A new species of electric ray of the genus Narcine Henle, 1834 from the South-Western Indian Ocean (Chondrichthyes: Torpediniformes: Narcinidae)

The Namibian linefishery is a multisector fishery, with recreational and commercial sectors targeting overlapping species. Prior to 1990, catch data were recorded for the boat sector, but little research was done on the population dynamics of any of the species. Hence, none of the stocks were assessed owing to the lack of parameter estimates for fisheries modelling. Current management regulations for the Namibian linefishery are therefore not based on scientific investigations but, for historical reasons, were adapted from South African legislation for similar species. From 1994, linefish research in Namibia focused on the life history and population dynamics of silver kob Argyrosomus inodorus and West Coast steenbras Lithognathus aureti, important species in both commercial and recreational sectors. A roving-roving creel survey was used to determine effort and catches of recreational shore-anglers. Both species were assessed using yield-per-recruit models. Based on the results, reduced daily bag limits and the implementation of size limits for the recreational sector were proposed. For the commercial sector, the introduction of a total allowable catch would seem to be the best option. Results from an economic survey indicated that the recreational sector earns six times more for the country annually than the commercial sector.     

Suggested mechanism for the chronic pollution by oil of beaches east of Cape Agulhas,South Africa

The distribution of tar balls over the Agulhas Bank during 1977 and 1978, although variable, was consistent with the oceanography of the region. Mesoscale patches of tar-ball concentrations appeared to move onto the Agulhas Bank from the south between 20 and 21°E, under the influence of prevailing winds and currents. Substantial concentrations were observed inshore and offshore in the region east of Cape Agulhas. Data of a hydrological and meteorological nature, on ship's drift and from drift cards have shown the existence of a zone of surface divergence extending southwards from Cape Agulhas. Surface water east of Cape Agulhas (20°E) tends to move slowly eastwards and northwards and water west of the same longitude has a predominantly westerly component. The surface currents east of Cape Agulhas would tend to move the tar balls, which accumulate in eddies in the area, on shore, whereas west of Cape Agulhas on-shore flow is less pronounced. The residence time of tar balls on the Agulhas Bank appears to be substantial, in the range 2—8 weeks. It is concluded that the beaches between Cape Agulhas and Mossel Bay are highly susceptible to pollution by oil and tar balls.     

Long-distance migration of the rock lobster Palinurus delagoae off South Africa and Moçambique

Long-distance migration patterns of deep-water rock lobster Palinurus delagoae were investigated using tagrecapture data obtained over a period of 6 years (1995–2000). Of 7 654 animals tagged, 363 (4.7%) were recovered from South African and seven (0.1%) from Moçambican waters. Lobsters remained at large for up to 3.2 years and migrated distances of up to 495 km. Some 48.3% of juvenile lobsters (carapace length <65 mm) but only 2.1% of larger lobsters migrated further than 20 km. Movements were mostly north-eastwards (91.7% of migrants), for both sexes, and the migration rate of the fastest 5% of migrants was 0.43 km day^?1. P. delagoae seems to have evolved long-distance, counter-current migrations as a retention mechanism to maintain its populations off both South Africa and Moçambique. The resource needs to be managed jointly between the two countries.     

Life history of white stumpnose Rhabdosargus globiceps (Pisces: Sparidae) off South Africa

Fishery-dependent and fishery-independent distribution analyses together reveal four discrete areas of white stumpnose Rhabdosargus globiceps abundance between Port Nolloth and the Kei River off the Cape Province of South Africa: the Western Cape (Saldanha Bay), the South-Western Cape, the Southern Cape and the South-Eastern Cape. On the basis of migratory patterns determined from tagging and catch data, and on differences in growth rate and size-at-maturity, it is concluded that these areas of abundance represent four separate stocks. Each stock apparently disperses offshore in winter (to c. 130 m depth) and concentrates inshore (<60 m depth) in response to ocean ographic patterns during summer. Growth rate and size-at-50% maturity (L ₅₀) increased clinally from the South-Eastern Cape through to the South-Western Cape, and in all three regions males matured at larger size than females. Sizes at maturity for male and female R. globiceps were respectively 18.6 and 15.3 cm (fork length, FL) in the South-Eastern Cape, 22.1 and 18.1 cm in the Southern Cape and 24.3 and 23.6 cm in the South-Western Cape. The fitted Von Bertalanffy growth equations for the three regions were: L_t = 349 (1?e^{?0.114(t+3.60)}) mm for the South-Eastern Cape; L_t = 337 (1?e^{?0.207(t+1.05)}) mm for the Southern Cape; and L_t = 379 (1?e^{?0.290(t+0.16)}) mm for the South-Western Cape. Maximum ages recorded in each region were 21 years for the South-Western Cape, 20 years for the Southern Cape and 10 years for the South-Eastern Cape. Lack of older fish in the South-Eastern Cape sample, attributed to inadequate sample size, has probably resulted in overestimates of both L _∞ and K in this region. Spawning is from August to February, with a peak in spring (September–November). Early juvenile R. globiceps recruit into estuarine and surf-zone marine nursery areas at around 2–5cm (±3 months), but move progressively farther offshore with growth; those trawled deeper than 50 m east of Cape Agulhas were predominantly adults (20–35 cm FL). Because of cooler water temperatures west of Cape Agulhas, adults there are found from the surf zone to depths of only 20 m in summer.     

