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 Objective Seaward migration and early marine residence are periods of high mortality in the lifecycle of Pacific salmon Oncorhynchus spp. The conservation of these species requires knowledge of habitat use patterns during early life to address survival bottlenecks. Using new miniaturized transmitters (V3 307-kHz tag; Innovasea Systems), we investigated the association between biological and environmental factors and the residence behavior of subyearling Chinook Salmon Oncorhynchus tshawytscha in a fjord estuary in British Columbia. Methods Tagged wild subyearling Chinook Salmon (n = 49; 67–95 mm) were experimentally released into an estuary. Using time-to-event analysis and model selection, we evaluated the association between biological (size, growth, day of release) and environmental (temperature, salinity, depth, discharge, tide direction) factors and estuary residence patterns. Result Of the 49 fish released, 36 were inferred to have exited the estuary successfully. In this sample of subyearling Chinook Salmon, the median residence duration was estimated to be 11.2 days (95% CI = 6.5–15.5). 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 that made multiple trips upstream and downstream. Within the estuary, most movements occurred at night (76%), regardless of tide direction. Conclusion This study provides the first direct measure of estuary residence duration in wild subyearling Chinook Salmon and a valuable description of movement ecology in this age-class of fish. While this was a small study in a single year, our results indicate this estuary is likely a stopover habitat for larger subyearlings. 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.