Temperature effects on the growth and maturation of Antarctic krill (Euphausia superba)

This paper describes growth and maturation of individual Antarctic krill for a complete annual cycle in an artificially controlled environment. Antarctic krill were kept individually and maintained under temperature regimes of −1, 1 and 3 °C, while light conditions simulated natural seasonal conditions for the 14 months of the experiment starting in April. The growth increment (GI), instantaneous growth rate (IGR), daily growth rate (DGR), intermoult period (IMP) and maturity scores (MS) were tracked by measuring the consecutively moulted exoskeletons and modelled using linear mixed models (LMMs). There was a clear seasonal trend in growth and maturity in the three different temperature treatments. Males generally reached peak growth rates and matured earlier than females, but females showed higher growth rates overall. Negative growth occurred towards the end of January for both males and females, coinciding with the regression of external sexual characteristics. There was, as expected, a significant decline in IMP with increasing temperature, which was moderated by increasing trends with total length (TL) and month for the −1 °C treatment. For the 1 and 3 °C treatments there was no relationship with TL and the season effect declined after October and December, respectively. For krill with a TL of 40 mm in October, the predicted mean IMP values at −1, 1 and 3 °C were 28 days, 24 days and 19 days, respectively. There was some evidence to suggest that 1 °C was optimum for growth, as the IMP was significantly lower than at −1 °C but the difference in GI between the two temperatures was not significantly different from zero, while all growth variables were significantly lower for the 3 compared to the −1 °C treatment. For the first time, this study has been able to confirm, by fitting a bivariate LMM, that compensation mechanisms do exist between IMP and IGR and to quantify the degree of compensation at both the between- and within-animal levels. This will allow modelling of growth trajectories in the wild to quantify the distribution of length given age.