Snoek Thyrsites atun in South African waters : aspects of its biology, distribution and fishery

Dudley, Sheldon Francis John

Snoek Thyrsites atun in South African waters : aspects of its biology, distribution and fishery

Master Thesis

1987

Publisher

University of Cape Town

Department

Department of Biological Sciences

Faculty

Faculty of Science

Abstract

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.