North Atlantic right whale (Eubalaena glacialis) recovery strategy: chapter 8

1.4.4. Basic biology and ecology

Natural history

While techniques have been developed to determine the age of toothed whales and some baleen whales, none have been demonstrated as effective for North Atlantic right whales (Kraus and Rolland 2007). Back-calculation from first birth records suggests that North Atlantic right whales routinely live longer than three decades.  The oldest known North Atlantic right whale was an adult female who lived to be at least 70 years old based on photographic records from 1935 to 1995 (Hamilton et al. 1998). The mean age of female sexual maturity is not known, but the mean age at first parturition is currently about 10 years (Kraus 2002). The youngest known mother gave birth to her first calf at age five (Knowlton et al. 1994).  Sexual maturity in males, estimated from a few known paternities, is about 15 years of age (Frasier 2005). Brown et al. (1994) used genital morphology and genetics to infer that the sex ratio in this population is approximately 50/50.

Most calves are born in the coastal waters of northern Florida and Georgia (Kraus et al. 1986b).  Since 1980, the number of calves observed each year has varied from a low of one calf in 2000 to a high of 31 calves in 2001; however there is no apparent trend.  Between 1980 and 1992, there were 11-12 calves born per year on average (Knowlton et al. 1994).  Since some cows with newborn calves are missed during the winter surveys off Georgia and Florida, complete assessment of a year's calf production requires surveys in the northern feeding areas, particularly Cape Cod Bay, the Great South Channel and the Bay of Fundy. At least two females have had calves over a period of 28 years, suggesting that the reproductive lifespan of North Atlantic right whales is at least that long.

The reasons why mothers and calves choose relatively shallow, nearshore habitat in low latitudes for calving grounds are unknown. The advantages provided by this habitat may include avoidance of predation (i.e. killer whales, Orcinus orca), relative warmth to conserve energy, reduced exposure to surface turbulence, easier orientation and navigation, and fewer disturbances from boisterous approaches by courting males. This same habitat is apparently not critical to females that are not pregnant, adult males, or most juveniles, as these classes are underrepresented on the calving ground. The advantages of this habitat do not persist beyond the calving season, as North Atlantic right whales are almost never seen off the U.S. east coast south of Cape Hatteras (ca. lat 35ºN) between late spring and late autumn (Winn et al. 1986).

The mating system of North Atlantic right whales is not fully understood, but appears to be shaped in large part by the prolonged spacing of calves (3-5 year intervals).  Such spacing means that the effective adult sex ratio is roughly one ovulating female to every four males, leading to significant competition among males for mating opportunities.  Courtship is the most energetic behaviour that has been observed with this species. Courtship groups (referred to as “surface-active groups” (SAGs)) may include 40 animals or more, as multiple males try to get close enough to mate with the focal female (Kraus and Hatch 2001).  Males appear to compete for positions next to the female, which are best for taking advantage of each mating opportunity when the female surfaces to breathe (Kraus and Hatch 2001).  Based upon the limited data available, it appears that the female may have intercourse frequently during a courtship bout with several different males, and this may result in a substantial degree of sperm competition among competing males.  Social factors, such as the need to participate in the activities of “surface-active groups”, may also influence an individual’s movements.

The seasonal timing and duration of observed courtship activities, from August through October, is puzzling.  Calving is first observed in December in the waters off Georgia and Florida and extends through early March.  The gestation period for right whales in the North Atlantic is unknown.  Best (1994) estimated a 12 month gestation period for southern right whales based on whaling data. Although the seasonal timing of the observed courtship for North Atlantic right whales is not consistent with this estimate, it is possible that courtship in the Bay of Fundy is merely “foreplay,” and that conception occurs elsewhere during December.  However, observed mating behaviour is indicative of strong competition among males, and participation in SAGs is likely expensive energetically. Thus an alternative hypothesis for this “foreplay” behaviour is that individuals require practice to develop the ability to compete successfully. The resolution of these questions will require better knowledge about the wintering habitats of North Atlantic right whales, and improved methods for evaluating pregnancy in North Atlantic right whales.

