Sablefish Predation on Juvenile Pacific Salmon in the Coastal Marine Waters of Southeast Alaska in 1999

Abstract

AbstractAt‐sea observations of predation by age‐1 and older (age‐1+) sablefish Anoplopoma fimbria on juvenile Pacific salmon Oncorhynchus spp. were combined with laboratory studies to determine gastric evacuation rates and were used to estimate summer predation impact in the northern region of Southeast Alaska. In June and July 1999, up to 63% of sablefish examined from trawl catches in strait habitat had each consumed one to four juvenile pink salmon O. gorbuscha, chum salmon O. keta, or sockeye salmon O. nerka. In two laboratory experiments, field‐captured sablefish were acclimated without food in compartmentalized flow‐through tanks with conditions manipulated to reflect the photoperiod and temperature regimes of summer. These sablefish were each offered one whole, preweighed juvenile chum salmon; consumption events were observed, and the sablefish were then sacrificed at predetermined time intervals. Prey biomass remaining in the stomach of each sablefish predator was weighed, and an exponential model of the decline in percent biomass over time was used to yield instantaneous evacuation rates of 0.049 at 12°C and 0.027 at 7°C. From field data combined with model‐derived estimates of meal frequency, we estimated that 0.8‐6.0 million juvenile salmon were consumed by age‐1+ sablefish in the 500‐km2 area of Icy Strait during a 33‐d period. Moreover, a 10‐year time series of catches indicated that 1999 was a year of unusually high age‐1+ sablefish abundance and relatively low juvenile salmon abundance. We speculate that sablefish predation in 1999 could have affected abundance of out‐migrating juveniles and contributed to low harvests of returning adult pink salmon in 2000 and adult chum salmon in 2002. Our results suggest that sablefish predation on juvenile Pacific salmon can occur during episodic strong year‐classes of sablefish and may affect adult salmon returns.