Dynamics of the pelagic component of Namibian hake stocks

A study was undertaken to investigate the spatial, diurnal and interannual dynamics of the pelagic component of Namibian hake stocks. Data were derived from trawl surveys conducted on board the R.V. Dr Fridtjof Nansen during the period 1991–1999. In addition to trawl surveys, acoustic methods were applied concurrently to quantify fish biomass off the bottom (above the headline). The pelagic component of hake was variable in both time and space – it was denser in the north and at depths of 200–500 m. Pelagic densities also correlated well with the density of adult fish (≥30 cm) at the bottom, but not with that of young hake (>18 and <30 cm) at the bottom. Pelagic densities were lowest at midday and highest at night, early morning and late afternoon densities being intermediate between the two. Incorporation of acoustic methods allows for at least partial account to be taken of diurnal variation in determining catchability. The fraction of the biomass off the bottom was not the same each year, thus affecting assessment and management advice for the stock, even if the survey results are treated only as relative indices of abundance. More field studies are needed to investigate the size range and species of hake active in vertical migration.     

Sharks caught in the protective gill nets off Natal,South Africa. 4. The bull shark Carcharhinus leucas Valenciennes

A total of 772 bull sharks Carcharhinus leucas was caught in Natal's protective "shark nets" between 1978 and 1990. Confusion in distinguishing C. leucas from C. amboinensis resulted in their catch data being combined from 1966, when data collection began, to 1977. The catch rate of the species pair declined until 1977, recovered until the mid 1980s, but subsequently declined again. The trend in catch rate of C. leucas alone for the period 1978–1990 was similar, with minima of 0,70 sharks·km-net^?1·year^?1 in 1978 and 0,95 in 1990, and a maximum of 2,08 in 1986. Recaptures of six tagged sharks suggest that the species is not highly migratory. Catches, particularly of immature sharks, were highest at the northernmost beaches. Most bull sharks were caught in summer and in turbid water (mean water clarity 2,0 m). The sex ratio of the catch was 1 male to 1,3 females. Sizes ranged from 74 to 213 cm precaudal length, with modes of 141–145 cm (males) and 171–175 cm (females). Size at maturity for both sexes was between 180 and 190 cm. The mating season was prolonged but with a summer peak. Seven gravid females were examined; the mean litter size was 8,7 embryos and size at birth was approximately 55 cm. Fluke infections were observed on 9 per cent of animals examined. As size increased there was a shift in diet, in terms of frequency of occurrence, from teleost to elasmobranch predominance. There was a high incidence of benthic and demersal species in the stomachs. Minor prey groups included mammals, birds, turtles, molluscs and crustaceans. Scavenging appeared to be important.     

Fish response to the annual flooding regime in the Kavango River along the Angola/Namibian border

The results of the first seasonal survey of the fish of the Kavango River flood-plain along the Angola/Namibia border are reported. The river experiences peak flooding from February through June, with the 375-km long floodplain extending up to 5 km across. The floodplain was sampled five times in 1992 by seine, fish traps and rotenone. The data indicated a pronounced structural and functional response of the fish community in relation to the alternating flood and drought conditions in the river. Catch per unit effort and diversity were highest during months of peak flooding (May and June), and lowest during the month of least flow (November). The reproductive strategies of K-selected piscivorous cichlids and tigerfish were in advance of flooding. Many r-selected invertivores, especially cyprinids, were in relative synchrony with flooding and the stimulation of littoral zone plant growth, while other invertivores lagged the cyprinids. Herbivores had lowest relative abundance during peak flooding; this seemingly inverse relationship with the invertivores should not be interpreted as replacement, but rather the swamping of the system with young-of-the-year r-selected invertivores. The data support the Flood Pulse Concept, which hypothesizes that flooding is the major "driver" of productivity in lowland or floodplain rivers.     

A Bayesian state-space model for mixed-stock migrations,with application to Northeast Atlantic mackerel Scomber scombrus

Management of fisheries that exploit mixed-stock populations relies on assumptions made concerning stock structure and mixing in different areas. To address the problems of accounting for uncertainty when formulating scientific advice for the management of highly migratory fish stocks, management decisions need to be based upon assessment models that represent plausible alternative hypotheses for stock structure and migration patterns of the exploited populations. We present a multi-stock, multi-fleet, multi-area, seasonally structured Bayesian state-space model in which different stocks spawn in spatially different areas and the mixing of these stocks is explicitly accounted for in the absence of sufficient tagging data with which to estimate migration rates. The model is applied to the Northeast Atlantic mackerel Scomber scombrus population, accounting for the annual spawning-feeding-overwintering migration patterns of the three spawning components, together with uncertainty in the extent to which the southern component migrates north to feed and overwinter, and consequently the extent to which it mixes with the other components and is subject to exploitation. The model allows the effect of exploitation on the individual components to be assessed, and the results suggest that the fishing mortality of southern spawning adults was insensitive to the extent to which they migrated north.