Mortality and biological reference points for the king mackerel ( Scomberomorus commerson) fishery off Natal, South Africa (based on a per-recruit assessment)

Abstract

Mortality estimates for the king mackerel ( Scomberomorus commerson) and biological reference points derived from a yield and a spawning biomass-per-recruit assessment for the fishery off Natal, South Africa are presented. Total mortality was estimated from a catch curve and a technique requiring age-at-full requirement and mean age of fully recruited fish. The estimates of total mortality ( Z) differed between these techniques and an average Z of 0.75 year −1 was assumed. The natural mortality rate ( M) was also estimated from two different techniques and yielded different values. An average M = 0.5 year −1 was therefore assumed. The per-recruit model was based on three different growth curves and was sensitive to the choice of growth parameters. A growth curve assuming annual periodicity of the otolith bands predicts a virtual collapse of the fishery when the fishing mortality rate ( F) equals M. At the current age-at-first-capture it also predicts that the spawning biomass will be 50% of its unexploited level at a fishing mortality rate of 0.1 year −1. This scenario is considered unrealistic for the Natal king mackerel fishery. The per-recruit analyses based on the other two growth curves show no indication of biological overexploitation at the current fishing mortality rate (0.25 year −1). Values of M from 0.3 to 0.4 and from 0.5 to 0.7 year −1 show little variation in the biological reference point estimates. The current spawning biomass is probably at a minimum value of 35% of its unfished level. Setting a minimum size above the length-at-maturity will result in short term deceases in yield but with substantial gains in spawning biomass-per-recruit. However, such a management strategy will result in socio-economic problems because at least 90% of the catch currently being taken will no longer be accessible.