Use of Morphological and Physiological Indices to Characterize Life History Diversity in Juvenile Hatchery Winter-Run Steelhead

AbstractSteelhead Oncorhynchus mykissexhibit significant life history plasticity, including variation in freshwater residence time, age of smoltification, and maturation that are influenced by exceeding specific thresholds in size or body energy stores. Fish that do not reach minimum thresholds for smoltification or exceed thresholds for maturation may “residualize” in freshwater. Understanding life history diversity is important in steelhead hatchery programs. The objective of this investigation was to characterize life history diversity in Hood River, Oregon, hatchery winter-run steelhead prior to release. We used both morphological and physiological indices to assess degree of smoltification in both sexes and maturation in males. Smoltification was categorized by measuring size, visual smolt index, and gill Na+/K+-ATPase activity. All females were determined to be immature. Male reproductive development required histological staging of testes. Other indices of maturation—including gonadosomatic index, plasma 11-ketotestosterone, and pituitary fshband lhband testis ifg3and amhgene expression—were also employed, with varying degrees of success. Five phenotypes were described: (1) immature male and female parr; (2) precociously mature male parr; (3) male smolts in early to mid-spermatogenesis that are likely to spawn after <1 year in the ocean; (4) males that might have attempted maturation, but failed; and (5) immature male and female smolts that could mature after >1 year in the ocean. Over three brood years, we estimated that 3–4% of the population could potentially residualize due to failure to smolt or precocious male maturation, 1–12% could mature after <1 year in the ocean (one salt male), and 84–95% could mature after >1 year in the ocean (two to three salt males and females). Based on previous studies conducted in the Hood River basin, the residualism rate estimated for this hatchery population is lower than that of naturally rearing Hood River steelhead.Received November 2, 2016; accepted February 14, 2017 Published online May 11, 2017