Proceedings of the Third World Fisheries Congress: Feeding the World with Fish in the Next Millenium—The Balance between Production and Environment
Diversity Patterns of Exploited Demersal Fish Communities on the Continental Shelf and Slope in the Greek Seas (Northeastern Mediterranean)
Mary Labropoulou, Costas Papaconstantinou
Reductions in catch rates and the mean size of caught fish is well documented in world fisheries (Pitcher 1996). Consequently, new approaches to the study of exploited populations have been suggested, including the study of the fish assemblage structure in relation to environmental variables and the characterization of seasonal changes to improve management practices. Despite this progress, basic descriptions of demersal fish faunas comprising both commercially exploited species and nontargeted components are not yet widely available for many coastal and offshore regions, although their value with respect to understanding possible fishing effects may be high (Rogers et al. 1999). As pointed out by Caddy and Sharp (1988), this type of study is a necessary step toward understanding the dynamics of multispecies stocks. Such work then can be extended to descriptive community dynamics to find general patterns that may be associated with particular environmental conditions and fishing effort.
Changes in species composition with depth on continental shelf and slope are well established for many areas (e.g., Haedrich et al. 1980; Carney et al. 1983; Abelló et al. 1988; Hecker 1990; Cartes and Sardá 1993; Koslow 1993; Smale et al. 1993; Cartes et al. 1994; Sardá et al. 1994; Gordon et al. 1995; Fariña et al. 1997a). Physical and biological factors have been discussed as causes of faunal zonation with depth. Hydrographic conditions, the steepness of the continental slope, and substrate type are among the major physical factors considered. Resource availability, predator–prey relationships, and interspecies competition are the most important biological factors reported. However, most studies have examined megafaunal assemblages, and little attention has been paid to the structure of the highly exploited demersal fish assemblages, apart from the degree to which different fish species are zoned with depth.
More recent studies of fish community structure have focused on patterns of spatial and temporal variation in composition, abundance, and distribution of demersal fish assemblages of the continental shelf and slope at several latitudes (Markle et al. 1988; Bianchi 1991, 1992a, 1992b; Fujita et al. 1995; Fariña et al. 1997b; García et al. 1998; Moranta et al. 1998). Changes in species composition and size structure of demersal fish communities in response to fishing have been demonstrated (Haedrich and Barnes 1997; Sainsbury et al. 1997; Zwanenburg 2000). One key challenge appears to be to incorporate ecosystem objectives within fisheries management, including measurable indicators such as ecosystem diversity, species diversity, and ecologically dependent species (Gislason et al. 2000).
Information about soft-bottom fish assemblages is particularly scarce in the eastern Mediterranean region, where demersal fishes are heavily exploited as principal targets or as bycatch. In Greek waters, demersal fishes of the continental shelf and slope are subjected to an intensive fishery carried out by trawl, gill-net, and longline fleets. The gill nets and longlines catch a few species, whereas the trawls exploit a multispecies fishery, targeting several demersal and benthic species. The results of experimental trawl fishing in the Greek seas indicate that commercially important demersal and inshore stocks suffer from overfishing. As a result, commercial catches consist of mainly immature individuals and various noncommercial species that are discarded (Stergiou et al. 1997).