Modelling growth of Antarctic krill. I. Growth trends with sex, length, season, and region

Abstract

Growth trends of Antarctic krill with sex, length, season, and region as independent variables were modelled with a linear mixed model (LMM) using 10 yr of accumulated instantaneous growth rate (IGR) measurements. A model of inter-moult period (IMP) as a function of temperature, required to convert IGR to daily growth rate, was fitted to data from published constant-temperature rearing studies; this model was used to predict seasonal IMP using a model of the average sea surface temperature seasonal trend for each region. Smaller krill exhibited higher growth rates and, in general, we observed a progressive decrease in IGR and daily growth with increasing size (from over 0.2 mm d -1 down to 0.05 mm d -1 for 20 and 50 mm krill, respectively, in December in the Indian Ocean sector of the Southern Ocean). IGR decreased from summer to autumn, with small to negative values predominating across all length classes by autumn. December was a period of rapid growth in the Indian Ocean sector of the Southern Ocean, whereas a similar peak in growth rates appeared to occur a few months earlier in the southwest Atlantic sector. Significantly lower growth rates were exhibited by females in January and February (e.g. 0.063 and 0.050 mm d -1 for 40 mm females) compared to males (0.119 and 0.090 mm d -1 for 40 mm males) in the Indian Ocean sector. Since these estimates were based on growth measurements of individual animals, they may reflect the occurrence of sex-differentiated growth patterns in the natural environment. Growth rate differences between sexes suggested that growth rate estimates based on composite length frequency distributions without sex differentiation may be problematic.