Decline of the Steller Sea Lion in Alaskan Waters Untangling Food Webs and Fishing Nets (2003) / Chapter Skim
Currently Skimming:
5. Fisheries
5. Fisheries
Pages 77-111
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From page 77... ...
Groundfish fisheries are of particular interest with respect to Steller sea lions. These fisheries are the main focus of this chapter because groundfish species comprise a large fraction of the sea lion diet (see Chapter 4~.
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From page 78... ...
For Pacific cod, the largest fishery takes place in the Bering Sea, although significant fisheries occur in the Gulf of Alaska and Aleutian Islands as well. Given the diversity and complexity of commercial fisheries in the vast area covered by the Gulf of Alaska, Aleutian Islands, and Bering Sea, it is not possible to completely describe each fishery in this report.
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From page 79... ...
Finally, management measures aimed at reducing potential competition between fisheries and Steller sea lions are summarized. Fishery History with Reference to Steller Sea Lion Trends Alverson et al.
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From page 80... ...
This heavy exploitation occurred during a period when Steller sea lion populations were considered healthy and Pacific ocean perch rarely appeared in the sea lion diet. After a period of pulse fishing, Pacific ocean perch were overfished in the Gulf of Alaska, and Russian fleets moved south to exploit rockfish stocks off Oregon and Washington.
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From page 81... ...
Most groundfish fisheries off Alaska are managed by the National Marine Fisheries Service (NMFS) under federal fishery management plans adopted by the North Pacific Fishery Management Council (NPFMC)
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From page 82... ...
In the late l990s this fishery was increasingly constrained by regulations to avoid possible adverse effects on Steller sea lions. As a precautionary measure, the pollock fishery in the Aleutian Islands has been halted since the 1998 season.
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From page 83... ...
Declines of Steller sea lions appeared to begin in the eastern Aleutian Islands where nonpup index counts declined 61% during 1977-1985. Indeed, a significant pollock fishery first developed there during 1980-1985.
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From page 84... ...
In the mid-1980s to early 1990s, a large pulse fishery for pollock developed in the Donut Hole in the central Bering Sea and a much smaller fishery developed near Bogoslof Island (north of the eastern Aleutian Islands) beyond the eastern Bering Sea shelf (see Figure 5.5~.
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From page 85... ...
(2002) reviewed fishing vessel participation in the federally managed groundfish fisheries of the Gulf of Alaska, Aleutian Islands, and Bering Sea for 1992-2000.
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From page 86... ...
registered with the state of Alaska in the region occupied by the western stock of Steller sea lions, which excludes southeastern Alaska and regions north of Bristol Bay, 1969-2001. Vessels must register with the state to fish for salmon, herring, crabs and other invertebrates, and groundfish (state waters only)
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From page 87... ...
Vessels participating solely in federally managed groundfish fisheries and delivering to offshore processors are not required to register with the state, and these vessels are underrepresented in the database. Participation was summarized for state-registered vessels for the region occupied by the western stock of Steller sea lions (see Figure 5.6b)
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From page 88... ...
The F40% harvest strategy has been a source of some confusion, regarding its effect on the fish biomass and prey availability to Steller sea lions. It is not a fishing rate that depletes the entire fish stock to 40% of the level that would occur in the absence of fishing.
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From page 89... ...
Only pollock, cod, and Atka mackerel were considered here because of their apparent importance to the Steller sea lion diet. In the Gulf of Alaska, estimated biomass of pollock of age 2 and greater increased from 431,000 mt in 1961 to 4.1 million mt in 1981 and declined in the 1990s to levels typical of the 1960s and early 1970s (see Figure 5.7a)
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From page 91... ...
,........ ,~,, 85 90 95 00 Year FIGURE 5.8 Aleutian Islands and Bering Sea Atka mackerel exploitation, 19772001.
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From page 92... ...
With the shift from foreign to domestic groundfish fisheries in the 1980s, the fraction of pollock, cod, and Atka mackerel landings in Steller sea lion critical habitat increased (see Figure 5.10~. Critical habitat is defined by the 20-nm radii around sea lion rookeries and important haulouts, and three aquatic foraging areas in Shelikof Strait, Bogoslof Island area, and Sequam Pass.
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From page 94... ...
For example, from January to April 1985 effort was concentrated mostly on the southeastern Bering Sea shelf in a relatively small area north of Unimak Island. From January to April 1986 nearly all effort occurred in the Aleutian Basin but shifted from May to August 1986 to the shelf north of 56° N with most effort on the northwestern portion of the shelf north of 58° N
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From page 95... ...
in Shelikof Strait, and this area appears to be an important foraging area for Steller sea lions. It is uncertain whether these major shifts in fish removals adversely affected the number and density of pollock schools and the foraging success of Steller sea lions in the region.
