Swimming depths and water temperatures encountered by radio‐archival‐tagged Chinook Salmon during their spawning migration in the Yukon River basin

Abstract

Objective Historically, Chinook Salmon Oncorhynchus tshawytscha have supported important fisheries throughout the Yukon River basin, but dramatic declines in abundance since the late 1990s have resulted in smaller returns, severe reductions in harvests, and difficulties in meeting escapement goals. These observations coincide with major climatic changes in the northern Pacific, characterized by a general warming trend throughout the region. Our objective was to document the migratory patterns of the fish in relation to the environmental conditions encountered in order to assess the impact of climate change and help manage the returns. Methods We used radio‐archival tags to track the distribution and movements of adult Chinook Salmon returning to the Yukon River to spawn. The tags were equipped with sensors that recorded the swimming depth of the fish and water temperatures encountered during the upriver migration. Spawning ground surveys and fishery returns were used to recover the tags to download the sensor data. Result Ninety‐five (71.4%) of the 133 tags tracked upriver were recovered, including 35 (26.3%) returned by fishermen and 60 (45.1%) retrieved on the spawning grounds. Upriver movements were characterized by continuous and highly variable fluctuations in depth throughout the migration, ranging from <5 m to >20 m in the lower river and progressively less as fish moved upstream into shallower waters. Swimming depth was not influenced by time of day. Temperatures encountered by the fish were generally warmer in 2004, but this pattern was not consistent throughout the basin and was driven by conditions in the lower main stem, with temperatures frequently >18°C and periodically exceeding 21°C. There was no obvious behavioral response to the warm conditions, with comparable movements and survival rates when conditions were cooler. Temperatures in terminal tributaries often exceeded the upper range generally considered optimal during spawning (13°C), but signs of impaired behavior or prespawning mortality were not observed. A thermal diel pattern was evident as fish left the main stem and approached their spawning grounds, with temperatures declining from early evening to early morning and increasing during daylight hours, suggesting that assessments based on average daily temperature may not adequately reflect exposure to suboptimal conditions. Conclusion Although the fish during our study frequently encountered temperatures associated with adverse effect on salmon, impaired behavior and increased mortality were not evident. However, the current warming trend occurring throughout the northern Pacific is predicted to continue and may impact salmon populations more severely. Our findings provide a baseline for comparing past conditions and migratory patterns with those of present and future returns. Radio‐archival tags not only provided site‐specific information, but substantially increased the number of tags recovered, with a recovery rate considerably higher than reported for most archival tag studi...

Adult spawners: A critical period for subarctic Chinook salmon in a changing climate

“…Among Yukon River Chinook salmon, no studies have been designed to estimate overall spawning success across the freshwater adult period. Only one large scale radio tagging study has been conducted in the region and it did overlap with a warm year (2004) when archival tags demonstrated that individuals often experienced water temperatures >18°C and occassionally >21°C, but there was no evidence of en route mortality (Eiler et al, 2014(Eiler et al, , 2015(Eiler et al, , 2022. Spawning ground surveys to estimate egg retention and prespawn mortality have occurred occasionally in a few tributaries and reveal that egg retention is common with 45% of females retaining eggs (reviewed by Twardek et al, 2022).…”

“…Moreover, the existence of thermal refugia alone may be insufficient if changes to hydrologic flow regimes results in fragmentation of available habitats (Tonina et al, 2022). The mainstem Yukon River does not appear to have thermal refugia (Eiler et al, 2022) and may help explain why Yukon River salmon have been particularly vulnerable to a warming climate.…”