Common
Cetaceans
Humpback
Whales
Fin Whales
Minke Whales
Northern Right Whales
Sei Whales
Atlantic White-sided Dolphins
Pilot Whales
Harbor Porpoises
Humpback
Whales
General
Description and Status
Humpback whales, Megaptera novaeangliae, are moderately large
baleen whales reaching lengths of up to 60 feet. Humpbacks are commonly
sighted in the Stellwagen Bank area and are easily recognized by their
long pectoral fins and distinctive black and white markings on the ventral
side of the tail. Despite complete protection from commercial hunting
since 1966, humpbacks are still classified as "endangered".
The western North Atlantic population is currently estimated at 5,505
animals (Katona and Beard 1990), although some researchers regard this
figure as low.
Distribution
and Abundance
Humpback whales occur mainly in the Stellwagen Bank area during spring,
summer and autumn with a few animals remaining through the winter (CeTAP
1982, Clapham et al. 1993). During the late fall and early winter, most
humpbacks leave this area and migrate to mating and calving grounds
in the West Indies (Clapham and Mattila 1988, Katona and Beard 1990).
Humpbacks begin returning to the Stellwagen Bank vicinity approximately
mid-March (Clapham et al. 1993).
Humpback
whales are individually identified using variations in the ventral fluke
patterns, dorsal fin shape and scarring (Katona and Whitehead 1981).
Investigations based on photo-identified individuals in the Stellwagen
area have shown a high rate of annual return (approximately 80% of the
whales seen in one year are also observed the next) together with a
high rate of within season occurrence (Clapham et al. 1993). Such studies
have also reported considerable variation in both seasonal and yearly
occurrence of individuals in this region which is probably due to fluctuations
in the abundance of food (Mayo et al. 1988a, Payne et al. 1990, Clapham
et al. 1993). Spatial distribution of humpbacks on Stellwagen Bank appears
to vary from year to year as well as within a season; however, when
clumping of animals was observed by Mayo et al. (1988a), it occurred
largely along the sloping edges of the Bank. The occurrence of upwellings
along the sloping edges together with the bottom topography appear to
generate great productivity in the area (Meyer et al. 1979) which may
create conditions for successful foraging. Studies suggest that the
distribution of humpbacks on Stellwagen Bank is strongly influenced
by the amount of food available on or near the Bank (Mayo et al. 1984,
Patev 1987).
Feeding
Humpback whales utilize the Stellwagen Bank area primarily for feeding
(Mayo et al. 1988a, Payne et al. 1990). Humpbacks have been described
as opportunistic feeders preying on small, schooling fish as well as
krill (Katona et al. 1993). On Stellwagen Bank the principal food source
is sand lance, Ammodytes (spp.) (CeTAP 1982, Overholtz and Nicolas
1986, Payne et al. 1986). In this area, humpbacks employ many different
feeding techniques including surface lunges, bubble columns and bubble
clouds (as described in Hain et al. 1982). Both solitary and cooperative
feeding has been observed. Clapham (1993) found frequent changes in
group size during foraging with group size tending to increase when
larger food patches were immediately available.
Reproduction
Data from photo-identification studies conducted on this population
have suggested that the Stellwagen region is an important nursery ground.
Mature females are generally observed more frequently in years when
accompanied by a calf than in non-calf years (Clapham and Mayo 1987).
The gestation period for humpbacks is nearly 12 months (Chittleborough
1958). Calving intervals observed for this population are approximately
every 2 to 3 years with most females attaining sexual maturity by the
age of 6 (Clapham and Mayo 1990, Clapham 1992). Mother and calf pairs
can remain together for up to a year and though it is unknown for certain
when weaning occurs, data from this area indicate that some females
may wean their calf during the autumn while still on the feeding grounds
(Baraff and Weinrich 1993). The high return rate of calves to this area
after having been separated from their mothers for at least one year
(Clapham and Mayo 1990) supports the belief that the fidelity to a feeding
ground by individuals is maternally directed (Clapham and Mayo 1987).
Group
Size and Composition
Most sightings of humpbacks in the Stellwagen Bank area are of single
animals or pairs (Mayo 1982, Clapham 1993). Large groups occur infrequently
and are usually associated with surface feeding activity (Clapham 1993).
