Fast growth inferred for northern Benguela shallow-water hake Merluccius capensis using annual survey- and monthly commercial length-frequency distributions

The hypothesis that fast growth rates demonstrated for 3-21-month-old Merluccius capensis continue for older fish is tested. Cohort-specific growth rates of M. capensis between 6 and 65 cm total length were described using a von Bertalanffy growth function (VBGF) fitted to annual/bi-annual research survey length-frequency distributions (LFDs) from 1995 to 2011 and monthly commercial LFDs from 1998 to 2011. These data were combined with cohort analysis of younger fish from seal scat data to provide cohort-specific growth rates for cohorts hatched from 1994 to 2008. The fitted VBGF gave Lt (cm) = 108.6*{1-exp[-0.199*(t(y) + 0.025)]}. M. capensis grow between 1.4 and 0.8 cm month−1 from ages 1 to 5 years, varying by 0.2 cm month−1 among cohorts. This is about twice as fast as growth rates previously estimated from otolith methods, which therefore need to be re-evaluated. It follows that M. capensis should have higher natural mortality rates and a greater stock productivity than previously believed, emphasizing the need to review this in the current stock assessment approaches.