Walleye pollock egg distribution and mortality in the western Gulf of Alaska

Abstract

ABSTRACTWe examine the distribution and mortality of walleye pollock (Theragra chakogramma) eggs in the western Gulf of Alaska. Most pollock eggs were found in mid‐water, with low proportions in the neustonic and epibenthic layers during all years of sampling. A silhouette camera towed through a high egg density region provided new information on small‐scale spatial distributions and provided density estimates at two depth layers similar to those of depth‐discrete net sampling. Annual egg production curves and natural mortalities were estimated for 1987‐92 based on the abundance of several cohorts relative to their production rate. Production during 1989 and 1990 was lower than in the other four years but 1988 was the only year to show markedly different (higher) mortality than the rest. Fertilization rate was generally very high (>99%) but several collections early in the season contained a substantial fraction of unfertilized eggs. Invertebrate egg predation was mainly due to euphausiids and was variable among locations and years. Egg cannibalism by adult pollock on the spawning grounds was inconsequential (< 1% for all years) compared to invertebrate predation.