Variability in trophic positions of four commercially important groundfish species in the Gulf of Alaska

- We examine ontogenetic variability in trophic levels of groundfish using δ15N.
- We model variations in mean trophic level (TL) among years with fish length.
- We estimate TL of catch and biomass using length frequency data.
- Trophic levels remained stable over time, except for walleye pollock.
- Including length of catch in TL estimates may lead to earlier detection of declines.

We examined trends in nitrogen stable isotope data as a proxy for trophic position (mean trophic level, TL) of commercial and survey catches as an ecosystem-based indicator of sustainability of four groundfish species in the Gulf of Alaska. From 2000 to 2004, walleye pollock (Gadus chalcogrammus), Pacific cod (Gadus macrocephalus), arrowtooth flounder (Atheresthes stomias), and Pacific halibut (Hippoglossus stenolepis) were collected from the waters surrounding Kodiak Island, Alaska. Several analyses of covariance (ANCOVA) models were tested to detect variations in mean TL among years with fish length as a covariate. Best-fit models were selected using the Akaiki Information Criterion to estimate trends in mean TL of commercial catch using length-frequency data from onboard fishery observers for each target species. Then, linear regression models were used to estimate mean TL of commercial catch over 1990-2009 and the mean TL of population biomass over 1984-2007 based on length-frequency data and biomass estimates from trawl surveys conducted by National Marine Fisheries Service and from historical catch data. The TL of catch for each species except walleye pollock remained stable over the time frame of the study. Walleye pollock TLs became increasingly variable after 1999. Similar trends in mean TL were observed for the survey biomass of walleye pollock. Additionally, there was an observed decrease of the occurrence of higher TL Pacific halibut over time. While the decline had no impact on overall TL estimates during 1990-2009, a continued decline may affect mean TL in the future. Overall, length seems to be the most important factor in estimating a species' TL. Therefore, including relationships between length of catch and TL estimates could lead to an early detection of TL declines that may be associated with unsustainable fishing mortality.