Aquaculture CRSP Aquanews Vol. 18 No. 4 ~ Fall 2003

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Graduate’s Corner

he Aquaculture CRSP congratulates Nguyen Thanh Long on the successful defense of his M.S. thesis at the Asian Institute of Technology, Thailand, in August. CRSP Principal Investigator Yang Yi served as his major professor. Long is presently a Lecturer and Researcher at the Department of Fishing and Fisheries Resource Management, College of Aquaculture and Fisheries, Can Tho University. In 1985 he received his Bachelor of Science in Fishing Technology from Can Tho University, and went on to participate in several important programs involving the development of aquaculture in Vietnam. We wish him every success in his future endeavors.

STOCKING RATIOS OF HYBRID CATFISH (CLARIAS MACROCEPHALUS X C. GARIEPINUS) AND NILE TILAPIA (OREOCHROMIS NILOTICUS) IN AN INTENSIVE POLYCULTURE

(Abstract of Nguyen Thanh Long’s M.S. thesis)
An experiment on intensive polyculture of hybrid catfish and Nile tilapia was conducted in fifteen 5-m2 outdoor cement tanks at the Asian Institute of Technology for 91 days, to determine the appropriate stocking ratio of Nile tilapia (Oreochromis niloticus) to hybrid catfish (Clarias macrocephalus x C. gariepinus) and effects of different stocking ratios on the growth of both Nile tilapia and hybrid catfish, water quality, nutrient utilization efficiency and economic returns. There were seven stocking ratios (treatments) of Nile tilapia to hybrid catfish. Hybrid catfish fingerlings of 25–26 g size were stocked at 100 fish per tank (20 fish/m2), while Nile tilapia fingerlings of 20–23 g size were stocked at 0, 5, 10, 15, 20, 25 and 30 fish per tank (0, 1, 2, 3, 4, 5 and 6 fish/m2), giving stocking ratios of Nile tilapia to hybrid catfish as 0:100, 5:100, 10:100, 15:100, 20:100, 25:100, and 30:100, respectively. The control (0:100) had three replicates, while other treatments had two replicates each. No fertilizers were applied to the tanks. Floating pelleted feed (30% crude protein) was given twice daily at 0830 and 1530 h 6 days per week. Feed rations were calculated based only on hybrid catfish biomass estimated by biweekly fish sampling and observed survival rate. Feeding rates of 5% and 3% body weigh per day were used for hybrid catfish smaller than 100 g and larger than 100 g, respectively. All tanks were aerated by airstones throughout the experimental period. The addition of Nile tilapia into hybrid catfish tanks did not cause any significant effects on survival, growth and yields of hybrid catfish (P > 0.05). Survival of Nile tilapia was not significantly different in the polyculture treatments (P > 0.05). With increasing stocking ratios of Nile tilapia to hybrid catfish, mean tilapia weights at harvest decreased linearly, while both net and gross tilapia yields increased linearly (P < 0.01). FCRs based on either hybrid catfish alone or combined hybrid catfish and Nile tilapia were not significantly different among all treatments (P > 0.05). Combined net yields were not significantly different among all treatments (P > 0.05), however, combined gross yields increased linearly with increasing stocking ratios of Nile tilapia to hybrid catfish (P < 0.05). To produce 1 kg of live hybrid catfish and Nile tilapia generated 18–22 g nitrogen (N) and 3 g phosphorus (P) as metabolic waste. Approximately 34–40 % of total nitrogen and 52–55% of total phosphorus input from the feed were released in the tanks. There were no significant differences in nitrogen utilization efficiency among the catfish monoculture and polyculture of hybrid catfish and Nile tilapia (P > 0.05), however, phosphorus utilization efficiency was significantly lower in the monoculture than in the polyculture (P < 0.05). The addition of Nile tilapia into hybrid tanks did not improve water quality. Water quality fluctuated largely among experimental tanks and deteriorated towards the end of the culture period in all treatments. Compared to hybrid catfish monoculture, the ratio of added return to added cost was highest in the polyculture at 10:100 ratio of Nile tilapia to hybrid catfish, intermediate at 5:100 ratio, and lowest at higher ratios (15–30:100). This study has demonstrated that the intensive polyculture of hybrid catfish with Nile tilapia is feasible and that the appropriate ratios of Nile tilapia to hybrid catfish are 5–10:100.

Congratulations are also in order for Melinda C. Clarke, who successfully defended her M.S. thesis in August at The University of Michigan. CRSP Principal Investigator Jim Diana served as her major professor. We wish her every success in the future.

SHRIMP AQUACULTURE BROWNFIELDS: SOCIAL, ENVIRONMENTAL AND ECONOMIC ISSUES DETERMINING REHABILITATION OPTIONS

(Abstract of Melinda M. Clarke’s M.S. thesis)
Intensive shrimp culture in Thailand has fueled the country’s economic development, but has left areas of abandoned and degraded ponds. Thailand is dependent on agriculture, but has exhausted availability of cultivable land, making it necessary to rehabilitate abandoned land. This study examined the scale of shrimp pond abandonment as well as related environmental, economic and social issues.

Data were collected using semi-structured questionnaires in Chanthaburi, Chachoengsao and Samut Sakhon provinces in Thailand. Shrimp farmers, head villagers and other stakeholders were interviewed about social development of villages throughout the shrimp boom, local employment preferences, and how local people viewed land abandonment. All interviewed groups were asked common sets of questions about these issues. Data generated from these questions were analyzed comparing frequency distributions using a chi square test to determine if responses differed from a random distribution. Field observations of alternative uses for failed shrimp farms were used to understand viability of conversion options.

The study documented a number of conversion options and measured significant areas of abandoned and converted land. At the time of the study 26% of the area of land surveyed was still active in shrimp culture, 55% had been converted to other uses, and 14% had been abandoned. Respondents indicated that villages had developed in positive ways in the decade during which shrimp culture proliferated. They indicated improved environment, employment, government representation, health, and opportunity for children. They did not recognize land abandonment as a community-wide problem. A preference for on-farm income generation and land ownership over wage labor was demonstrated. Preference for type of on farm employment varied by study site as did demonstrated conversion options. Although the conversion pattern after failed shrimp culture was expected to be linear, the observed pattern demonstrated a cycle of repeated conversion and return to shrimp culture.

Results suggested that agencies wishing to address pond abandonment must work to understand how abandoned land is perceived at the local level. Research into alternative uses for those ponds should be focused on those activities that are most congruous with local perception and preference to enhance likelihood of adoption and dissemination of practices

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