Patterns of Iteroparity in Wild Snake River Steelhead

Abstract

AbstractSome anadromous salmonids are iteroparous (repeat spawners); however, postspawn individuals are often not considered in management plans because of the lack of long‐term studies of this life history. The balance of factors affecting survival between spawns and fecundity of repeat spawners should determine the percentage of iteroparous individuals in a population. Steelhead Oncorhynchus mykiss are capable of iteroparity, but Snake River populations in the interior Columbia River basin in the Pacific Northwest have the lowest iteroparity rates. Managers are interested in augmenting the incidence of iteroparity as a conservation measure. However, there is little information about repeat‐spawning steelhead to guide and evaluate management. In this study, we elucidated patterns and characteristics of repeat‐spawning steelhead in the Snake River basin. Repeat spawners were overwhelmingly female and were found in all 10 stocks. Most iteroparous fish first spawned after 1 year in the ocean (51.4%) and skipped a year between spawns (55.4%). Growth between spawns declined for longer, older steelhead. Survival from the first spawning migration to postspawn emigration increased during the study. However, survival in the Columbia River estuary and Pacific Ocean declined such that overall survival to second spawn varied little. Small females were most likely to survive to a second spawning migration. We hypothesized that iteroparity in Snake River steelhead was constrained by the energetic costs of the spawning migration (distance, elevation, timing, dam passage) combined with a lack of timely postspawn recovery. Hence, survival to second spawning migration is low for Snake River steelhead and most postspawn steelhead needed a full year to recover. Management programs intended to augment the incidence of iteroparity should account for (1) factors that increase stress and metabolic demand on spawning and postspawn fish and (2) factors that increase the time it takes for emigrating kelts to effectively refuel and recover.