comparison of each strain indicated monosex reached a larger final size than mixed-sex fish, but that hormone treatment resulted in a smaller difference in final size than in
pond-reared fish. The mean DWG of fish in all treatments ranged from 1.8 0.18 to Ž
. 2.3 0.15 g Table 3 . The survival of mixed-sex and monosex stocks in the three
strains ranged between 94.4 and 96.7 and was not significantly different. New-season fry stocked in cages grew poorly compared to fish stocked in ponds
Ž
y1
. 0.8–0.9 g day
. There was no effect of hormone treatment and the effect of strain was less distinct. The survival of fish in all cages was very high however, ranging from
97.8 to 100. 3.3. Sexual maturation of fish
Fry were found in ponds stocked with mixed-sex, overwintered fish from the fourth month onwards. Mouth brooding females were also detected in cages stocked with
mixed-sex fish from the fourth month onwards. In ponds stocked with new-season mixed-sex fingerlings, there were no fry observed during the period of experiment, and
no mouth brooding females found at harvest.
3.4. Water quality parameters Ž
The physical and chemical parameters of pond water during both experiments Table .
4 were maintained within the tolerance range of Nile tilapia, as reported by several Ž
. authors Jiazhao, 1991; Popma and Lovshin, 1996 . Levels of net primary productivity
Ž .
did decline from August to November Table 4 explaining the relatively poor perfor- mance of fish in E2. The new-season fry, reared in cages grew particularly slowly
despite similar water quality to ponds stocked at the same time.
4. Discussion
The potential role of overwintered tilapia fingerlings in Northern Vietnam was clearly indicated by the first experiment. Fish of all three strains, both mixed or monosex,
reached a size of 300–400 g within a culture period of 6 months in either cages or Ž
. ponds. Tilapia of more than 200 g attract premium prices in Hanoi Cao, 1998 .
Growth of overwintered individual fish compared well with other studies. Diana et al. Ž
.
y1
1991, 1994 observed individual growth of 1.2 g day in hand-sexed male tilapia
raised in ponds receiving high rate of fertilizers and 1.7–2.0 g day
y1
in fertilized ponds Ž
. with supplemental feeding. Siddiqui et al. 1989 reported growth rates of 1.7–1.8 g
day
y1
in Nile tilapia reared in out-door tanks. Both mixed-sex and monosex overwin- tered fry grew well in ponds and cages. Among the three strains, GIFT strain reached a
larger individual size in both cages and ponds at final harvest than either the Thai or Viet strains. This is perhaps to be expected given that this strain has been selected for
growth performance in a wide range of environments. However, this growth differential is much less than reported for this selected strain compared to unselected controls
Ž
. Eknath, 1995 . The comparable performance of the Viet strain with the two recently
introduced strains suggests that it should have been promoted more widely in the past. The value of monosex tilapia has been linked both to the population control affected by
Ž .
male only stocks and the faster individual growth of males Mair and Little, 1991 . Using monosex seed can control the stunting problem caused by competition for food
that otherwise occur between recruits and stocked fish. The comparison of performance Ž
. between monosex and mixed-sex groups of the three strains E1 showed that on
average, monosex fish grew more than 10 faster than mixed-sex fish in both ponds and cages. Faster growth of monosex tilapia has been related to the lack of energy
expenditure in egg production and mouth brooding by females and lower energy
Ž .
expenditure on courtship by males Macintosh and Little, 1995 . Stunting of mixed-sex treatment was not observed however. Although the period before the tilapia reach sexual
Ž .
maturity is short, which is about 3 to 6 months Hepher and Pruginin, 1982 , the fish in E1 which were at least 7 months old at stocking did not appear to reach sexual
maturation until after another 3 months at a size of around 200 g. The absence of a significant decline in growth rate during the onset of reproduction suggests that the
biweekly removal of recruits through seining to sample fish was sufficient to control any impact of recruitment on growth of the stocked fish. This indicates that culture of
monosex stock was not required to produce fish of 200 g and that even larger fish could be produced relatively easily through intermittent harvest of recruits. Stocking large
overwintered seed early in the season indicated potential for double cropping small fish Ž
. 200 g , or production of a single large sized fish over the whole season.
Ž New-season fry stocked as late as August also reached optimal marketable size 200
. g before cool temperatures retarded growth. This stocking and harvest schedule would
have important implications for the use of different strains and the value of hormone treatment. New-season fry showed little difference in performance of monosex com-
pared to mixed-sex, or between strains and a comparison of the growth of overwintered seed to a size of 200 g showed a similar situation.
The large differences in initial size and age, timing of stocking and culture duration prevent direct comparison of new-season and overwintered seed performance. An
assessment of overwintered and new-season fry performance can suggest practical choices for culturist, however.
The growth curves of fish in ponds and cages in both experiments were more or less linear indicating that the critical standing crop in either ponds or cages was not exceeded
Ž .
Diana et al., 1994 possibly due to the initially low stocking density and provision of a pelleted feed in addition to fertilization. The linear growth also suggests that overwin-
tered fish were not stunted. Ž
. New-season fingerlings E2 of the three Nile tilapia strains grew slowly in cages
compared to fish stocked in ponds. Several factors may have influenced this result. Heavy rains occurred from August to October 1997 increasing turbidity in ponds,
reducing primary productivity, and limiting the availability of natural food. It is possible that fish that had access to the whole pond were less affected by the reduction in natural
Ž .
food, which is considered more critical for juvenile fish Huet, 1994 compared to those confined in cages. The overwintered fish, which were larger at this stage and relatively
more dependent on the supplementary feed, continued to grow fast in the same cage-in-pond culture system.
A further major factor affecting the growth rates of the overwintered and new-season fry was the relative exposure to lower temperatures occurring in the latter part of the
growing season. Whereas overwintered fish grew during the period of optimal tempera- Ž
. tures early in the season May–August, 30.5–31.98C , new-season fry were stocked
Ž .
shortly before ambient temperature began to decline August–November; 31.9–24.98C Ž
. Table 4 .
Ž .
Survival of fish was higher in the protected environment of cages 94 than Ž
. ponds 65–86 , however. Regular seining of ponds was observed to cause mortality,
Ž but overall survival was in the range normal for pond culture of Nile tilapia Diana et
. al., 1994 .
5. Uncited references
