Greenside darter (Etheostoma blennioides) COSEWIC assessment and status report: chapter 8
Limiting Factors and Threats
There are several potential threats to greenside darter populations in Canada, but none appear to be imminent or currently having a significant impact on populations. Dalton (1991) suggested that the food, habitat and breeding areas of the greenside darter are specialized, and that any disturbance of these resources would reduce populations. It is likely that such disturbances would need to be large in magnitude to have a significant impact.
Impoundments exist on all of the river systems where greenside darter are found in Ontario. Impoundments can destroy habitat by flooding upstream riffles and reducing flows downstream (Dalton 1991). Bunt et al. (1998) found that an impoundment on the Grand River had rendered upstream habitat unsuitable; however, Reid (2004) found the greenside darter at several sites in Guelph Lake (a large impoundment in the Speed River portion of the Grand River drainage), demonstrating that lentic habitats can be colonized. Bunt et al. (1998) suggested that the Mannheim Weir played an important role in the creation and maintenance of habitat preferred by the greenside darter (unembedded cobble substrate that supported thick growths of Cladophora). Impoundments that lack fish passage facilities prevent greenside darter from moving upstream, but do not appear to restrict downstream movements. The discovery of greenside darter above barriers without fishways, suggests ongoing human-mediated transport within the Grand River watershed.
Sediment and nutrient inputs associated principally with agricultural activities have been identified as primary threats limiting aquatic species at risk in the Ausable, Sydenham and Thames river watersheds (Nelson et al. 2003; Staton et al. 2003; Taylor et al. 2004). These factors do not seem to have affected greenside darter populations which have maintained or expanded their range in these systems. The high nutrient levels in these watersheds may have, in fact, benefited greenside darter populations by promoting the growth of filamentous algae and other vegetation. Excessive sedimentation could impact greenside darter habitat by increasing imbeddedness of rock substrates and increased turbidity could result in decreased growth of aquatic vegetation.
Contaminants associated with industrial activity and agricultural runoff have the potential to kill greenside darter outright, or to affect their insect food supply (Dalton 1991). At least four separate chemical or fertilizer spills have occurred within the Ausable, Grand, Sydenham and Thames river watersheds in the last 7 years that have resulted in fish kills (A. Dextrase, unpubl. data). Although the impacts of these spills are localized and short-lived, they can be significant. A chemical spill into the Ausable River at Exeter in April 2005 caused a fish kill along a 5.1 kilometres (km) reach of the river. A sub-sample of 60 metres (m) of river revealed 242 dead fishes of 20 species including 7 greenside darter (S. Staton, personal communication 2005). If the sampled section of stream was representative of the entire kill zone, then more than 700 greenside darter may have been lost to this spill. Chronic levels of contaminant inputs, currently present, do not appear to have negatively affected greenside darter populations. Greenside darter populations in the Detroit River and Lake St. Clair are vulnerable to contaminant runoff and spills associated with large urban centres, shipping and the chemical industry upstream along the St. Clair River. Cannon et al. (1992) found that greenside darter were present in a Pennsylvania stream that had been impacted by acid mine drainage (elevated iron and sulfate levels, heavy coating of precipitate on substrate), but that other sensitive benthic fishes were absent.
Large urban centres are present within the range of the greenside darter in the Thames River (London) and Grand River (Brantford, Cambridge, Guelph, Kitchener-Waterloo). Growth of these urban communities is proceeding rapidly – the population of the Grand River watershed is projected to increase by 30% over the next 20 years (Grand River Conservation Authority 2005). Urban expansion has the potential to degrade or destroy habitat, and to increase contaminant inputs into these systems.
The introduced round goby is a potential threat to greenside darter populations in Ontario. It was first found in North America in the St. Clair River in 1990 (Jude et al. 1992), has since spread to each of the Great Lakes, and has become locally abundant in some areas. Predation and competition from the round goby has been implicated in declines of mottled sculpin (Cottus bairdii), and possibly logperch, populations in the St. Clair River (French and Jude 2001), but impacts on greenside darter populations have not been specifically studied. Ontario ranges of the round goby and greenside darter currently overlap in the Detroit River, Pefferlaw Brook and Lake St. Clair. The round goby has the potential to invade most of the river systems that currently support greenside darter populations.
Page details
- Date modified: