Harbour porpoise (Northwest Atlantic population) COSEWIC assessment and status report: chapter 9
Biology
General
Compared to other cetaceans, this species has a relatively early age at sexual maturation and high fecundity (Read and Hohn 1995). Nevertheless, the limited lifespan and production of a single young per pregnancy impose constraints on the potential rate of increase (Caswell et al. 1998).
Reproduction
Most information on the life history of harbour porpoises in eastern Canada comes from research conducted on the relatively well-studied subpopulation in the Bay of Fundy and Gulf of Maine (Fisher and Harrison 1970; Gaskin et al. 1984; Read 1990a; Read 1990b; Read and Gaskin 1990; Read and Hohn 1995). Richardson (1992) examined porpoises killed in bottom-set gillnets off eastern Newfoundland during the summer months and concluded that their reproductive biology was, in general, very similar to that in the Bay of Fundy. There are no published descriptions of the reproductive biology of female harbour porpoises from the Gulf of St. Lawrence.
Reproduction in all populations studied to date is seasonal, with ovulation and conception limited to a few weeks in the late spring or early summer (Börjesson and Read 2003). Gestation lasts for 10-11 months followed by a lactation period of at least 8 months. In many populations, most mature female porpoises become pregnant each year and thus spend most of their adult lives simultaneously pregnant and lactating (Read 1999). In the Bay of Fundy, for example, mean age at sexual maturation for female porpoises was estimated to be 3.44 years of age and the annual pregnancy rate was estimated to be 0.86 (Read 1990b; Read and Gaskin 1990). Estimates of age at sexual maturation (3.1 years) and pregnancy rate (0.76) were similar in Newfoundland (Richardson 1992). At birth, porpoise calves are approximately 75 cm long and weigh about 6 kg (Börjesson and Read 2003). While nursing, the calves grow rapidly and triple their body mass by 3 months of age (Read 2001), by which time they have started taking solid food (Smith and Read 1992).
Males exhibit pronounced seasonal variation in testicular size and activity, with peak sperm production occurring around the period of ovulation (Fontaine and Barrette 1997; Neimanis et al. 2000). The testes are large, reaching 4% of body mass during the peak breeding season, suggesting that male porpoises are sperm competitors (Fontaine and Barrette 1997). In Newfoundland, male porpoises matured at 3.0 years of age (Richardson 1992). In the Bay of Fundy, age at sexual maturation for male porpoises was estimated to be 2.6 years (Neimanis 1996).
Survival
There are no estimates of the annual survival rates of this species in any portion of its range. There are no data on survival of known individuals and samples of age distributions come primarily from strandings of dead animals or bycatches, both of which are known to be biased (Caswell et al. 1998). Nevertheless, it is clear that the species is relatively short-lived compared to other odontocetes and few individuals live past their teens (Richardson 1992; Read and Hohn 1995). The maximum reported lifespan is 24 years, derived from counts of dentinal growth layers in thin, decalcified and stained sections (Lockyer 1995).
Attempts to estimate the potential rate of increase have been thwarted by a lack of information on survival rates (Caswell et al. 1998). Estimates of the maximum potential rate of increase, derived using survival rates from a variety of other large mammals with similar life histories, ranged from 4% (Woodley and Read 1991) to 10% (Caswell et al. 1998), imposing considerable uncertainty in our understanding of the potential for populations to withstand anthropogenic sources of mortality.
Harbour porpoises are preyed on by white sharks (Carcharodon carcharias) (Arnold 1972) and killer whales (Orcinus orca) (Jefferson et al. 1991). There are no estimates of the numbers of porpoises consumed by these predators, nor are there estimates of the rates of natural mortality for any population. Furthermore, we know very little about the abundance or trends of abundance of these predators. Little is known about the role of disease in the natural mortality of harbour porpoises. Each spring, however, many emaciated, dead juveniles are found stranded along the U.S. east coast between New York and North Carolina, apparently having starved to death (Cox et al. 1998). In addition, in some parts of their range, harbour porpoises are killed by bottlenose dolphins Tursiops truncatus (Ross and Wilson 1996).
Physiology
The species is well adapted to cold water and is seldom found in water warmer than 16°C (Gaskin 1992). It maintains homeothermy in a cold, conductive environment using a variety of physiological and anatomical adaptations, including a 1.5-2 cm thick layer of lipid-rich blubber (Koopman 1998; Koopman et al. 2002; McLellan et al. 2002).
Movements/dispersal
Very little is known of the movements of harbour porpoises in Newfoundland or the Gulf of St. Lawrence. In the western Bay of Fundy, 25 porpoises were equipped with satellite-linked radio transmitters between 1994 and 2002 (Read and Westgate 1997; Westgate and Read 1998; Read and Westgate unpublished data), providing a large amount of information on the movement patterns of individuals in this subpopulation. These individuals travelled more than 50 km in a single day and had home ranges that encompassed the entire Gulf of Maine, an area of many thousands of km2 (Read and Westgate 1997). The movements of these tagged porpoises were variable, and the only general tendency was that they moved southward into the Gulf of Maine during autumn.
