Community ecology of the metazoan parasites of snoek Thyrsites atun (Euphrasen, 1791) (Perciformes: Gempylidae) off South Africa

The parasite assemblage of snoek Thyrsites atun, a commercially important fish in the Benguela system, was examined over a one-year period. In all, 16 parasite taxa including eight new host records (Bolbosoma vasculosum, Caligus coryphaenae, Caligus dakari, Corynosoma australe, Hatschekia conifera, Nothobomolochus fradei, Rhadinorhynchus cadenati, Tentacularia coryphaenae) and four new locality records (B. vasculosum, C. dakari, Molicola uncinatus, Pseudoterranova sp.) were recovered from 210 specimens. The dominance of larval helminths in the component community suggests that T. atun occupies an intermediate position in the food web. The ‘nestedness metric based on overlap and decreasing fill’ (NODF) indicated that the parasite assemblage exhibited no nestedness (NODF = 73.3, p = 0.103). Generalised additive mixed modelling results indicated that host length is the main determinant of parasite species richness in T. atun. PERMANOVA and ANOSIM suggested that infracommunity structure varied little with respect to the sex, length, seasonality and capture region of the host. The stability and randomness in parasite acquisition, as indicated by the lack of nestedness of the parasite assemblage, can be ascribed to the opportunistic feeding behaviour and nomadic movement of T. atun in the southern Benguela. The homogeneity of the community structure of long-lived endoparasites suggests that a single T. atun stock occurs off South Africa.Keywords: host traits, multivariate analysis, parasite infracommunity, population structur

The effect of marine protected areas on an exploited population of sex-changing temperate reef fish: an individual-based model

The effect of two marine protected areas (MPAs) on roman Chrysoblephus laticeps (Sparidae), an exploited reef-fish species inhabiting the South African temperate south coast, was simulated with a spatially explicit, individual-based model based on geographically correct habitat distribution. The model domains were small compared with the dispersal range of pre-recruits, and recruitment was held constant. Adult fish movement rules were based on the results of high-resolution markand-recapture and telemetry studies. Fish densities, age-structure and life-history parameters were derived from comprehensive underwater surveys and biological analyses. The model included the effect of fishing on size-at-sex change. The results indicate a recovery of fish abundance-per-recruit, size frequency and sex ratio to, or close to, pre-exploitation levels within the protected areas 10 years after the implementation of both MPAs. Results suggest that, for resident species such as roman, even small MPAs (6 km2) can offer protection. The small exchange of post-recruit fish into the adjacent areas results in negligible improvement (1%) of catches. The MPAs support more and larger spawning fish, which may improve recruitment into adjacent fished areas.Keywords: Chrysoblephus laticeps; fisheries management; individual-based model; marine protected areas; South Africa, spilloverAfrican Journal of Marine Science 2008, 30(2): 337–35

Effects of fishing on a temperate reef community in South Africa 2: benthic invertebrates and algae

The effect of fishing on benthic reef communities was studied by comparing reefs within and outside the Goukamma Marine Protected Area (MPA) on the South African warm-temperate south coast. Although study sites were similar in terms of bathymetry and oceanographic conditions, benthic reef communities were significantly different with higher abundance of algae and crinoidea at fished sites. Crinoidea, the principal food source of roman Chrysoblephus laticeps, was scarce at protected sites where this species was most abundant. Low algal abundance at protected sites was negatively correlated with grazer abundance within the fish communities encountered there, suggesting a high potential for coexistence of these trophic guilds. Other fish species showed dietary overlap with roman, and their abundance was likely controlled by interspecific exploitative competition for food. The study suggests that a moderate rate of  fishing can alter benthic communities through indirect effects on temperate reefs in South Africa. Keywords: benthic reef community; exploitative competition; fishing effects; foodweb; indirect effects; South AfricaAfrican Journal of Marine Science 2009, 31(2): 253–26

