Dept. of Biology,
Wilfrid Laurier University,
Humans make many demands on a riverine system, demands which often change drastically the nature of the communities present. I would like to mention three different types of demands and to varying degrees discuss their permanent effects on a riverine community. The construction of dams is a response to one set of human needs. Dean Fitzgerald has already mentioned in detail some consequences of dam construction. A second demand is the need to treat waste and the third is the need for recreational facilities. I would like to reiterate some of these effects and perhaps present some additional changes.
Dams are constructed for flood control, a very human concern, as well as for water quality improvement, (which at times may be debatable), and for human recreation, such as fishing, swimming and boating. The construction of a dam has three effects.
Ignoring the reservoir, since the topic is stream communities, we can look at upstream effects and downstream effects.
2. An increase in some species due to upstream migration from the reservoir. Here again there are at least two causes for this. Some species are more suited to the reservoir habitat, may reach high populations, and are forced upstream in the search for space. Secondly some individuals will migrate upstream if the reservoir is drawn down seasonally.
2. Increase in some species due to overflow from the dam. Associated with this is the creation of a & new type of habitat at the base of the dam - the plunge pool. To illustrate these two points I would like to refer to a study by Tanya Mackie on Baden Creek (Table 1). Note the plunge pool community is dominated by largemouth bass, from the reservoir, whereas the typical community for this stream is dominated by suckers and minnows. In general, a plunge pool community in this area is dominated either by members of the sunfish family or of the perch family, whereas the stream is typically dominated by minnows or minnows and suckers. The plunge pool associated with Laurel Creek reservoir is somewhat atypical in that dominant species is brown bullhead. Carp are also present in spring.
Another set of human needs is the treatment of human waste. This involves the use of a sewage treatment plant. The outflow of such plants in this region is typically into a river. The various processes of the plant are designed to ensure that the released water is both disease and substance free. However pathogenic bacteria are living organisms, and what can kill bacteria can destroy other life forms.
The Laurel Creek sewage outflow provides a good example of the effects on a fish community. The outflow is into the Grand River just below the Bridgeport bridge. Table 2 illustrates the results of a study by Julie Kruse.
What is seen is that a biological desert is created for some distance downstream of the outflow and that 250 m downstream there is little recovery. The rocks in this zone are similarly devoid of any organisms. The Grand is a rather large river, however, so one can imagine how much more severe the effects would be on a smaller stream, e.g. if the Waterloo plant flowed out into Laurel Creek.
The effects of both of these activities change the proportions of the different species in a stream but do not create new assemblages of fish.
A third need is recreation. A popular form of recreation is fishing. To meet fishermen's demands for increased fisheries resources, stocking of new species is often undertaken. Since about 1989 there has been a programme to stock brown trout and walleye in the middle Grand R. drainage.
Brown trout are a well-known piscivore with a voracious appetite. In many rivers into which brown trout have been introduced it has become the most significant predator in terms of total brook trout consumed. In addition brown trout and brook trout spawning activity and other ecological requirements are essentially the same, including an overlap in their breeding times. This has resulted in statements in the literature such as "native brook trout have disappeared from many waters in eastern North America, especially streams, over the past 100 years and have been replaced by the recently naturalized European brown trout." In the middle Grand basin two important brook trout streams susceptible to brown trout invasion are Carroll and Swan Creeks. The upper Canagagiuge populations are protected by dams such as the North Woolwich and Floradale dams. Brown trout have been caught at the base of Laurel Creek dam, indicating that the species has considerable ability to overcome a number of obstacles in its way.
The effect of the brown trout extends beyond its effects on the brook trout populations. In the middle Grand there is one species, silver shiner, listed as vulnerable by COSEWIC, and one species, the black redhorse sucker, listed as threatened. Both of these species are susceptible to brown trout predation. Studies in the United States have implicated the brown trout in the reduction of sucker populations, in one case this involved a species considered threatened. One conclusion of such studies is that large brown trout can significantly alter abundance of native fish through predation.
The introduction of the walleye has had other very important serious effects. Several other species apparently were also introduced along with the walleye. The black crappie, which previously was known in the Grand just to Brantford, and the green sunfish, previously considered uncommon in some tributaries such as the Nith River, are both now common in the middle Grand, whereas populations of the previously common pumpkinseed appear to have declined.
Although it may be argued that the black crappie and green sunfish spread may not be related to the introduction of the walleye, there is a third species whose explosion in the middle Grand provides a valuable marker, illustrating the spread of an accidentally introduced species. This is the greenside darter, a species considered vulnerable, and never recorded in the Grand River system before 1989. Today, it has spread throughout the middle Grand System (Fig. 1) and in some locations has become a dominant species: e.g. the Conestogo River at the St. Jacob's dams and at the Mannheim weir on the Grand (Bunt, pers. comm.). The link with the greenside darter and the walleye introductions is one of timing and of the source of the walleye used for stocking. The walleye used for stocking were collected from the Thames R. (Timmerman, pers. comm.) the river system which previously had the best populations of this darter. Also green sunfish and black crappie are common in this system. The great increase in populations of all three species occurred after 1989, the year recent stockings were initiated.
With the intentional introduction of the brown trout and walleye, and the unintentional introduction of the greenside darter and probably green sunfish and black crappie, the original fish community has changed to an artificial human-created community. This has occurred in just six to seven years.
The Grand River has been designated as a Heritage River but it appears that the original fish community is fast disappearing in the middle Grand region through human activities.
This work was supported in part by the Tri-Council Secretariat Green Plan Study of the Grand River.