Turbidity-induced changes in feeding strategies of fish in estuaries

The aim of this study was to determine the effect of turbidity on the feeding strategies of fish in estuaries. Three species representing different feeding guilds were selected for the investigation. These were Elops machnata (representative piscivore), Pomadasys commersonnii (a macrobenthivore) and Atherina breviceps (a planktivore). The stomach contents of these fish were examined from a clear and a turbid estuary and some experimental work was carried out on A breviceps to test the hypothesis that turbidity affects feeding behaviour. Turbidity was found to have no effect on size selection of prey, but feeding rate, particularly of visual predators, was reduced at higher turbidity levels. This was caused by a decrease in the reactive distance of the fish. It would appear that in order to optimize the aquisition of food under different turbidity conditions fishes have the ability to change their feeding strategies. Visual predators are more affected by turbidity than are macrobenthic feeders

Comparative trophic ecology of Cape anchovy Engraulis encrasicolus and Natal anchovy Stolephorus holodon off South Africa’s east coast

The distributional ranges of Cape anchovy Engraulis encrasicolus and Natal anchovy Stolephorus holodon presently overlap at their eastern and western extremes, respectively, off South Africa’s east coast. These species might become more strongly sympatric as a consequence of changed distributions caused by climate change, possibly leading to intensified interspecific competition for trophic and other resources. A comparative analysis of the trophodynamics of the two species, which set out to assess elements of current niche partitioning between them, was conducted through morphological, dietary and stable-isotope analyses of specimens caught off South Africa’s east coast. Cape anchovy have larger gillraker spacing, gain the majority of their dietary carbon from large (>3 000 μm total length) planktonic crustaceans and occupy a higher trophic level than Natal anchovy, which have smaller gillraker spacing and gain the majority of their dietary carbon from planktonic crustaceans of two size classes (900–1 400 μm and 2 800–3 200 μm total length). These results indicate an element of trophic partitioning between the two species that will mitigate against interspecific competition.Keywords: Engraulidae, gillraker morphology, prey items, resource partitioning, stable isotopes, stomach contents, trophodynamics, western Indian Ocea

Biology, fishery and management of sardines (Sardinops sagax) in southern African waters

Sardines have been a major contributor to the pelagic fishery off southern Africa for several decades. The South African and Namibian stocks are separate, and seasonal hydrology, in conjunction with age-related behaviour, produces the observed size-distribution patterns. Sardines are relatively fast-growing and spawn repeatedly over the spring and summer months. Eggs and larvae are transported from the Agulhas Bank to the west coast by the north-flowing jet current at the shelf edge. Sardines are omnivorous, microphagist filter-feeders and are important prey for many piscivorous fishes, birds and mammals. There have been major fluctuations in sardine population biomass in southern Africa which have been associated with environment, regime shifts, recruitment and fishing, South African and Namibian sardine catches peaked in the 1960s, declining dramatically thereafter, and the pelagic fishing industry switched to anchovy. During the 1990s, sardine biomass increased but, although South African catches rose, the Namibian stock collapsed again. The annual South African total allowable catch is set through an operational management procedure that takes into account biomass estimates from comprehensive hydroacoustic surveys and the sardine by-catch in the anchovy fishery

Spatial variation in meristic and morphometric characteristics of sardine Sardinops sagax around the coast of southern Africa

Spatial variability in phenotypic characteristics within a fish population may be used to infer the existence of multiple stocks, and knowledge of the population structure of exploited species is important for their sustainable management. In this study we investigated geographic variability in meristic (vertebral count) and morphometric (body shape) characteristics of sardine Sardinops sagax from three southern African regions: Namibia, the South African west coast, and the South African south coast. There were significant regional differences in vertebral counts, with Namibian fish tending to have more vertebrae than South African fish, but with no difference between sardine off the South African west and south coasts. Body shape was characterised using morphometric landmarks, and shape changes were investigated using geometric morphometrics, with influences on fish shape assessed using multivariate regressions. To remove the effects of allometry, the 22 shape variables (represented by Procrustes coordinates) were regressed against fish size (represented by centroid size). There was a significant allometric effect, which was removed by using the residuals of these regressions in further analyses. A multivariate multiple regression was applied to the 22 size-corrected shape variables and to three covariates: condition factor, stomach fullness and fat stage. The significant influence of these covariates was removed by using the residuals from this regression in a canonical variate analysis, where corrected shape variables were classified in relation to region, year and sex. The Mahalanobis distances from the canonical variate analysis differed significantly for most combinations of classifier variables, with clear separations among the three regions. Sardinops sagax off Namibia had thicker bodies and more contracted tail regions than South African fish, whereas S. sagax off the South African south coast had thinner bodies and smaller heads than those off the west coast. These results support the hypothesis of multiple sardine stocks around southern Africa.Keywords: body shape, canonical variate analysis, management, population structure, stock discrimination, vertebral coun