Optimum harvest time in aquaculture: An application of economic principles to a Nile tilapia, Oreochromis niloticus (L.), growth model.

R.R. Springborn & A.L. Jensen, School of Natural Resources, University of Michigan, Ann Arbor, Michigan, USA

W. Y. B. Chang, Division of International Programs, National Science Foundation, Washington DC, USA and Center for Great Lakes and Aquatic Sciences, University of Michigan, Ann Arbor, Michigan, USA

C. Engle, Department of Agriculture, University of Arkansas at Pine Bluff, Pine Bluff, Arkansas, USA

11 February 1994, CRSP Research Report 94-61

Abstract A simple method is presented for determining the optimum time to harvest fish and the effect of fertilization type on optimum harvest time for aquaculture. Optimum harvest time was similar for either maximizing fish yield or maximizing profit of fish harvested (price of fish times fish yield minus fish production cost), because the daily change in fish production cost was low for the low-input Nile tilapia, Oreochromis niloticus (L.), production system in Thailand. At a harvest time of 150 days for an organic fertilization treatment compared to an inorganic fertilization treatment fish yield increased from 1.505 t/ha to 2.295 t/ha, and profit of fish harvested increased from 15,657.1 baht/ha (US$ 590.8/ha) to 25,127.5 baht/ha (US$ 948.2/ha). For the organic treatment, optimum harvest time occurred at 191 days, with a fish yield of 2.328 t/ha and a profit of 25,520.5 baht/ha (US$ 963.0/ha), compared to the inorganic treatment where optimum harvest time occurred at 105 days with a fish yield of 1.536 t/ha and a profit of 16,035.4 baht/ha (US$ 605.1/ha).

This abstract was excerpted from the original paper, which was published in Aquaculture and Fisheries Management 23:639-647, 1992.

Instantaneous mortalities and multivariate models: applications to tilapia culture in saline water.

Kevin D. Hopkins, College of Agriculture, University of Hawaii at Hilo, 523 W. Lanikaula St., Hilo, Hawaii 96720-4091, USA

Daniel Pauly, International Center for Living Resources Management, MC P.O. Box 2631, 0718 Makati, Metro Manila, Philippines

11 February 1994, CRSP Research Report 94-62

Abstract The "per cent mortalities" commonly used by aquaculturists do not allow separation of the different components of fish mortality between stocking and harvesting in aquaculture experiments. It is shown that "instantaneous" or exponential mortalities, as used in fish population dynamics, have the properties required for such separation, especially when used in conjunction with a multiple regression model. Examples drawn from tilapia experiments conducted in seawater tanks in Kuwait and brackishwater ponds in the Philippines are presented.

This abstract was excerpted from the original paper, which was published in Multivariate Methods in Aquaculture Research: Case Studies of Tilapias in Experimental and Commercial Systems, p. 105-111. M. Prein, G. Hulata, and D. Pauly (eds.). ICLARM Studies and reviews 20, International Center for Living Aquatic Resources Management, Manila, Philippines, 1993.

Production of Oreochromis niloticus fry for hormonal sex reversal in relation to water temperature.

B.W. Green and D.R. Teichert-Coddington, Department of Fisheries and Allied Aquacultures, Alabama Agricultural Experiment Station, Auburn University, Alabama 36849-5419, USA

11 February 1994, CRSP Research Report 94-63

Abstract Recently hatched tilapia fry 9 to 11 mm total length (TL) are preferred for hormonal sex reversal because they are most likely to be sexually undifferentiated. Thirty-three trials were conducted in Honduras between September 1988 and March 1990 to quantify the effect of water temperature on Oreochromis niloticus fry production in earthen ponds for hormonal sex reversal.Two 0.05-ha ponds were simultaneously stocked with brood fish in each trial; generally, one pond was harvested after 17 days, the other after 20 days (range 16 to 21 days). Fry production was evaluated in relation to degree-days from the threshold temperature of 15°C. Harvests averaged 86,000 fry/0.05 ha. A total of 4,897,000 fry were produced, of which 4,363,000 fry were of appropriate size for hormone treatment. No fry production occurred at less than 140 degree-days; fry production increased significantly with increased degree-days above this level. Above 195 degree-days percent of the population retained by a 3.2-mm vexar-mesh grader (too large for androgen treatment) increased significantly with increased degree-days. Fry retained by the grader averaged 14.2-mm TL, while fry not retained averaged 9.5 mm TL. No significant linear relationship between degree-days and number of fry not retained by the grader was observed between 140 to 280 degree-days. However, production appeared to peak at about 210 degree-days.

This abstract was excerpted from the original paper, which was published in Journal of Applied Ichthyology 1993. 9:230-236, 1993.

An economic analysis of fish production in a subsistence agricultural economy: The case of Rwanda

Carole R. Engle, Department of Agriculture, University of Arkansas at Pine Bluff, Pine Bluff, AR 71601, USA

Marcie Brewster, ATTRA, University of Arkansas, Fayetteville, AR 72701, USA

Felix Hitayezu, Rwanda

6 June 1994, CRSP Research Report 94-64

Abstract A survey of 55 fish farmers in Rwanda provided data to analyse the economics of fish production in a subsistence agricultural economy. Survey results demonstrated that fish are primarily a cash crop and secondarily a source of food for the family.

Net cash returns to labour per 100 m² varied from FRW 510 at high altitudes to FRW 1308 at low altitudes and represented approximately 7% and 29%, respectively, of average annual income on high- and low-altitude farms.

Labour was the primary resource utilized in fish farming and yielded cash returns that
were competitive with daily wage rates even though realistic employment opportunities were scarce. Income from fish farming represented a significant proportion of total cash income in addition to its food value for the household.

This abstract was excerpted from the original paper, which was published in Journal of Aquaculture in the Tropics. 8:151-165, 1993.

Effect of fertilization frequency on the production of Nile tilapia (Oreochromis niloticus)

Christopher F. Knud-Hansen, Ted R. Batterson, Department of Fisheries and Wildlife , Michigan State University, East Lansing, Michigan 48824, USA

30 September 1994, CRSP Research Report 94-65

Abstract An experiment was conducted to determine the most efficient frequency of urea and triple superphosphate (TSP) fertilization of earthen ponds stocked with Nile tilapia (Oreochromis niloticus). There were five treatments consisting of the following fertilization frequencies: daily, twice per week, weekly, twice every 3 weeks, or once every 2 weeks. All ponds received the same total fertilization inputs for the entire growout period. Net fish yield (NFY) was not correlated to fertilization frequency, but strongly linearly related to net primary productivity (NP) (r²=0.90, P<0.001). NP was related to low inorganic carbon availability and/or inversely related to light availability. The latter was reduced by inorganic suspended solids. Urea and TSP input rates resulted in soluble nitrogen (N) and phosphorus (P) accumulation in all ponds, with greater accumulations in more turbid ponds. As NP was not limited by either N or P, varying the frequency of urea or TSP inputs had no effect on either NP or NFY. The frequency of urea and TSP fertilization may become a management concern only when the availability of either N or P limits phytoplankton productivity.

This abstract was excerpted from the original paper, which was published in Aquaculture 123:271-280, 1994.