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C. East Africa

C. East Africa



Masculinization of Nile Tilapia (Oreochromis niloticus) through Immersion in 17a-Methyltestosterone or 17a-Methyldihydrotestosterone

Interim Work Plan, Africa Study 2

William L. Gale, Martin S. Fitzpatrick, and Carl B. Schreck
Oregon Cooperative Fishery Research Unit
Department of Fisheries and Wildlife
Oregon State University
Corvallis, Oregon, USA

Abstract

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.

Maximum Voluntary Feed Intake and Growth of Nile Tilapia Fry as a Function of Water Temperature

Work Plan 7, Africa Study 4

Joyce R. Newman and Thomas J. Popma
Department of Fisheries and Allied Aquacultures
Auburn University, Alabama, USA

Abstract

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.

Characterization of African Soils and Site Evaluation

Work Plan 7, Africa Study C

Wayne K. Seim and James R. Bowman
Department of Fisheries and Wildlife
Oregon State University
Corvallis, Oregon, USA

Abstract

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.

Reproductive Efficiency of Nile Tilapia (Oreochromis niloticus) and Red Tilapia (Oreochromis spp.) and a Comparison of Fry Growth and Susceptibility to Being Sex Reversed

Interim Work Plan, Africa Studies 6 and 7

Edwin S. Smith and Ronald P. Phelps
Department of Fisheries and Allied Aquacultures
Auburn University, Alabama, USA

Abstract

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.

Growth and Efficiency of Sex Reversal of Nile Tilapia (Oreochromis niloticus) Fed 17a -Methyltestosterone-Treated Feed Stored Under Different Storage Regimes

Interim Work Plan, Africa Study 5

Edwin S. Smith and Ronald P. Phelps
Department of Fisheries and Allied Aquacultures
Auburn University, Alabama, USA

Abstract

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%.