Ecosystem Approaches for Fisheries Management, Part 3

Ecosystem Approaches for Fisheries Management 211 Alaska Sea Grant College Program • AK-SG-99-01, 1999 Ecosystem Impacts of the KwaZulu-Natal Reef Fishery, South Africa: An Exploratory Model M. Verónica Toral-Granda University of Cape Town, Percy FitzPatrick Institute of African Ornithology, Rondebosch, South Africa Coleen L. Moloney Marine and Coastal Management, Cape Town, South Africa Jean M. Harris KwaZulu-Natal Nature Conservation Service, Durban, South Africa Bruce Q. Mann Oceanographic Research Institute, Durban, South Africa Abstract The linefishery in the subtropical waters off KwaZulu-Natal, South Africa, targets a variety of species associated with rocky reefs at depths of up to 200 m. Populations of many of these reef fish are severely depressed by past and current fishing. Current management objectives aim to rebuild these stocks and to conserve the ecosystems, but means of implementing these objectives are unclear. This paper presents a mass-balance, ecosystem model of the KwaZulu-Natal reef ecosystem. It divides the reef community into 18 groups, and quantifies biomass, production, and consumption for each group. Flows among groups are calculated using a diet composition matrix that changes according to the relative productiv- ity of food groups. Model results indicate that community biomass is domi- nated by suspension-feeders. Detritus is assumed to be an important source of food for consumers, but this needs to be substantiated. The biomass and production of selected groups were changed in a series of experi- ments to investigate the impacts of the fishery. The results were evalu- ated in terms of ecotrophic efficiencies, and sometimes were unexpected 212 Toral-Granda et al. — Ecosystem Impacts of a Reef Fishery because of factors such as redirected predation from previously abundant groups onto other prey groups. The model was sensitive to changes in biomass of the fish groups, including that of the group that was historically dominant in the fishery. Given the large historical changes, the current reef ecosystems are probably markedly different from the pristine state. However, it is difficult to predict the likely impacts of past and future changes on single species. Future management should aim to main- tain unfished areas of representative reef, and future data collection should be directed at comparative studies of protected and unprotected reef communities, to complement the ongoing fisheries data. The model, despite its limitations, is guiding future research so that limited resources can be directed appropriately to achieve management goals. Introduction Off KwaZulu-Natal, South Africa, the continental shelf is narrow, reaching a maximum width of 45 km in the central area and narrowing to 3-11 km in the north and south (Garratt 1984; Fig. 1). Scattered rocky reefs occur along the shelf in depths of up to 200 m (Fig. 1). The shelf waters are warm (21-26ºC) and the rocky reef ecosystems support diverse, subtropical communities containing more than 1,000 fish species belonging to 150 fami- lies (van der Elst 1988). Many of the demersal reef fish species are targeted by a diffuse, mostly small-scale linefishery that operates in the coastal areas (Garratt 1986), and supports recreational, subsistence, and com- mercial sectors. The mean annual landings of the linefishery over the last decade have been approximately 1,270 t (Mann-Lang et al. 1997). Fishing effort by all sectors of the KwaZulu-Natal reef fishery increased dramatically from the turn of the century. In early years, the large en- demic reef species were important in the catches, including species such as seventy-four (Polysteganus undulosus), red steenbras (Petrus rupestris), and other sparids that together made up 60-80% of the total catch by weight. With the fishing down of these previously abundant species to very low levels, there have been marked shifts in the species composition of the catch (Garratt 1986, van der Elst 1988, van der Elst and Adkin 1988, van der Elst and de Freitas 1988, Mann-Lang et al. 1997). Small, less fa- vored species are now caught, particularly slinger (Chrysoblephus puniceus) and soldier (Cheimerius nufar). In addition, demersal migrants have be- come more important in the catches. Most of the species currently caught by the linefishery are now considered threatened or endangered. Management of the KwaZulu-Natal linefishery has been carried out primarily by means of species-specific restrictions, including bag limits, size limits, and closed seasons (Mann-Lang et al. 1997). However, in a multispecies, multi-user fishery of this sort, it is difficult to predict the effects of single-species measures on the dynamics of other species in the ecosystem. Furthermore, there are concerns about the impacts that the Ecosystem Approaches for Fisheries Management 213 Figure 1. The distribution of rocky reefs on the continental shelf off KwaZulu- Natal, South Africa. After Garratt (1984). 