Most groups are small and tend to stay together for only a brief period
of time; however, some stable associations (other than mother and calf
pairs) have been recorded (Weinrich, 1991, Clapham 1993). Clapham (1993)
found immature whales of both sexes tended to be alone and that overall
most humpbacks associated with animals of their own maturational class
(adult males and females pairs, in particular, were very common although
these were found to be over-represented statistically). Weinrich and
Kuhlberg (1991) found that adult females were more social than males
and tended to pair with animals of either sex whereas males appeared
to prefer associating with only females when forming groups of 2 or
3 animals. Kinship appeared not to be important in the association patterns
and groupings among humpbacks in this area and dominant or territorial
behaviors seemed to be absent from thispopulation as well (Clapham 1993).
Overall, the group size and dynamics of humpbacks in the Stellwagen
region is, as with other species, believed to be a function of ecology
(Clapham 1993).
Fin
Whales
General
Description and Status
Fin whales, Balaenoptera physalus, are the largest baleen whales
regularly observed in the Stellwagen Bank area. Their maximum length
is approximately 70 feet. Fin whales are recognized by their large,
streamlined bodies and unique asymmetrical coloration (the lower right
side of the jaw is white and the lower left side black). Fin whales
have been protected from commercial hunting in the western North Atlantic
since 1971 but are still considered "endangered". Reliable
population estimates for the western North Atlantic are not available.
Distribution
and Abundance
Fin whales are most abundant in the Stellwagen Bank area in the spring,
summer and autumn, although there have been periodic sightings during
the winter (CeTAP 1982). Overall, it is assumed that, like many other
baleen whales, fin whales migrate from high-latitude feeding grounds
to lower latitude areas for breeding and calving (Mackintosh 1966).
Distribution studies of this species have been facilitated by the ability
to identify individuals. The technique includes looking at variations
in natural markings, mainly the shadings on the right side of the body,
along with differences in dorsal fin shape and scarring (Mayo 1982,
Agler et al. 1990). Individuals have a high re-sighting rate in this
area with approximately half of the fin whales observed in one year
being seen again the following year (Seipt et al. 1990). It also appears
that many individuals remain in the Stellwagen vicinity for extended
periods each season; however, individuals re-sighted outside the area
indicate that some animals may undertake extensive movements while on
the feeding grounds (Seipt et al. 1990). Data from this area suggest
that the fin whale's distribution pattern is similar to that of the
humpback (Seipt et al. 1990). Overall, however, the temporal and geographical
distribution of fin whales remain poorly understood.
Feeding
Fin whales are often observed feeding in the Stellwagen Bank area. They
forage on a variety of fish including herring, Clupea harengus,
and sand lance, Ammodytes (spp.); the latter being considered
the primary food item in this area (Overholtz and Nicolas 1979, Payne
et al. 1990). Often when fin whales feed, they will circle the prey
presumably to corral the fish thus making it easier to capture. Also,
it is common to find fin whales rolling to one side while lunging through
a patch of food. It has been suggested that the unique coloration of
the fin whale's jaw may play a role in their foraging (see Tershy and
Wiley 1992).
Reproduction
The Stellwagen region appears to be an important nursery ground for
fin whales. Mother and calf pairs are regularly sighted in this area.
Presently, no specific calving grounds are known for fin whales, although
stranding data suggest that calving could occur around the Cape Hatteras
area or possibly in nearby latitudes from October through January (Hain
et al. 1992). Whaling data from Norway indicate most calves are born
between November and January (Haug 1981). The gestation period is approximately
one year and nursing is believed to last about 6 to 7 months (see Lockyer
1984 for review). Information on sexual maturity rates varies depending
on geographical location, although whaling data from Iceland have suggested
possibly 8 years of age for females (Lockyer 1981). Data from the Gulf
of Maine on individual females returning to the area with more than
one calf over the years have shown a minimum birthing interval of 2
years (Agler et al. 1993). Individual fin whales show a high return
rate to their maternal summer ranges suggesting that fidelity to specific
feeding grounds is determined matrilineally (Clapham and Seipt 1991).
Group
Size and Composition
Group structure or composition for fin whales in the Stellwagen region
is poorly described. In the Stellwagen Bank area, Mayo and others (1988a)
found that most fin whale sightings were of single animals with a few
observations consisting of pairs and trios. They reported that groups
of 4 or more individuals were extremely rare. Studies looking at whaling
data have indicated possible segregation of sex and/or age classes by
this species (JonsgŒrd 1966, Haug 1981) and there is evidence that similar
segregation may occur in the Gulf of Maine (Agler et al. 1993).
Minke
Whales
General
Description and Status
Minke whales, Balaenoptera acutorostrata, are the smallest of
the baleen whales to occur in the Stellwagen Bank area. Their maximum
length is approximately 30 feet. Minkes are generally recognized by
their fast speed, small size and quick surfacing behavior. They rarely
show the visible spout commonly exhibited by confamilials and have a
tendency to raise the snout out of the water when surfacing for a breath.
