Abstract- Three cohorts of Pmae- 

 K.s iikIii'k.^ were sampled with abeam 

 trawl at 6-week intervals over 2 years 

 in the St. Lucia Lake system. Shrimp 

 recruited to the system over Septem- 

 ber-November (spring) and March- 

 May (autumn) and overwintered in 

 the estuary. Mean growth rates of 

 cohorts over the size range 7.25- 

 17.66 mm carapace length (CL) ranged 

 from 0.032 to 0.0,58 mm CL per day 

 with the autumn cohorts exhibiting 

 the slowest growth and longest resi- 

 dency times. Up to 26.5°C, growth 

 rates were positively correlated with 

 water temperature. Growth was neg- 

 ligible at temperatures between 19 

 and 22°C. Shrimp emigrated from 

 the estuary between the sizes of 18 

 to 25 mm CL. The onset of emigra- 

 tion appeared to be related to declin- 

 ing water temperatures. 



Growth and Emigration of 

 Penaeus indicus H. Miine-Edwards 

 (Crustacea: Decapoda.-Penaeidae) 

 in the St. Lucia Estuary, 

 Southern Africa 



Mark C. Benfield 



Department of Biology, University of Natal, Durban 4001, South Africa 

 Present address: Department of Marine Biology, Texas A&M University at Galveston 

 PO Box 1675, Galveston, Texas 77553-1675 



Jaap R. Bosschieter 



Department of Biology, Liniversity of Natal, Durban 4001, Soutti Africa 



Present address Cotton Lane off Paradise Road, Simon's Town 7995, Soutti Africa 



Anthony T. Forbes 



Department of Biology, University of Natal 

 Durban 4001, Soutti Africa 



Manuscript accepted 12 September 1989. 

 Fishery Bulletin, U.S. 88:21-28. 



The commercial importance of pen- 

 aeid shrimp is well documented with 

 1987 United States commercial land- 

 ings valued in excess of $515 million 

 (U.S. Department of Commerce 

 1988). Shrimp fisheries in the Indo- 

 Pacific region generate considerable 

 revenue, and India is the region's ma- 

 jor shrimp supplier. Penaeus indicus 

 H. Milne-Edwards, 1837 is numer- 

 ically the most important species in 

 the Indian shrimp fishery (Silas et al. 

 1984) and likely formed the majority 

 of the $56.9 million of shrimp ex- 

 ported to the United States by India 

 in 1987 (U.S. Department of Com- 

 merce 1988). 



The distribution of P. indicus ex- 

 tends along the east African coast in- 

 to the inshore waters of Natal, South 

 Africa (Champion 1983). In common 

 with many inshore penaeids, it util- 

 izes estuaries during the juvenile 

 phase of its life cycle and migrates 

 offshore for subsequent maturation 

 and reproduction (Garcia and Le 

 Reste 1981). The proximity of St. 

 Lucia to the southern limit of distri- 

 bution of this essentially tropical 

 species suggested the desirability 



of investigating the relationship be- 

 tween the growth of P. indicus and 

 temperature as well as the factors 

 responsible for onset of offshore 

 emigration. 



The St. Lucia Lake system is the 

 largest estuary in southern Africa 

 and provides a nursery habitat for 

 P. indicus and several other penaeid 

 species (Joubert and Davies 1966, 

 Forbes and Benfield 1986a). The 

 Natal Parks Board has operated a 

 bait fishery for shrimp in the system 

 since 1952 (Joubert and Davies 1966), 

 with annual landings of approximate- 

 ly 16 t. Penaeus indicus ty])kn\\y 

 constitutes 80-90% of this catch 

 (Forbes and Benfield 1986a). An off- 

 shore South African commercial fish- 

 ery also depends largely on P. indi- 

 cus (Forbes and Benfield 1985). 



Most penaeid shrimp exhibit rapid 

 growth which is frequently corre- 

 lated with water temperature. Little 

 information is available on the 

 growth of the St. Lucia penaeids. 

 Joubert and Davies (1966) conducted 

 four surveys over a 1-year period and 

 provided estimates of growth for P. 

 indicus and two other numerically 



21 



