Do freshwater growth opportunities and migration barriers limit recovery of imperiled anadromous Oncorhynchus mykiss populations?
Marston, Gary Wayne
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Steelhead (Oncorhynchus mykiss) populations in the Puget Sound Distinct Population Segment are far below their historic abundance and are currently listed as Threatened under the Endangered Species Act. Despite the declines in steelhead abundance, resident O. mykiss populations appear to be relatively abundant and little is known about how resident O. mykiss above anadromous barriers interact with steelhead downstream. This study evaluated the dispersal and gene flow of above-barrier Oncorhynchus mykiss in the Duckabush and Hamma Hamma Rivers, Washington into below-barrier reaches that were accessible to anadromous salmonids. As well as the abundance, sex ratio and annual survival rate of O. mykiss above and below the barriers, and their influence on gene flow. We also evaluated the annual and summer growth potential of Oncorhynchus mykiss at various life stages (age 1-2, age 2-3 and age 3-4) above and below anadromous barriers in rivers to determine whether growth was limited by environmental conditions within the watershed, and if so, whether the greatest influence over growth was the availability of food or the thermal regime. Although a three year mark-recapture study did not detect any dispersal from the above the barrier populations into the below-barrier populations, microsatellite DNA analysis detected gene flow from the above-barrier populations into the below barrier populations. Gene flow was more pronounced in the Duckabush River, where 90% of the below-barrier fish were assigned to the above-barrier population. The Hamma Hamma River populations were more distinct, with 41% of the below-barrier population assigning to the above-barrier population. Abundance was estimated for O. mykiss ≥130 mm FL in the Duckabush River, with 150 trout per km in the lower below-barrier reach, 907 per km in the middle reach below the barrier and 1,165 per km above the barrier. Catch per unit effort in the Duckabush River above the barrier was 5.4 fish per angler hour compared to 2.0 in the Hamma Hamma River above the barrier, suggesting much lower relative abundance in the Hamma Hamma River. These results indicate that the gene flow from the above-barrier populations into the below-barrier population is strongly tied to abundance. Annual survival rates were low ranging from 5.7% to 21.4%, with 2013-2014 generally having better survival than 2014-2015. Survival declined with age in the middle (below-barrier) and above-barrier reaches, but improved with age in the lower reaches. Bioenergetic modeling was used with empirical data on growth, diet, and thermal experience to estimate the feeding rate, growth trajectory and the amount of food required to attain the growth observed between life stages in the, middle and lower reaches below the barrier and above the barrier in both watersheds. Model outputs indicated that consumption rates were low throughout the watersheds, ranging from 20% (age-2) to 32% (age-4) of their maximum consumption rate (%Cmax) annually. Food was insufficient to support adequate growth for age-2 and older O. mykiss in the middle and upper reaches in the Duckabush and Hamma Hamma Rivers. Food availability and quality appeared to limit growth of juvenile O. mykiss in the middle and upper reaches in the Duckabush and Hamma Hamma Rivers with little scope for growth beyond age-2, making these populations vulnerable to density-dependent effects on growth and production. Prey supply and quality were less limiting in the lower reaches below the barriers, largely due to marine subsidies from spawning salmon; however it appears that other factors might limit the populations in these reaches. These results suggest that high mortality rates in freshwater caused by delayed smoltification due to low growth rates may be significant factors limiting these steelhead populations. With steelhead facing declines across their range, this study highlights the importance of identifying factors limiting growth and survival during critical life stages in freshwater.
- Fisheries