A Quantitative Approach for Targeted Subsampling of Juvenile Fish to Obtain Adequate Description of Diet

Kathryn Mier , Department of Commerce, NOAA Alaska Fisheries Science Center, Seattle, WA
Nissa Ferm , Department of Commerce, Contractor for NOAA Alaska Fisheries Science Center, Seattle, WA
Investigating the trophic ecology of juvenile fish species is an important component of the Alaska Fisheries Science Center Recruitment Processes Program in order to understand the influence of prey selectivity and availability on survival during early life. In order to standardize protocols for sampling juvenile Walleye Pollock for diet analyses, a procedure was developed to optimize subsample size per station sampled using prey curves, i.e., number of prey species vs. number of fish processed. The majority of published diet studies use prey curves post analysis which often shows inadequate sampling.  Using historic diet data from 5 years and 3 habitats of juvenile Walleye Pollock from the central Bering Sea, projected prey curves and their confidence intervals are created using the Chao2 estimator from EstimateS software. The minimum number of fish to process in order to adequately describe the diet for each year and habitat is then determined from these prey curves and sampling effort is distributed accordingly. Other issues address stratification by predator size, sample vs. individual-based data, incidence vs. abundance data and power.