Harbour porpoise (Northwest Atlantic population) COSEWIC assessment and status report: chapter 11

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Limiting factors and threats

• Hunting
• Incidental mortality in fisheries (Bycatch)
• Habitat degradation

Hunting

Archaeological examination of coastal middens indicates that porpoises were exploited by Aboriginal peoples of eastern Canada prior to the arrival of Europeans, although the number of porpoise bones in these middens is quite small. Pinnipeds are much more commonly encountered in these archaeological excavations and were likely much more important in the diet of these people (D. Johnston, pers. comm.). Harbour porpoises were hunted by Aboriginal people in parts of eastern Canada during the 19th and early 20th centuries (Leighton 1937). The number of animals taken was not recorded, but in the Bay of Fundy, several hundred porpoises were likely taken each year. Hunters worked from canoes on calm days, when it was possible to follow and approach porpoises; shotguns were used to wound or kill the animals. The blubber and mandibular fat pads were rendered for oil and the meat was used for human consumption (Leighton 1937). A small hunt by members of the Passamaquoddy tribe in Maine continued sporadically into the late 20th century, with the last animals taken in 1997 (Waring et al. 2001). Porpoises are still taken occasionally by Aboriginal hunters in the northern part of their range in eastern Canada and by non-Aboriginal residents of Newfoundland, Labrador and perhaps Quebec. For example, a 96-cm porpoise was shot by a hunter in Pangnirtung Fjord in October 1988 (D. Pike, pers. comm.).

Incidental mortality in fisheries (Bycatch)

The most important recent threat to harbour porpoises in eastern Canada is bycatch in commercial fisheries. Most of this bycatch occurs in bottom-set gillnets used to capture groundfish, such as cod (Gadus morhua); this bycatch has existed since gillnets were first introduced into North American fisheries in 1880 by Spencer Baird, then United States Commissioner of Fish and Fisheries. In the first report of the efficacy of these nets, Collins (1886) noted that “…in addition to the various species of Gadidae which have been taken, porpoises (locally called “puffers”)…have been caught…”.

Substantial bycatches of harbour porpoises occurred in the past few decades throughout eastern Canada and in the U.S. portion of the range of the Bay of Fundy-Gulf of Maine subpopulation (see Stenson 2003 for a thorough review). The magnitude of this threat has changed considerably in recent years in eastern Canada and the Gulf of Maine because of the depletion of groundfish stocks and subsequent reductions in fishing effort. However, “As fish stocks in these areas recover, fishing effort will increase, likely resulting in increased levels of bycatch of harbour porpoise unless mitigation measures are taken or alternate methods of fishing used…” (Stenson 2003:284).

Large bycatches of harbour porpoises occurred in Newfoundland and Labrador during the 1970s and 1980s. Most estimates of the total annual bycatch, extrapolated from phone surveys and fisherman logbooks, were in the low thousands (J. Lien, in Department of Fisheries and Oceans 2001). As acknowledged by Lien, “Asking fishermen for numbers of animals incidentally captured and adding them up does not necessarily make good estimates” (in Department of Fisheries and Oceans 2001). Nevertheless, it is clear that harbour porpoises were a common bycatch in Newfoundland and Labrador during this period, primarily in groundfish gillnets.

Patterns of groundfish gillnet fishing effort changed dramatically after the moratorium on fishing for NAFO Subdivision 2J3KL cod in 1992 and other subsequent groundfish closures, although the actual effects of these changes in fishing practices on porpoise bycatches have not been documented. Porpoises are taken in sentinel groundfish gillnet fisheries in Newfoundland and Labrador (designed to monitor depleted cod stocks). The bycatch in 2002 in the nearshore sentinel cod gillnet fishery was estimated using combinations of fishing effort and bycatch rate multipliers derived from reports by sentinel fishermen and on-board fishery observers (Lawson et al. 2004). Lawson et al. (2004) concluded that 1,500-3,000 porpoises were caught in this fishery in 2002. A significant gillnet fishery also exists for lumpfish (Cyclopterus lumpus), in which approximately 15,000 harp seals (Phoca groenlandica) have been taken per year since 1994 (Walsh et al. 2001). This fishery is known to take harbour porpoises, but there are no published estimates of numbers. Logbook data exist from the past decade, and it may be possible to use these data to estimate the annual mortality of harbour porpoises in this fishery (B. Sjare, pers. comm.). Additional Newfoundland fisheries known to take harbour porpoises as bycatch are the nearshore fishery for Greenland halibut (Reinhardtius hippoglossoides) and a shelf-edge fishery for monkfish (Lophius americanus) and skate (Raja sp.) (Lawson et al. 2004). Widespread fishing for herring (Clupea harengus) and groundfish such as winter flounder (Pseudopleuronectes americanus) to be used as lobster bait may also contribute to porpoise mortality (Lawson et al. 2004). Although recent efforts to estimate the magnitude of porpoise bycatch in the nearshore cod fishery constitute a significant improvement over the situation that had existed in Newfoundland for decades, a comprehensive estimate that reflects the actual scale of fishery removals from this porpoise subpopulation is still badly needed.

