some systems the ecosystem effects of fishing have not fully developed or worked their way through the ecosystem. Although there is little information on ecosystem effects of fishing in the open ocean, those ecosystems that have been affected by fishing—estuarine, coastal, semienclosed, and continental-shelf ecosystems—provide most of the world's fishery products and many other services.
In addition to fishing, anthropogenic impacts and environmental changes continue to occur and are important. Anthropogenic impacts, which include contamination with toxic and other chemicals, habitat alteration and destruction, and introduction of exotic species, can be identified and in some cases managed. Most natural environmental changes cannot be managed in the traditional sense. Indeed, many are not even predictable in a precise way. However, they are becoming better understood in a general way and must be taken into account in any sensible management program.