Early marine growth and consumption demand of juvenile pink salmon in Prince William Sound and the northern coastal Gulf of Alaska

Although early marine growth has repeatedly been correlated with overall survival for Pacific salmon (Oncorhynchus spp.), we currently lack a mechanistic understanding of the timing, magnitude, and source of stage-specific mortality periods. Pink salmon (O. gorbuscha) are a key plankton consumer in Prince William Sound and the Gulf of Alaska, and hatchery production significantly increases their abundance in this region. This study combined within-season back-calculations of growth and bioenergetics techniques to examine interannual variability in the growth performance and consumption demand of the average hatchery, average wild, and surviving hatchery juvenile pink salmon during the first summer at sea in Prince William Sound and the northern coastal Gulf of Alaska among years corresponding to low marine survival (3% in 2001 and 2003) and high marine survival (9% in 2002, 8% in 2004). Juvenile pink salmon were consistently larger throughout the summer and early fall of 2002 and 2004 than in 2001 and 2003, indicating that larger, faster-growing juvenile pink salmon experienced higher survival. All cohorts ate a larger proportion of their theoretical maximum consumption and consumed more prey during 2002 than during 2001 and 2003 while feeding predominantly on the pteropod Limacina helicina. Unmarked "wild" juvenile pink salmon were significantly larger than hatchery fish during low-survival years, but no significant difference was observed during high-survival years. Pink salmon that survived to adulthood were larger at circuli, grew faster, and consumed more prey than the average juvenile. The localized standing stock biomass of key prey exceeded the daily consumption demand of juvenile pink salmon during July--August; however, estimated prey biomass was not enough to sustain the high level of consumption required to satisfy observed growth. The high percentage of prey biomass consumed, low feeding rates during May--July, a mid-summer decrease in circulus spacing and growth efficiency, and the fact that growth and consumption rates were much higher for all cohorts in high-survival years and for surviving cohorts in all years indicate that pink salmon are food limited in Prince William Sound and the coastal Gulf of Alaska during their first summer at sea.