Information on bycatches of harbour porpoises in the Gulf of St. Lawrence comes from questionnaires mailed to fishermen in 1989, 1990 and 1994 (Fontaine et al. 1994; Larrivée 1996; Department of Fisheries and Oceans 2001) and again in 2000 and 2001, and from on-board observer programs covering both commercial and sentinel fisheries through 2002 (Lesage et al. 2004, in press). Although there are many acknowledged problems with the analysis and interpretation of these data, it is generally accepted that annual bycatch mortality in the 1980s and early 1990s was in the low to mid-thousands. Most bycatches historically occurred during summer in groundfish gillnets set along the lower north shore and along the coasts of the Gaspé Peninsula and Baie des Chaleurs (Fontaine et al. 1994). As in Newfoundland, there has been considerable change recently in the commercial fisheries in the Gulf of St. Lawrence, with large-scale decline and recruitment failure of groundfish stocks leading to fishery closures. With the overall decline in fishing effort, the porpoise bycatch has declined, perhaps by 24-63% since the late 1980s, but it remains “non-negligible” (i.e. in the low thousands; Lesage et al. 2004) and has been judged to be an ongoing source of concern in terms of the porpoise population’s ability to sustain it (Lesage et al. in press).

Bycatches of harbour porpoises in commercial fisheries in the Bay of Fundy have been documented since the early 1980s (Gaskin 1984; Read and Gaskin 1988). As in other areas of eastern Canada, the largest bycatches occur in groundfish gillnet fisheries. The magnitude of this bycatch was estimated in recent years, by DFO in Canada and in the U.S. by the National Marine Fisheries Service. These agencies place independent observers aboard a sample of fishing vessels, so that a bycatch rate can be estimated. This bycatch rate is then extrapolated to the entire fishery using some metric of total fishing effort (see Bravington and Bisack 1996; Bisack 1997; Trippel et al. 1996; and Waring et al. 2001 for more details). Estimates of harbour porpoise bycatches generated for the Bay of Fundy, Gulf of Maine and Mid-Atlantic States through 2001 are presented in Table 3. All of these bycatches from the Bay of Fundy and Gulf of Maine and the majority of bycatches from the Mid-Atlantic states are believed to have been taken from the Bay of Fundy-Gulf of Maine harbour porpoise subpopulation (Table 3). The most recent aggregate estimate of annual bycatch for this subpopulation is 477 (CV = 0.17) (National Marine Fisheries Service 2005).

Data are taken from Bravington and Bisack (1996), Bisack (1997), Trippel et al. (1996), Waring et al. (2001) and Trippel and Shepherd (2004). Data are not available (N/A) prior to 1993 for the Bay of Fundy or 1995 for the Mid-Atlantic. Totals are only provided for years in which estimates are available for all three areas.

As in Newfoundland and the Gulf of St. Lawrence, there have been profound changes in fishing effort in the groundfish gillnet fishery in the range of this southern subpopulation. In the Bay of Fundy, a variety of fisheries conservation measures have been used to reduce fishing mortality on cod and other groundfish, including temporal fishery closures. In 1995, a Harbour Porpoise Conservation Strategy for the Bay of Fundy was implemented by the Department of Fisheries and Oceans (1995). Under this strategy, a cap of 110 bycaught harbour porpoises per year was set for the Bay of Fundy, after which the fishery would be closed. Time-area fishing closures have been used as a fisheries conservation measure in the Gulf of Maine, together with a host of other tools designed to conserve and rebuild overfished stocks of cod and other groundfish. These measures have significantly reduced fishing effort in both Canadian and U.S. fisheries. In addition, in U.S. waters of the Gulf of Maine and Mid-Atlantic states, harbour porpoise bycatches are now regulated under two Take Reduction Plans (see below). Taken together, all of these conservation measures have significantly reduced the bycatches of harbour porpoises from the Bay of Fundy-Gulf of Maine subpopulation over the past few years.

