Lacustrine growth of juvenile pink salmon and a comparison with sympatric sockeye salmon

Abstract

The growth rates of naturally sympatric juvenile pink Oncorhynchus gorbuscha and sockeye Oncorhynchus nerka salmon were compared in a common lacustrine environment in south-west Alsaka, an unusual opportunity given the normal disparity in freshwater residence time of these two species. Fork length (LF) frequency distributions of juvenile pink salmon caught in the lake during the summer in 1991 and 1999–2003 indicated a growth rate of 0·54 mm day−1, 54% greater than the estimated growth rate of juvenile sockeye salmon sampled from 1958 to 2003 (0·35 mm day−1). Examination of daily growth rings on otoliths indicated that pink salmon in Lake Aleknagik grew an average of 1·34 mm day−1 in 2003 but sockeye salmon grew only 0·63 mm day−1(average specific growth rates were 3·0 and 1·8% day−1, respectively). Pink salmon increased from c. 32 mm LF and 0·2 g at emergence to 78 mm LF and 3·0 g within 3–4 weeks. After experiencing these rapid growth rates, the pink salmon appeared to leave the lake by late July in most years. The diets of pink and sockeye salmon in the littoral zone of the lake were very similar; >80% of the stomach contents consisted of adult and pupal insects and the remainder was zooplankton. This high degree of diet overlap suggested that the observed differences in growth rate were not attributable to variation in prey composition.