Reported benefits of this method, however, have been anecdotal to date. There is a need for scientific tests in order to understand not only the effect of this practice on the fish, but also to obtain insight into the basic ecological changes that a pond undergoes when manure is added. Recently, researchers at the Dor research center in Hof Hacaramel, Israel, tested manure additions to fish ponds.
The basic (or normal) treatment used was a daily dose of chicken manure; initially the dose equalled 50 kg of dry matter per hectare, but this was increased by 25 kg a day until a level of 175 kg per hectare was reached. This quantity was standardized at the station more than a decade ago. Although the amended dry matter exceeded recommendations by previous authors, it was found to be in agreement with recommended levels of carbon. Two other treatments were employed, measuring half and double the normal treatments of added manure (50 kg/ha).
The researchers tested a mixture of four fish species in their experimental ponds: common carp, grass carp, silver carp, and hybrid tilapia. A 10,000 fish/ha (or 250-kg-of-fish/ha) initial stocking level was used. After three and a half months (the experiment ran from mid-July to early November), the fish were harvested.
Surprisingly, the manure treatments had little effect on fish growth rate. The only species that was significantly affected was the common carp whose growth was faster in the double dosage treatment compared with the half dosage. This could be attributed to different feeding habits; the common carp is a bottom feeder in contrast to the other species, which feed primarily on phytoplankton. Even though the double dosage represented a three-fold increase in manure when compared to the half-dosage treatment, common carp growth was only increased by 22% (with respect to the lowest dosage) and nitrogen ammonium only increased by 26%.
Besides the relative effect on fish production, the researchers also hoped to answer questions regarding the effect of the manure on the ecology of the pond. Various parameters in the water and sediments were measured and then linked to the important aspects of autotrophic (algal) and heterotrophic (bacterial) activity. Generally the manure additions affected sediment properties more than those of the water. The half dosage was found to be sufficient to support both algal and bacterial populations in the water while the double dosage did not significantly increase these populations. Generally as organic loading (i.e. manure) and, in turn, anaerobic conditions increased, heterotrophic activity decreased. When averaged over the whole experiment, the half-dosage treatment translated to 83 kg of dry matter per day; this daily amount contained 30% carbon and 3.1% nitrogen.
These results are preliminary and may be more meaningful in the future by adding a control or zero-manure treatment to the study. There was also a hint at the end of the report that fish yields were unusually low during the year of the experiment, which was tentatively attributed to the use of a new herbicide on the research center grounds which found its way into the water.