Of the 14 tagged porpoises monitored between 1994 and 1997, ten moved from Canadian to U.S. waters and two of these ten then returned to Canada in the same year before their tags ceased transmitting (Westgate and Read 1998). It is clear that the subpopulation of porpoises in the Bay of Fundy and Gulf of Maine is transboundary in nature; management and conservation actions must take this fact into account. It should be possible to use these telemetry data, together with sightings data collected during abundance surveys, to calculate the proportion of this subpopulation present in the U.S. or Canada during the summer months, but such an analysis has not been conducted.
One tagged individual, a pregnant and lactating female accompanied by a small dependent calf, was tagged in the Bay of Fundy during mid-July and travelled to the Gulf of St. Lawrence, where it spent the remainder of the summer. This was the only tagged individual to have left the range of the Bay of Fundy-Gulf of Maine subpopulation, as defined above. It was also tagged earlier (by approximately two weeks) than the other individuals.
Nutrition and interspecific interactions
Information on the diet of harbour porpoises comes almost exclusively from examination of prey remains in the stomachs of bycaught and dead, stranded animals. The diet includes a variety of small fishes and cephalopods, usually < 30 cm in length (Read 1999).
In Newfoundland, the diet of bycaught porpoises consisted mainly of small fish such as capelin, Atlantic herring, sand lance and horned lantern fish (G. Stenson, pers. comm.).In the Gulf of St. Lawrence, the diet of porpoises killed in groundfish gillnets was examined by Fontaine et al. (1994). Herring and capelin accounted for the majority of caloric intake; redfish, mackerel, cod, and squid were also consumed. There is significant regional variation in diet in both Newfoundland and the Gulf of St. Lawrence. In the latter area, capelin was the dominant prey in the northeastern Gulf but porpoises from the Gaspé region consumed mostly herring.
In the Bay of Fundy and Gulf of Maine, porpoises feed primarily, but not exclusively, on juvenile Atlantic herring of age classes 2, 3 and 4 (Recchia and Read 1989; Gannon et al. 1998). This primary prey item is augmented with juvenile gadids and other small groundfish. In the Bay of Fundy, porpoise calves begin to take solid food during the late summer by feeding on euphausiid crustaceans (Smith and Read 1992).
Due to their small size and limited energy reserves, harbour porpoises have a limited capacity for fasting. The blubber is lipid-rich, but only part of this lipid store is available during times of food shortage (Koopman 2001; Koopman et al. 2002; McLellan et al. 2002). Consequently, individual porpoises must feed frequently to maintain body condition. This may also help explain the tight ecological association observed between this species and lipid-rich prey such as capelin and herring throughout eastern Canada.
The primary prey of harbour porpoises exhibits large fluctuations in abundance caused by natural recruitment cycles and the effects of commercial fisheries. In the Bay of Fundy and Gulf of Maine, the abundance of herring has varied widely over the past three decades, as stocks were overfished and subsequently recovered. Read (2001) examined the effects of this variation in prey biomass on the reproductive biology of female porpoises and particularly on the size of calves produced by females during these three decades. Surprisingly, female porpoises produced significantly larger calves during the decade (1980s) when prey biomass was lowest. There were no effects of variation in herring biomass on the body condition or fecundity of mature females during these three decades.
Behaviour/adaptability
Little is known about the behaviour of harbour porpoises, in part because it is difficult to identify individuals in the field. Observations of a small number of naturally marked females in the Bay of Fundy indicated that their social groupings are fluid and that individual porpoises may use the same areas in successive years (Watson 1976). Porpoises tagged together and equipped with satellite transmitters in the Bay of Fundy did not remain together after release (Read and Westgate 1997).
Harbour porpoises are usually observed in small groups of a few individuals, or alone, although larger aggregations of several hundred animals have been reported on occasion (Hoek 1992). Such large aggregations are temporary and likely driven by unusual concentrations of prey. As noted above, the mating system of this species likely involves sperm competition (Fontaine and Barrette 1997; Neimanis et al. 2000).
Harbour porpoises do not adapt readily to a captive environment and are seldom kept in oceanaria. Several live-stranded, rehabilitated juveniles, however, have been maintained for years in captivity, and observations of these individuals have provided considerable insight into the biology of the species (Read et al. 1997). Some live-stranded juveniles have been released successfully after periods of rehabilitation that lasted for months or years (Westgate et al. 1998).
In general, harbour porpoises are shy animals, and intensive human activities in coastal waters may adversely affect their populations.
Page details
- Date modified: