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Freshwater fishes [Diversity] [Biogeography] [Hydrographic units] [Water masses] [Depth] [Sediments] [Wetlands] [Local richness] [Rarity] [Conservation status] [Diversity and conservation] |
Saltwater fishes [Diversity] [Biogeography] [Salinity gradient] [Aquatic physiographic units] [Distance from the shore] [Depth] [Sediments] [Local richness] [Rarity] [Conservation status] [Diversity and conservation] |
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| Saltwater fishes of the St. Lawrence: diversity and conservation |
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The marine section of the St. Lawrence, which extends from the brackish-water estuary to the edge of the Gulf, forms a semi-closed system that can be distinguished in part from the bordering marine waters of the Northwest Atlantic (see articles cited in Therriault 1991, including Chadwick and Sinclair 1991). At the biogeographical scale, however, there is a strong link between these two regions. The position of the marine part of the St. Lawrence in the Northwest Atlantic coastal province (Longhurst 1998), at the border of the Acadian and Labrador biogeographical regions (Ray 1988), determines in large part the fish species present, with northern species mixing with southern ones. Although most species are resident, some occasional visitors occur (Ryder and Scott 1994). Owing to the lack of a systematic review of some of the more difficult groups, no endemic marine species were found in the region, which underwent successive glaciations. The St. Lawrence is also the largest estuary system on the east coast of North America, extending more than 1000 km from the brackish-water estuary to the Strait of Belle Isle. The Gulf, at its widest, extends roughly 500 km from north to south. In this vast region, mixing occurs between the marine waters of the continental shelf and the fresh water of the St. Lawrence and its many large tributaries. These conditions have led to the establishment of a upstream-downstream diversity gradient, determined largely by the salinity gradient, and a north-south diversity gradient, which reflects in part climatic differences and in part the stratification of water masses. The harsh weather conditions in the region cause the stratification of the region's water masses into three layers. The permanent cold intermediate layer, sandwiched between the surface and deep layer, washes over most of the sediments in the southern Gulf. The northern Gulf contains the Laurentian and Esquiman channels, glacial-cut channels that reach over 500 m deep and are located in the deep layer. These differences are partly responsible for the north-south diversity gradient, and also explain the marked differences in species assemblages by depth. Northern species are associated with the cold intermediate layer, while very diverse species, including some extremely rare visitors, are associated with the deep layer. The surface layer is the one most subjected to seasonal climatic variations, particularly along the north-south axis. Diadromous species, often found near shore, prefer the surface waters and depend greatly on the accessibility of tributaries and the habitat conditions found there. Several pelagic and benthic species are also abundant in the surface layer, serving as prey for seabirds such as the Northern Gannet, murres and Razorbill (ICES 1997; Schneider 1994; Wright and Begg 1997). Species dwelling in deeper waters made accessible to birds by commercial fishing must also be added to this list; an example is the Atlantic cod for the Herring Gull (Chapdelaine and Rail 1997). Trophic interactions with birds are not limited to saltwater fish but include macroinvertebrates, eaten mainly by eiders and scoters or by various shorebirds. Owing to the region?s harsh climate, the marine part of the St. Lawrence is also the most southerly coastal zone on the continent to be intensively affected by ice in winter. The ice cover provides, particularly in the case of anchor ice along the shore, a form of environmental protection in that it prevents significant modifications in the coastal zone for such things as aquaculture. The growing problems caused by salmon farming are well known, including the pollution of sheltered sites and the creation of hybrids between the cultured fish and local wild strains. Land use managers must consider all of these characteristics, even if the development in question is only local. Every local area is part of an extensive hierarchy ranging from the biogeographical province to a mosaic of microhabitats, and development in the marine environment must take into account all these scales. Maps of local richness, rarity and conservation status provide valuable information in determining critical areas in terms of management. Water current dynamics, particularly in terms of the transport of pelagic larvae and sediments, must also be taken into account in defining the location, size and protection status of areas in question. These concerns must also be considered in the establishment of marine protected areas and, in the case of coastal sties, terrestrial and marine considerations must be integrated. The differences between pelagic and benthic ecosystems in the estuary for example (Laprise and Dodson 1993), should also be examined. The Atlantic cod has recently been listed as vulnerable on the Canadian endangered species list (COSEWIC 1998), due to the collapse of cod stocks. However, this has not directly threatened the richness of the river's ichthyofauna. The strong decline of a key species such as cod may result in changes in the relative abundance of other species and the partial reorganization of food webs through discontinuities and synergisms (Ray 1996). Many coastal marine environments around the world (Gray 1997) have been more deeply affected by environmental problems, and the St. Lawrence has not been identified as a region at risk on the international scale. However, all our effort must be concentrated on maintaining this situation and adequately protecting the most sensitive components of the system. Jacques Leclerc |
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