A Survey of Steelhead Age and Iteroparity Rates from a Volunteer Angler Program in the Willamette River Basin, Oregon

Abstract

AbstractThe iteroparous life history of steelhead Oncorhynchus mykiss can provide genetic variability and increase recruitment, thereby sustaining populations. Steelhead are difficult to survey given that (1) they spawn during high river flows, which can flush carcasses away; (2) they spend a short time on the spawning grounds; (3) nonspawning fish are elusive; and (4) adults that do not die after spawning can evade surveyors. An angler volunteer program was implemented in Oregon to monitor steelhead populations. Recreational anglers collected scales from 2,216 steelhead in the Willamette River basin during 1981–1994; the scales were used to estimate ages and iteroparity rates. A freshwater age of 2 and a saltwater age of 2 were the most common, followed by saltwater ages 3 and 4. The incidence of iteroparity was greater among female steelhead than among males and was greater for winter‐run steelhead than for summer‐run fish. Wild fish had a higher incidence of repeat spawning than hatchery fish, which may be partly attributable to hatchery practices (e.g., recycling the fish through the fishery or using them for spawning). These results agree with previously published information for steelhead. The overall iteroparity rate of 3.4% for Willamette River basin steelhead appears to be moderate in comparison with other populations; for instance, iteroparity in Willamette River steelhead was much lower than rates for Oregon coastal stocks, was similar to rates for lower Columbia River stocks, and was higher than rates for interior Columbia River stocks. I offer the following hypotheses to explain these geographical trends in iteroparity: (1) Willamette River basin steelhead have a greater distance to migrate than coastal stocks and iteroparity may be selected against by barriers to adult and juvenile passage, (2) similar iteroparity rates for Willamette River basin and lower Columbia River steelhead stocks suggest similarity in migratory conditions, and (3) by contrast, the interior Columbia River stocks have a longer migration distance and also experience challenges associated with the hydropower system.Received December 12, 2014; accepted July 30, 2015