Overfishing in inland waters reduces biodiversity and threatens health

Threats to freshwater fisheries and associated biodiversity largely unrecognized, authors state

Systematic overfishing of fresh waters occurs worldwide but is largely unrecognized because of weak reporting and because other pressures can obscure fishery declines, according to an article in the December 2005 issue of BioScience. Although the status of inland waters and their fish species should be of broad concern, threats to freshwater fisheries and associated biodiversity have received scant attention from conservation groups and the media, according to author J. David Allan of the University of Michigan and his colleagues. Allan and colleagues refer to the relative lack of attention to fresh waters as a “dangerous” imbalance, since freshwater ecosystems are by some accounts more threatened than marine ecosystems.

Fishing is a crucial source of livelihoods in developing nations, and in 2000 constituted an estimated 15.3 percent of human consumption of animal protein. About 1 billion people rely on fish as their primary protein source. Landings from inland waters have increased more than fourfold since 1950, mainly in developing nations, though in developed countries, in contrast, catches have decreased. But catch statistics are difficult to interpret for inland species because they may exclude recreational and illegal fisheries, and because landings are dispersed. Moreover, overfishing may not immediately cause declines in the total catch––and its weight may even transiently increase. Overfishing leads to numerous changes in both the target species and other species. Larger individuals and species are often successively replaced in the catch by smaller ones. At the same time, some fish populations respond to fishing pressure with reductions in mean size at maturation.

Allan and colleagues identify two main types of overfishing. One leads to marked declines in catch per unit effort and size of individuals captured. The second type is characterized by sequential declines of species and depletion of individuals and species of large size, especially piscivores. These types are illustrated by case studies of fish declines in Australia, in the lower Mekong River of Southeast Asia, in the Great Lakes of North America, and in the Oueme River of the Republic of Benin.

Allan and his coauthors warn that overfishing of inland waters has the potential for severe impacts on human health, particularly in developing countries. For example, fish consume the vectors of important diseases such as schistosomiasis. The authors also conclude that there is ample evidence of the global importance of overfishing as a threat to inland water biodiversity. They recommend that management of inland fisheries should be guided by sustainability of yields, maintenance of biodiversity, protection from habitat degradation and other anthropogenic stressors, and provision of socioeconomic benefits to a broad spectrum of consumers.