Griffiths: Life history of Jhyrsitcs ntun 



691 



29° 



■gi-ia^ tV^Js Orange River 



^ ^PortNollotti 



30" 



31 



32° 



33° 



34' 



35' 



36' 



Region 



■> Hondel<lip Bay 



Region 1 



Region 2 '. Bird fsland —,,1 Lambert's Bay 



SOUTH AFRICA 



SI Helena Bay 

 (Cape Columbine 



Malgas Island 

 Region 3 Dassen Island ', V\|CAPE TOWN .^ 



Cape 

 St Francis port 



Danger Point 



N 



t 



Region 4 



■:,-'', 72Mile^ank 



"',[;, Agulhas Bank 

 \- Region 5 



Region 6 



Mill I 



15° 



16° 



19° 20° 21° 22° 23° 24° 25° 26° E 



Figure 1 



Map of the seven regions in which snoek iThyrsites atiin I were sampled, and the locahties mentioned in the 

 text. Snoek caught by hook and hne were, with the exception of the 72-mile bank ( region 5), taken within 15 

 km of the coastline, whereas those that were trawled were caught farther from the shore at water depths 

 between 150 and 600 m. 



Hurst, 1988; Hurst and Bagley, 1987; 1989) have been 

 extensively studied, knowledge of the hfe history of the 

 Benguela snoek is surprisingly fragmented, in spite of its 

 socioeconomic and ecological importance and long history 

 of exploitation. An extensive seasonal migration between 

 the northern and southern regions of the Benguela system 

 has been postulated (Davies, 1954; Crawford and De Vil- 

 liers, 1985; Crawford et al., 1987); however, this migration 

 was based largely on anecdotal reports of line catches and 

 remains to be confirmed. Preliminary observations of go- 

 nad maturity (Davies, 1954; Nepgen, 1979a) and trends 

 in the abundance of eggs and larvae (Olivar and Fortuno, 

 1991; Olivar and Shelton, 1993) suggest that spawning 

 occurs in winter and spring, but the spawning season has 

 not been accurately described. Three previous studies on 

 the diet of snoek in South African waters (Nepgen, 1979b, 

 1982; Dudley, 1987) excluded juveniles (<75 cm fork 

 length) because nursery grounds were not sampled, and 

 inadequate sample sizes precluded meaningful compari- 

 sons of snoek offshore and closer to the coast, and spatial 

 comparisons of offshore diet. 



The objective of this study was to provide information 

 on the life history of South African T. atun, including sea- 

 son and localities of spawning, condition, nursery areas, 

 juvenile growth, size-at-maturity, diet, and patterns of 

 juvenile and adult distribution. 



Materials and methods 



The South African coastline between the Orange River and 

 Cape St Francis was divided into seven sampling regions 

 (Fig. 1). Biological (September 1994--January 1998) and 

 length-frequency (January 1985-December 1997) data 

 were collected in each region from fish caught by 1) the line 

 fishery, 2) the demersal trawl fishery, and 3) research trawl 

 sui-veys of pelagic and demersal fish conducted by Marine 

 and Coastal Management. The trawl fishery operates 

 farther offshore than the line fishery; consequently all fish 

 sampled deeper than 150 m were trawl caught, whereas the 

 bulk of those from shallower depths were caught by hook 

 and line (except for a small number caught by trawl during 

 research cruises). 



Demersal biomass surveys (DBS), based on the swept- 

 area method, are designed to provide annual indices of 

 biomass for the resources exploited by the South African 

 hake-directed trawl fishery, and methods have been fully 

 described by Badenhorst and Smale (1991). DBSs are 

 subdivided into two areas: the Namibian border to Cape 

 Agulhas (west coast), and Cape Agulhas to Port Alfred 

 (south coast). In each survey, area was divided into four 

 depth zones (0-50 m, 51-100 m, 101-200 m, and 200-500 

 m) which, in turn, were subdivided into blocks of 5 x 

 5 nautical miles. The blocks trawled each survey were 



