A summary of the assessment and management approach applied to South African abalone (Haliotis midae) in Zones A-D

Abstract

The management of abalone stocks worldwide is complicated by factors such as poaching combined with the difficulties of assessing a sedentary (but not immobile) resource that is often patchily distributed. The South African abalone Haliotis midae fishery is faced with an additional problem in the form of a movement of rock lobsters Jasus lalandii into much of the range of the abalone. The lobsters have dramatically reduced sea urchin Parechinus angulosus populations, thereby indirectly negatively impacting juvenile abalone, which rely on the urchins for shelter. The model developed for abalone is an extension of more standard age-structured assessment models because it explicitly takes spatial effects into account, incorporates the ecosystem change effect described above and formally estimates illegal catches using a novel index, the Confiscations Per Unit Policing Effort (CPUPE). The model is simultaneously fitted to CPUE and Fishery-Independent Abalone Survey (FIAS) abundance data as well as several years of catch-at-age (cohort-sliced from catch-at-length) data for the various components of the fishery as well as for different strata. A basic tenet of fisheries modelling is to not go beyond the information content of the data. The model developed involves the efficient use of data to allow a model of greater complexity (as was essential in this instance) than usual. It has provided the basis for management advice over recent years by projecting abundance trends under alternative future catch levels.