Penney, Z. L., and C. M. Moffitt. 2014. Histological Assessment of Organs in Sexually Mature and Post-spawning Steelhead Trout and Insights into Iteroparity. Reviews in Fish Biology and Fisheries 24:781-801. DOI 10.1007/s11160-013-9338-2.
Like other anadromous salmonids, steelhead trout (Oncorhynchus mykiss) fast during freshwater spawning migrations, but little is known about the changes that occur in vital organs and tissues during this process. Stocks of steelhead show a wide range in the proportion of iteroparity, and efforts to recover stocks at risk have explored tools to understand the limiting factors affecting iteroparity. Intuitively, recovery and repeat-spawning is ultimately dependent on restoring lost somatic energy via the re-initiation of feeding. Using Snake River steelhead as a model we examined the gastrointestinal tract of kelts to document if freshwater feeding was occurring (active energy intake), and (2) used histological analysis to quantitatively assess the cellular architecture in the pyloric stomach, ovary, liver, and spleen in sexually mature and in migrating kelts to evaluate organ function and the capacity for iteroparity. We found 38% of the kelts contained food or fecal material in the gastrointestinal tract, and feeding was more likely in kelts in good condition. Histological assessments of the pyloric stomach found a significant renewal of villi in migrating kelts, associated with feeding. No vitellogenic oocytes were observed in sections of kelt ovaries, but perinucleolar and early/late stage cortical alveolus stages were present suggesting that most Snake River steelhead would likely exhibit skip-spawning (alternate years) strategies rather than consecutively (spawn annually). We documented a negative relationship between the quantity of perinucleolar oocytes in steelhead ovaries and fork length of fish suggesting that larger steelhead may invest more heavily into a single spawning event. Liver and spleen tissues rarely displayed significant cellular deterioration or loss of function during maturity or kelt emigration. Together, these findings indicate that the physiological processes causing rapid senescence and death in semelparous salmon are not the same in steelhead, and recovery begins in freshwater.