Migration and movements

There are five known seasonal, high use areas for North Atlantic right whales.  Winn et al. (1986) proposed a six-phase model to explain the seasonal north-south movements of right whales in the western North Atlantic.  Most adult females give birth in coastal waters of the southeastern U.S. between Brunswick, Georgia, and Cape Canaveral, Florida, during the winter months (Kraus et al. 1986b).  Males and non-calving females are rarely seen in that area, and their whereabouts during the winter remain largely unknown (Kraus et al. 1988).  They may be scattered widely along the eastern U.S. coast to at least as far north as Cape Cod Bay (Winn et al. 1986) and the central Gulf of Maine (Northeast Fisheries Science Center unpublished data).  There are records of adult and juvenile North Atlantic right whales of both sexes in Cape Cod Bay during the winter and spring, but the number of animals observed annually accounts for less than 30% of the known population (Hamilton and Mayo 1990). 

There is a northward migration in the late winter and early spring from the calving ground, with some mother calf pairs moving along the shore.  In the spring, aggregations of North Atlantic right whales are observed feeding and socializing in the Great South Channel east of Cape Cod, and in Cape Cod and Massachusetts Bays (Winn et al. 1986, Hamilton and Mayo 1990, Kenney et al. 1995).  Directed movements are made in June and July to the feeding grounds in the lower Bay of Fundy and on the western Scotian Shelf where a very high proportion of the known population in seen in at least August and September (Winn et al. 1986). From October, a steady southward migration occurs, with some animals passing through the Gulf of Maine and off Cape Cod (Winn et al. 1986). Recent data have expanded our understanding of the migration and movement patterns of North Atlantic right whales.  For example, sightings of North Atlantic right whales in the Bay of Fundy (Laurie Murison, pers. comm.) and in the southeastern Gulf of St. Lawrence (Jack Lawson, pers. comm.) have been reported into late December as have recordings of their vocalizations on Roseway Basin (Mellinger et al. 2007).  Summer and autumn sightings of North Atlantic right whales along the Gaspe Peninsula are indicative of transits between the known habitat areas in Atlantic Canada and the Gulf of S. Lawrence (Canadian Whale Institute and NEAq unpublished data).

1.4.5. Habitat requirements[1]

The North Atlantic right whale is primarily found in coastal and shelf waters. The habitat needs of right whales can be inferred from the seasonal distribution of the population and the types of activities observed in the areas that the animals frequent. Judging by the evidence of segregation among the various classes of whales, habitat requirements appear to differ considerably, depending on a whale’s age, sex, and reproductive status (Brown 1994).

Virtually the entire population moves north for the summer; however since nothing is known about how right whales navigate, it is difficult to judge the importance of oceanographic or topographic features along the migratory corridor(s).  Winn et al. (1986) found that calf sightings, even in the higher latitudes, were significantly closer to shore than non-calf sightings. The advantages of a near shore migration would, presumably, diminish by late autumn, when most first-year calves are probably about to be weaned (Hamilton et al. 1995). Opportunities to forage en route might also influence the course taken from one destination to the next.

Defining habitat requirements on the feeding grounds is more straightforward, at least conceptually. Several investigators have attempted to define critical threshold densities of prey necessary for efficient feeding by right whales (see feeding behaviour below). It is assumed that the distribution and movements of right whales during much of the year are driven primarily by the distribution of the large zooplankton that constitutes their prey.

Winn et al. (1986) attempted to define preferred habitat by comparing the distribution of sightings (effort corrected) and behaviour (feeding and socializing) with various environmental factors (water depth, sea surface temperature, and sea floor relief). Their overall conclusions were that right whales preferred water 100 to 150 m deep, usually but not always, over steep bottom slopes. Preferred surface temperatures were mainly in the range of 8 to 15ºC. Gaskin (1987) hypothesized that right whales find preferred feeding conditions in frontal zones between well-mixed and stratified water masses (Murison 1986). Right whales are rarely seen in areas where the sea surface temperature is higher than 18ºC (Kraus et al. 1993).

In the Bay of Fundy, right whales are found mainly in the upper Grand Manan Basin, in waters 90 to 240 metres deep, with surface temperatures of 11 to 14.5ºC and weak thermoclines (Murison and Gaskin 1989, Gaskin 1991). However, it would appear that depth and temperature features are, at best, only proxies for other more directly relevant factors. The driving force is more likely the formation and maintenance of dense concentrations of calanoid copepods, which are, in turn, governed by physical features and processes, such as frontal boundaries, vertical stability and stratification in the water column, and bottom topography (Woodley and Gaskin 1996). When right whales first arrive in the Bay of Fundy in early summer, their distribution tends to be dynamic, with animals often occurring in shallow water close to shore. Similar spreading out occurs in the autumn, when they are sometimes seen in very shallow areas (e.g., around Campobello Island, the ledges south of Grand Manan). These trends are consistent with the lack of dense copepod patches in the Grand Manan Basin, which develop through the summer to a peak in late autumn and then decline (Murison 1986). Outside the Bay of Fundy, satellite-monitored right whales have shown an affinity for edges of banks and basins, upwellings, and thermal fronts.

