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Possible decline in the half-pounder life history among Trinity River steelhead (Oncorhynchus mykiss)
Steelhead, Oncorhynchus mykiss, exhibit a wide variety of life histories, including a unique “half-pounder” life history, primarily found in Northern California and Southern Oregon. A half-pounder is an immature steelhead that returns to freshwater within about four months of ocean entry. I estimated the frequency of the half-pounder life history among wild and hatchery adult steelhead from the Trinity River using scale analysis. Hatchery smolts currently are released as age-1 smolts from Trinity River Hatchery and are substantially larger than their wild counterparts. Wild smolt ages range from 1 through 3, with most smolts ages 1 or 2. To determine whether age- or size-at-ocean entry affects the tendency for an individual steelhead to exhibit half-pounder life history, I examined over 2,000 scale samples from wild and hatchery adult steelhead collected intermittently over a 27-year period from 1982 through 2009. Age at ocean entry of wild steelhead smolts appeared to be strongly associated with the proportion of adult steelhead returning that have undergone a half-pounder run. Wild smolts entering the ocean as age-1 smolts were more likely to undergo a half-pounder migration than age-2 or age-3 smolts. Based on logistic regression analyses, the size at the end of freshwater zone (prior to estuary or ocean entry) was inversely correlated with half-pounder proportions among returning wild adults. Thus, wild age-2 smolts that were smaller at the end of the freshwater zone were more likely than larger steelhead smolts of the same age to undergo the half-pounder migration. The tendency of hatchery smolts to mature as half-pounders was generally quite low (10-20%), especially in recent years, and appears related to the large size at which steelhead have been released from Trinity River Hatchery since the mid-1990s.