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The culture performance of monosex and mixed-sex
new-season and overwintered fry in three strains of
ž
/
Nile tilapia Oreochromis niloticus in northern
Vietnam
Nguyen C. Dan
a,b,), David C. Little
b,ca
( )
Research Institute for Aquaculture No.1 RIA-1 , Dinh Bang, Tien Son, Bac Ninh, Viet Nam
b
Aquaculture and Aquatic Resources Management Program, School of EnÕironment, Resources and
( )
DeÕelopment, Asian Institute of Technology AIT , P.O. Box 4, Klong Luang, Pathum Thani, 12120, Thailand
c
Institute of Aquaculture, UniÕersity of Stirling, Stirling FK9 4LA, Scotland, UK
Accepted 4 October 1999
Abstract
Ž .
Two experiments E1, E2 to evaluate the performance of new-season and overwintered fry in
Ž . Ž .
three strains Viet, GIFT and Thai of Nile tilapia Oreochromis niloticus were conducted at
Ž .
Research Institute for Aquaculture No.1 RIA-1 in northern Vietnam. The performance of both mixed and monosex fish of the three strains was also compared. The experiments were carried out
Ž . Ž .
from May to November E1 and August to November 1997 E2 with overwintered and new-season fry, respectively. Each experiment included a comparison of production in fertilized
Ž 2. Ž 3. Ž 2.
earthen ponds 6=300 m and cages 18=1.2 m suspended within a single pond 1200 m . In the pond trials, the three strains were stocked communally as either monosex or mixed-sex
Ž .
groups, whereas treatments strain=sex were separated into replicate cages. Individual size of
Ž . Ž . Ž
fish stocked was 11.8"0.8 g E1 and 9.4"0.4 g E2 . Floating pellets were fed at 5% b.wt.
y1 .
day , 30% crude protein initially, and declined to 2% by the third month of culture. Water quality and fish growth were monitored monthly.
Overwintered fingerlings, both mixed and monosex, of all three strains performed well. Daily
Ž . y1
weight gain DWG ranged between 1.8 and 2.3 g day in both ponds and cages. Among the
Ž .
three tilapia strains, the GIFT fish attained a significantly P-0.01 larger individual final weight
Ž387.7"21.0 g in ponds; 410.6"35.1 g in cages compared to the Thai 351.6. Ž "26.1 g in
)Corresponding author. Research Institute for Aquaculture No.1 RIA-1 , Dinh Bang, Tien Son, Bac Ninh,Ž .
Vietnam. Tel.:q84-4-878-0408; fax:q84-4-827-3070.
Ž .
E-mail address: [email protected] N.C. Dan .
0044-8486r00r$ - see front matterq2000 Elsevier Science B.V. All rights reserved. Ž .
. 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
ŽP-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 DWGs0.8–0.9 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.q2000 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
ŽBehrends et al., 1990 . The Nile tilapia requires a minimum temperature of 20. 8C 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
ŽBehrends et al., 1990 or underground warm water Cruz and Ridha, 1994 and in green. Ž .
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.
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
2.1. Experimental duration and location
Ž .
Two experiments E1, E2 were conducted in both ponds and cages during the
Ž .
growing season of 1997 at Research Institute for Aquaculture No. I RIA-1 located near
Ž .
Hanoi, northern Vietnam. Overwintered Nile tilapia seed E1 were stocked earlier and
Ž . Ž . Ž
cultured longer 15 May to 15 November 1997 than new-season seed E2 5 August to
.
25 November 1997 .
2.2. Origin of Nile tilapia strains
The three strains of Oreochromis niloticus used in the two experiments were Thai
Ž .T , GIFT G and Viet V strains. The Thai strain originated from Egypt Pham et al.,Ž . Ž . Ž .
1998 but experimental stocks were derived from fish transferred from the Asian
Ž .
Institute of Technology AIT , Thailand in 1994. The GIFT strain was derived from the
Ž .
International Center for Living Aquatic Resources Management ICLARM , Philippines
Ž . Ž .
in May 1994 ICLARM, 1998 . The Viet V strain has been maintained in northern
Ž
Vietnam since its introduction from Taiwan, via southern Vietnam, in 1977 ICLARM,
.
1998 .
2.3. Production of oÕerwintered and new-season fry
Natural spawning in similar hapa-in-pond systems produced the progeny of each strain for both experiments. Eggs and yolk-sac fry were harvested after the same 5-day
Ž .
period and artificially incubated to the first feeding stage Macintosh and Little, 1995 .
