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 . Ž .
Ž . Ž
T , GIFT G and Viet V strains. The Thai strain originated from Egypt Pham et al., .
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 day
y1
on days when water temperature exceeded 168C. Survival rate of fry after overwintering in
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 m
2
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 area s 1200 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 fishrm
2
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 day
y1
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
Ž .
urea and TSP were applied weekly to all experimental ponds, after soaking in water and broadcasting, at rates of 4 kg ha
y1
day
y1
of N and 1 kg ha
y1
day
y1
of 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
. initial weight, divided by experimental period days , based on individual measurements.
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. biomass s
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 CaCO Boyd and Tucker 1992
3
U
and standard EDTA titrant, measured weekly
U
Ž .
Concentration of phenate method, measured twice monthly
APHA 1980 Ž
. ammonia NH –N
3
Ž .
Concentration of cadmium reduction procedure, measured twice
APHA 1980
U
Ž .
nitrate NO –N monthly
3
U
Ž .
Concentration of nitrate diazotizing method, measured twice monthly
APHA 1980 Ž
. NO –N
2
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
