Griffiths: Life fiistory of Thy/site'; atun 



705 



Figure 13 



Annual frequency of occurrence (%F; dotted line) and mean mass (solid line) of 

 Thvrsites atun in the stomachs of Cape gannets from Bird Island (circles) and 

 Malgas Island (squares), 1978-97. n=number of gannet stomachs examined. 



1.15 



1.05 m 



0.95 



Figure 14 



Mean monthly relative condition (Kn) and the proportion of adult Thyrsites 

 atun (>75 cm) without intestinal fat reserves, in the southern Benguela Eco- 

 system (1994-97). 



deklip Bay (Shelton and Hutchings, 1990; Shannon and 

 Nelson, 1996). 



Snoek hatch about 2 days (50 hours) after fertilization 

 (De Jager, 1955), are phytoplanktivorous from first feed- 

 ing (3.5 mm and 3-4 days after hatching) until 8 mm long 

 (standard length), after which they prey largely on the 

 larvae of other fishes (Haigh, 1972). Total abundance of 

 fish larvae in the southern Benguela is highest in spring 

 and summer (Shelton, 1986); snoek larvae spawned dur- 

 ing the winter and spring months are therefore assured of 

 an abundant supply of food. Following northward advec- 

 tion in the jet current, young-of-the-year anchovy, round 

 herring, and sardine, which share the northern nursery 

 area with snoek, move shoreward across the shelf, with a 

 concomitant increase in size (Armstrong et al., 1987; Roel 

 and Armstrong 1991; Hampton, 1992; Roel et al., 1994; 

 Barange et al., 1999; Van der Lingen and Merkle, 1999). 



They arrive at the coast from February through through 

 June at total lengths of 5-11 cm, and, unlike snoek, which 

 are found on the nursery grounds for 2-3 years, immedi- 

 ately begin a southwards migration back onto the Agulhas 

 Bank, where they were originally spawned. Coupled with 

 earlier spawning, rapid growth ensures that 0-year-old 

 snoek are large enough (ca. 35 cm TL) to partake with the 

 older juveniles in this seasonally abundant food source. 

 Substantial increases in line catches during the period of 

 clupeoid recruitment suggest that juvenile snoek follow 

 their prey inshore. Differences in the size and age com- 

 position of catches north and south of Cape Columbine, 

 however, indicate that they do not move southwards with 

 them and onto the Agulhas Bank. The large contribution 

 of lanternfish to the diets of snoek <50 cm is attributed to 

 the fact that most of these fish were sampled during the 

 second half of the year, i.e. when juvenile clupeoids (i.e. 



