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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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| Freshwater fishes of the St. Lawrence: diversity and conservation |
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On a worldwide scale, large rivers have been greatly affected by losses in biodiversity (Allan and Flecker 1993; Hugues and Noss 1992). Modifications such as the control of water levels or the loss of a connection between the channel and floodplain are some of the main disturbances affecting temperate fluvial ecosystems (Bayley 1995; Sparks 1995). In North America, roughly thirty fish species have gone extinct in the last century (Miller et al. 1989), and roughly one third of species are of concern (Hugues and Noss 1992). Several of these species inhabit large rivers. In the St. Lawrence and Great Lakes system, pelagic assemblages in the Great Lakes (such as ciscos and lake trout) have been the most affected. In this light, the freshwater section of the St. Lawrence appears to be less degraded that many other temperate river systems, despite the worrisome status of a few species such as the copper redhorse and redfin pickerel. The results presented here are of direct relevance in efforts to conserve the freshwater species of the St. Lawrence. Maps of local species richness, rarity and conservation status permit identification of the more important areas using to these criteria. Based on the different ecological divisions, some priority units can be identified, along with a fairly accurate idea of their relative importance. The division of the study area into hydrographic units along the longitudinal axis of the river and into depth classes along the transversal axis of the river seem to be the most significant divisions. This does not mean, however, that other factors are not important. Their influence can be demonstrated by finer analyses of habitat selection, particularly in the case of zones of aquatic vegetation. Our results not only help define the spatial dimensions of fish communities in the freshwater part of the St. Lawrence, but clearly indicate two main types of freshwater species. One type consists of large, highly mobile species that are long-lived and late maturing sexually, which frequent the channel. The other type consists of many small species that are less mobile, and restricted to shallow littoral areas with or without vegetation. All these species depend to different degrees on habitats such as spawning and feeding grounds and overwintering habitats (Schlosser 1991), interwoven in an aquatic environment that may become locally limiting (oxygen and temperature, see Ryder and Kerr 1989). Spawning grounds are often a limiting factor, particularly for lithophilic spawning species that lay their eggs on gravel or rock. Over 50% of the river's species are lithophilic spawners (Pageau 1984). Roughly 20% of species are phytophilic spawners (they lay their eggs on vegetation) and depend largely on littoral habitats, for which access and environmental conditions are determined by water levels and the state of the riparian shoreline. Several recent studies have shown the importance of historical and regional factors in the establishment and maintenance of the local diversity of freshwater fishes (Angermeier and Winston 1998; Kelso and Minns 1996; Mandrak, 1995). Therefore, just taking into account local factors to predict the richness and structure of local communities is not enough. Instead, data at several scales must be studied by placing local habitats in a proper environmental perspective. Jacques Leclerc |
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