Occasional
Cetaceans
Sei
Whales
Blue Whales
Orcas
White-beaked Dolphins
Risso's Dolphins
Common Dolphins
Bottlenose Dolphins
Sei
Whales
General
Description and Status
Sei whales, Balaenoptera borealis, are similar in size and shape
to fin whales and thus are often mistaken as such. However, sei whales
are smaller, reaching a maximum length of 50 feet and are uniformly
colored about the head. Sei whales also tend to have a tall, thin and
profoundly hooked dorsal fin which usually appears simultaneously with
the blow. There are no recent population estimates for the western North
Atlantic, although the species is listed as "endangered."
Distribution
and Abundance
Very little is known about the distribution of sei whales. In the western
North Atlantic, their range is believed to be from Greenland to the
waters of southern New England (Payne and Selzer 1986). Decreased sightings
in northern areas together with strandings off the southeastern coast
of the United States in late winter and early spring suggest sei whales
migrate southward during the fall (Mead 1977). Prior to 1986, virtually
no sightings of sei whales were recorded in the Stellwagen region (Mayo
et al. 1988a). In 1986, an influx of this species occurred which was
attributed to the great availability of copepodite or euphausiid prey
in the area at that time (Payne et al. 1990, Schilling et al. 1992).
In recent years, again, only infrequent sightings have been reported
for this area (Mayo et al. 1988a, Center for Coastal Studies unpublished
data), although another influx was recorded in nearby offshore waters
in 1994. Sei whales are known for sudden appearances into an area followed
by an absence of sometimes several years (Jonsgard and Darling, 1977).
Photographic-identification
of individual sei whales has been conducted on a limited basis using
the dorsal fin shape, variations in pigmentation and scarring (Schilling
et al. 1992). Studies based on photo-identified animals during the 1986
influx showed evidence of short-term seasonal residency by individuals
(Schilling et al. 1992). Overall, however, information regarding the
abundance and occurrence of sei whales in the Stellwagen region is lacking.
Feeding
While in the Stellwagen Bank area, sei whales have been observed feeding.
Sei whales skim feed as right whales do, preying primarily on plankton
species such as Calanus (spp.) or krill (Mitchell 1974, Watkins
and Schevill 1979). However, the presence of ventral pleats on the throat
and chest area suggest that sei whales can also gulp-feed like other
piscivorous species such as humpbacks and fin whales (Nemoto 1970, International
Whaling Commission 1977, Nemoto 1977).
Reproduction
Mother and calf pairs have been observed in the Stellwagen region. Nearly
all information on sei whale reproduction comes from whaling data. Such
data from Iceland suggest that this species breeds and calves during
the winter months and has a gestation period of just under a year (Lockyer
and Martin 1983). Whaling data from Norway indicate mating occurs in
late autumn and winter and report a calving interval of 2 years (Jonsgård
and Darling 1977). Currently, little is known about where sei whales
breed or calve.
Group
Size and Composition
The CeTAP (1982) study found the average number of sei whales in a group
was 3. Schilling and others (1992) reported that sei whales observed
in the area were in groups ranging from 1 to 6 individuals with a mean
group size of 1.8. Data based on photo-identified individuals showed
that individual associations were transient, usually lasting less than
one day (Schilling et al. 1992). Generally, little is known about the
sex or age classes occurring in the Stellwagen region, however, there
is some indication that both temporal and spatial segregation by reproductive
class may occur. During the 1986 influx of sei whales, mother and calve
pairs were observed to arrive later than other whales (Mayo et al. 1988a)
and tended to associate less frequently with other individuals (Schilling
et al. 1992).
Blue
Whales
General
Description and Status
The blue whale, Balaenoptera musculus, is the largest whale in
the world, reaching lengths of up to 100 feet. Besides their overall
size, blue whales can be distinguished by their tall spout, at times
reaching 30 feet or more into the air, and by their mottled blue skin.
In 1965, the International Whaling Commission established complete protection
from hunting for blue whales. Presently, no reliable estimate exists
for this species in the western North Atlantic, however, blue whales
are considered one of the most "endangered" of the great whales.
Distribution
and Abundance
Like most baleen species, blue whales are believed to spend the summer
in high-latitude feeding areas and move to lower latitudes during the
fall (Leatherwood et al. 1976). Little information is available regarding
their winter range, however, in the North Atlantic, it is thought that
they travel to mid-temperate latitudes and perhaps extend south into
the tropics (Leatherwood et al. 1983). Although the distribution of
blue whales in the North Atlantic is not well understood, they are frequently
observed off the eastern coast of Canada. Prior to 1986, no sightings
of blue whales had been recorded in the Gulf of Maine and since then
only a few animals have been observed (Wenzel et al. 1988, Center for
Coastal Studies unpublished data). The occasional occurrence of this
species in the Stellwagen Bank region appears to be due to the presence
of euphausiids (krill) in the area (Wenzel et al. 1988).
Feeding
Blue whales sighted in the Stellwagen Bank area have been observed doing
horizontal surface lunges while feeding on krill which is their principal
prey item (Wenzel et al. 1988).
Reproduction
Reproductive biology and behavior is, for the most part, unknown for
blue whales in the western North Atlantic. However, whaling data from
the Southern Hemisphere indicate that females reach sexual maturity
around the age of 5 and have an 11 month gestation period (see Lockyer
1984 for review). Females calve approximately every 2 years and wean
usually by 7 months (Mackintosh and Wheeler 1929).
Group
Size and Composition
Information regarding the population structure or group composition
of blue whales in the western North Atlantic is scant. Sears and others
(1981) reported that blue whales observed off the Mingan Islands, Canada
were most often single animals or in pairs, although small groups of
as many as 5 animals were occasionally sighted.
Orcas
General
Description and Status
Orcas, Orcinus orca, commonly called killer whales, reach a maximum
length of 30 feet. They are easily recognized by their striking black
and white coloration. Mature males are quickly identified by their very
tall dorsal fins which can reach up to 6 feet in height. Presently,
there are no conclusive estimates regarding abundance in the western
North Atlantic.
Distribution
and Abundance
In the North Atlantic, orcas are thought to range from the polar ice
down to the Lesser Antilles and Gulf of Mexico (Leatherwood et al. 1976,
Schmidley 1981). Katona and others (1988), after examining numerous
records of orca sightings and strandings, reported that there may exist
a small population along the eastern coast of the United States which
migrate seasonally. The movements of orcas found off the eastern seaboard
are not well defined. They may make latitudinal migrations during the
course of the year or perhaps move back and forth between inshore and
offshore areas (Katona et al. 1988). Within the Gulf of Maine, orca
sightings are most common from mid-July to September and have been recorded
on Jeffrey's Ledge southward to Stellwagen Bank (Katona et al. 1988,
Mitchell and Reeves 1988). In the Stellwagen Bank region, orcas tend
to be sighted during the late summer and fall (Center for Coastal Studies
unpublished data). Overall, orcas are considered uncommon in the Gulf
of Maine and are only infrequent visitors to Stellwagen Bank.
Feeding
Mayo (1982) reported that in Cape Cod Bay and on the southwest edge
of Stellwagen Bank, orcas have been observed feeding on bluefin tuna,
Thunnus thynnus, which is thought to be their principal prey
item in the area. Leatherwood and others (1976) noted that the arrival
of orcas in the waters of Cape Cod coincided with that of bluefin tuna
suggesting that their distribution is influenced by the presence and
abundance of that prey. Orcas are considered opportunistic feeders,
feeding on both baleen and toothed cetaceans, seabirds, pinnipeds, marine
turtles, schooling fish and squid (Katona et al. 1988).
Reproduction
Little information is available regarding the reproductive biology and
behavior of orcas in the western North Atlantic. Whaling data from Norwegian
coastal catches suggest that females attain sexual maturity at approximately
8 years and males at about 15 years (Christensen 1984). The data also
indicated that mating occurs throughout the year with a peak between
October and December. Christensen (1984) reported a birth rate of a
calf every 3 years. Data on the gestation period is considered inconclusive,
however, it has been estimated anywhere from 12 to 16 months. Reproductive
information from studies based on individual identification of orcas
in the coastal waters of British Columbia and Washington State indicated
that females usually give birth to their first viable calf around 15
years of age and average a calf every 5 years (Olesiuk et al. 1990).
Group
Size and Composition
Group sizes recorded in the Stellwagen Bank and Cape Cod Bay region
ranged from 1 to 20 animals (Center for Coastal Studies unpublished
data). The largest group reported in the southern Gulf of Maine contained
at least 40 animals (Katona et al. 1988). Information regarding the
social organization of orcas in the western North Atlantic is unavailable;
however, studies based on individual identification conducted off the
northwestern coast of North America report that matrilineal groups are
the basic unit of social organization (Bigg et al. 1990). Groups contain
both males and females and animals of mixed ages.
White-Beaked
Dolphins
General
Description and Status
White-beaked dolphins, Lagenorhynchus albirostris, reach a maximum
length of 10 feet. Recognition of this species from the field is usually
done by sighting the white back. They have a short, blunt beak which
on the majority of animals is white in color. Presently, there are no
population estimates for white-beaked dolphins although they appear
to be abundant within their range (Klinowska 1991).
Distribution
and Abundance
White-beaked dolphins are found only in the North Atlantic and range
from Greenland to Cape Cod (CeTAP 1982). Generally, their distribution
is more northerly than that of white-sided dolphins (Katona et al. 1977).
In the southern Gulf of Maine, white-beaked dolphins occur April through
November although sightings are more frequent during the spring in the
Stellwagen Bank and Cape Cod Bay area (CeTAP 1982, Center for Coastal
Studies unpublished data). Today, white-beaked dolphins are infrequent
visitors to the Stellwagen region though it is believed that they were
more common nearly 40 years ago. Together with the decline in white-beaked
dolphin sightings in recent years, has been an increase in sightings
of white-sided dolphins (Katona et al. 1983).
Feeding
White-beaked dolphins forage on squid, Illex spp., and fish with
the former being the principal prey item (Katona et al. 1993).
Reproduction
Very little is known regarding the reproductive biology and behavior
of white-beaked dolphins. It is thought that mating occurs between July
and October with most births taking place May through August (see Evans
1987 for review).
Group
Size and Composition
The group size for white-beaked dolphins can range from a few animals
to several dozen (Katona et al. 1993). In the Stellwagen Bank area,
groups were observed containing up to 30 or 40 animals (Center for Coastal
Studies unpublished data). In some locations, group sizes of up to several
hundred animals has been recorded with group size tending to increase
when food is immediately available (Klinowska 1991). Groups containing
both sexes as well as mixed ages have been reported (see Evans 1987).
Risso's
Dolphins
General
Description and Status
Risso's dolphins, Grampus griseus, commonly called Grampus whales,
are large sized dolphins reaching lengths of up to 13 feet. They are
easily recognized in the field by their gray colored bodies which are
usually covered with numerous white scars and scratches. They lack any
noticeable beak and have a bulbous forehead with a unique vertical groove
(Katona et al. 1993). Risso's dolphins are frequently active, tending
to breach and lobtail as well as spyhop. Current population estimates
for the western North Atlantic are unavailable, although they are believed
to be abundant within their range (Leatherwood et al. 1983).
Distribution
and Abundance
In the western North Atlantic, Risso's dolphins range from Newfoundland
to the Lesser Antilles and Gulf of Mexico (Fritts and Reynolds 1981,
Leatherwood et al. 1976). The CeTAP (1982) study found this species
to be most common along the continental slope extending south of Cape
Cod to Cape Hatteras, North Carolina. They have been observed infrequently
in the Stellwagen Bank region and generally occur during the late summer
and fall (Center for Coastal Studies unpublished data). Overall, this
species is rarely found in the Stellwagen Bank area.
Feeding
Risso's dolphins are believed to feed almost exclusively on squid,
Illex spp. (Leatherwood et al. 1976).
Reproduction
Very little information exists on the reproductive biology and behavior
of Risso's dolphins in the North Atlantic. There is some evidence that
calves may be born in all seasons although, in the North Atlantic, calving
may occur mainly April through September (see Evans 1987, Baird and
Stacey 1991)
Group
Size and Composition
The CeTAP (1982) study found the average group size for Risso's dolphins
was about 17 animals, although as many as 200 have been observed. In
the Stellwagen Bank area, groups consisting of a few animals to more
than 60 have been recorded (Center for Coastal Studies unpublished data).
Data based on photographically identified animals indicate that this
species forms fairly cohesive social groups in which individuals remain
together for extended periods of time (Kruse 1987).
