Food habits and trophic ecology of two common skate species, Raja rhina and Bathyraja interrupta, in Prince William Sound, AK

Jenny Kemper

Email: jkemper@mlml.calstate.edu

In Prince William Sound (PWS), skates have historically been taken as bycatch in groundfish fisheries. Markets, declines in more commonly targeted stocks, and increases in unused skate biomass from groundfish fisheries are reasons for an emerging skate fishery within PWS that is set to begin in March 2009. This directed fishery will target longnose (R. binoculata) and big (R. rhina) skates and, if the fishery is successful, expansion to other common skate species may occur.

Skates play important trophic roles within benthic marine communities (Ebert et al., 1991; Orlov, 2003), and as upper trophic level predators, may influence the distribution and abundance of co-occurring species. Processes that influence food webs and energy transfer are necessary to understand trophic relationships and the dynamics of marine ecosystems. Food habit information concerning skates within PWS is non-existent. Dietary information is necessary to understand the mechanisms responsible for community regulation and potential community-level effects of targeting local skate species. Diet composition information and trophic level estimates of the two most common skates on the outer shelf and upper slope of PWS, the longnose skate, R. rhina, and Bering skate, B. interrupta, will provide important information about their prey spectrum, trophic roles, and trophic dynamics within the local benthic assemblage.

Project objectives are to: 1) determine the diets of R. rhina and B. interrupta in PWS through analysis of stomach contents, 2) examine temporal and size-based dietary differences within species, 3) use stable isotope analysis to corroborate stomach content analysis, 4) determine and compare size-specific trophic levels within and between species in PWS using stomach content and stable isotope analysis, 5) examine interspecific diets among PWS skate species to determine the degree of dietary overlap, and 6) compare diet composition of R. rhina and B. interrupta with that of skates from other regions of the Gulf of Alaska.

Literature Cited:

Ebert, D.A., P.D. Cowley, & L.J.V. Compagno. 1991. A preliminary investigation of the feeding

ecology of skates (Batoidea: Rajidae) off the west coast of southern Africa. South African Journal of Marine Science, 10: 71-81.

Orlov, A.M. 2003. Diets, feeding habits, and trophic relations of six deep-benthic skates (Rajidae) in the western Bering Sea. Journal of Ichthyology and Aquatic Biology, 7(2): 45-60.