Kokanee–sockeye salmon hybridization leads to intermediate morphology and resident life history: implications for fisheries management

Abstract

Stocking programs designed to return extirpated species to their historical range have become increasingly prevalent, punctuating the need to better understand the risks posed to recipient ecosystems. Here, we investigated the genetic and biological consequences of an anadromous sockeye salmon (Oncorhynchus nerka) stocking program in Skaha Lake, British Columbia, where substantial levels of hybridization and introgression with the native freshwater resident ecotype (kokanee) have been detected. We genetically assigned 543 individuals (adult spawners, age-0 juveniles) to estimate stock proportions (pure-stock sockeye, pure-stock kokanee, or hybrid) between 2010 and 2017, with a subset undergoing otolith microchemistry analysis to determine migratory life history and maternal ancestry. The proportion of hybrid spawners varied from 5% to 20% across sampling years, while that of hybrid age-0 juveniles remained relatively constant (∼11%). Hybrid spawners exhibited intermediate size relative to pure stocks, with the vast majority being nonanadromous (92%) and of resident maternal ancestry (76%). Our results provide empirical support for previously hypothesized mechanisms of hybridization between O. nerka life-history forms and underscore the importance of continued monitoring of stocking programs to quantify long-term fitness impacts of introgression and refine management strategies.