Small numbers of harbour porpoises are taken in other fisheries throughout eastern Canada, including surface drift net fisheries for herring and mackerel and weir fisheries for herring, particularly in the Bay of Fundy. Mortality in the latter fishery has been reduced to a few porpoises each year because of a co-operative program run by biologists and fishermen on Grand Manan Island, New Brunswick (Read, unpubl. data). Substantial bycatches occurred in commercial salmon gillnet fisheries in Newfoundland, Labrador and West Greenland in the past but are now presumably much reduced. The commercial salmon fishery in Newfoundland was closed in 1992. In Labrador, the salmon quota was reduced throughout the 1990s until the fishery was closed completely in 1998 (G. Stenson, pers. comm.). The scale of the commercial salmon fishery in Greenland declined steadily through the1980s and 1990s; it was closed in 1997, reopened for one year in 2001, and remains closed (G. Stenson, pers. comm.). The ongoing bycatch of porpoises in small-scale inshore fisheries for local fish and shellfish consumption and in some offshore commercial fisheries is uncertain because of the lack of monitoring.

Habitat degradation

Other potential threats to the species include loss of habitat due to the use of acoustic harassment devices (AHDs) around salmon mariculture sites in the Bay of Fundy (Strong et al. 1995). Concern has been expressed regarding the proliferation of high amplitude acoustic harassment devices (AHDs) used to deter pinnipeds from approaching salmon mariculture sites in the Bay of Fundy and elsewhere (Taylor et al. 1997). These devices produce high intensity sounds at frequencies within the hearing range of harbour porpoises.  During experiments conducted in the Bay of Fundy, no porpoises approached within 645 m of an active, commercial AHD, and porpoise densities were reduced significantly in its vicinity (Johnston 2002). Experiments with AHDs and harbour porpoises in British Columbia demonstrated similar results (Olesiuk et al. 2002), and reductions in the occurrence of other odontocete cetaceans in the vicinity of active AHDs have also been documented (Morton 2000, Morton and Symonds 2002) These devices have been used widely in the mariculture industry in the Bay of Fundy (Johnston and Woodley 1998). Thus, there is potential for habitat exclusion of harbour porpoises in this region. In the past, concern was expressed over the level of anthropogenic organochlorine contamination (OC) in harbour porpoises (e.g. Gaskin 1992). Recent data exist on OC loads in all three subpopulations in eastern Canada (Westgate et al. 1997). Polychlorinated biphenyls (PCBs) and chlorinated bornanes are the dominant contaminants. Generally, concentrations of OC contaminants increase in a north to south gradient with porpoises in the Bay of Fundy and Gulf of Maine exhibiting the highest levels. Westgate et al. (1997) also reported that levels of PCBs and dichloro-diphenyl-trichloroethanes (DDTs) had decreased significantly from those documented by Gaskin et al. (1971, 1976, 1983). Recent concentrations of OCs are similar to contemporary levels reported in other harbour porpoise populations (Westgate et al. 1997). It is still unclear what proximate or ultimate effects these OC burdens have on harbour porpoises. The harbour porpoise is one of the indicator species used by the IWC Scientific Committee in its “Pollution 2000+” program, which is designed to provide information on the effects of pollutants on the health of cetaceans. Results of this program are not yet available.

Habitat degradation and loss caused by petro­leum exploration and production is a potential threat in several areas of the range of this species in eastern Canada, especially in parts of the Gulf of St. Lawrence and along the Scotian Shelf. Acoustic harassment or displacement could occur during seismic exploration, particularly if such activities occur relatively close to shore, in preferred feeding areas, or within migration corridors. There have been no studies of the effects of these activities on harbour porpoises.

Finally, the primary prey species of harbour porpoises, particularly herring, are exploited by commercial fisheries throughout eastern Canada; thus, the potential exists for depletion of these prey resources through overfishing. At this time, however, there is no evidence that the population biology of any harbour porpoise population has been affected by fishing for prey.