Figure 1. The release of a radio-tagged Chinook salmon in the Yukon River.
Adult Chinook salmon migrate to spawning areas throughout the Yukon River basin – a massive, transboundary river system and a major producer of Chinook salmon in the northern Pacific Rim. Some of these fish travel over 3,200 km and take over 60 days to reach their final destination, making it one of the longest freshwater migrations on record. Chinook salmon returns to the Yukon River and other rivers in western Alaska have declined dramatically since the late 1990s. This trend has resulted in the closure of commercial fisheries, severe restrictions in subsistence harvests, and difficulties in meeting regional and treaty-related goals. These declines have also increased concerns over salmon bycatch in the Bering Sea walleye pollock fishery. Recent reductions in fish size and shifts in age composition to younger fish have also been observed. In addition to salmon studies in the marine environment, the Center’s Auke Bay Laboratories has also conducted research to determine the run characteristics of Yukon
River Chinook salmon. During 2002-2004, we captured and tagged over 2,800 adults with radio transmitters (Fig. 1), and track them upriver using satellite-linked tracking stations and aerial surveys.
Fish traveling to spawning areas in the Upper Yukon and Tanana River (Fig. 2) dominated the returns, comprising over 70% of the total run. Substantially fewer fish returned to the other regional areas within the basin, but their collective contribution was still appreciable. Most regional components consisted of several principal stocks and a number of small, spatially isolated populations. Regional and stock composition estimates were similar across years even though differences in run abundance were reported, suggesting that these differences were not related to regional or stock-specific variability. Run timing within the basin was relatively compressed compared to Chinook salmon returns in river drainages farther south (California-British Columbia), with most stocks passing through the lower river over a 6-week period ranging from 16 to 38 days. Although lower basin stocks consisted primarily of later run fish, run timing was similar for middle and upper basin stocks, limiting the use of timing information for management.
Figure 2. Map of the Yukon River showing the regional areas and the major tributaries of the drainage. The location of the tagging site and tracking stations are also indicated.
Pronounced differences in upriver movements were observed among the regional components of the return. Fish traveling to tributaries in the lower river were uniformly slower (28-40 km/day) than those traveling farther upstream (52-62 km/day). Upper river fish exhibited three distinct migratory patterns (Fig. 3), and stocks returning to the same regional area displayed similar migratory patterns. Individual fish within a stock showed substantial variation, but tended to reflect the regional pattern. Movement rates were substantially faster and the percentage of atypical movements considerably less than reported in southern drainages, and may reflect the pristine conditions within the Yukon River basin, wild origins of the fish, and discrete run timing of the returns.
Figure 3. Migratory patterns of Chinook salmon stocks returning to different regions within the Yukon River basin. The movement rates of fish that have left the main river are indicated (*).
Physical features of the basin explained over 70% of the variation in movement rate, with river gradient and the distance remaining to reach spawning areas exhibiting the strongest relationships. The biological and behavioral characteristics of the fish (fish size, run timing, etc.) were less of a factor (explaining ~ 18% of the variation), and may reflect stabilizing selection on long-distance migrants. The information from this study on stock structure, timing, and migratory patterns provides numerous insights into the current status and management of salmon returns. The effect of climate change on fish populations is also an increasing concern, particularly in the Arctic due to polar amplification. Salmon returns with extended migrations in many northern rivers could be adversely affected by prolonged exposure to elevated water temperature and its effect on swimming performance, stamina, and reproductive success. The baseline information provide by this study will be useful for assessing the impacts of changing environmental conditions on salmon returns in the Yukon River and other large drainages in western Alaska.