Ž y1
The number of brood fish used for fry production was 642 214 brood fish strain with
.
the size ranging from 162 to 250 g and the ratio of male:female was 1:1 . The size of
2 Ž . 2 Ž .
breeding hapa was 40 m 4 hapas and size of rearing hapa was 5 m 24 hapas . Total number of seed harvested for each breeding time was about 60,000 swim-up fry.
Ž .
Batches of fry were then divided into two groups. One group monosex was fed a fine fry feed treated with 17a-methyltestosterone dissolved in food grade ethanol using
Ž . Ž
standard sex reversal methods Macintosh and Little, 1995 . The second group
mixed-.
sex was treated in the same way, including the addition of ethanol to the food, but no hormone was added.
Ž .
Overwintered fish E1 were produced in early October 1996 and held in deep hapas suspended in a single deep pond from December 1996 to March 1997. The size of E1 fry before overwintering was 1.11"0.07 g. During overwintering, fry were fed with a pelleted feed containing 30–35% crude protein at a rate of 1–2% fish biomass dayy1 on
Ž
hapas-in-ponds was 78.7–90.7% in the cold season 1996–1997 Nguyen, 1997,
unpub-. Ž .
lished data . New-season fry E2 were produced from brood fish spawned at the end of April 1997 and reared until early August to reach a fingerling size of 10 g prior to stocking in ponds and cages. Survival rate, of fry to fingerlings, exceeded 90%.
2.4. Experimental design and set up
Twelve earthen ponds used for communal stocking of the three strains were similar in shape and size, with a surface area of 285–295 m2 and water depth of 1.5 m. The six
Ž .
ponds stocked with overwintered fry E1 on 15 May 1997, and other six ponds stocked
Ž .
with new-season fry E2 on 5 August 1997. A total of 36 net cages were used for stocking of the strains separately in both experiments. Nylon net, aluminium-frame
Ž .
cages 1.0=1.0=1.2 m; 1.2 cm mesh were suspended 40 cm off the bottom of a
Ž 2 .
single large pond areas1200 m ; 1.4 m deep , with a freeboard of 20 cm above the water surface. River water was added to all the ponds monthly, via an irrigation canal, to replace water losses due to seepage and evaporation and maintain 1.0-m water depth in the cages.
Fingerlings of the three strains were stocked at 1.5 fishrm2 in earthen ponds, and 30
3 Ž
fishr1 m cage. Individual fish of the three strains were identified by fin clipping T,
.
left pelvic; G, right pectoral; V, anal before communal stocking in ponds. Treatments were assigned based on blocks of two adjacent ponds, stocked with mixed-sex and monosex fingerlings respectively. This has been found to be the optimal design to
Ž .
reduce the effects of heterogeneity in earthen pond experiments Darmi, 1996 . The
Ž . Ž .
three strains monosex and mixed-sex were randomly assigned to 18 3=3=2 treatment cages, within three blocks located along a central axis of the pond. A randomized block design was used to quantify possible environmental differences between different parts of the large, wind-affected pond. Fish in both pond and cage-in-pond systems were fed twice daily, 6 daysrweek with a floating pelleted feed containing 30% crude protein. Feeding rate was set at 5% of fish biomass dayy1 in the
first month, 3% in the second month and 2% thereafter, adjusted biweekly based on sampled mean weight and survival. Recruits produced from reproduction in the commu-nally stocked ponds were removed during monthly sampling by seine net. Fertilizers
Žurea and TSP were applied weekly to all experimental ponds, after soaking in water.
and broadcasting, at rates of 4 kg hay1 dayy1 of N and 1 kg hay1 dayy1of P.
At initial stocking, fish were counted and batch weighed. The initial mean weight of fish was also estimated by individually weighing a sample of 30 fish from each strain before stocking in each pond or cage. Monthly and at final harvest, a sample of 30 fish of each strain in each pond was individually weighed. All fish were measured individu-ally in each cage monthly. At final harvest, all fish were batch weighed and counted after draining of ponds. Males and females in each pond or cage were counted and weighed separately.
Methods and timing of water quality sampling and are presented in Table 1.
2.5. Data analyses and interpretations
Ž y1.
Individual fish growth rate g day was determined as mean final weight minus
Ž .
Table 1
Ž
Methods of analysis of chemical and physical water quality of experimental units ponds, water quality
.
sampled from a minimum of three locations per pond; cages, one sample per cage Parameters Method, frequency and timing of sampling Source
Ž .
Temperature and DO meter, measured twice daily at 6:00 a.m. YSI model 58
Ž .
dissolved oxygen DO and 3:00 p.m.
