Contributions to the assessment of demersal fish stocks off Namibia

Abstract

In Northern Benguela, the two hake species M. capensis and M. paradoxus with similar visual appearance have a common habitat and overlapping depth and latitudinal distribution. These species can be found in the same hauls in what can be referred to as a mixed fishery. The challenges in mixed fisheries are that it is often not possible to obtain Maximum Sustainable Yield from all stocks at the same time. Species-specific assessments are important for management and sustainable utilization of the fisheries resources. Since the two species are similar and not distinguished by the commercial fisheries, methods should be developed to separate the species in commercial landings. This thesis has developed splitting algorithms to split the two hake species in the commercial landings using observer and survey data (Paper I). The results support the previous findings: M. capensis had a shallower and more northern distribution than M. paradoxus, while M. paradoxus dominated the annual hake catches. Based on the results from Paper I, the thesis proceeds to develop a species-specific abundance at each commercial haul (Paper II). We develop a generalized additive model (GAM) to identify factors that are affecting catch rates or catch-per-unit- of effort (CPUE) of M. capensis and M. paradoxus for each tow (Paper II). Standardized catch rates for M. paradoxus are significantly higher than catch rates of M. capensis. Furthermore, seasonal migration patterns are described for the first time and are found to correspond to the spawning areas and season. We conclude on hake species-specific studies by implementing a data-rich statespace assessment model (SAM) (Paper III) for two independent single-species assessments, one for M. capensis, and the other for M. paradoxus. M. capensis showed a higher spawning stock biomass than M. paradoxus, while fishing mortality was higher on M. paradoxus. Finally, in (Paper IV) a biomass dynamic model (SPiCT) was applied to a data-rich Cape monkfish and a data-moderate West Coast sole with a perspective of evaluating their use in Northern Benguela with a particular focus on assessing demersal resources off Namibia. Overall, the thesis work has constructed state-of-the-art single-species assessment models of hake and monkfish, using SAM, has explored the applicability of data-moderate assessment methods, specifically SPiCT, to monkfish and applied it to west coast sole, and has examined the differences in distribution between the two hake species, thus addressing the species splitting problem which is a prerequisite for separate assessments of the two species. Thus, the work has contributed to the advance of the state-of-the-art with respect to fisheries assessment and sustainable management of the living resources off Namibia and in the Benguela current system.