. Ž
ponds; 373.2 30.4 g in cages and Viet strains 359.5 33.2 g in ponds; 350.3 33.4 g in .
cages . Overall, monosex fish of the three strains grew significantly faster than mixed-sex fish Ž
. P - 0.05 .
Ž .
y1
DWG of new-season fingerlings E2 in ponds ranged from 1.6 to 1.8 g day , showing slower
growth compared to overwintered fingerlings. There was no significant difference in the growth between the three strains, and between mixed and monosex fish in ponds. Growth of all three
Ž
y1
. strains was significantly slower in cages than ponds overall DWG s 0.8–0.9 g day
; P - 0.01 . The results suggest that the performance of overwintered seed compare well with new-season fry.
These results are discussed in the context of developing appropriate strategies for tilapia seed productions for northern Vietnam. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Tilapia; Overwintering; Growth; Survival; Sex
1. Introduction
One of the problems of tilapia culture in sub-tropical regions is their poor tolerance of seasonally low temperatures, such as occur in northern Vietnam. Nile tilapia in general
does not grow at temperatures below 168C and exhibits poor survival if water tempera- Ž
. tures fall below 108C for more than few days Chervinski, 1982 . Growth and reproduc-
tion are adversely affected at temperatures of less than 208C and 228C respectively Ž
. Behrends et al., 1990 . The Nile tilapia requires a minimum temperature of 208C to
Ž .
spawn Hauser, 1977 . Seasonally cold temperatures in northern Vietnam thus affect both hatchery and food fish production as, between December and March, water
temperatures drop to between 118C and 208C. Under these conditions, mortalities can occur, especially in shallow ponds, and breeding activities are completely curtailed.
Conditions are highly suited to raising tilapia between April and December, when temperatures range from 258C to 358C. However, as spawning only begins in April, seed
are not normally available until June–July, reducing the duration of the production season. Overwintering of late-spawned tilapia fry may be a possible way to make
fingerlings available early in the following grow-out season.
Overwintering of tilapia fry has been successfully carried out using heated facilities Ž
. Ž
. Behrends et al., 1990 or underground warm water Cruz and Ridha, 1994 and in green
Ž .
houses Jiazhao, 1991 , but these resources are generally unavailable in northern
Ž .
Vietnam. Overwintering late-season fingerlings in deep hapas 3.0 m suspended in deep Ž
. Ž
. ponds 3.5 m has been demonstrated Nguyen, 1996, unpublished data . However, the
long-term effects of overwintering on subsequent growth and reproduction have not been established.
Nile tilapia production is still undeveloped in northern Vietnam but recent initiatives may stimulate rapid expansion. Traditional fish culture is based on polyculture of carps
in earthen ponds; small-sized tilapias, where present, have had low market acceptance. Recently, several new ‘improved’ strains have been introduced, as well as monosex
technology, to overcome this problem. Interest in the development of cage culture is also growing. The relevance of these innovations to the needs of farmers and consumers in
northern Vietnam are considered in the current study. The purpose of this study was to
evaluate and to compare the performance of mixed-sex and monosex stocks of three Ž
. Ž
. strains of Nile tilapia. Their performance as overwintered E1 and new-season fry E2
was investigated in ponds, and cage-in-pond systems.
2. Materials and methods
