Modelling ecosystem effects of reduced prawn recruitment on the Thukela Bank trawling grounds, South Africa, following nursery loss

Abstract

Penaeid prawns are targeted in the Thukela Bank shallow-water trawling grounds in south-eastern Africa. Based on previous studies, these targeted prawns are assumed to primarily use 2 large estuaries (St. Lucia and Richards Bay/Mhlathuze) as nurseries on the KwaZulu-Natal (KZN) coast, which comprise over 80% of the estuarine habitat in the region. However, St. Lucia has been closed to the Indian Ocean since 2002 due to continued separation from the Mfolozi River, cessation of dredging required to open the inlet, reduced freshwater flow and drought con- ditions. To study the effects of prawn trawling and prawn nursery loss on the ecosystem, Ecopath with Ecosim models of the Thukela Bank were constructed. Models were fitted to total catch (t) for 1990 to 2009, and biomass trends were estimated by scaling Ecosim feeding interaction parame- ters to prey trophic level. Prawn trawling was modelled using fishing effort data. Prawn recruit- ment was modelled via the 'hatchery stocking' function in Ecosim simulating nursery loss along the KZN coast. Various prawn recruitment and trawling effort levels were simulated from 2010 to 2040. Decreasing prawn recruitment positively affected benthic fish and carnivorous benthos bio- mass and negatively affected prawn, commercial crustacean and benthopelagic fish biomass. Negative effects were exacerbated and positive effects were decreased by high versus low trawl- ing effort. The results suggest that indirect anthropogenic and environmental factors, such as recruitment from estuarine nursery areas, can impact offshore fisheries catches. Therefore, when modelling and managing marine ecosystems, it is important to couple processes between adjacent systems, particularly for critical life-history stages.