The Decline of the Steller Sea Lion in Alaskan Waters: Untangling Food Webs and Fishing Nets (2003)Ocean Studies Board
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The Alaskan population of Steller sea lions has declined more than 80 percent in the last 30 years. Large-scale fishing effects on the food supply of Alaska's Steller sea lions do not appear to be the main reason for their current, continuing decline. This report concluded that a combination of other possible factors, including entanglement or injury by fishing gear, illegal shooting, predation by killer whales, subsistence harvesting, and disease, may be causing the decline. Establishing a series of carefully monitored open and closed fishing areas is the best way to measure fishing's impact on sea lion survival.
- Option No. 5. Micromonitor and manage localized interactions between sea lions and fisheries to reduce mortality where and when it occurs in the future. This option would require expansion of all basic monitoring activities (abundance, prey fields, mortality agent distribution) around key rookeries to pinpoint times and places of increased mortality so that appropriate management measures could be taken.
- Although no hypothesis can be excluded based on existing data, top-down sources of mortality appear to pose the greatest threat to the current population. Investigations of top-down sources of Steller sea lion mortality should be increased to evaluate the proportionate impact of these factors on the population decline.
- Existing data on the more recent period of decline (1990-present) with regard to the bottom-up and top-down hypotheses indicate that bottom-up hypotheses invoking nutritional stress are unlikely to represent the primary threat to recovery.
- In the existing body of information on Steller sea lions, there is no conclusive evidence supporting either the bottom-up or the top-down hypotheses.
- Multiple factors probably contributed to the widespread population decline in the 1980s, including incidental and deliberate mortality associated with fishing activities, but elucidation of the complete spectrum of causes and consequences is unlikely because of gaps in the available data.
- Option No. 1. Wait and see, maintaining current closures indefinitely. Recent management actions, including area closures, may be sufficient to reverse or reduce the rate of population decline.
- Option No. 2. Eliminate direct fishery impacts with greatly expanded closures. This would require closing the Atka mackerel fishery in the Aleutians and reducing the main pollock fishing areas in the southern half of the eastern Bering Sea.
- Option No. 3. Establish spatial management units consisting of two sets of closed and open areas where each treatment area is centered on a rookery. The western population would be divided into management regions with at least two closed and two open rookeries per region. The closed treatment units would be subject to fishery closures, and the open units would have sea lion-related fishery restrictions removed.
- Option No. 4. Implement a titration experiment where restrictions on fisheries (such as area closures) are increased progressively over time until a positive response is achieved. This option is a variation on the strategy used during the 1990s. Fishery regulations continue to become more restrictive as long as the sea lion population continues to decline.
- Resolution of this conflict requires management that not only improves chances for the recovery of Steller sea lions but also facilitates scientific study of the efficacy of these protective measures.
- Steller sea lions are found along the North Pacific rim from California to Japan with about 70% of the population living in Alaskan waters.
- The Alaskan population declined precipitously during the 1970s and 1980s and continued to decline at a slower rate during the 1990s.
- The committee has identified five general management options that might be taken to address the potential impacts of groundfish fisheries on sea lions and recommends monitoring priorities to assess the efficacy of each option.
- The first category, the bottom-up hypotheses, includes potential causes that would affect the physical condition of sea lions such as (1) large-scale fishery removals that reduce the availability or quality of prey species; (2) a climate/regime shift in the late 1970s that changed the abundance or distribution of prey species; (3) nonlethal disease that reduced the foraging efficiency of sea lions, and (4) pollutants concentrated through the food web that contaminated fish eaten by sea lions, possibly reducing their fecundity or increasing mortality.
- The hypotheses proposed to explain the decline of the western stock fall into two categories.
- The second category, the top-down hypotheses, encompasses factors that kill sea lions independently of the capacity of the environment to support the sea lion population. These include (1) predators such as killer whales (or possibly sharks) that switched their prey preference to sea lions; (2) incidental takes of sea lions through capture or entanglement in fishing gear; (3) takes of sea lions in the subsistence harvest that were higher than estimated; (4) shootings of sea lions that were underestimated in the past and present, and (5) pollution or disease that increased mortality independently of effects on nutrition.
- To resolve questions about the impact of the fisheries on Steller sea lion survival, the preferred option is No 3 because it is the only approach that directly tests the role of fishing in the decline.
- Under option No. 1, the most valuable monitoring information would be derived from annual reference rookery and haulout ounts and new demographic data from branded pups.
- Under option No. 2, monitoring of fish population dynamics, both locally and at the stock level, would be required to determine the effects of the fisheries on stock distribution and fish community composition.
- Under option No. 3, the most critical monitoring needs would be detailed local Steller sea lion censuses and spatial analyses of fish population changes for each experimental unit in the overall design.
- Option No. 4. This approach requires monitoring of sea lion population trends, but results could be confounded by the lack of baseline data and natural environmental variability.
- to fully exclude fisheries as a contributing factor to the continuing decline.