Variation in growth among individuals and over time: A case study and simulation experiment involving tagged Antarctic toothfish

Organisms in the marine environment are likely to exhibit variation in growth rates among individuals, and this variation may be persistent (particular individuals growing faster/slower throughout their entire lifetime) or transient (particular individuals growing faster in one year than another year). Understanding variation in growth is necessary when interpreting data regarding size (length or weight) in population models, or when estimating growth given data for tagged individuals. In this study, we explicitly model persistent and transient variation in growth rates among individuals in a wild marine population of Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea, in addition to sex-specific differences in average growth rates. The model is implemented using maximum marginal likelihood estimation and validated using a simulation study. The code is distributed as a publicly available package TagGrowth in the R statistical environment. Using simulated data, we show that we can accurately estimate parameters representing the magnitude of persistent and transient variation in growth rates, and that parameters estimated in these models are reasonably precise given the case study sample sizes (315 individuals tagged and recaptured over 10 years). The case study application suggests that transient variation among individuals accounts for up to half of the total variability in Antarctic toothfish. We conclude by recommending further research to additionally estimate temporal and spatial variation in growth rates. Estimating the relative magnitude of multiple sources of growth variation will improve our ability to assess the sensitivity of existing population models to growth variation, as well as to understand the range of variation exhibited by wild marine populations.