Common
Dolphins
General
Description and Status
Common dolphins, Delphinus delphis, also known as saddlebacks,
are recognized by a distinct hourglass-shaped pattern on both sides
of the body and a very pointed snout. Their length can reach up to nearly
8 feet and they are commonly observed bowriding alongside vessels. Currently,
the population size in the western North Atlantic is unknown, but in
the coastal waters of North America they appear to be abundant (Gaskin
1992).
Distribution
and Abundance
In the Atlantic, common dolphins range from eastern Canada to Venezuela
(Leatherwood et al. 1976). In the western North Atlantic, this species
is generally found along the continental slope area between Nova Scotia
and Cape Hatteras, North Carolina (Gaskin 1992). Very few sightings
have been recorded in the Stellwagen Bank region, however, during the
winter and spring of 1994, 12 animals stranded off the shores of Cape
Cod (Center for Coastal Studies, unpublished data). Overall, common
dolphins appear to be very infrequent visitors to the Stellwagen Bank
area.
Feeding
Common dolphins are considered to be opportunistic in their foraging.
They feed on a variety of schooling fish and squid, Illex spp.,
usually depending on what is locally abundant or available (Klinowska
1991, Katona et al. 1993).
Reproduction
Information regarding the reproductive biology of common dolphins indicate
that sexual maturity occurs around 5 to 7 years of age (see Perrin and
Reilly 1984 for review). The gestation period is approximately 11 months
and females are thought to calve about every 2 years (Gaskin 1992).
Group
Size and Composition
CeTAP (1982) reported finding common dolphins in group sizes ranging
from 1 to as many as 2,000 animals but that the mean group size was
55 animals. Group sizes of 20 to 25 animals have been recorded in the
Cape Cod Bay and Stellwagen Bank area (Center for Coastal Studies, unpublished
data). Little information exists regarding group composition or structure,
however, there is some evidence that common dolphins segregate by sex
and age classes (Mitchell 1975).
Bottlenose
Dolphins
General
Description and Status
Bottlenose dolphins, Tursiops truncatus, reach lengths of up
to 12 feet. Their coloration is generally bluish gray or brown on top
of the body and lighter below. Two forms of this species have been described:
inshore and offshore populations which appear to be very distinct (Hersh
and Duffield 1990). The total abundance estimate for bottlenose dolphins
off the northeast United States is 10,000 to 13,000 animals (Kenney
1990).
Distribution
and Abundance
The general range of bottlenose dolphins inhabiting the eastern coastal
waters of the United States is from Florida to the southern edge of
Georges Bank (Kenney 1990, Mead and Potter 1990). Sightings of bottlenose
dolphins within the Gulf of Maine appear to be extralimital and usually
occur during the late summer and fall (Payne and Selzer 1986). The occurrence
of this species is considered very rare in the Stellwagen Bank and Cape
Cod Bay region. However, Wiley and others (1994) reported an apparent
two year residency of two bottlenose dolphins in Cape Cod Bay. Overall,
bottlenose dolphins do not appear to utilize the Stellwagen Bank region
to any significant extent.
Feeding
Bottlenose dolphins are considered opportunistic feeders eating a variety
of fish, squid, Illex spp., and invertebrates (CeTAP 1982, Leatherwood
1975) The CeTAP data indicate that they mainly feed beneath the surface,
however, bottlenose dolphins have shown great diversity regarding their
feeding techniques depending on the type of prey and surrounding location
(see Shane et al. 1986).
Reproduction
Information regarding the reproductive biology of bottlenose dolphins
suggests that females reach sexual maturity between the ages of 5 and
12 and calve about every other year (Katona et al. 1993). The gestation
period is approximately 12 months and nursing can continue for nearly
2 years (see Perrin and Reilly 1984 for review).
Group
Size and Composition
Group sizes of bottlenose dolphins range from 1 to over a 100 animals,
however, they are more commonly found traveling in pods containing 2
to 15 individuals (Shane et al. 1986). The Center for Coastal Studies
(unpublished data) recorded group sizes of 20 to 50 animals in the Stellwagen
Bank region. Research conducted off western Florida (see Wells et al.
1987) indicates that adult females show more site fidelity than adult
males and that calves of either sex tend to stay within the maternal
group at least until sexual maturity is reached, if not longer. Wells
and Scott (1990) have observed associations between mothers and their
calves continuing for up to 10 years. Data from several studies indicate
that bottlenose dolphins form relatively permanent social groups based
on sex and age (Shane et al. 1986).
continue
to next section