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From page 96... ...
96 DECLINE OF THE STELLER SEA LION IN ALASKAN WATERS :.' ~ | m a b C Sept.-Dec. 1978 84 hauls \ r .
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From page 97... ...
No significant differences in the biomass of juvenile or adult pollock were detected in response to commercial fishing in which 27 vessels harvested 2,850 mt of pollock from Barnabas trough. Moreover, analysis of pollock school dynamics (e.g.,
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From page 98... ...
Other Steller sea lion prey species may be more susceptible to localized depletion. Evidence of seasonal, localized depletion of Atka mackerel by the trawl fishery in 1992-1995 has been reported by Fritz (1999~.
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From page 99... ...
The diet of Steller sea lions in the Aleutian Islands is dominated by Atka mackerel, but depletion of pollock could further reduce diet diversity, which appears to be inversely correlated to the rate of decline (Merrick et al., 1997~. Aside from pollock, other instances of overfishing occurred prior to the rapid decline of the sea lions in the 1980s and involved fish species that are less important components of the sea lion diet (see Chapter 4~.
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From page 100... ...
In general, the fishery competition hypothesis is not supported by comparisons of trends in Steller sea lion populations and fish biomass in the Gulf of Alaska, Aleutian Islands, and eastern Bering Sea. The 1970s was a decade of high sea lion abundance despite relatively low groundfish abundance.
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From page 101... ...
Federal fishery management plans provide frameworks for state management authority for scallop and salmon fisheries statewide and for crabs in the Bering Sea and Aleutian Islands region. For other species the state manages fisheries within territorial and EEZ waters without the auspices of federal fishery management plans.
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From page 102... ...
, Aleutian Islands, and Bering Sea. Catches include those from reduction fisheries prior to 1960, offshore foreign trawl fisheries in the Bering Sea during 1960-1980, and recent sac roe fisheries since 1970.
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From page 103... ...
, a period of decline of Steller sea lions in the Aleutian Islands. Since the late 1980s, herring biomass slowly declined (Funk and Rowell, 1995~.
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From page 104... ...
Smaller fisheries were prosecuted in lower Cook Inlet and the Aleutian Islands. As red king crab fisheries declined through the 1970s in the Gulf of Alaska, a large fishery developed in Bristol Bay in the eastern Bering Sea.
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From page 105... ...
Not shown are commercial crab fisheries in southeastern Alaska and relatively small crab fisheries for golden king crab in Prince William Sound, hair crab in the Bering Sea, and red king crab in Norton Sound. SOURCE: Data from Shareef Siddeek, Alaska Department of Fish & Game, Juneau, personal communication (2002~.
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From page 106... ...
Inferences must be drawn from landings data. The largest red king crab fisheries in the Gulf of Alaska and Aleutian Islands occurred in the 1960s and 1970s, before the steep decline of Steller sea lions.
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From page 107... ...
The apparent importance of salmon in the diet of sea lions depends on area, season, and data collection methods. On average, from the 1950s to the 1980s salmon were found in 5.6 to 7.1% of the stomachs of the eastern stock of Steller sea lions and just 1.1 to 1.8% of those collected from the western stock (National Marine Fisheries Service, 2000, see Figure 4.5~.
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From page 108... ...
Most pink salmon are caught in southeastern Alaska and Prince William Sound, with significant landings off Kodiak Island. By far the largest sockeye salmon fishery occurs in Bristol Bay, but significant catches are taken from southeastern Alaska to the Alaska Peninsula.
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From page 109... ...
The issue of sea lion shootings by fishermen is considered in Chapter 6. MANAGEMENT MEASURES TO MITIGATE POTENTIAL ADVERSE FISHING EFFECTS A number of management measures have been implemented in attempts to mitigate potential adverse effects of fishing on Steller sea lions since they were first listed as threatened under the Endangered Species Act in 1990 (North Pacific Fishery Management Council, 1999, 2002; National Marine Fisheries Service, 2001~.
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From page 110... ...
. A 4-year schedule is implemented to reduce the percentage of harvest in sea lion critical habitat in the central and western Aleutian Islands from 80% (1999)
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From page 111... ...
· Several new measures are instituted to spread out the catch of Pacific cod throughout the year. Obviously, the potential need for additional fishery restrictions depends on the chances that fishing adversely affects Steller sea lions and the degree to which these effects may have been mitigated by measures already implemented.
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