214 Toral-Granda et al. — Ecosystem Impacts of a Reef Fishery shifting species composition will have on other species and on the reef ecosystem as a whole. This paper attempts to understand the complex trophic relationships that occur in the rocky reef ecosystem. The limited available data from the region are synthesised in an ecosystem model, using a mass balance approach similar to that used in the Ecopath software (Christensen and Pauly 1992). The model is necessarily speculative and is used primarily as a tool to aid understanding about the possible past and future ecological effects of fishing on the rocky reef ecosystem. The model is also used to highlight gaps in knowledge. Despite its limitations, the model is useful in synthesising information, and it is being used to guide future research and management in the context of an ecosystem approach. Methods Ecosystem Model Structure The biological component of the rocky reef ecosystem is represented by 18 groups, aggregated on the basis of trophic function: 1 detrital, 2 primary producer, 5 invertebrate, and 10 vertebrate groups (Fig. 2). Each group contains a large number of species. Detritus In deep reefs, light availability limits photosynthesis, and it is assumed that the energetic basis for the system production comes mainly from detritus. This includes material from the overlying water that has sunk through the water column, as well as imported material, much of which presumably derives from land via the many rivers in the region (Fig. 1). The detrital pool receives contributions from all dead organic material, and the excretions and egestions of all living components of the ecosystem. It consists of fecal material, dead plants and animals, exoskeleton molts, particulate organic matter, dissolved organic matter, and bacteria. Detritus is consumed by many of the groups in the model. Primary Producers Primary production is derived from two main sources: macroscopic algae and phytoplankton. Macroscopic algae are attached to the rocky substra- tum, and are grazed by vertebrates and invertebrates. Phytoplankton pro- vide food for some suspension-feeding, reef-dwelling organisms as well as for pelagic species that are migrant visitors to the reef system. Invertebrates All small, invertebrate heterotrophs suspended in the water column are represented by a single zooplankton category (which also contains fish lar- vae). This aggregation necessarily results in an over-simplification of the food web, because zooplankton communities typically contain herbivores Ecosystem Approaches for Fisheries Management 215 Figure 2. Diagrammatic representation of the major components of the KwaZulu- Natal rocky-reef ecosystem, and their relative positions in the water column. Model groups can be further aggregated into the seven groups shown in the key. 216 Toral-Granda et al. — Ecosystem Impacts of a Reef Fishery and carnivores with a large range of sizes and life-styles. The remaining four invertebrate groups are all associated with the rocky reef, and are distinguished on the basis of their main feeding mode and diet: suspension-feeders, deposit feeders, grazers, and carnivores. Vertebrates Of the 10 vertebrate groups, 8 consist of bony fish, 1 represents sharks, and 1 represents marine mammals and seabirds. The fish groups are sepa- rated using four linked criteria: (1) habitat (reef-dwelling species or pelagic “migrants”); (2) feeding mode and diet; (3) size; and (4) status in the fishery (whether the species are currently or were previously important in the catches). These criteria result in two pelagic groups (small and large pelagic fish) and six reef-dwelling groups: planktivores, herbivores (i.e., grazers), small carnivores, large carnivores (i.e., feeding mainly on invertebrates), and two groups of large piscivores (feeding mainly on fish), one containing species that are commonly caught at present and the other containing those species that were important historically but that are currently scarce. Representative species are indicated for each of these groups (Fig. 2). Ecosystem Flows and Data Sources The mass balance for each component of the ecosystem is given by: Consumptioni = Productioni + Respirationi + Egestioni (1) Annual consumption (Q) and production (P) were derived from biomass (B) estimates using constant annual Q:B and P:B ratios: Consumption (gDW × m–2 × y–1)= Q:B (y–1) × Biomass (gDW × m–2) (2) Production (gDW × m–2 × y–1) = P:B (y–1) × Biomass (gDW × m–2) (3) Egestion (non-assimilated food) was assumed to represent 20% of consumption throughout, and respiration was calculated by difference from equation (1). All egested material was assumed to enter the detrital pool. The losses to consumers from each category i were calculated as: 18 Q =×∑ × Losses ofijB to consumers j DCij = (4) j 1 B j where DCi,j is the diet composition, expressed as the fraction by weight that food item i forms of consumer j ’s diet. Diet compositions are known for only a few of the species that make up the KwaZulu-Natal rocky reef community.

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