Salish Sea Marine Survival Project researchers compared an array of growth rate indices in current use for measuring salmon growth by testing each index on laboratory-held fish with known growth rates. They found that plasma insulin-like growth factor 1 (IGF1) concentration was strongly correlated with juvenile chinook salmon growth rate, while muscle RNA:DNA ratio and mean spacing of the outer two circuli of fish scales were moderately correlated with juvenile chinook salmon growth rate. Plasma IGF1 is causally related to growth and a strong indicator of relative recent growth rate; however, for small fish samples must be taken lethally. Muscle tissue samples can be taken non-lethally, and scale sampling is minimally invasive. Both these indices can be used to assess growth rates, with the caveats that thermal regime changes can impact muscle RNA:DNA ratios and that scale metrics represent different periods of growth for fish growing at different rates. Read more: Duguid et al. (2018) Assessing indices of growth for field studies of juvenile salmon: an experiment and synthesis. DOI: 10.1002/mcf2.10020
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