The Influence of Smolt Age on Freshwater and Early Marine Behavior and Survival of Migrating Juvenile Sockeye Salmon

Abstract

AbstractSockeye Salmon Oncorhynchus nerka smolts typically experience high mortality during out‐migration through freshwater and the early marine environment. Due to tag burden (tag mass relative to fish mass) constraints, telemetry studies investigating migration survival have focused on tracking hatchery smolts or larger individuals within wild populations. However, hatchery and larger wild smolts may not represent the majority of migrants. We used miniaturized acoustic transmitters (VEMCO V4) to track age‐1 Sockeye Salmon smolts (n = 200) for the first time over approximately 950 km from Chilko Lake, British Columbia, Canada, through the coastal environment by using large‐scale receiver arrays. We compared their survival with that of concomitantly tracked age‐2 smolts (n = 100) by using VEMCO V7 acoustic transmitters. Freshwater survival of tagged age‐1 smolts was double the age‐2 survival (56% versus 28%), potentially due to higher proportional tag burdens on age‐2 smolts. Age‐1 smolt cumulative survival was 16% to the last array capable of detecting age‐1 smolts (944 km from release) and was within the range of cumulative survival reported for age‐2 Chilko Lake Sockeye Salmon smolts in previous telemetry studies. Age‐2 smolt minimum cumulative survival to the final arrays (944 and 1,041 km) was about 2% and was lower than survival in previous years. Although survival differed between age‐classes in 2016, both experienced similar trends in landscape‐specific survival relative to age‐2 smolts tracked in previous years. Survival was particularly low in the upper Fraser River tributaries for age‐1 and age‐2 smolts (76% and 53%, respectively, over 13 km from release) and after ocean entry for age‐1 smolts (36% over 217 km). Migration rates of both age‐classes among landscapes were within the range observed for age‐2 smolts in earlier studies. Our study highlights the importance of expanding research to include smaller age‐1 smolts and provides survival estimates for an indicator population of Fraser River Sockeye Salmon.