Dark times for dageraad Chrysoblephus cristiceps: evidence for stock collapse

The declaration of a state of emergency in the South African linefishery in 2000 has had a positive impact on a few overexploited endemic seabreams (Family Sparidae). However, the population of the reef-dwelling dageraad Chrysoblephus cristiceps has been unresponsive to progressively stricter management regulations. After decades of unsustainable fishing mortality the species is in a critical state. In this paper, a method based on standardised probability of capture is used to develop an index of relative abundance for rare species, such as dageraad, which is more robust to changes in output control regulations than conventional methods. The results show a severe decline in dageraad capture probability in the south-west region of South Africa, from 8% in 1985 to <0.1% in 2011. The east region experienced a more rapid decline, from 24% in 1999 to <0.1% in 2011. A spatial comparison of the distribution of historical and recent dageraad probability of capture along the South African coastline indicates a range contraction commonly associated with the collapse of a population. The once-widespread species is now largely limited to a few locations in the Eastern Cape province and dageraad is mostly absent across its former western distribution. Although the historical decline has commonly been attributed to commercial overexploitation, the recreational sector had an estimated 3–5 times greater impact on the dageraad population than the commercial sector in recent years. Given that previous management regulations have been unsuccessful in arresting the decline of dageraad populations, assertive management is necessary to ensure the species’ sustainability. Rehabilitation of dageraad stocks will require a further decrease in fishing mortality, in conjunction with maintaining an appropriate marine protected area (MPA) network.Keywords: CPUE, fisheries management, linefish, marine protected area, South Africa, Sparidae, stock assessmen

Life-history parameters of white stumpnose Rhabdosargus globiceps (Pisces: Sparidae) in Saldanha Bay, South Africa, with evidence of stock separation

White stumpnose Rhabdosargus globiceps were sampled over 24 months in Saldanha Bay on the west coast of South Africa. The species is a rudimentary hermaphrodite, with < 1% of mature fish having simultaneously developed ovaries and testes. Spawning activity extended from September to March and peaked in October and February. Mature males developed spawning colouration in spring, which can be used as an indicator of spawning activity. The amount of abdominal fat correlated with the spawning cycle in both sexes, suggesting that abdominal fat reserves are used in the development of gonads. Seasonal variation in the condition index suggests a slightly greater investment in reproduction by males than females. Females grew slightly faster than males and attained a larger size. The Saldanha Bay fish grew more rapidly and matured earlier than fish from populations on the south coast of South Africa. Age-at-50% maturity was 2+ for females and 1+ for males, and size-at-50% maturity was 222 mm (fork length) for females and 194 mm for males. It is necessary to use area-specific growth and life-history parameters for the assessment of the stock status of white stumpnose in South Arica.Keywords: growth; life history; reproduction; Rhabdosargus globiceps; Saldanha Bay; stock separationAfrican Journal of Marine Science 2010, 32(1): 23-3

An alternative method for estimating the status of resident reef fish stocks, based on differential fishing effort across a marine reserve boundary

The stock status of roman Chrysoblephus laticeps was estimated in the Goukamma, a temperate South African marine protected area (MPA). Standardised catch per unit effort (CPUE) from a controlled angling survey on both sides of the MPA border was employed to extrapolate the CPUE at zero fishing mortality. Converted into biomass, the estimate (61% of unexploited biomass) lay midway between those of two biomass-per-recruit (B/R) models for the same population based on angling and diving surveys (i.e. 52% and 69% of unexploited biomass respectively). The extrapolated CPUE at zero fishing mortality (4.4 fish angler-hour–1) in this study compared well with the mean CPUE of 4.6 fish angler-hour–1 determined during a concomitant survey in the core area of the nearby Tsitsikamma MPA — the oldest, and one of the largest, MPAs in Africa. Extrapolations of CPUE have the potential to deliver reliable and consistent estimates of stock status and could offer a practical alternative to conventional B/R models. Keywords: biomass-per-recruit models, Chrysoblephus laticeps, CPUE extrapolation, fisheries management, marine protected area, stock statusAfrican Journal of Marine Science 2011, 33(1): 107–11

Comparison of the effects of different linefishing methods on catch composition and capture mortality of South African temperate reef fish

The effects of hook size, bait type, water depth and fishing effort on the selectivity and capture mortality of reef fish were investigated in a standardised fishing experiment on the temperate south coast of South Africa. A total of 2 406 fish was caught at 273 stations, distributed according to a random-stratified design. Generalised linear models separated the influences of environmental factors such as depth, temperature and season from the effects of fishing methods on catch rates and capture mortality. Dominant reef predators and larger individuals within species were more frequently caught on large hooks and sardine Sardinops sagax bait. The main cause for capture mortality was gut-hooking, aggravated by increased capture depth. Large roman Chrysoblephus laticeps were more frequently gut-hooked than small individuals. More roman were caught early in the fishing session and the average size of captured roman increased slightly during fishing sessions. The selection of legal-sized target fish can be increased by about 23% and immediate capture mortality of undersized fish can be reduced by 50% by using large hooks and sardine bait and by fishing for less than one angler-hour per station in water shallower than 20m. The results are useful for planning fisheries independent catch per unit effort surveys and for developing rules for catch-and-release competitions.African Journal of Marine Science 2007, 29(2): 177–18