The magnitude and impact of by-catch mortality by fishing gear.

Abstract

AbstractMost fishing operations trap organisms that are not the primary fishing target, and are commonly referred to as the by-catch. It may include small individuals of the target species, or other species with little or no commercial value. The problem is widespread, with a global estimate of approximately 20 million t, equivalent to about a quarter of the total world landings. Shrimp fisheries tend to generate the largest quantities of by-catch, and fisheries for small pelagics the least. By-catch rates in mixed demersal and large pelagic fisheries are intermediate. By-catch arises because fishing gears have imperfect selection properties, but the problem is made worse by economic pressures resulting from overexploitation. This leads to inefficient use of resources and changes in the abundance of both target and non-target species. Some by-catch species, including certain fish, reptiles, birds and mammals, may be threatened with extinction. Raised public awareness means that these conservation issues increasingly influence fishery management. Much of the by-catch is simply discarded at sea. While not intended, the imposition of regulations such as minimum landing sizes and catch restrictions may encourage discarding. Most discards do not survive, but the material provides food for other organisms, especially scavengers, whose abundance may increase. Technical conservation measures, which involve modifications to fishing gear or practices, offer an effective means of reducing by-catch. For trawls, these include grids and square mesh panels that sort animals by size, allowing a part of the catch to escape. For fixed gear, methods can be used to prevent the capture of large animals such as birds and mammals. The successful use of these devices, however, depends on overcoming gear handling constraints and the short-term economic losses often associated with their use. By-catch is just one component of the total mortality of species affected by fishing. Hence by-catch is not an isolated issue. Addressing the problem requires consideration of the broader question of resource management, including the target species. Success in reducing by-catch requires that chronic problems of excessive exploitation must be tackled, and this remains a major challenge worldwide.