690 



Abstract— Snoek [Thyrsites atun) is 

 a valuable commercial species and an 

 important predator of small pelagic 

 fishes in the Benguela ecosystem. The 

 South African population attains 50'7f 

 sexual maturity at a fork length of 

 ca.73.0 cm (3 years). Spawning occurs 

 offshore during winter-spring, along 

 the shelf break (150-400 m) of the 

 western Agulhas Bank and the South 

 African west coast. Prevailing currents 

 transport eggs and larvae to a primary 

 nursery ground north of Cape Colum- 

 bine and to a secondary nursery area 

 to the east of Danger Point; both shal- 

 lower than 150 m. Juveniles remain on 

 the nursery grounds until maturity, 

 growing to between 33 and 44 cm in the 

 first year (3.25 cm/month i. Onshore- 

 offshore distribution (between 5- and 

 150-m isobathsi of juveniles is deter- 

 mined largely by prey availability and 

 includes a seasonal inshore migration 

 in autumn in response to clupeoid 

 recruitment. Adults are found through- 

 out the distribution range of the spe- 

 cies, and although they move offshore 

 to spawn — there is some southward 

 dispersion as the spawning season pro- 

 gresses — longshore movement is appar- 

 ently random and without a seasonal 

 basis. Relative condition of both sexes 

 declined dramatically with the onset 

 of spawning. Mesenteric fat loss was, 

 however, higher in females, despite a 

 greater rate of prey consumption. Spa- 

 tial differences in sex ratios and indi- 

 ces of prey consumption suggest that 

 females on the west coast move inshore 

 to feed between spawning events, but 

 that those found farther south along 

 the western Agulhas Bank remain on 

 the spawning ground throughout the 

 spawning season. This regional differ- 

 ence in female behavior is attributed to 

 higher offshore abundance of clupeid 

 prey on the western Agulhas Bank, as 

 determined from both diet and rates of 

 prey consumption. 



Life history of South African snoek, 



Thyrsites atun (Pisces: Gempylidae): 



a pelagic predator of the Benguela ecosystem 



Marc H. Griffiths 



Department of Environmental Affairs and Tourism 



Branch Marine and Coastal Management 



Pnvate Bag X2 



Roggebaai 8012 



Cape Town, South Afnca 



E-mail address, mgriffitia'sfriwcapegovza 



Manuscript accepted 27 Mav 2002. 

 Fish. Bull. 100:690-710 (2002). 



Snoek (Thyrsites atun) is a medium- 

 size, pelagic predator (max. size 9 kg; 

 Nepgen, 1979a) inhabiting the coastal 

 waters of the temperate Southern 

 Hemisphere; it is found from the sur- 

 face to the seabed, to depths of 550 

 m (Kailola et al., 1993). Occurring off 

 southern Africa, Australia, New Zea- 

 land, the east and west coasts of south- 

 ern South America, Tristan da Cunha, 

 and the islands of Amsterdam and St. 

 Paul (Nakamura and Parin, 1993), 

 snoek have successfully colonized envi- 

 ronments as diverse as oceanic island, 

 west coast upwelling, and subtropical 

 convergence ecosystems. It is an impor- 

 tant food fish throughout much of its 

 distribution, supporting moderate fish- 

 eries (<1000 metric tons ItJ/yr) off 

 southern Australia, Chile, and Tristan 

 de Chuna, and substantial fisheries 

 010,000 t/yr) off New Zealand and 

 Southern Africa (Andrew et al.. 1995; 

 FAO. 1997). 



Southern African snoek have been 

 recorded from northern Angola to Algoa 

 Bay on the South African east coast but 

 are mostly found between the Cunene 

 River and Cape Agulhas, i.e. in the 

 Benguela ecosystem. Thyrsites atun has 

 been an important commercial species 

 in this system since the early 1800s, 

 caught initially with hand lines but al- 

 so trawled after 1960 (Crawford, 1995). 

 Total catch peaked at about 81,000 t 

 in 1978 but dropped substantially with 

 the exclusion of foreign trawlers from 

 the Namibian fishing grounds in 1991 

 (FAO, 1978, 1981, 1990, 1997). Current 

 annual-catch ranges between 14,437 

 and 22,920 t (1991-95), and 93% of it 

 is made in South African waters (FAO, 

 1995). Thyrsites atun is far the most 



important species caught by the 

 South African commercial line fishery^ 

 (comprising 39^?^ of the 1986-97 catch 

 (National Marine Linefish System-] ); 

 it is also targeted by recreational an- 

 glers, for which catch statistics are not 

 available. Around 40% of the South Af- 

 rican catch (1990-96) is made by com- 

 mercial handline fishermen and 60% 

 by trawlers (Demersal Commercial 

 Data Base^). However, the commercial 

 line-catch may be under-reported by 

 as much as 75% (Sauer et al., 1997); 

 therefore the total catch could be 

 substantially larger. In addition to its 

 fishery significance, T atun is a major 

 predator of anchovy (Engraulis japoni- 

 cus) and sardine (Sardinops sagax) 

 in the southern Benguela ecosystem 

 (Wickens et al., 1992) and has been 

 implicated in top down effects on both 

 prey and consequently zooplankton 

 populations (Verheye et al.. 1998). 



Life-history information is funda- 

 mental to identifying and assessing 

 fish stocks, as well as to formulating 

 management strategies for their sus- 

 tainable use. Whereas snoek stocks 

 off Australia (Blackburn and Gartner, 

 1954; Blackburn, 1957; Grant et al., 

 1978) and New Zealand (Mehl, 1971; 



' The South African commercial linefishery 

 consists of about 2500 vessels (5.5-15 m 

 long) that operate on the continental shelf 

 using handline or rod-and-reel. 



- National Marine Linefish System. 2000. 

 Unpubl. data. Linefish Section, Marine 

 and Coastal Management. Private Bag X2 

 Roggebaai 8012. Cape Town. South Africa. 



^ Demersal Commercial Data Base. 2000. 

 Unpubl. data. Demersal Section. Marine 

 and Coastal Management. Private Bag X2 

 Roggebaai 8012, Cape Town, South Africa. 



