Browsing by Author "Dudley, Sheldon Francis John"

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    Shark nets in KwaZulu-Natal : an evaluation of catches and alternatives
    (1996) Dudley, Sheldon Francis John; Griffiths, Charles L
    Protective gillnets (shark nets) have been successful in reducing the frequency of shark attacks on the coast of KwaZulu-Natal (KZN), South Africa. This is achieved primarily through a reduction in numbers of large sharks. The nets also take a by-catch of dolphins, sea turtles, batoids and teleosts.Catch rates of most shark species declined initially but have shown no trend since the mid-1970s. Turtle and teleost stocks do not appear to be threatened by net mortalities, but there is concern about the sustainability of catches of the humpback dolphin. Certain batoids may have declined despite a high release rate. A published contention that shark netting has resulted in a proliferation of small sharks through reduced predation is re-examined and considered to be exaggerated. Reduced predation on dolphins, as a result of shark netting, is estimated. Considerably less fishing effort is applied in the shark control programs of New South Wales and Queensland, Australia, than in that of KZN. On the basis of a comparison of factors such as the nearshore physical environments and trends in shark catch and catch rate, it is concluded that the number of nets used in KZN could be reduced. To test whether a 70 cm mesh would continue to capture potentially dangerous sharks, while at the same time reducing by-catch, a gamma distribution model was used to determine length-specific selectivities in 50.8 cm and 70 cm mesh nets respectively. A reduction in relative selectivity from 81 to 25 for a shark of 1.6 m PCL would result from an increase in mesh size from 50.8 to 70 cm. Despite a probable reduction in catch of dolphins .and certain other by-catch species, the introduction of the larger mesh would constitute an unacceptable reduction in levels of bather safety. Baited lines, or drumlines, were tested as possible alternatives to gillnets. They demonstrated greater species selectivity for sharks, including a higher catch of two of the target species, Carcharhinus leucas and Galeocerdo cuvier, and also a reduced by-catch of nonshark animals. The probability of the bait being scavenged, or a shark being caught, was modelled in relation to a number of physical factors. Although there were insufficient data for a quantitative comparison of catch rates between nets and drumlines, the results indicated that an optimal solution may be to deploy a combination of nets, using the existing 50.8 cm mesh, and drumlines, using 14/0 shark hooks.
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    Snoek Thyrsites atun in South African waters : aspects of its biology, distribution and fishery
    (1987) Dudley, Sheldon Francis John; Field, John G; Crawford, Robert J M
    The snoek Thyrsites atun is an important fish predator in the southern Benguela region. It is exploited by both a handline and a demersal fishery. A survey of the Cape line fishery revealed that snoek line fishing effort is changing from the traditional harbour-based line-boat to the nomadic ski-boat. It was widely claimed that snoek catches are declining and that migration patterns are changing. Snoek constitute a by-catch of the hake-directed demersal fishery but nevertheless are seasonally important. The principle prey of snoek caught by handline off the Cape Peninsula were anchovy and mantis shrimp. Snoek caught in midwater trawls offshore were feeding primarily on anchovy, with pilchard, euphausiids and amphipods also important. Snoek trawled demersally had a more diverse diet, dominated by redeye roundberring, lightfish, lanternfish, hake, buttersnoek and euphausiids. Snoek were caught in the demersal environment throughout daylight hours, but may come off the bottom at night. The small degree of overlap between the diet of snoek caught demersally offshore and that of snoek caught in the pelagic zone, both inshore and offshore, indicates that snoek do not seem to move extensively on a diurnal basis between the two zones. Over the period 1970 to 1985 availability of snoek to the handline fishery was strongly seasonal, with catches peaking from May to July, although the traditional winter snoek run is a declining phenomenon along the South African coast. At Dassen Island, for which catch data have only been available since 1981, peak months were from November to January. The snoek seems to move offshore from July and is trawled demersally until September. The presence of snoek larvae offshore between June and September indicates an offshore spawning migration. With the exception of the summer presence in the region of Dassen Island, snoek appear to be present in the southern Benguela region between April and September. Handline catches of snoek have declined markedly since 1978, but demersal catches have remained more stable.