Ž .
pH digital pH meter, measured twice weekly at Pye model 290 6:00 a.m. and 3:00 p.m.
Ž .
Chlorophyll-a and acetone extraction and measuring absorbance APHA 1980 phytoplankton biomass by using spectrophotometer. Ph. biomasss
U
chlorophyll-a=67, measured weekly
Ž .
Alkalinity volumetric analysis using phenolphthalein and Boyd and Tucker 1992 methyl indicators and standard sulphuric acid
U
titrant, measured weekly
Ž .
Hardness Eriochrome black-T indicator, standard CaCO3 Boyd and Tucker 1992
U
and standard EDTA titrant, measured weekly
U Ž .
Concentration of phenate method, measured twice monthly APHA 1980
Ž .
ammonia NH –N3
Ž .
Concentration of cadmium reduction procedure, measured twice APHA 1980
U
Ž .
nitrate NO –N3 monthly
U Ž .
Concentration of nitrate diazotizing method, measured twice monthly APHA 1980
ŽNO –N2 .
Ž .
Dissolved ascorbic acid procedure, measured twice Boyd and Tucker 1992
U
orthorphosphorous monthly
Ž .
Total phosphorous persulphate digestion method, measured twice Boyd and Tucker 1992
U
monthly
U
Water samples are taken at 7:00 a.m.
Feed conversion rates were calculated as the weight of feed given divided by biomass of fish harvested.
Comparison of growth rate of the treatments was analyzed for both experiments using factorial analysis for which a significance level of P-0.05 was set. When significant differences between means was detected, a LSD range test was used to compare mean
Ž .
values P-0.05 .
3. Results
3.1. Grow-out of oÕerwintered fry and new-season fry in ponds
Both strain and use of hormonal sex reversal affected individual final size of fish
Ž .
stocked as overwintered seed in ponds P-0.05 . The GIFT strain grew fastest
Ž2.1"0.13 g dayy1. and attained a final size 11.1% larger than the Thai and Viet
Table 2
Ž . Ž .
Data of stocking and harvest parameters of overwintered I and new-season II seed of three strains of Nile
Ž
tilapia raised communally as either monosex male or mixed-sex groups in earthen ponds mean values"s.e.,
.
replicatess3
Strains At stocking At harvesting Daily Survival
Ž .
weight gain % Number Mean weight Number Mean weight Žg dayy1.
Ž .g Ž .g
( )
Experiment 1 oÕerwintered seed — 184 days
ab a
Thai mixed-sex 435 10.7"0.7 305 350.0"28.6 1.8"0.15 70.1
bc a
GIFT mixed-sex 435 10.5"0.8 358 375.3"35.7 2.0"0.19 82.3
a a
Viet mixed-sex 435 10.5"0.9 330 338.7"37.6 1.8"0.20 75.8
ab a
Thai monosex 435 10.7"0.7 320 353.1"23.5 1.9"0.12 73.6
c a
GIFT monosex 435 10.3"0.8 316 400.0"26.3 2.1"0.13 72.9
c a
Viet monosex 435 9.9"1.2 337 380.3"28.7 2.0"0.15 77.5
( )
Experiment 2 new-season seed — 112 days
a a
Thai mixed-sex 435 9.0"0.4 339 199.5"20.4 1.7"0.18 77.3
ab a
GIFT mixed-sex 435 9.2"0.4 322 201.3"21.1 1.7"0.17 74.0
a a
Viet mixed-sex 435 9.3"0.4 342 197.0"22.5 1.7"0.16 78.6
b a
Thai monosex 435 9.1"0.5 310 211.9"16.9 1.8"0.16 72.4
a a
GIFT monosex 435 9.0"0.3 339 194.2"20.4 1.7"0.16 77.9
a a
Viet monosex 435 9.1"0.3 333 192.0"19.3 1.6"0.17 76.6
Ž .
different Table 2 . Factorial analysis indicated that new-season fry stocked in ponds showed no significant differences in final size that were related to strain or hormone
Table 3
Ž . Ž .
Data of stocking and harvest parameters of overwintered I and new-season II seed of three strains of Nile
Ž
tilapia raised communally as either monosex male or mixed-sex groups in cages mean values"s.e.,
.
replicatess3
Strains At stocking At harvesting DWG Survival
y1
Žg day . Ž .% Number Mean weight Number Mean weight
Ž .g Ž .g
( )
Experiment 1 oÕerwintered seed — 184 days
a a
Thai mixed-sex 90 12.7"0.8 86 373.0"34.1 1.9"0.19 95.6
ab a
GIFT mixed-sex 90 11.3"0.6 87 391.4"33.3 2.1"0.18 96.7
a a
Viet mixed-sex 90 12.4"0.5 85 337.1"33.5 1.8"0.18 94.4
ab a
Experiment 2 new-season seed — 112 days
ab a
Thai mixed-sex 90 9.5"0.4 90 102.0"10.6 0.8"0.09 100.0
treatment. A paired comparison indicated that the final size of monosex Thai strain was
Ž .
larger than mixed-sex P-0.05 however.
