assessments and commercial fisheries indicate that the overall levels of fish biomass in the federal fishery management areas are large relative to the current population of Steller sea lions. However, additional research on seasonal fish movements and migrations, effects of fishing on fish school size and distribution, and sea lion foraging ecology are needed to more fully resolve the issue of localized depletion.
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From page 126... ...
. Although all of these values can be modeled or estimated based on body size or other mammal species, unless they are actually measured on Steller sea lions, it will be not be possible to know if Steller sea lions balance the overall equation of demand and supply similarly to other species.
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From page 127... ...
and shifts in dietary diversity correlate with Steller sea lion population trends. However, this correlation may be coincidental rather than an indication of a causal link between diet and population decline.
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From page 128... ...
Predation on Steller Sea Lions (Killer Whales and Sharks) Ecology has been slow to consider the role of predation in population regulation and food web dynamics, despite early examples (Elton, 1927; Brooks and Dodson, 1965; Paine, 1966)
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From page 129... ...
Finally, because large sharks are rarely if ever observed, it is doubtful that they could occur in sufficient abundance to account for declining Steller sea lion populations in the Aleutian Islands and southeastern Bering Sea. The third candidate predator, the killer whale, attacks and consumes a diverse array of marine mammals and fish, including many species of large and small cetaceans, pinnipeds, and sea otters (Matkin et al., 1999~.
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From page 130... ...
Hence, the role of killer whale predation in the historical decline is indeterminate, but further research on killer whales should indicate whether predation is preventing the recovery of the remaining sea lion population. Subsistence Harvests Alaska natives have hunted Steller sea lions since prehistoric times.
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From page 131... ...
. Reports by subsistence hunters indicate that Steller sea lions declined in abundance in the Aleutian Islands and Bering Sea during the 1800s.
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From page 132... ...
Shooting of Steller sea lions was legal prior to the Marine Mammal Protection Act of 1972. However, after passage of this act, fishermen were allowed to continue to shoot sea lions that were destroying their gear or causing a threat to human safety.
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From page 133... ...
Experimental Harvests of Steller Sea Lions Experimental harvests were conducted from 1959 until 1972 as a more cost-effective alternative to earlier bounty programs for controlling sea lion populations (Thorsteinson et al., 1961~. The harvested animals were to be used as food for humans, feed for fox farms in Alaska, and feed for mink ranches in the Pacific Northwest.
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From page 134... ...
Lack of systematic reporting of sea lion kills by fishermen makes it impossible to provide reliable estimates of the impact of shooting on the population (see Box 6.3~. Incidental Takes and Entanglement in Fishing Gear Incidental mortality of Steller sea lions in lost (ghost)
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From page 136... ...
In Bering Sea fisheries, most incidentally caught Steller sea lions were males, whereas in the Gulf of Alaska and probably in the Aleutian Islands most takes were females and subadult males (Loughlin and Nelson, 1986; Perez and Loughlin, 1991~. To estimate total incidental mortality, observed sea lion takes must be adjusted for sea lion survival rate, fraction of observer days spent recording sea lion takes, and fraction of the fleet with onboard observers.
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From page 137... ...
and 3 a.m. During the l990s, incidental takes of Steller sea lions were monitored for domestic groundfish trawl, pot, and longline fisheries conducted in the Gulf of Alaska, Aleutian Islands, and Bering Sea (Angliss et al., 2001~.
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From page 138... ...
,1 ~ 3 ~ ~ ~\ ~& ~/~N ~ ,< ~ /, -/ ~0 o~0 ~J; ~ / r\; ) l~ ~ G~ ~ (~_~' ,~J 1 984 (I ~;^ PA CIF/C OCEA N 158° W 157° 156° 155° 154° 153° 152° 151° 60° N 59o s80 57° 56° FIGURE 6.3 Generalized areas of incidental catch of Steller sea lions during the pollock joint-venture trawl fishery in Shelikof Strait in 1982-1984.
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From page 139... ...
sea lions by time of day (hour) compared to percent of Steller sea lions seen near processing vessels, as reported by observers in the joint-venture pollock fishery in Shelikof Strait in 1983-1984.
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From page 140... ...
If these estimates are representative of total mortality from all fisheries, then fishery-induced mortality explains only a minor portion of the decline of the western Steller sea lion population. However, extrapolations of total kill rates depend heavily on assumptions about unobserved foreign and domestic vessels and fisheries during these years.
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From page 141... ...
The importance of infectious disease epidemics in causing declines of marine mammal populations is unclear because few die-offs have been investigated sufficiently to determine their cause, and it is often difficult to accurately determine host population numbers. Recent epidemics in marine mammals have caused dramatic mortality, but the effects on host population numbers vary.
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From page 142... ...
To definitively eliminate the hypothesis that a viral epidemic caused the rapid decline of Steller sea lions would require more comprehensive tests of banked serum samples and molecular tests for the presence of disease agents in preserved tissue samples. Endemic diseases could inhibit recovery of the Steller sea lion population.
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From page 143... ...
, and antibodies to this organism are widespread in adult Steller sea lions, especially females (Burek et al., 2001~. Further studies should be directed at determining the prevalence of infection in Steller sea lion populations and the effect of infection on reproduction and survival.
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From page 144... ...
Levels of PCBs and DDTs were higher in Steller sea lions than in ringed and harp seals from Arctic waters but were comparable to levels in gray seals from the east coast of Canada and lower than in California sea lions with normal gestation periods. Both gray seals and California sea lion populations are currently increasing.
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From page 145... ...
Further epidemiological studies focusing on associations between contaminant levels in tissues of individuals and life history parameters, coupled with determination of the significance of reproductive failure and infectious disease in the dynamics of Steller sea lions are needed to determine whether contaminants could play a role in limiting sea lion recovery. WEIGHT OF EVIDENCE Steller sea lion behavior, physiology, life history, and environment can be analyzed with regard to how they would be expected to change under each of the eight hypotheses described above using a simple positive or negative response variable (Bowen et al., 2001~.
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From page 146... ...
For this purpose, characteristics of sea lions in the eastern population are assumed to be representative of the western population prior to the start of the decline. The rationales for the expected response and data sources for the observed response are described in Box 6.4 Table 6.2 lists relevant behavioral, physiological, and life history metrics of Steller sea lions, and features of the associated ecosystem for which data are available.
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From page 150... ...
The possibility remains that local depletion of some fish stocks, such as Atka mackerel, may have occurred in Steller sea lion habitat, but there is less support for local depletion of pollock stocks. Changes in the abundance of forage fish species related to the regime shift in the late 1970s may have affected sea lion fitness, but these effects do not appear sufficient to account for the large mortality of sea lions in the 1980s.
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From page 151... ...
Also, it is possible that predation by killer whales was higher than previously estimated. Therefore, even during the rapid decline of the western population, it is likely that a combination of topdown and bottom-up forcing mechanisms were responsible for the high mortality of Steller sea lions.
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