Assessing Indices of Growth for Field Studies of Juvenile Salmon: An Experiment and Synthesis

The hypothesis that size‐selective mortality in the first marine year is a major regulator of recruitment in Pacific salmon Oncorhynchus spp. has led to interest in assessing the recent growth of field‐caught fish. Understanding differences in relative growth across years, regions, habitats, and prey fields may provide insights into factors influencing survival. Plasma insulin‐like growth factor 1 (IGF1), muscle RNA : DNA ratio (RD), and scale circulus spacing have all been used as indices of recent growth in juvenile salmonids. We concurrently assessed these growth rate indices in a laboratory study of postsmolt, young‐of‐the‐year, ocean‐type Chinook Salmon O. tshawytscha. We synthesized results with previous work to inform selection of appropriate growth rate indices for field studies on juvenile salmonids. Muscle samples suitable for RD analysis were obtained nonlethally and without subsequent growth impacts, even for very small juvenile salmon (75–99 mm FL). Plasma IGF1 concentration was strongly correlated with growth rate (R² = 0.79), while logₑ(RD) and mean spacing of the outer two circuli were moderately correlated with growth rate (R² = 0.47 and 0.44, respectively). Relationships between the two biochemical indices and growth rate were independent of body size at the start of the experiment. Conversely, initially larger fish formed wider circuli for a given growth rate. Insulin‐like growth factor 1 and RD responded to a change in growth conditions within 4 and 6 d, respectively. Rate of circulus formation varied positively with growth rate, meaning that outer circulus spacing indexed shorter periods of growth in faster‐growing fish. Our results confirm the value of plasma IGF1 as an index of recent growth in juvenile salmon. Although scales (marginal circulus spacing) and white muscle (RD) can both be sampled nonlethally, scale sampling presents a number of practical advantages.