Recruitment modelling for Antarctic krill (Euphausia superba) stock assessments considering the recurrence of years with low recruitment

Antarctic krill (Euphausia superba) is a keystone species in the Southern Ocean food web, and, as such, it is crucial to effectively manage the krill fishery to ensure its long-term sustainability. Setting precautionary catch limits for krill relies on sampling and population modelling. Krill stock projections are developed with the generalised yield model (GYM), which provides an assessment for stock status under current harvesting scenarios and various levels of uncertainties. One of the fundamental components of the GYM is the simulation of recruitment. De la Mare (1994) presents a proportional recruitment model for estimating krill recruitment based on length-frequency distributions collected from field surveys. The de la Mare (1994) function uses estimates of the mean and variance of proportional recruitment from survey data to determine the scaling of natural mortality and the distribution of random recruits that reproduce the observed mean and variance estimates. Here we evaluated de la Mare’s (1994) proportional recruitment function and found that for large variations in recruitment the function does not accurately reproduce the observed mean proportion of recruits and its variance. The deficiencies within the de la Mare (1994) function were reviewed and two alternative methods were provided, which can support a wider range of values and possible scenarios, such as years of extremely low recruitment.