Mapleleaf mussel (Quadrula quadrula) COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

Many mussel species within the Great Lakes have had local populations destroyed by the establishment of the invasive zebra mussel (Dreissena polymorpha) (Schlosser et al. 1996, submitted). Zebra mussels attach to the unionid mussel shell and impair burrowing, movement, feeding, respiration and other physiological activities (Haag et al. 1993, Baker and Hornbach 1997). This impairment results in the death of the unionid mussel. Because the zebra mussel has such explosive population growth they can effectively wipe out entire unionid mussel populations in a very short time. The evidence indicates that Quadrula quadrula has been extirpated from Lake Erie, Lake St. Clair and the Niagara and Detroit rivers as a result of infestations by the zebra mussel. Recent reports of zebra mussel from the Fanshawe Reservoir of the Thames River indicates the populations of Q. quadrula in this river may be threatened by this invasive species (Maskant 2004). The zebra mussel must be considered a serious threat to Q. quadrula and all unionid mussel species. Strayer and Fetterman (1999) identify non-point source input of nutrients, sediment and toxins as the major threats to unionid mussels, with agricultural activities as the major contributor. Agricultural activity comprises 75% of land use in the Grand River basin and inputs of silt and nutrients from agricultural activities can be expected to increase. Agriculture comprises about 80% of land use within the Sydenham River basin and the Thames River basin. Nutrients have been recorded to be consistently above accepted standards in the Sydenham basin. Dams and siltation are of concern in the Thames watershed. The pattern is the same in the Ausable river with approximately 80% of the land being used for agriculture, and suspended solids and nutrient levels exceed federal guidelines. There is also an expectation of increasing human populations within these watersheds resulting in increased urbanization with concomitant effects on water quality.

Agricultural activity comprises the majority of land usage in the Assiniboine, Red and Roseau river basins. Nutrient inputs have been increasing dramatically in recent years. The developing hog industry also represents a substantial potential threat to water quality resulting from the increasing use of liquid manure as fertilizer. As the number of hogs continues to increase there will be increasing strain to dispose of the manure that is produced. Damage to river banks and the riparian zone is also of concern, but generally in a much more localized focus. General decline in water quality from these agricultural sources as well as urban and industrial inputs can also result in the accumulation of metals in the tissues of mussels which is a source of additional stress and toxicity (Pip 1995). An additional threat was a brief commercial clam harvest from the Assiniboine in the early 1990s (Pip, 2000) and suspected poaching for commercially valuable species including Quadrula quadrula (Erickson, pers. comm.).

The major natural limitation to distribution and abundance of freshwater mussels is the availability, distribution and abundance of the fish hosts required for successful completion of the life cycle. If the fish host is absent then there can be no recruitment. If a mussel species is long-lived, persistence of adult populations can lead to the illusion of healthy mussel population despite the lack of recruitment due to a missing fish host. Because Q. quadrula is relatively long-lived, it is evidence for recruitment that would indicate a healthy viable population, not just the presence of adult mussels. The known host fishes for Q. quadrula include the channel catfish and flathead catfish. Channel catfish is a common species in Ontario and in Manitoba and this should not limit the possibility for successful reproduction by Q. quadrula. However, fish populations should be monitored to ensure mussel reproduction is possible.

In summary, the major threats in Ontario are derived from the zebra and quagga mussels and habitat (water degradation) associated with agriculture and urbanization. In Manitoba the major threats are related to decreasing water quality resulting from agricultural activities. Neither the zebra mussel nor the quagga mussel has been recorded in Manitoba waters.

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