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INTRODUCTION

Loss of biodiversity is of increasing concern around the world. Most attention has been on terrestrial environments and in particular tropical rainforests where both habitat destruction and species diversity are high. In the aquatic environment conservation has focused on freshwater environments some of which have suffered dramatic and irreversible damage (Micklin 1988, White 1988). In North America 27 species of freshwater fishes in three genera have become extinct during the past century (Miller et al. 1989). For the marine environment the species extinction list is restricted to mammals and birds and the list of threatened or endangered species is dominated by mammals and birds with a few anadromous fishes (Meffe 1987, Upton 1992). Marine fishes that are slow growing and have low fecundity are vulnerable to overfishing: several species of shark may soon be endangered if current exploitation levels are not reduced (Manire and Gruber 1990), while the common skate has been brought to the brink of extinction by commercial fishing (Brander 1981).

Biodiversity has been broadly defined as ‘the degree of nature's variety’ (McNeeley 1988) and as ‘the variety of life and its processes’ (Hughes and Noss 1992) and encompasses all species of plants, animals and micro-organisms and their ecosystems (Kapoor-Vijay 1992). Biodiversity is recognised at four levels:

  1. genetic diversity: the sum total of information in the genes of individual organisms of a species,
  2. species diversity: the number and frequency of organisms in a given area,
  3. ecosystem diversity: the variety of ecological processes, communities and habitats within a region, and
  4. landscape diversity: the spatial heterogeneity of the various land uses and ecosystems within a larger region from 100 to 10 000 000 km sq. (Noss 1983, Norse et al. 1986, OTA 1987).

Species and ecosystem diversity are high in the marine environment, there are more phyla and classes represented in the marine than terrestrial environment with organisms ranging from unicellular plants and animals through to whales. There are approximately 20 000 species of teleost accounting for nearly half of the vertebrate species. Some 9 000 teleost species are exploited and for 22 species the global catch is in excess of 100 000 tonnes per annum. Around 58% of fishes are marine and although the oceans cover 2/3 of the planet most marine fish are found in coastal waters which represent less than 10% of the planet's surface.

It has been suggested that the coastal zone is being altered as fast as tropical forests, and simply knowing which ecosystems have more or less species is misleading (Ray 1988). The relative lack of knowledge concerning the loss of marine diversity is in part due to the remoteness and difficulty of monitoring marine habitats. Loss of forest diversity occurs through simplification, fragmentation, and selective destruction (Norse 1990) especially when management is focused on a few species (Cairns and Lackey 1992). Similar processes are operating in coastal seas through land reclamation, pollution and over harvesting. Productivity in land-based farming is highest where systems have been simplified and similar trends are emerging in coastal environments with the development of aquaculture and enhancement (Ray 1988).

Genetic conservation of aquatic resources has centred on freshwater and anadromous resources where the problems of habitat alteration, introductions, and overfishing are greatest and most urgent (Ryman 1981, 1991, Meffe 1987, Skaala et al. 1990, Hindar et al. 1991, Minckley and Deacon 1991, Nyman 1991, Moyle and Leidy 1992, Cloud and Thorgaard 1993). Increasing attention is being directed towards the loss of genetic diversity in the marine environment and reviews of this subject have been presented by FAO 1981, 1993, Polunin 1983, Nelson and Soule 1987, Ray 1988, Carlton 1989, PDT 1990, Upton 1992, Munro 1993, Policansky 1993.

The focus of this paper is on genetic diversity in the marine environment and the possible impact of fishing. Genetic changes due to exploitation have been reported in a wide variety of fisheries from the Arctic Ocean to tropical African lakes. Reports of genetic responses to exploitation were first made in the 1970s, before direct methods for measuring genetic diversity, such as allozyme electrophoresis, were developed. Much of the evidence for genetic changes in fish stocks is based on life history characters, such as growth rate, size at age, and size or age at first maturity. These characters are influenced by both genetics and the environment. The complex and poorly understood relationships between the genetic components of life history characters and the non genetic responses of these traits to changes in population density and environmental parameters make it difficult to seperate the genetic and non genetic impact of fishing on natural populations. Therefore it is necessary to outline the major factors producing changes in fish stocks before discussing genetic diversity and the evidence for the genetic impact of fishing.


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