1.4.6. Feeding behaviour

Right whales swim through the water with their mouths open, filtering zooplankton through their baleen. Baleen is a series of keratinous plates that hang down from the whales’ upper jaw. Because of the morphology of their baleen, and the need to swim slowly to efficiently push large quantities of water through their baleen, right whales are limited to a feeding on a narrow range of zooplankton species (Baumgartner et al. 2007). Right whales locate aggregations of prey at the surface (skim feeding) or at depth (down to at least 200 metres).  In the Bay of Fundy, right whales sometimes swim near to the bottom as evidenced by the fact that they surface with mud on their heads.  Right whales in the Bay of Fundy, tracked with timed depth recorders, were recorded on feeding dives to depths of 80-175 metres and lasting for 5 to 14 minutes (Baumgartner and Mate 2003). 

Right whales in the western North Atlantic feed on a variety of organisms but seem to depend most heavily on the later oil-rich developmental stages (C-IV and C-V, and adults) of the copepod Calanus finmarchicus (Murison and Gaskin 1989, Mayo and Marx 1990, Kenney and Wishner 1995, Baumgartner et al. 2003a, Michaud and Taggart 2007). This dependence is evidenced by the copepod hard parts found in faecal material (Kraus and Prescott 1982, Murison 1986, Kraus and Stone 1995) and by the high density of copepods found in the immediate vicinity, or exactly on the paths, of feeding right whales (Murison and Gaskin 1989, Mayo and Marx 1990).  As well, evidence is provided by the fact that spring, summer, and autumn aggregations of right whales occur primarily in areas with high densities of these copepods (Kenney et al. 1986, 1995, Michaud and Taggart 2007, Wishner et al. 1988).

Copepods form dense concentrations both vertically and horizontally where tides, winds, or prevailing currents form convergences or where water parcels of different temperature, salinity, and density meet to form fronts (Wishner et al. 1988, Kenney and Wishner 1995). Major changes have been observed in the spring and summer distribution of right whales over the last 20 years, suggesting that they respond to changes in prey density.  For example, they appeared to favor of the Bay of Fundy over Roseway Basin between 1993 and 1997 (NEAq, unpublished data) and no right whales were found during surveys of the Great South Channel in 1992 (Kenney 2001). 

Zooplankton organisms are not homogeneously distributed, but instead usually occur in "patches" in the water column. When sampling in the discrete layers targeted by whales in the Bay of Fundy, Baumgartner and Mate (2003) and Baumgartner et al. (2003b) found copepod abundances that were several orders of magnitude above background levels. The spatial variability in their occurrence was associated with a layer of mixing at the bottom and their depth of dive suggested that they are targeting dense concentrations where conditions help to form discrete, vertically compressed layers of copepods.  Concentration may be further enhanced as zooplankton seek preferred intensities of light or other physical factors during diurnal vertical migrations.  The mechanisms underlying the formation and maintenance of zooplankton patches are not well understood and are an active area of research both in the right whale research community and in the broader field of biological oceanography.

Right whales are, therefore, highly dependent upon a narrow range of prey, which occur in highly variable and spatially unpredictable patches in the Atlantic ecosystem.  The four known northern feeding habitats apparently have conditions that are conducive to the creation of highly concentrated patches of copepods.  There is, however, substantial inter-annual variability in copepod production, and thus in right whale abundance, in each of these areas (Brown et al. 2001, Kenney 2001).  Right whales may adapted to this unpredictability with a large caloric buffer in the form of blubber (Moore et al. 2001) and an ability to travel long distances in relatively short periods of time (Mate et al. 1997, Slay and Kraus 1999, Kraus 2002, Baumgartner and Mate, 2005).

[1] Unless otherwise specified, where used in document here on in, the term ‘right whale’ will serve as a shortened version of the North Atlantic right whale.

Page details

Date modified: