Comparing Non-Lethal Measures of Condition and Nutritional Status in Snake River Steelhead Trout to Histological Assessments of the Liver, Spleen, Gastro-Intestinal Tract, and Gonad

Steelhead trout (Oncorhynchus mykiss) are iteroparous and many individuals will emigrate downriver after spawning as kelts.  Kelt survival rates to the ocean are difficult to quantify and repeat-spawning rates in Snake River steelhead trout are speculated to be less than 2%.  In 2009 and 2010, we lethally sampled sexually maturing and kelt steelhead trout.  All fish were visually assessed for external condition and categorized as good, fair, or poor.  Blood samples were collected from the caudal vessel and a necropsy of internal organs conducted.   We found that good condition steelhead trout possessed higher nutritional plasma factors than poor condition steelhead trout.  Steelhead trout categorized in fair condition occupy an ambiguous range of nutritional factors that typically transcend both good and poor categories. Our histological assessments of liver and spleen show well-defined differences in cell microstructure between poor and good condition steelhead trout.  In particular, the liver was useful in further separating fair individuals into good, fair or poor categories based on hepatocyte structure and the presence or absence of glycogen vacuoles.  Histological assessments of the gastro-intestinal tract did not always correspond to the external condition of fish, blood plasma, or condition of the liver and spleen.  Rather, the gastro-intestinal tract appears to be predominantly affected by feeding activity.  The majority of sexually maturing steelhead trout did not appear to be feeding, but 16% of kelts in 2009 and 21% of kelts in 2010 were identified as having food items in their stomach.  The gastro-intestinal tracts of feeding individuals showed a slight resurgence of goblet cells and tissue layers in the pyloric stomach and ascending intestine.  Additional analysis, of ovary tissues from female kelts also showed the presence of numerous stages of oocyte development suggesting that many females were fully capable of re-spawning.  Histological assessment of vital organs provides useful information regarding fish condition that may not be always be apparent via external condition or non-lethal measures of nutrition, such as blood plasma.  Using histology we can better understand the physiological processes that occur during steelhead trout sexual maturation and kelt emigration and thereby gain insight to the factors that may influence successful repeat-spawning.