Minkes have a white band going across each flipper and pale shadings
running along their sides.
Minkes
are considered non-endangered and are still hunted for scientific research
under the provisions of the International Whaling Commission. Presently,
Norway is the only country hunting minke whales for commercial purposes.
Reliable population estimates for the western North Atlantic are not
available.
Distribution
and Abundance
Minke whales are often observed in the Stellwagen Bank area. Generally,
they arrive in the early spring and may remain as late as December (Murphy
in press). In the autumn and winter months, sightings are scarce or
absent which may reflect a migration to lower latitudes during this
time. The winter distribution of minkes is still largely unknown, although
sightings have been reported in the southwestern North Atlantic (Mitchell
1991, Mattila and Clapham 1989, Mattila et al. in press). The CeTAP
(1982) study reported a few winter sightings in the western Gulf of
Maine and southeast of Cape Cod. Maximum abundance appears to occur
between July and September, although there is considerable variation
in occurrence both seasonally and annually (Murphy in press). Yearly
changes in abundance of minke whales has also been reported in other
areas and have been linked to fluctuation in the availability and distribution
of prey (Sigurjonsson 1982, Larsen and Oien 1988).
Studies
based on photographic identification of individual minkes conducted
off the western coast of the United States have shown that individuals
returned to the same summering grounds from year to year (Dorsey, 1983,
Dorsey et al. 1990). Presently, it is unknown whether the minkes observed
in the Stellwagen Bank region return to this area from year to year,
or if they represent transient individuals from one or more populations.
Feeding
Surface feeding by minke whales has rarely been observed in the Stellwagen
region. Since sympatric confamilials such as humpbacks and fin whales
are commonly observed feeding in this area (Overholtz and Nicolas 1979,
Payne et al. 1986), it is generally believed that minkes also use Stellwagen
Bank for feeding. It may be that minke whales exploit prey at greater
depths or perhaps employ a foraging technique that does not utilize
the surface for catching prey. In the North Atlantic, minkes have been
reported to forage on a variety of small schooling fish (Jonsgard 1951,
Katona et al. 1977, Sears et al. 1981) and many observers assume that
sand lance, Ammodytes (spp.), are their principal prey item in
this area.
Reproduction
Few mother and calf pairs have been recorded in the Stellwagen region.
The scarcity or absence of minke whale calves is common in other areas
of the North Atlantic (JonsgŒrd 1951, Sergeant 1963, Mitchell and Kozicki
1975). North Atlantic minkes are believed to calve between November
and March after a 10 month gestation and to nurse for only 4 to 5 months
(JonsgŒrd 1951). The location of the calving grounds is unknown, although
minke calves have been observed in the Caribbean and southwestern North
Atlantic (Mattila and Clapham 1989, Mitchell 1991).
Group
Size and Composition
Most sightings of minke whales in the Stellwagen region are of single
animals (CeTAP 1982, Murphy in press). Neither sex nor age class of
minke whales is known in this area but the majority of stranded animals
have been immature with the sex ratio approximately even. Overall, the
social structure and group composition of minkes observed in the Stellwagen
Bank area are unknown.
Northern
Right Whales
General
Description and Status
Northern right whales, Eubalaena glacialis, reach a maximum length
of 50 feet and are easily recognized by their distinctive "V-shaped"
blow. Right whales lack a dorsal fin and have raised patches of roughened
skin on the head and lip area called calosities. Though internationally
protected from commercial hunting since the 1930's, the western Northern
right whale population is currently estimated to have less than 350
animals making it the most endangered of the great whales (NMFS 1991).
Unfortunately, the right whale has not shown a significant increase
in numbers, having an estimated population increase as low as 0.5% (Finn
1992). The main cause of the right whale's initial decline is believed
to be over-hunting, however, several reasons have been indicated for
its slow recovery. Such reasons include human impact such as vessel
strikes, entanglement in fishing gear and degradation of critical habitats
(Reeves et al. 1978, Kraus 1990). Low reproductive rates and inbreeding
may also be contributing factors (Brown et al. 1994).
Distribution
and Abundance
Northern right whales occur mainly off the eastern coast of the United
States and Canada (CeTAP 1982). Their general pattern of distribution
shows right whales in the southern Gulf of Maine during the spring moving
northward to the Bay of Fundy and Scotian Shelf for the late summer
and fall (Winn et al. 1986). In the late fall, right whales leave the
northern areas with pregnant females migrating to the waters off Georgia
and Florida where they calve (Kraus et al. 1986a, Winn et al. 1986).
