Evaluating the influence of environmental and biological factors on migration behavior and residence duration of wild subyearling Chinook Salmon in a fjord estuary using miniature acoustic transmitters

Abstract

Seaward migration and early marine residence are high mortality periods in Pacific salmon lifecycles. The conservation of these species requires knowledge of habitat use patterns during early life to address survival bottlenecks. Using new miniaturized transmitters (V3‐307 Hz, Innovasea Systems, Inc), we investigated the association between biological (size, growth, day of release) and environmental (temperature, salinity, depth, discharge, tide direction) factors and the residence behaviour of sub‐yearling Chinook Salmon Oncorhynchus tshawytscha in a fjord estuary in British Columbia. Wild sub‐yearling Chinook Salmon (n = 49; 67 ‐ 95 mm) were experimentally released into the estuary of which 36 exited successfully. Median residence duration was 11.2 days (95% CI: 6.5 ‐ 15.5). Using model selection and time‐to‐event analysis, we found tide direction and salinity to be important factors influencing the probability of a fish leaving the estuary. Fish were 75% more likely to leave the estuary on an ebb tide. Higher salinity also increased the probability of a fish leaving the estuary. Fish successful in leaving the estuary displayed either direct or indirect movement patterns in the estuary, with most fish (71%) being indirect movers making multiple trips upstream and downstream. Within the estuary, most movements occurred at night (76%) regardless of tide direction. This study provides the first direct measure of estuary residence duration in wild sub‐yearling Chinook Salmon, as well as a valuable description of movement ecology in this age‐class of fish. While this was a small study in a single year, it indicates this estuary to be a stopover habitat for larger sub‐yearlings. Studies indicating the use of habitats by specific populations, as presented here, are integral to the design of habitat‐based conservation measures for mobile species.