Comparing Laboratory and at-Sea Analysis of Stomach Contents in Alaska

In recent Alaska Fisheries Science Center surveys, we have conducted Stomach Content ANalysis at sea (SCAN) and analysis in the laboratory for several groundfish species.  This provided us with the opportunity to compare our ability to identify key prey types between the two methods.   Preliminary analyses indicate that identification of jelly-prey (Cnidaria and Ctenophora) and salp-prey (pelagic Tunicata and Larvacea) differs between the two methods for some zooplanktivores.  SCAN more frequently identified jelly-prey in the diets of Dark rockfish, Dusky rockfish, Prowfish, Sablefish.  Dissolution of jelly-prey tissues during the preservation process is likely the main factor in the differences observed.  Salp-prey was more frequently identified in the laboratory for Sablefish and Walleye pollock.  Tissue consistency and size of salp-prey may make them more easily detected in the laboratory.  Preliminary analyses also indicate that the identification of fish-prey to species was higher in SCAN of Pacific cod, Pacific halibut, Walleye pollock, and Arrowtooth flounder.  A double-blind test of visual identification using genetics confirmed accurate identification of common fish-prey at all levels of digestion.  Genetic identification of fish-prey that were not identifiable in the SCANs and subsequently in the laboratory, indicated that these fish-prey were different species than the visually identified fish-prey.