Marine fisheries cause changes at three levels of organization of biodiversity. At the genetic level, fisheries increase mortality and act as an agent of directional selection, affecting age distribution, age and length at maturity, and growth rates, among other life history characteristics. Of particular concern are genetic effects induced by marine aquaculture and ocean ranching, where fish husbandry and large scale releases decrease genetic diversity, selecting for particular traits that may not be best suited to long-term environmental variability. At the species level, fisheries affect species composition in fished ecosystems and consequently interactions among fished species and their prey. Direct effects of fishing on species diversity are caused by removals of target species, by-catch of unwanted species, physical disturbance, and discards. Fisheries often begin on large predators and decreased populations may lead to increased abundance of prey species, which may themselves become subject to fisheries. Long term fisheries can lead to system dominance by species with certain life history characteristics (early maturation, small maximum size reached rapidly, high fecundity and relatively short longevity); these characteristics describe small pelagic fishes, which often become major components of mature fisheries. Other components of the system are also affected; as an example, trophic inputs from fishery discards in the North Sea have caused long-term changes in the abundance and species composition of seabirds. Finally, through effects of by-catch, habitat alteration, and altered energy flow, fisheries impact the diversity of marine habitats and the function of ecosystems. Although several instances of fisheries mediated habitat destruction have been documented, ecosystem-level biodiversity changes are often the most poorly understood. Ecosystem models have the potential to evaluate these changes.
Biodiversity is an important issue not only to protected resources and habitat, but also to sustainable fisheries; it should be of growing concern to fishery managers. While many of the effects of marine fisheries on biodiversity are obvious, evidence is circumstantial or lacking for some of the potentially most devastating effects. On the time scale of decades, however, fisheries can change genetic, species, and ecosystem diversity from levels that have been achieved over millions of years through natural selection. There is little doubt that such changes in biodiversity will decrease the resiliency of species, communities, and ecosystems to respond to natural perturbations that occur on longer time scales. Understanding how fisheries and other anthropogenic changes affect marine biodiversity is necessary to develop strategies that will ameliorate future ecological changes. Fisheries have major impacts upon marine biodiversity; the long-term sustainability of these fisheries may be dependent upon the very diversity being changed. (G.Boehlert (831) 648-8447)