Biostatistical implications of individual variability in growth in rainbow trout and Atlantic salmon

Abstract The relationship between specific growth rate and initial body weight in fish was analysed by multivariate analysis using data sets from growth studies on rainbow trout (initial weight between 10 and 30 g, n =546) and Atlantic salmon (initial weight between 10 and 20 g, n =355). We demonstrate that individuals can be pooled into different classes according to their growth characteristics. Classes with similar initial mean weight can have very different mean specific growth rate. Indeed, we found the mean specific growth rate ratio between classes with similar initial mean weight to be between 1.5 and 5.8 in rainbow trout and between 1.2 and 1.9 in Atlantic salmon. We conclude that the constitution of replicates with homogeneous initial weight is not assurance of homogeneous specific growth rate among replicates. The consequences of such variability in growth on experimental designs and power of analysis-of-variance models are discussed.