Harbour porpoise (Northwest Atlantic population) COSEWIC assessment and status report: chapter 14

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Summary of status report

In all areas of the range of this species, the most serious threat is incidental mortality (bycatches) in commercial fisheries. Bycatches have occurred primarily in gillnet fisheries for groundfish, although their magnitude has diminished in recent years, primarily because of conservation measures designed to promote the recovery of fish stocks. Additional potential threats to the species come from anthropogenic modification of habitat, although the magnitude of this threat will have less direct impact than bycatch mortality.

No estimates of total abundance are available for the Newfoundland-Labrador and Gulf of St. Lawrence regions although there are partial estimates for the latter (in 1995-1996); there are no estimates of total bycatch mortality from either area.

Most information on this species in eastern Canada pertains to the southernmost subpopulation in the Bay of Fundy. A current estimate of abundance and a decade-long time series of bycatch estimates are available for this subpopulation. Several past estimates of abundance are available, but these surveys did not cover comparable areas, so it is not possible to derive a trend. During the 1990s, large bycatches (numbering in the thousands) occurred in the U.S. and Canadian ranges of this subpopulation, spurring several management initiatives, primarily in U.S. waters, to reduce bycatches to sustainable levels. Porpoise bycatches also declined as a result of management measures designed to promote the recovery of depleted groundfish stocks in the Bay of Fundy and Gulf of Maine. Current bycatch levels are less than the allowable limits under the U.S. Marine Mammal Protection Act and have been reduced to the extent that the Bay of Fundy-Gulf of Maine subpopulation has been removed from the list of candidate species under the U.S. Endangered Species Act. A population viability analysis (PVA) indicates that the recent levels of bycatch pose little or no threat to the future viability of this subpopulation.

Harbour porpoise bycatches will increase significantly if and when groundfish stocks recover and gillnet fisheries expand in eastern Canada. Management measures exist under U.S. legislation to ensure that future bycatches in U.S. fisheries should not endanger the Bay of Fundy-Gulf of Maine subpopulation, but no similar measures exist in Canadian law.

To ensure that future bycatches do not threaten harbour porpoises in eastern Canada, the following scientific information is required, particularly for the Gulf of St. Lawrence and Newfoundland-Labrador:

1. Unbiased estimates of abundance;
2. Unbiased estimates of the magnitude of bycatch, from independent observer programs; and
3. Improved understanding of population structure and dispersal rates.