Estimating Recent Growth Rates of Atlantic Salmon Smolts Using RNA‐DNA Ratios from Nonlethally Sampled Tissues

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.