Abstract
Efficient supplementary feeding will utilize not only the feed added to the pond, but also natural food. Feeding improves the growth rate of fish in a pond only when the fish are food limited. Natural foods may provide all of the necessary nutrition for normal growth of small fish, and if growth begins to decline at a larger size, supplementary feeding should yield higher growth rates. One experiment compared the use of fertilizer alone, feed alone, and a combination of feed and fertilizer for growth and production of Nile tilapia (Oreochromis niloticus). Ponds receiving feed and fertilizer had higher growth rates than ponds with feed alone, and both showed considerably higher growth rate than ponds with fertilizer alone. Feeding was necessary to grow fish to a size of over 200 g in a reasonable amount of time. A second experiment tested the possibility that feeding at reduced ration and allowing the consumption of natural food might give a comparable growth rate to feeding alone. In this experiment, full satiation ration, 75% satiation ration, or even 50% satiation ration resulted in similar mean weights at harvest, about 400 g after 150 days. These results indicate that it may be efficient to simply feed the fish at about half of their maximum ration and allow them to eat natural food for the other half of their growth, if the pond is managed well. Economic analysis showed that fertilization resulted in $1,891 ha-1 year-1 of profit, feed only lost almost $3,000 ha-1 year-1, 75% feeding lost $258, 50% feeding yielded $1,765 ha-1 year-1, and 25% feeding yielded $155 ha-1 year-1. If the growth rate were linearly extrapolated until the fish reached 500 g and were then harvested, the 75% feeding would yield $4,835 ha-1 year-1 and the 50% feeding $5,865 ha-1 year-1. A third experiment was conducted to evaluate the timing when first feeding should occur. In this case, fish in ponds were fed once they reached 50, 100, 150, 200 or 250 g in size. The fish grew similarly before they were fed, and then grew again similarly after feeding, with a growth rate under fertilization of about 1 g day-1, and a growth rate under feeding of about 3 g day-1. Fish fed first at 50 or 100 g reached the same ultimate size by the end of the experiment and had similar production levels. Fish first fed at 150 or 200 g also showed similar results and, finally, fish fed at 250 g never reached the targeted size. These results indicate that it is most efficient to wait until the fish reach 100 g before commencing artificial feeding. Economic evaluation showed an annual net profit of $3,700 ha-1 year-1 when the fish were fed first at 50 g, $6,160 at 100 g, $4,800 at 150 g, $1,800 at 200 g and $3,600 at 250 g. Clearly, efficient feeding systems produce higher incomes than less efficient systems, and fertilization alone, if the fish reach a targeted size that has a higher economic value.
Correlations between characteristics of some free-living Chlorella sp. and their ability to form stable symbioses with Hydra viridis
