Foraging differences between male and female harbor seals present challenges for fisheries management

A 2015 article published in the Marine Ecology Progress Series identifies intraspecific differences in diet between harbor seals in the Salish Sea, suggesting implications for marine reserve management. 

Harbor seal photographed by Andreas Trepte. Available through a Creative Commons Attribution Share Alike 2.5 license.
Harbor seal photographed by Andreas Trepte. Available through a Creative Commons Attribution Share Alike 2.5 license.

Summary

We used stable isotopes to reconstruct the contribution of various food items (primarily fish) to the diet of harbor seals.  Our results indicate diet differences between males and females and between large and small seals.  It appears that females forage more in nearshore environments and males forage farther from shore.  While female harbor seals appear to consume less salmon than males, they may have a secondary positive indirect effect on salmon because sculpins appear to be a more common diet item for female seals, and sculpins and other cottids are major predators of salmon eggs and fry. It is possible that female harbor seal consumption of sculpins may improve conditions for salmon, while male seals may have an opposite effect. In terms of designing marine reserves, many established marine reserves in the Pacific Northwest are located in close spatial proximity to seal haul-out sites. If smaller females forage closer to haul-out sites, then they may have a greater impact, either positive or negative, on prey populations in these areas. Given this possibility, future studies should assess the positive and negative impacts of this differential mortality on fisheries and on marine reserves. --Summary by Scott Pearson for the Encyclopedia of Puget Sound.

Citation

Bjorkland, R.H., Pearson, S., Jeffries, S.J., Lance, M.M., Acevedo-Gutiérrez, A., Ward, E.J. (2015). Stable isotope mixing models elucidate sex and size effects on the diet of a generalist marine predator. Marine Ecology Progress Series. Vol. 526: 213–225, 2015 doi: 10.3354/meps11230.

 

About the Author: 
Rhema H. Bjorkland1,5,*, Scott F. Pearson2, Steve J. Jeffries2, Monique M. Lance2, Alejandro Acevedo-Gutiérrez3, Eric J. Ward4: 1Fisheries Resource Assessment and Monitoring Division, Northwest Fisheries Science Center, National Marine Fisheries Service, National Oceanic and Atmospheric Administration, 2725 Montlake Blvd. East, Seattle, WA 98112, USA 2Washington Department of Fish and Wildlife, Wildlife Science Division, 1111 Washington St. SE, Olympia, WA 98501, USA 3Department of Biology, Western Washington University, 516 High St. MS9160, Bellingham, WA 98225-9160, USA 4Conservation Biology Division, Northwest Fisheries Science Center, National Marine Fisheries Service, National Oceanic and Atmospheric Administration, 2725 Montlake Blvd. East, Seattle, WA 98112, USA 5Present address: US EPA Office of Pollution Prevent and Toxics, 1200 Pennsylvania Ave NW, Washington, DC 20460, USA