Modelling growth of Antarctic krill. II. Novel approach to describing the growth trajectory

Abstract

Von Bertalanffy (VB) growth models for Antarctic krill have been calibrated from population-level data consisting of modal lengths obtained from a time sequence of length frequency samples. We developed an alternative approach to predicting the trajectory of length over time, using a step-growth function that combines models of instantaneous growth rate (IGR) at moult calibrated from direct measurements of individual pre- and post-moult krill sampled from the wild with a model of temperature-dependent inter-moult period (IMP). Using summer and early autumn data for juveniles and males sampled from the Indian Ocean sector of the Southern Ocean, we modelled IGR as a function of pre-moult length and season using linear mixed models (LMM) incorporating cubic smoothing splines. We generated a number of growth trajectories starting from an Age 1+ yr mean length, for different scenarios of winter and spring growth. We then provided convenient parametric approximations to these step trajectories for use in population dynamic modelling systems using either a punctuated-growth model based on an F-distribution or a seasonal-growth VB model. Alternatively, step-trajectories could be used directly. Our models indicated that, when allowing for shrinkage, at the start of the sixth winter after hatch (Age 5+ yr) the mean length of krill for the Indian Ocean sector was close to 53 mm, compared to 56 mm obtained from studies for the southwest Atlantic Ocean sector. In contrast, when shrinkage was not allowed for, a slightly higher mean length was obtained for krill from the Indian Ocean sector (57.5 mm) compared to krill from the southwest Atlantic Ocean sector.