FRESHWATER FISHES
E. J. Crossman and E. Holm
HUMAN IMPACTS ON FISHES
The fish fauna of the Ecozone has changed dramatically since the Europeans first arrived in the 1600's. Changes in the aquatic community structure have resulted from a variety of human activity including exploitation, habitat alteration, pollution, dams, canals, and introduction of non-native species. Often these factors act together to decimate native species and reduce the overall biodiversity of native aquatic ecosystems.
Unregulated or inadequately regulated commercial fishing quickly reduced populations of the lake sturgeon, and many species of native trout and whitefishes.
Species diversity is affected by habitat destruction which includes removal of ground water, land clearing and consequent sedimentation and water turbidity, more drastic variability of water levels, channelization, removal of gravel and sand, nutrient enrichment and addition of toxic contaminants. Populations of species which migrate up streams to spawn have been reduced or extirpated because dams have blocked access to spawning areas. In the 1800's, dams were built usually to provide a head of water to operate a mill. A secondary effect was the pollution caused by the organic waste material dumped into the water by the function of this mill. Species have been unintentionally introduced through man-made canals (e. g. Welland Canal, the Erie Barge Canal and the Trent Canal) or transported and introduced with ballast water. Human interest in only a limited number of recreational species results in constant pressure for the authorized introduction of additional non-native species. Unauthorized introductions have come from bait buckets, aquariums, and fish markets that import live fishes for human food.
Case History #1: Interaction of Native and Introduced Species
Recent examples of unintentional introductions include two members of the Goby family native to the Black and Caspian Seas in Europe which have been transported and inadvertently introduced with discharged ballast water of international vessels. The tubenose goby, Proterorhinus marmoratus, was the first to be discovered in April 1990 in the St. Clair River followed shortly by the round goby, Neogobius melanostomus (Fig. FF-2), discovered in June 1990 (Jude et al. 1992; Crossman et al. 1991). The round goby has reached extremely high densities in some areas and is a nuisance to anglers because it is it strips dew worms from fishing lines or is the only fish that can be captured. Seine hauls in Lake St. Clair by the Fisheries Management Unit (Don MacLennan, pers. comm.) and others (Barry Myler, Tarandus Associates, pers. comm.) found them numerous and at some locations the round goby was the most abundant species captured. The effects of this population explosion on the total biodiversity is not fully understood yet, but it is becoming apparent that the round goby will cause a significant decline of the native mottled sculpin and possibly logperch and other darters through competition for food, space, spawning areas, or by direct predation on these species (Jude et al. 1996). These two species are an excellent example of the unpredictable consequences of introductions. One might have predicted from the rare occurrences of the introduced tubenose goby that the round goby would respond similarly and exert a negligible impact on native species.
Case History #2: Erosion and the extirpation of native species
Erosion and consequent siltation of streams, resulting from several causes including agricultural practices, has been considered the number one pollutant of streams and rivers. This is largely the result of detrimental effects on the aquatic habitat and survival of many types of organisms (Kerr 1995, Waters 1995). Egg survival of species such as trout is reduced when silt impedes the circulation of oxygenated water among interstices of gravel. Suspended silt reduces visibility of food items and hinders growth of aquatic plants. If erosion is controlled, with better land use practices, we should see a resurgence of the approximate 60 species in the Ecozone that are sensitive to sedimentation and turbidity. Alternatively, if erosion continues or increases, species will continue to decline or be extirpated.
A useful indicator species for some systems in the Ecozone is the little known eastern sand darter (Ammocrypta pellucida) (Fig. FF-3) found in Canada only in the Mixedwood Plains Ecozone. Both the abundance of this species, and the number of locations at which it occurs have been reduced everywhere in its limited range. In the Lake Erie Lowland Ecoregion (No. 135 of Appendix 1), it has probably been extirpated from the Ausable River, a Lake Huron tributary, and three tributaries of north central Lake Erie. Recent surveys by the ROM indicate that the eastern sand darter is still extant in the Thames and Sydenham rivers but compared to numbers captured in a 1923 collection in the Thames River (Hubbs and Brown 1929), the species is now much less abundant where it does occur. Recent collecting in the St-Laurent Lowlands ecoregion by the Ministère de l'Environnement et de la Faune suggests that it has declined or been extirpated from several river systems where it was more common 20-30 years ago. There, poor water quality other than siltation may also be an important factor in the decline of this sensitive species.
Case History #3: A native species reducing its range
The cisco (Coregonus artedi), a cool water species, has declined dramatically in the Great Lakes and other lakes in the Mixedwood Plains Ecozone such as Lake Simcoe. Inadequately regulated fishing, assessment of populations, and habitat alteration combined with sea lamprey predation and interspecific competition with, or predation by, the introduced rainbow smelt to decimate this species in Lake Simcoe and the Great Lakes. Some Great Lakes are showing a moderate resurgence of the cisco but it is declining rapidly in Lake Simcoe and its fate will likely soon be the same as that of the lake trout and lake whitefish which have ceased to reproduce naturally in that lake.
Case History #4: A native species expanding its range
The black crappie (Pomoxis nigromaculatus) (Fig. FF-4), a warmwater species, is a highly edible and therefore important sportfish which can reach a length of 14 inches and a weight of two pounds. It was discovered only very recently (1993) in Lake Simcoe and lakes in the Trent River system. Its advent there would appear to result from natural expansion as opposed to introductions. In those lakes it has become abundant in a very short period of time. Where it occurs, it is frequently an ecologically dominant species and its numbers can only be controlled by a large population of predators such as pike, muskellunge, yellow perch and fish-eating birds. Liberal angling regulations are often not enough to control some runaway populations. Overdominance of this species will be deleterious to the original populations of other sunfishes.
