Gravel chub (Erimystax x-punctatus) COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

The gravel chub is considered pollution-intolerant, has narrow habitat requirements, and populations are confined to areas where there is sufficient current to keep the bottom free of silt (see Becker 1983). The species is susceptible to elevated turbidity and increased siltation (Becker 1983). Siltation, or sedimentation, is the filling-in of lakes and stream channels with soil particles, usually as a result of erosion on adjacent land. Turbidity, on the other hand, is a principal physical characteristic of water and is an expression of the optical property that causes light to be scattered and absorbed by particles and molecules rather than transmitted in straight lines through a water sample. It is caused by suspended matter or impurities that interfere with the clarity of the water such as finely divided inorganic and organic matter, soluble coloured organic compounds, and plankton and other microscopic organisms. Typical sources of turbidity include: waste discharges; surface runoff, especially from areas that are disturbed or eroding; algae or aquatic weeds and products of their breakdown, humic acids and other organic compounds resulting from decay of plant material; and high iron, or other mineral concentrations which may give rise to discolouration. Siltation is the key limiting factor for the gravel chub, which requires silt-free substrates. Turbidity may also be limiting depending on the source and current flow. In areas of low current, sedimentation of suspended matter from turbid waters may occur.

Increased siltation was associated with its extirpation from many parts of Ohio (Trautman 1981) and Wisconsin (Becker 1983). In Iowa and Wisconsin, pesticides, sewage and other point-source discharges have also been identified as potential causes for extirpations (Schmidt 2000).

Similar habitat changes in the Thames River drainage may have caused extirpation of the gravel chub in Canada. In 1923, Brown described the Thames River as clear, with a fast current at capture sites and with sand and gravel substrates at capture depths of up to 5 feet (Holm and Crossman 1986). The report of the ROM 1985 collection efforts suggests a shift in environmental conditions adverse to the species as silt and clay was evident at all sites and the water was quite turbid (Holm and Crossman 1986). Turbid conditions were also measured in 2005 during Fisheries and Oceans Canada sampling of the Thames River, adjacent to Muncey (Marson et al. 2006). Holm and Crossman (1986) also found an increase in the abundance of species such as the spotfin shiner (Notropis spilopterus), known for their tolerance to turbidity and siltation (Trautman 1981). In addition, less tolerant species such as the mimic shiner (N. volucellus) and the eastern sand darter (Ammocrypta pellucida) were absent or in reduced abundance from previous collections (Holm and Crossman 1986). However Dextrase (pers. comm. 2008), found eastern sand darter relatively abundant throughout these reaches of the Thames in 2006 surveys.

The impoundment of riffles is considered a serious threat to gravel chub populations in the United States (Becker 1983, NatureServe 2007). Dams alter upstream and downstream habitat conditions and act as barriers, fragmenting populations and limiting re-colonization (NatureSeerve 2007; Edwards et al. 2007). Most dams in the Thames River watershed are either in the upper watershed or tributaries to the lower and middle Thames River. The most downstream barrier along the mainstem of the Thames River is the Springbank Dam (situated more than 40 km upstream of Muncey in the city of London). When stop-gates are in place from mid-May to early November, it is a barrier to fish passage and creates a small upstream run-of-the-river type impoundment (55 hectares). With the exception of filling the reservoir in mid-May and draining the reservoir in November, the dam has little effect on downstream flows (Reid and Mandrak 2006). Therefore, historic gravel chub habitats in Canada are not expected to be affected by dams.

Silt loads arising from agricultural and urban activities may be the most significant threats to species such as the gravel chub, with narrow habitat requirements for silt-free waters of low turbidity. The Thames River watershed drains an area with one of the highest levels of agricultural land use in the province, and perhaps in all of Canada. Seventy-eight percent of land use in the upper watershed is under agricultural production and 88% in the lower watershed (Taylor et al. 2004). Much of the land is systematically tile drained; storm water run-off and tile drainage lead directly to large-scale soil deposits in the river via municipal drains and tributaries. Additionally, livestock grazing and tillage to the stream edge have destroyed riparian vegetation and contributed to bank erosion and sediment loading (Bailey and Yates 2003). The most heavily impacted areas are upstream of former gravel chub collection sites.

Nutrient loading from manure and fertilizers, manure spills; sewage treatment effluents and domestic sewage systems are also a cause for concern (UTRCA 1998; Taylor et al. 2004). There are at least 15 sewage treatment plants with varying treatment levels discharging wastewater into the Thames River and bacteria levels are often above acceptable provincial standards (100 E. coli/100 ml). Since 1998 periodic algal blooms (leading to reduced dissolved oxygen levels) resulting from nutrient loading and chemical spills (usually oil and fuel), have resulted in episodic fish kills (UTRCA 1998).

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