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COSEWIC assessment and status report on the American Eel in Canada

Rescue effect

If the American eel became extirpated or severely depleted in one or more FEAs in Canada, the possibility of a rescue effect can be addressed in two scenarios:

1.     Eels become extirpated or severely depleted in Canada, but there is no substantial change in American components. In this scenario, young eels from the Sargasso Sea, primarily of American parentage, would continue to colonize the whole continental rearing area, including eastern Canada. External components would thus "rescue" the species in Canada. However, egg production in the Sargasso Sea might be substantially reduced due to the lack of Canadian-reared spawners, so total recruitment of young eels to Canada and elsewhere could be much less than normal, and recruitment towards range limits could be reduced if migration is density dependent.

2)    Eels become extirpated or severely depleted in both Canada and the United States. In this scenario, egg deposition in the Sargasso Sea would be drastically lowered, and recruitment of young eels would likely fall to low or negligible levels in all rearing areas, including Canada. The rescue effect for eels in Canada would therefore be limited, particularly if migration is density dependent.

Because of panmixis, progeny of eels reared in Canada colonize the full range of the species in random fashion. Therefore, in the case of a population decline that affects both Canada and the United States, reproductive output from Canadian-reared eels would "rescue" United States components as much as reproductive output from United States-reared eels would "rescue" Canadian components.

There are widespread concerns about the conservation status of the American eel in the United States. These concerns have led to the current examination by the United States Fish and Wildlife Service (USFWS) of the American eel as a candidate for listing under the Endangered Species Act (USFWS 2005). Additionally, the USFWS is considering a proposal to list the species in Appendix III of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). Conservation concerns have also prompted the Atlantic States Marine Fisheries Commission to prepare a stock assessment for the species, which is currently underway.