Length-dependent growth and mortality within each cohort cannot be ignored in stock assessment: a case study of Antarctic krill Euphausia superba

Many marine species have different growth and mortality rates within a cohort due to length variability among individuals at the same age, but this fact has rarely been considered in stock assessment models as an assumption, which produces uncertainty in the results. Antarctic krill Euphausia superba , an important Southern Ocean fishery species, is characterized by length-at-age uncertainty and broad length distribution within the same cohort, making it a representative species to explore the effect of intra-cohort growth and mortality variability. We applied the age-structured catch-at-length (ACL) model, which assumes uniform growth and mortality within cohorts, and the age-and-length-structured statistical catch-at-length (ALSCL) model, which accounts for length-dependent growth and mortality variability within each cohort. Using 4 scenarios with different growth parameter estimations, we examined whether length-dependent growth and mortality variations affect assessment results. Our results revealed significant differences between the 2 models in cohort dynamics, including spawning stock biomass, relative abundance, growth patterns, and total mortality. The ALSCL model provided more accurate estimates for individuals over 45 mm and was less sensitive to growth parameter estimations. These differences highlight the impact of length-dependent growth and mortality within the same cohort of krill. Ignoring this assumption in stock assessment may bias estimates of growth potential and older age group abundance, affecting management advice based on spawning stock biomass. For species like krill, which are difficult to age or exhibit high growth variability within the same cohort, we recommend improving age-and-length-structured models to extend application in assessment and management.