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The use of all-male populations increases the efficiency and feasibility
of tilapia aquaculture. The objective of this study was to examine the efficacy
of a short term immersion procedure for masculinizing Nile tilapia
(Oreochromis niloticus). Two synthetic androgens were evaluated, 17a-methyldihydrotestosterone (mestanolone) and 17a-methyltestosterone. Exposures of fry at 10 and 13 days post fertilization for 3 hours
to 17a-methyldihydrotestosterone at 500 µg/l produced sex ratios greater than 93 percent male.Immersions in 17a-methyldihydrotestosterone at 100 µg/l and
17a-methyltestosterone at 500 or 100 µg/lwere unsuccessful at producing all-male populations. Neither steroid
treatment affected mortality nor growth of fry. When compared to current
techniques for steroid-induced sex inversion of tilapia, short term immersion
in 17a-methyldihydrotestosterone
(500 µg/l) shortens the treatment period and reduces possible worker exposure
to anabolic steroids.
Temperature and growth rate have long been suspected to influence timing
of gonadal differentiation and efficacy of sex reversal, but little research
data is currently available. However, before these investigations can be
conducted, more information is needed on appetite and growth response of
tilapia fry grown at different temperatures. As maximum voluntary feed intake
(satiation) of fry fed a powdered feed cannot be measured using standard
techniques, for this study satiation was defined as the feed rate beyond which
growth no longer increased. One trial was run at each of the following three
temperatures: 30°C,
26°C and 22°C. For each trial, treatments of four
replicates were fed at seven feed rates, ranging from 10-28% BW/d at
30°C, 7-25% BW/d at 26°C and 4-22% BW/d at
22°C. Fish were sampled every four days and the trials completed at
28 days (30°C and
26°C) or 32 days (22°C). Fish at 30°C
increased 412-884 mg in 28 days, depending on feed rate, with overall growth
rates of 14.7-31.6 mg/d. At 26°C, they increased 161-513 mg with
growth rates of 5.6-18.3 mg/d, and at 22°C gain was 66-288 mg at
rates of 2.1-9.0 mg/d. In these trials, the highest feed rates used at each
temperature did not exceed maximum voluntary feed intake for the smallest sizes
of fish, and fish at the lower feed rates did not reach a size sufficient for
adequate comparison with fish fed at the higher rates. Further trials are
planned to obtain the needed data points.
On-site PD/A CRSP research in Africa ended in April, 1994 as a result of
war and civil violence in Rwanda. Continued lack of security has prompted the
initiation of a search for a replacement African site. Study 3 objectives
included the development of site selection criteria, collection of data and
evaluation of promising sites, and characterization of African soils. Prime and
companion site evaluation criteria were developed with input from the Technical
Committee and ME. Information on potential sites and USAID planning documents
for all Sub-Saharan African countries were used to identify promising countries
and sites. Promising sites were identified at Sagana, Kenya; Malawi (Bunda
College station and Domasi Experimental Fish Farm) and several sites in
Zimbabwe. Travel approval was requested for the most promising sites in Malawi
and Kenya. Travel to Malawi was not approved by USAID Malawi; the trip to Kenya
was completed November, 1994 by Seim and Egna. Sagana Fish Culture Farm, about
100 km northeast of Nairobi met most selection criteria for a new site, but
support from the Kenya Department of Fisheries was not encouraging. A Kenya
policy of one international project per site made location there uncertain in
light of the current Belgian studies at Sagana. New leadership in the
Department of Fisheries is now enthusiastic about CRSP presence at Sagana and
would support a direct agreement with the CRSP should that site be selected.
Further investigation of Sagana appears warranted. That station is quite large,
having some 40 ponds now in operation with a total water area of about 15.3 ha,
with additional land area and water capacity in reserve. Information on other
potential sites in Africa was also received and Malawi, Zimbabwe, Zambia and
Tanzania remain under consideration. The selected site will be chosen both for
local and regional impact in Africa. Countries south of Kenya are organized
within SADC (Southern African Development Community). Ties with SADC will be
sought regardless of the prime site location to extend the regional impact of
the CRSP activities in Africa. Site evaluation and development planning
continue under the Interim Work Plan. Soils were collected from two African
sites; at least two cores were taken from five locations at two sites in Kenya.
Additional soils from Rwanda were also characterized. Soils were characterized
for CEC, sand, silt, and clay content, pH, exchangeable bases, base saturation
and lime requirement. Sagana pond soils were quite high in clay content and low
(5.1) in pH for a pond not recently in use, near neutrality for ponds more
recently in use.
Red tilapia, a cross between O. niloticus, O. aureus, and O.
mossambicus, were compared with Nile tilapia, O. niloticus, in
regard to brood reproductive efficiency and fry growth, survival, feed
conversion, and sex reversibility. In four trials, brooders of each type were
stocked in separate ponds and allowed to spawn for 215-230 degree-days (13-19
days) using two ponds per fish type. Fry were collected by a complete harvest
of the pond collecting fry from the harvest basin. Fry were graded (>14 mm,
<14 mm) and enumerated. Fry (>14 mm) from both types of brooders were
stocked at 4000/m 3 in outdoor hapas and fed a feed containing 60 mg/kg 17
-methyltestosterone. Hormone treatment periods were 0, 14, 21, and 28 days.
After 28-d of confinement, fry were harvested and growth and survival
determined. Fry were cultured an additional 58 days and the sex determined by
gonadal squash. Brood survival was similar for both types of fish (>95%).
There was no difference in the number of fry/kg of brood produced by each brood
type. Fecundity was correlated to temperature with greater fry production the
higher the temperature. Red x Red brooders gave an average of 77% red and 23%
wild type (black) fry. Harvest survival of fry from both brood types were
similar. There was no difference in the percent males obtained from
hormone-treated fry of either parent line. Mean fry survival and growth was
similar for fry of both sources.
Effects of storage conditions of 17a-methyltestosterone-treated rations and the effects on fish growth and sex reversal were
evaluated using rations held under six conditions. A common source of feed was
treated with 60 mg 17a-methyltestosterone/kg, stored frozen and then held under
one of the following conditions: 26 days at ambient conditions before use; 7
days at ambient conditions before use; 0 days at ambient conditions before
use;
2 months at 40°C and 0 days at ambient conditions before use; 2
months at 40°C and 7 days at ambient conditions before use; and 2
months at 40°C and
26 days at ambient conditions before use. These rations were fed to O.
niloticus fry for 28-d. Fry with an initial length of 10.4 mm were stocked
at
4000/m3 in outdoor hapas and fed at 15, 12, 8, and 4% BW per day during weeks
1, 2, 3, and 4 respectively. Growth and survival were determined at 4000/m 3 in
outdoor hapas and fed at
15, 12, 8, and 4% BW per day during weeks
1, 2, 3, and 4 respectively. Growth and survival were determined after the 28-d
treatment period. Fry were cultured until a minimum size of 4 cm. Fish were
sexed using the gonadal squash technique and the percent male, female, and
intersex fish determined. Feed storage conditions had no effect on the percent
males produced. All hormone-treated feeds resulted in > 99% male
populations. Storage conditions had no effect on growth, feed conversion ratio,
or fish survival when fish were treated in fertile fish ponds. Mean fish weight
after 28-d of treatment was 0.9 g, the feed conversion ratio was <1, and
survival averaged > 55%.
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The Pond Dynamics/Aquaculture CRSP is funded under USAID Grant No. LAG-G-00-96-90015-00
and by
the participating US and Host Country institutions.
Questions for or about the Aquaculture CRSP? Comments about this site? Email ACRSP@oregonstate.edu.
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