Harbour porpoise (Northwest Atlantic population) COSEWIC assessment and status report: chapter 2

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Executive summary

Harbour Porpoise
Phocoena Phocoena

Northwest Atlantic Population

Species information

Harbour porpoises are among the smallest cetaceans and, in eastern Canada, few individuals exceed 1.7 m in total length. Like all phocoenids, harbour porpoises possess rounded heads that lack an external rostrum or beak. A small, triangular dorsal fin is located at approximately the middle of the back. The flanks are mottled grayish white, fading to almost white ventrally. A black cape extends over the dorsal and lateral surfaces, although its extent varies considerably among individuals and populations.


Distribution and habitat

Harbour porpoises are widely distributed over the continental shelves of the temperate Northern Hemisphere. In eastern Canada, they occur from the Bay of Fundy north to Cape Aston, at approximately 70° N. The southern range of the species extends to North Carolina. The species, true to its name, is sometimes found in bays and harbours, particularly during the summer. There are no quantitative estimates of trends in the extent of habitat for harbour porpoises in eastern Canada.


Subpopulation structure

Analyses of mitochondrial DNA, but not nuclear microsatellites, support the existence of three subpopulations of harbour porpoises in eastern Canada: Newfoundland-Labrador, Gulf of St. Lawrence, and Bay of Fundy-Gulf of Maine. This division is further supported by evidence from tissue levels of organochlorine contaminants and by life history studies.


Biology

Reproduction in all populations is seasonal, with ovulation and conception limited to a few weeks in early summer. Gestation lasts for 10-11 months followed by a lactation period of at least 8 months. Most mature female porpoises become pregnant each year. There are no estimates of the annual survival rates of this species, but it is short-lived compared to other odontocetes and few individuals live past their teens. 

In the Bay of Fundy, individual porpoises equipped with satellite-linked radio transmitters moved frequently between Canadian and U.S. waters. The population of porpoises in the Bay of Fundy and Gulf of Maine is transboundary in nature. The diet of harbour porpoises includes a variety of small fishes and cephalopods. At least some prey items are demersal, living on or near the sea floor; porpoises feeding on such items are at risk of entanglement in bottom-set gillnets.


Population sizes and trends

There are no range-wide estimates of the abundance of harbour porpoises in eastern Canada, nor are there any estimates for the Newfoundland-Labrador subpopulation. Aerial line transect surveys in the Gulf of St. Lawrence during the summers of 1995 and 1996 provided estimates of 12,100 (CV = 0.26) and 21 720 (CV = 0.38) porpoises, respectively, although the results of the two years’ surveys are not directly comparable because they covered different portions of the Gulf. Moreover, the survey design did not allow for correction of g(0), the probability of detecting an animal on the trackline; thus, both estimates are negatively biased. Aerial and shipboard line transect surveys were conducted during July-September in 1991, 1992, 1995 and 1999 in the Bay of Fundy and Gulf of Maine. All estimates were corrected for g(0). The most recent estimate (August 1999) of subpopulation size in the Bay of Fundy and Gulf of Maine was 89 700 (CV = 0.22).


Limiting factors and threats

The most important recent and current threat to harbour porpoises in eastern Canada is bycatch in fishing gear. Substantial bycatches of harbour porpoises occurred in the past few decades in eastern Canada and the U.S. portion of the range of the Bay of Fundy-Gulf of Maine DU. The magnitude of this threat has diminished since the 1990s due to the depletion of groundfish stocks and consequent reductions in fishing effort. In the United States, annual bycatch mortality for the Bay of Fundy-Gulf of Maine population was estimated at 2900 in 1990 compared with 417 (CV = 0.17) in 2005, the latter figure incorporating data from both Canada and the United States. In the Gulf of St. Lawrence, the bycatch also declined, perhaps by 24-63% from the late 1980s to early 2000s, but remains “non-negligible” (low thousands). In 2002 an estimated 1500-2000 harbour porpoises were bycaught in the “severely reduced” nearshore cod fishery around Newfoundland.

Harbour porpoises are hunted in Greenland for domestic meat consumption, and it cannot be ruled out that some or all of these animals are from a transboundary population shared with Canada – most likely the Newfoundland-Labrador subpopulation. There is no limit on numbers that can be taken by hunting in Greenland.


Existing protection or other status designations

The harbour porpoise is protected from certain activities under the Marine Mammal Regulations of the Fisheries Act of Canada. These regulations do not, however, have any provisions to address the bycatch of marine mammals in commercial fisheries. The range of the harbour porpoise extends into United States waters of the Gulf of Maine, where the species is protected under the Marine Mammal Protection Act. Under this legislation, the maximum allowable annual removal limit for porpoises in the Bay of Fundy and Gulf of Maine is 747. Two Take Reduction Teams have been formed in the U.S. to address the bycatch of harbour porpoises from the Bay of Fundy-Gulf of Maine population. Both teams recommended measures to reduce the bycatches of harbour porpoises in the U.S. that include: times and areas completely closed to gillnet fishing; times and areas in which acoustic alarms are required on groundfish gillnets; and a series of required modifications to the structure and use of groundfish gillnets. In January 1993 the U.S. government proposed listing the harbour porpoise population in the Bay of Fundy-Gulf of Maine as threatened under the Endangered Species Act because inadequate regulatory measures existed in Canada or the U.S. to address the bycatches of harbour porpoises. In January 1999, the National Marine Fisheries Service (NMFS) determined that the proposed listing was not warranted because bycatch reduction programs implemented in Canada and the U.S. were sufficient to ensure the population’s sustainability. This conclusion was supported by a Population Viability Analysis. In August 2001, the U.S. government published its intention to remove this population from the candidate list under the Endangered Species Act. The harbour porpoise is classified as Vulnerable in the IUCN Red List and on Appendix 2 of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES).

The relatively secure status of harbour porpoises in eastern Canada is due, in large part, to measures enacted to restore groundfish stocks rather than to conserve porpoises. It is likely that harbour porpoise bycatches will increase significantly if and when groundfish stocks recover in eastern Canada. The following scientific information is required, particularly for the populations in the Gulf of St. Lawrence and Newfoundland: unbiased estimates of abundance and bycatches, and an improved understanding of subpopulation structure. There are no provisions to address bycatches under the Marine Mammal Regulations of the Fisheries Act of Canada. Nor is there any other mechanism for developing scientific advice regarding the sustainability of bycatch levels. The present respite in bycatch mortality provides a unique opportunity to formulate and implement such a mechanism.

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