Freshwater Fish

fish: any of numerous cold-blooded strictly aquatic craniate vertebrates ... that have typically an elongated somewhat spindle-shaped body terminating in a broad caudal fin, [and] limbs in the form of fins.

Arguably, more than any other vertebrate group, fishes are tied to a single dominant feature of their habitat: water. With sleek bodies designed to reduce drag, fins for steering and propulsion, swim bladders to buoy them at any depth, and special current-sensing canals running the length of their bodies (lateral line system), fishes are exquisitely adapted to movement through the underwater world. But water provides more to fishes than just a physically supportive medium in which to navigate. Water temperature, chemical composition, speed, and pattern of flow influence everything from whether fishes can breathe to where conditions are suitable for them to deposit their eggs (spawn). The physical connections among water bodies determine the distribution of fish species and the potential for colonization by other fishes, which may be predators, prey, or competitors. A break in the flow between water bodies is one of the most important factors in generating new fish species: genetic differences among fish populations can accumulate over time when a conduit for the exchange and inter-breeding of individuals is blocked. Indeed, it may have been just these kinds of barriers, some half a billion years ago, that set one group of fishes on the path to land, as the ancestor of all modern-day tetrapods (mammals, birds, amphibians, reptiles).

Whether it is that fishes are older than other vertebrates or the fact that 70% of the planet is covered by water, fish are certainly the most species-rich vertebrate group, making up about half (25 000) of all vertebrate species worldwide. Of these, approximately 10 000 species are freshwater species, a somewhat surprising proportion given that only about 3% of the world’s water is fresh. Canada contains about one-quarter of the world’s fresh water and thus has a considerable responsibility for the protection of freshwater fish habitat. Within our streams, rivers, and lakes, there are 230 species of fishes. Two major groups of Canada’s freshwater fishes contain members familiar because of their prominence in recreational and commercial fisheries: the salmons and allies (Salmoniformes), including salmons, trouts, ciscoes, and whitefish; and the perches and allies (Perciformes), including the perches, basses, walleye, and drums. These two groups account for about 12% and 20%, respectively, of Canadian freshwater fish species. A third group, Cypriniformes, represents the largest assemblage (29%) of freshwater Canadian fish species. It includes the minnows, dace, chub, suckers, shiners, and redhorses. Several other familiar but smaller groups are the catfishes, pikes and pickerels, and sculpins, together accounting for another 15% of Canadian species.

While a few of these species (e.g., Lake Trout, White Sucker, Slimy Sculpin, Lake Whitefish) extend across almost the whole country, many species are concentrated in particular provinces and territories. As a result, species richness varies across provinces and territories - the highest species richness occurs in Ontario (155 species) and Quebec (114 species). Species richness falls steadily east of Quebec and west of Ontario, being lowest in Atlantic Canada and the northern territories (under 50 species). Reasons for high species richness in Ontario and Quebec may be related to the large number and extent of water bodies there, a relatively mild climate in parts, and the fact that two major drainage basins overlap in these provinces. Still, overlap of drainage basins can be only part of the explanation, as Ontario has 42 species that do not occur anywhere else in Canada. This is 15 more of such species than the next province, British Columbia (27 species), and at least 38 more than any of the other regions, including Quebec (four species). Ontario still retains its place as the most species-rich province for freshwater fish, even when an adjustment for its 19% non-native (Exotic) species is made. Some other regions have no Exotic species (e.g., Yukon, Labrador); however, the propor-tion of Exotic freshwater fish species is high (at least 10%) relative to other groups ranked in the report.

A significant number of Exotic species is one factor that threatens Canada’s native freshwater species in the Great Lakes. A notable example is the Sea Lamprey, a primitive parasitic “fish” that has contributed to the extinction within Canada of at least one species, the Deepwater Cisco, in the Great Lakes. In British Columbia, the Black Catfish is implicated in the extinction of two species of lake stickleback. A total of seven freshwater fish species are extinct in Canada, with an additional species, the Blackfin Cisco, extirpated from freshwater bodies in Ontario. Other Exotic species have less obvious effects on native fishes than the mortal wounds inflicted by Sea Lampreys. For instance, introduced species are often voracious predators, either eating native fishes or competing with them for their regular diet of invertebrates (e.g., worms, leeches, insects, snails) or other fishes. Other factors that can influence the distribution and abundance of freshwater fishes are related to habitat alteration (e.g., dam construction, impoundments), overexploitation, and pollution.

The overall effect of these impacts on freshwater fish species in Canada is not clear, because national general status rankings have not yet been completed for this group. However, on average, provinces and territories reported about 2% At Risk, 4% May Be At Risk, and 12% Sensitive general status ranks for freshwater fish in their region. The majority of freshwater fishes are ranked as Secure (average 53%), but it is important to note that the average proportion Undetermined (17%) was quite high, with a further 1% Not Assessed.

It might be wondered whether our lack of enough data to allow general status assessments of these Undetermined species is important. In this respect, it is worth recalling that these lesser-known species form part of ecosystems that support other wildlife, including predatory fish species. Many of these predators are of direct economic importance, forming part of the Canadian commercial freshwater fishery, which had a market value of over $147 million in 1998, and a recreational fishery (freshwater and marine) that had a value of over $4.9 billion in 1995. Economic impacts then join the roster of reasons to work in the future towards determining the general status of all Canadian freshwater fish species.