The survival rate of the three strains ranged from 72.4% to 78.6% showing no significant difference between strains.
3.2. Grow-out of oÕerwintered fry and new-season fry in cages.
Overwintered fry raised in cages showed a similar trend in effects of strain and hormone treatment on performance as fish stocked in ponds. GIFT fish achieved a
Ž .
significantly P-0.05 larger final size compared to Thai and Viet strain. A compari-son of mixed-sex fish revealed no differences between strain however. A paired
Table 4
Ž
Monthly average of physical and chemical water parameters in experimental ponds and cages mean
.
values"s.e. at RIA-1 in period from May to November 1997
Parameters Number of Experimental ponds Experimental cages
samples per Mean of Range of Mean of Range of experimental parameters parameters parameters parameters unit
Experiment 1 Ž .
Temperature 8C 344 29.4"0.8 24.6–32.0 29.6"0.8 24.8–31.3 y1
Ž .
DO mg O l2 344 4.4"0.7 3.1–9.6 4.5"0.6 2.4–6.2
pH 52 7.7–9.1 7.5–9.1
Ž .
Transparency cm 52 21.3"5.4 15.6–26.3 21.9"3.3 15.6–25.0 y1
Ž .
Orthorphosphorus mg l 12 0.02"0.02 0.01–0.05 0.02"0.02 0.01–0.06 y1
Ž .
Total phosphorus mg l 12 0.07"0.06 0.03–0.22 0.07"0.07 0.03–0.09 y1
Ž .
Nitrite mg l 12 0.07"0.07 0.01–0.29 0.13"0.13 0.01–0.2 y1
Hardness mg CaCO l3 24 56.8"8.0 33.3–70.0 66.9"8.6 61.9–68.5 y1
Ž .
Alkalinity mg CaCO l3 24 62.2"7.4 44.6–82.7 68.8"7.6 62.9–82.7 Phytoplankton biomass 24 2512.2 716.2–4774.3 2537.1 1838.5–3223.4
y3
Ž . Ž .
schlorophyll-a=67 mg m
Experiment 2 Ž .
Temperature 8C 220 28.1"1.0 24.4–31.3 29.0"0.9 24.4–31.6 y1
Ž .
DO mg O l2 220 3.8"0.5 4.1–4.3 4.2"0.4 3.1–4.5
pH 32 7.5–7.9 7.9"0.3 7.5–7.7
Ž .
Transparency cm 32 21.9"4.9 17.5–34.4 22.2"4.3 11.5–26.6 y1
Ž .
Orthorphosphorus mg l 8 0.05"0.03 0.01–0.09 0.02"0.01 0.01–0.05 y1
Ž .
Total phosphorus mg l 8 0.09"0.05 0.02–0.14 0.09"0.02 0.03–0.21 y1
Ž .
Nitrite mg l 8 0.07"0.06 0.02–0.14 0.02"0.02 0.02–0.11 y1
Hardness mg CaCO l3 16 57.1"15.7 24.5–92.8 62.7"9.7 42.7–82.5 y1
Ž .
Alkalinity mg CaCO l3 16 59.5"11.4 40.1–67.3 65.9"10.5 51.8–68.9
Phytoplankton biomass 16 1899.1 671.3–5372.7 2149.1 895.8–2237.8
y3
Ž .Ž .
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
Ž0.8–0.9 g dayy1.. 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 dayy1 in fertilized ponds
Ž .
with supplemental feeding. Siddiqui et al. 1989 reported growth rates of 1.7–1.8 g dayy1 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
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
Ž200 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
ŽDiana 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 .
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
ŽTable 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
Coche, 1982
Philippart and Ruwet, 1982
Acknowledgements
This study is a part of a research project to improve capacity for tilapia seed production in northern Vietnam under the AIT Aqua-Outreach Programme. The study
Ž .
was conducted at the Research Institute for Aquaculture No.1 RIA-1 , Bac Ninh, Vietnam. The authors acknowledge DANIDA for financial support and Dr. Graham C. Mair for his kind assistance. David Little was seconded to the Asian Institute of Technology by the Department for International Development, UK.
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