The consecutive (1-year spawning interval) and skip (≥2-year spawning interval) repeat spawning life histories are found throughout seasonally breeding iteroparous teleosts, including anadromous steelhead trout. Initiation of a reproductive cycle in salmonids is thought to be determined by energetic status during a seasonally defined critical period ~1 year prior to spawning, during which energy levels must exceed a threshold or spawning will be deferred for a future annual cycle. Evidence in steelhead and rainbow trout indicates that reproductive schedule is determined during the period following spawning. However, the relevant aspects of energetic status and the importance of energetic status at spawning versus post-spawning nutrition are not well understood. The growth hormone–insulin-like growth factor-I (GH–IGF-I) endocrine axis indicates metabolic status. During fasting, circulating IGF-I levels decrease and GH levels increase, indicating a catabolic state. Elevated GH levels stimulate lipolysis, enabling access to endogenous energy reserves, while reduced IGF-I levels curtail growth.
We hypothesized that reproductive schedule in female steelhead trout would be determined during the 10 weeks after spawning based on an interaction between metabolic status at spawning and post-spawning nutrition and conducted an experiment to investigate these effects. Females collected at spawning were either fed to satiation or fasted for 10 weeks before being returned to satiation feeding. Plasma samples and morphometric data were collected every 10 weeks until terminal sampling at 40 weeks. Plasma GH, IGF-I, and estradiol (E2) levels were measured using established and validated assays. Reproductive status was assigned based on complete separation of E2 levels into two groups at 40 weeks. Some fish in both the fed and fasted groups became reproductively active on a consecutive spawning schedule. Fasted females that became reproductively active had lower GH levels at spawning than fasted fish that became non-reproductive, indicating a less catabolic state before the fasting treatment was begun. Of the fish that were fed after spawning, reproductively active females displayed both a decrease in GH from spawning levels and lower GH than non-reproductive fed females at 10 weeks, suggesting greater feed consumption. GH increased strongly in fasted fish after 10 weeks of fasting, and then decreased during refeeding and recovery from spawning. IGF-1 levels were lower in fasted fish after 10 weeks of fasting, and then increased during refeeding and recovery from spawning. These results suggest that the GH-IGF-I axis operates to access stored energy and regulate growth in the post-spawning state. Our results show that both metabolic status at spawning and feeding during the first 10 weeks after spawning influence reproductive decisions in repeat spawning female steelhead.