Science - June 1, 2006

Fish waste for shrimp farming

Fish farmers can use the waste from their ponds to raise bacteria and shrimps, thus increasing their returns, according to research done by biologist Oliver Schneider.

The German PhD graduate grew bacteria in a reactor fed with nitrogen-rich wastewater that contained faecal waste from African catfish. The bacterial biomass that is produced was fed to shrimps (Litopenaeus vannamei). A comparative test was done using commercial shrimp feed. Immediately after feeding over eighty percent of the shrimps were present at the feeding places and they displayed a preference for the commercial feed. But after five to ten minutes the behaviour of the shrimps changed, and they went for the bacterial biomass. Schneider concluded that although there is a slight preference for the commercial feed, the shrimps do not mind the ‘bacterial ‘soup’ either.

‘Schneider has shown that the fish waste can be converted into a recyclable product. This can be incorporated into a sort of waste management in fish farming,’ says chair of Aquaculture and Fisheries, Professor Johan Verreth. He adds a couple of comments, however. ‘Lots more research is needed before the technology will be really user-proof. The bacteria that Schneider grew on the manure did not do really well, and to stimulate the bacterial growth he had to add extra carbon.’

The principle has prospects though. Farmed fish does not absorb all nutrients in fish feed, so farmers can make use of this by raising shrimps at the same time, which would eat up the food remains and the improved fish manure. Shrimps in the wild also eat all sorts of animal and plant remains.

It is not always practical to keep shrimps in the same pond as fish. Different fish and shrimp species require different water temperatures. A temperature rise of just one degree can be fatal for some types.

Schneider also devoted attention to characterising the bacteria populations that grow on fish manure. Different substrates in the reactor, such as sodium acetate or molasses, caused changes in the bacteria population, including increased pathogens. This is a matter that requires further research. / HB

Dr Oliver Schneider received his PhD on 24 May. His promotor was chair of Aquaculture and Fisheries, Professor Johan Verreth.