Modelling growth of Antarctic krill: a new approach to describing the growth trajectory

Von Bertalanffy (VB) growth models for Antarctic krill have in the past been calibrated from population-level data consisting of modal lengths obtained from a time sequence of length frequency samples. We develop 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 intermoult period. Using summer and early autumn data for juveniles and males sampled from the Indian Ocean sector we model IGR as a function of pre-moult length and season using linear mixed models incorporating cubic smoothing splines. We generate a number of growth trajectories starting from an age 1+ mean length for different scenarios of winter and spring growth. We then provide convenient parametric approximations to these step trajectories using either punctuated-growth or seasonal-growth VB models. Our models indicate that, allowing for shrinkage, age 6+ mean length for the Indian Ocean sector was close to 53 mm compared to 57 mm obtained from studies for the Atlantic Ocean sector.