Recovery Strategy for the Shortnose Cisco (Coregonus reighardi) in Canada
4.1 Threat assessment
|Threat/Attributes||Level of ConcernFootnote a||Extent||Occurrence||Frequency||SeverityFootnote b||Causal CertaintyFootnote c|
|Ecosystem Impairment||Invasive Species, Habitat changes||High||Widespread||Historic/Current||Continuous||Unknown||Unknown|
|Overexploitation (Historic threat only – contributed to decline but no longer affecting the species)||Commercial “chub” fisheries||High||Widespread||Historic||Continuous||High||High|
- Footnote A
Level of Concern: signifies that should the species still exist managing the threat and/or its effects is of (high, medium or low) concern for the recovery of the species. This criterion considers the assessment of all the information in the table.
- Footnote B
Severity: reflects the population-level effect (High: very large population-level effect, Moderate, Low, Unknown).
- Footnote C
Causal certainty: reflects the degree of evidence that is known for the threat (High: available evidence strongly links the threat to stresses on population viability; Medium: there is a correlation between the threat and population viability e.g. expert opinion; Low: the threat is assumed or plausible).
4.2 Description of threats
Overexploitation, ecosystem impairment, and introgressive hybridization have all been implicated in the demise of the Shortnose Cisco (Smith 1964, 1967, Berst and Spangler 1973, Todd and Stedman 1989, Parker 1988, Committee on the Status of Endangered Wildlife in Canada (COSEWIC) 2005).
As the Shortnose Cisco is believed to be extinct and thus no viable population exists, the overexploitation threat from the commercial fishery that contributed to the decline is no longer affecting the species. The overexploitation threat is an historic threat only but could become a current threat if a commercial chub fishery becomes active in the future.
Among the threats identified, overexploitation by the commercial chub fishery probably had the most immediate and profound effect on the Shortnose Cisco (Smith 1968, Christie 1973, Wells and McLain 1973, Parker 1988, COSEWIC 2005). In Lake Ontario, the species was abundant in the 1880s (Pritchard 1931) but, by the 1930s, that fishery had all but collapsed (Gray 1979). The last reported sighting for Lake Ontario was in 1964 (Gray 1979, Parker 1988, COSEWIC 2005). A similar pattern of overexploitation was observed in lakes Michigan and Huron with last reported sightings of the species at these locations in 1974 and 1985, respectively (Webb and Todd 1995). The overexploitation and eventual collapse of the Shortnose Cisco populations followed the same pattern as the collapse of other deepwater cisco populations in the Great Lakes including the Deepwater Cisco (C. johannae), Shortjaw Cisco (C. zenithicus), Blackfin Cisco (C. nigripinnis), Kiyi (C. kiyi), and Bloater (C. hoyi) (Smith 1968, Wells and McLain 1972, Todd and Smith 1992).
Commercial chub fishing, which historically included the Shortnose Cisco, no longer occurs within the Canadian waters of lakes Huron or Ontario (L. Mohr, pers.comm.). Level of concern associated with overexploitation was rated as high based on the historical fishery and would remain such if viable populations and chub fisheries existed. When active, the extent of commercial fishing had been widespread. The frequency of commercial fishing was continuous since at least the mid-1800s with its greatest impact prior to the 1970s. Severity of impact was historically high with a high degree of causal certainty (Stone 1944, Smith 1964, Wells and McLain 1972, Berst and Spangler 1973, Parker 1988, Webb and Todd 1995, COSEWIC 2005). One of the significant issues with the commercial chub fishery was that it was not managed based on individual species. After the larger species were selectively removed, gear size was reduced in order to target smaller individuals thereby maintaining the chub fishery as a whole (Stone 1944, Smith 1964). This led to the sequential removal of the smaller species from the fishery, and in some cases, the eventual collapse of the fishery as a whole (Smith 1964, Smith 1968, Wells and McLain 1972, Parker 1988).
Ecosystem impairment is the result of multiple stressors including changes in coastal and aquatic habitats, invasive species, contamination, changes in biotic communities, resource utilization, land use/cover, and climate change. The most important of these to the Shortnose Cisco was probably the introduction of invasive species (Brown et al. 1987). Currently, more than 185 aquatic invasive species are known to persist in the Great Lakes with new introductions likely to occur in the future (Environment Canada and US Environmental Protection Agency 2009).
Predation by the Sea Lamprey (Petromyzon marinus) is suspected of having contributed to the collapse of various fish populations including the Shortnose Cisco (Smith 1968, Berst and Spangler 1973). Competition with, or predation by, other invasive species including the Alewife (Alosa pseudoharengus) and Rainbow Smelt (Osmerus mordax) may have further contributed to the population decline or, at least, prevented its re-establishment (Berst and Spangler 1972, Wells and McLain, Parker 1989). The recent establishment of Dreissena mussels into the Great Lakes and the concurrent decline in the benthic amphipod Diporeia spp. may also have significant implications on the biotic communities of the Great Lakes (Dermot and Kerec 1997, Nalepa et al. 1998, Lozano et al. 2001, Mills et al. 2003, Dobiesz et al. 2005, Nalepa et al. 2006, National Oceanic and Atmospheric Administration [United States] (NOAA) 2006, Riley et al. 2008, Environment Canada and US Environmental Protection Agency 2009). The degree to which this might affect deepwater cisco species, which depend on Diporeia as an important food source, is unknown. Habitat changes including eutrophication, pollution and habitat degradation have also been suggested as potentially limiting re-establishment of deepwater cisco populations (Wells and McLain 1972, Colby et al. 1972, Christie 1973, Parker 1988). Little is known about the effects of the other ecosystem stressors listed above on the Shortnose Cisco. The level of concern assigned to ecosystem impairment is rated as high as it would likely preclude, or have precluded, the recovery of the Shortnose Cisco even if the principal threat of overexploitation was removed or mitigated. The extent of ecosystem impairment is described as widespread throughout lakes Ontario, Michigan and Huron where the Shortnose Cisco occurred. The occurrence of ecosystem impairment is both historic and current, and its frequency would be continuous. Severity and causal certainty are listed as unknown as most populations of Shortnose Cisco were already in serious decline due to overexploitation and there have been no studies dedicated to looking at specific ecosystem impacts on the Shortnose Cisco.
Introgressive hybridization between Shortnose Cisco and other deepwater ciscoes has been suggested as potentially hastening the extirpation of the species (Smith 1964, Todd and Stedman 1989, Webb and Todd 1995). Smith (1964) reported the apparent increase in different and unique forms of chubs in Lake Michigan as noted by local fishermen and suggested that future forms of cisco stocks might be different than those recognized in the past. The lack of genetic markers between cisco species makes it difficult to validate this threat. As such, other than the level of concern which is rated as “High” based on the historical references, all other attributes for this threat are deemed to be “Unknown”.
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