Ontogenetic considerations in the trophic level of commercial groundfish species in the Gulf of Alaska

Abstract

"Trends in trophic level (TL) estimates of commercial fishery catches are used as ecosystem-based indicators for sustainability, but these estimates often do not incorporate species-specific interannual and ontogenetic feeding patterns. This study provides a finer resolution of ontogenetic and temporal variations in the trophic position of four groundfish species in the central Gulf of Alaska (GOA), walleye pollock (Theragra chaleogramma), Pacific cod (Gadus macrocephalus), arrowtooth flounder (Atheresthes stomias), and Pacific halibut (Hippoglossus stenolepis), using stable isotope analysis to assess TL and diet source. Samples were collected from the northeastern side of Kodiak Island, Alaska, from 2000-2004. Several Analysis of Covariance models were tested, allowing TL to co-vary with length, to detect possible variation among years and seasons and to estimate TL of catch for each study species. For each species, TL increased with length. Significant annual differences in [delta]¹³C and [delta]¹⁵N were detected for all groundfish, indicating a lower TL, pelagic diet in 2003 and a higher TL, benthic diet in 2001. Overall, TL of GOA commercial catches appeared to remain stable over 1990-2009, with the exception of walleye Pollack after 1999. This study shows that including length data could lead to an earlier detection of decline in TL estimates"--Leaf iii