Abstract-We evaluated 4 potential indices obtained by nonlethal sampling for use in determining nutritional state and short-term growth rate in postsmolt Atlantic salmon (Salmo salar): the ratio of RNA to DNA, both RNA and DNA normalized to protein, and plasma levels of insulin-like growth factor 1 (IGF1). Fish reared in the laboratory for 27 days were fed, fasted, or refed. Short-term growth rates (7 to 23 day intervals) were calculated on a wetweight basis. RNA/DNA values were highly correlated to growth rates, responded rapidly to changes in food availability and were the best able to consistently distinguish between the fasted and fed treatments. RNA/ protein values were also well correlated with growth rate; however, within any one sampling day, feeding groups could not be differentiated with this index. DNA/protein increased during fasting but was neither strongly correlated with growth rate nor an accurate discriminator of nutritional state. IGF1 values were positively correlated with growth rates and responded rapidly with refeeding but changed little during the 3 weeks of fasting-a result that may have been influenced by sampling serially. We propose that RNA/ DNA is a useful nonlethal technique for estimating recent growth rates and for identifying the nutritional condition of individual postsmolt Atlantic salmon exposed to short-term changes in food availability.The ability to measure the growth rate of a fish can be a powerful tool for evaluating the survival potential of an individual. The ability to assess the nutritional state of a fish, whether the animal is feeding or fasting and for how long the fish has been in that state (hours to days to weeks) is also highly desirable information because variation in nutritive state leads to variation in growth rate. Currently, there are a limited number of nonlethal techniques available for estimating growth rates or nutritional state (or both) in fieldcaught juvenile fish. Longitudinal cohort analysis is a direct approach to assessing changes in size (growth); however obtaining multiple samples of the same cohort in the field can be challenging. Biochemical indices that indirectly yield estimates of growth rate or nutritional state have the advantage of providing estimates within a single sampling. This point estimate allows an investigation of the connectivity between nutritional state and environmental parameters on relevant temporal and spatial scales.In this study we evaluated 4 potential biochemical indices of short-term growth-rate or nutritional state in postsmolt Atlantic salmon (Salmo salar): the ratio of RNA to DNA (RNA/ DNA), both RNA and DNA on a protein basis (RNA/pro and DNA/pro, respectively) and circulating plasma insulin-like growth factor 1 (IGF1). Considerable effort has been directed toward hatchery-based restoration of Atlantic salmon to 8 rivers in Maine, where the population has been listed as endangered since 2009 under the United States Endangered Species Act (Federal Register, 2009). Restoration managers require tools to both ass...

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Four tissues that can be removed nonlethally were evaluated for their effectiveness in estimating recent growth rates of Atlantic salmon Salmo salar smolts using RNA-DNA ratios (R/Ds). Weight-based growth rates were calculated for smolts reared in the laboratory for 4 weeks. Gill, muscle, scale, and caudal fin tissues were sampled to evaluate sampling ease during fieldwork, the quantity of RNA and DNA available for analysis, and the correlation of R/D with growth rate. Muscle provided adequate tissue for nucleic acid analysis, was strongly correlated with recent growth rates, and could be easily sampled in the field using a biopsy punch. Gill tissue did not provide adequate RNA for analysis with an ethidium-bromide-based microplate fluorometric assay, and R/Ds from scale and fin tissues were poorly correlated with growth rate.