Effect of Ocean Conditions on the Cross-shelf Distribution of Walleye Pollock (Theragra chalcogramma) and Capelin (Mallotus villosus) Anne Hollowed, Chris Wilson, Phyllis Stabeno, and Libby Logerwell National Marine Fisheries Service Alaska Fisheries Science Center 7600 Sand Point Way NE Seattle, WA 98115 Pacific Marine Environmental Laboratory 7600 Sand Point Way NE Seattle, WA 98115
80% decline of the western stock of Steller sea lions 1997 declared endangered (ESA) Sease & Gudmundson 2002
Hypotheses to explain the decline Intentional and unintentional takes Climate change Disease, pathogens and toxins Predators Competition with groundfish fisheries National Research Council, 2003. Decline of the Steller Sea Lion in Alaskan Waters
5 Year Study Steller Sea Lion Study Site Eastside of Kodiak Island 25 day cruises in August 2000, August 2001 August 2002 August 2004 August 2006 Walleye Pollock Capelin
Key Questions Do commercial fishing activities influence the distribution and abundance of Steller sea lion prey species? Wilson et al. 2002 How do spatial distributions and abundance of prey impact Steller sea lion foraging success? How do fish respond to local bio-physical forcing? Distribution patterns of principal acoustic sign types: capelin, adult, age 1 Comparative approach Hollowed et al. In Press, Logerwell et al. in review
Pollock Study area Russia Alaska Bering Sea Kodiak Is. Pacific Ocean Gulf of Alaska
Longitude ( o W) Pollock Study site 58.0 Chiniak Trough (control) 57.5 Kodiak I. Ugak Bay Cape Chiniak Latitude (o N) 57.0 Dangerous Cape Barnabas Trough (treatment) 200 m 100 m 56.5 154 153 152 151
Methods Echo integration-trawl (EIT) methods Acoustics to assess distribution and abundance of fish Trawls to confirm species, size and age Multiple surveys ( passes ) before and after the start of commercial fishing in late August Sites open (treatment) and closed (control) to fishing
Oceanographic Measurements Recent study 4 Moorings CTD and trawl temperature probes Continuous underway SST Satellite images Satellite track drifters deployed throughout the spring and summer Historical surveys Water column profiles along selected transects Mooring 197x
Walleye Project history 2000 feasibility study (fishery closed) 2001 full survey 2002 full survey, virtually no fishing effort 2004 full survey 2006 full survey (data being processed)
Drifter Track
Front separates juvenile and capelin 2000 2001 2006 Prey field for sea lions foraging nearshore composed of juvenile
2004 Front weakened and capelin/age-0 nearshore Prey field for sea lions foraging nearshore composed of capelin and juvenile High prey diversity has been linked to high sea lion survival
Revised Hypotheses 1976/77 Regime shift altered the strength of the mid-trough front allowing intrusion of capelin into nearshore waters. Changes in the species diversity of available prey may have contributed to changes in condition of Steller sea lions. Merrick, R.L., M.K. Chumbley, and G.V. Byrd. 1987. Diet diversity of Steller sea lions (Eumetopias jubatus) and their population decline in Alaska; a potential relationship. Can. J. Fish. and Aquatic Sci 54:1342-1348.
Anderson & Piatt, 1999 Nearshore fish diversity decreased after late 1970s Front weaker?
Seasonal mean temperature Pre-1977 Inside Tidal Mixed Zone Pre-1977 Outside tidal Mixed Zone Temp 16 14 12 10 8 6 4 2 0 0 1 2 3 4 5 6 7 8 9 10 11 12 Month Surface 75 meters Temp 16 14 12 10 8 6 4 2 0 0 1 2 3 4 5 6 7 8 9 10 11 12 Month Surface 75 meters Post-1977 Inside Tidal Mixed Zone Post-1977 Outside Tidal Mixed Zone Temp 16 14 12 10 8 6 4 2 0 0 1 2 3 4 5 6 7 8 9 10 11 12 Surface 75 meters Temp 16 14 12 10 8 6 4 2 0 0 1 2 3 4 5 6 7 8 9 10 11 12 Surface 75 meters Month Month
Conclusions Front separates juvenile and capelin in many (but not all) years Historical data on water column properties suggests that water was slightly cooler at depth Diversity of nearshore species shifted in early 1980s. Mechanism underlying decline in nearshore prey availablity appears to be linked to frontal boundary.