Correlates of growth in juvenile three-spined sticklebacks: potential predictors of growth rates in natural populations

The possibility of using indices to quantitatively predict short-term growth rates of juvenile three-spined sticklebacks (Gasterosteus aculeatus L.) was assessed. A range of weight-specific growth rates (GW) was obtained in experiments with individual sticklebacks fed enchtraeid worms at different daily rations over 21 days. There was a strong, positive correlation between GW and the RNA : DNA ratio in white muscle (r2 = 0.90) and lipid concentration (as percentage dry weight) of the carcass (r2 = 0.92). There were smaller, but significant correlations between GW and the percentage of dry matter in the carcass (r2 = 0.67), and the residuals from the weight–length relationship (r2 = 0.49). Regressions relating growth to RNA : DNA ratio and lipid concentration offer a means of estimating short-term growth rates in natural populations. A test of such predictions using the results from a 56-day experiment on stickleback growth suggested that predictions from percentage lipid were biased and inefficient, while predictions from the RNA : DNA ratio were unbiased but inefficient. Predictions from percentage lipid were higher than from the RNA : DNA ratio.