Analyses of trends in volume of fish catches by type shows great shifts which are attributable to species depletion - hence, today, Norwegian fishing boats, for example, are harvesting cod instead of the herring which they netted in the North Atlantic up until the 1960s. In 1989, fish production peaked at 100 million tons globally. Of this tonnage, roughly 80 percent was captured fish and the remainder produced through aquaculture techniques. Since that peak, the production level has remained stable but the ratio of capture to aquaculture-derived fish has decreased slightly. The FAO estimates that there will be a shortfall of 25 million tons of fish already by the year 2000.
Aquaculture is a double-edged sword, however.
Its use as a production method has the potential to relieve some of the overwhelming pressures on natural fish species. All too often, however, the techniques used in aquaculture are, in and of themselves, harmful to the environment, especially in cases where market forces cause high competition between aquaculture projects. The destruction of mangroves and the salination of groundwater in areas where intensive shrimp farming is practiced is one well known example. Tilapia refers to a freshwater fish species indigenous to Africa. There are many different subspecies of tilapia, including T. rendalli, T. Zillii, S. mossambicus, S. melanotheron and Gambusia, which adds to confusion in some discussions.