No Guts No Glory: Using Histology to Assess the Capacity for Post-Reproductive Recovery in Snake River Steelhead Kelts

During freshwater spawning migrations steelhead trout begin fasting and rely on stored energy. In inland Snake/Columbia River steelhead, fasting can range from 6 to 12 months. Little is known about the effects that prolonged fasting have on organ systems of maturing steelhead. We lethally sampled sexually maturing and post-reproductive (kelt) steelhead and evaluated the histology of the gastrointestinal tract, liver, and spleen tissues. We found liver and spleen tissues showed well-defined differences in cell microstructure that were correlated with fish condition. The integrity of liver hepatocytes and abundance of vacuoles were generally related to higher energetic and nutritional status of kelts. Histological assessments of the gastrointestinal tract did not always correspond to the external condition of fish, but appeared to be predominantly affected by feeding activity in kelts. The gastrointestinal tracts of feeding individuals showed a resurgence of goblet cells and tissue layers in the pyloric stomach and ascending intestine suggesting that these metrics were correlated with ability to feed, digest, and assimilate consumed energy during emigration. We believe these histological metrics can be combined with measures from blood plasma and white muscle energy content to estimate the capacity for iteroparity in migrating steelhead kelts.