Few males have been sighted in the southeastern United States and currently
the winter distribution of both males and non-pregnant females remains
poorly understood (Kraus et al. 1986a, Winn et al. 1986). Right whales
are most abundant in the Cape Cod Bay and Stellwagen Bank region February
through April with most animals leaving the area by mid-May (although
sightings have occurred in all months of the year except December) (Hamilton
and Mayo 1990). During the 1986 summer season, however, an interesting
shift in distribution was observed. During that time, a major influx
of right whales occurred on Stellwagen Bank and in Cape Cod Bay with
whales remaining in the area through October (Mayo et al. 1988a, Hamilton
and Mayo 1990). This change in distribution was attributed to a major
shift in the prey resources (see Payne et al. 1986 for review) Studies
based on photo-identified individuals have begun to reveal much information
regarding this species' seasonal and annual movements. Right whales
are identified by their unique callosity patterns as well as by prominent
scarring (Kraus et al. 1986b). Many individuals show a high rate of
annual return to the Cape Cod area as well as a high rate of seasonal
residency (Hamilton and Mayo 1990). More than half of the individually
identified right whales in the North Atlantic have been sighted in the
Cape Cod Bay area (Marx 1992).
Feeding
While in the Cape Cod Bay and Massachusetts Bay area, right whales are
often seen feeding on dense patches of plankton. Surface skim feeding
is a commonly observed foraging technique in the Cape Cod Bay area and
entails the whale swimming with mouth open, filtering out large amounts
of zooplanktonic organisms from the seawater with their baleen. The
principal food item for the right whale is copepods, primarily Calanus
finmarchicus. In Cape Cod Bay, animals are known to also forage
on Pseudocalanus minutus, Centropages and larval barnacles
(Mayo and Marx 1990).
Reproduction
Right whales also utilize the Cape Cod Bay and Stellwagen Bank region
as a nursing ground (Hamilton and Mayo 1990). Mother and calf pairs
are frequently observed in the area with many females returning in later
years with another calf (Mayo et al. 1988b, Hamilton and Mayo 1990).
Not much is certain regarding the reproductive biology of the Northern
right whale. Gestation is thought to last approximately one year with
nursing probably continuing for the same amount of time (Katona et al.
1993). Observed calving intervals range from 2 to 7 years with the mean
being 3.67 years (Knowlton et al. 1994). In their study, Knowlton and
others (1994) found that the mean age at first parturition was 7.57
years (ages ranged from 5 to 10 years).
It is
thought that this area is used for courtship activity as well: surface
active groups which consist of two or more whales rolling and exhibiting
prolonged body contact have been observed on many occasions during the
late winter and early spring (Hamilton and Mayo 1990).
Group
Size and Composition
All classes of right whales have been observed in this area. Most sightings
are of single animals, although groups of up to 5 or more have been
recorded (Hamilton and Mayo 1990). No apparent differences in occurrence
or residency by age or sex class have been noted except for mother and
calf pairs which tend to arrive in the area last (Hamilton and Mayo
1990). Overall, more data are needed regarding the social structure
and composition of right whales utilizing the Stellwagen region.
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 (JonsgŒrd
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).
Atlantic
White-sided Dolphins
General
Description and Status
Atlantic white-sided dolphins, Lagenorhynchus acutus, are a playful
species often given to leaping in the wake of boats or riding in the
bow-wave. They range in length from 7 to 9 feet and can be found in
groups containing hundreds of animals (Mayo 1982). Although there are
no current population estimates for this species, it is thought to be
abundant throughout its range (Klinowska 1991).
Distribution and Abundance
This species of white-sided dolphins is found only in the North Atlantic
ranging from Greenland to the mid-Atlantic Bight near Chesapeake Bay
(Testaverde and Mead 1980, Katona et al. 1993). They are commonly found
in the Gulf of Maine and are often observed in the Stellwagen Bank area
spring through autumn (Belt 1987, Selzer and Payne 1988). White-sided
dolphins have been sighted throughout the year in the Stellwagen region,
but peak abundance appears to be July through September (Belt and Weinrich
1985). Using variations in natural markings and scarring, a small number
of animals have been individually identified. Data regarding these individuals
have shown that several animals returned annually to the southern Gulf
of Maine as well as exhibited brief periods of residency within a season
(Mayo 1982, Belt and Weinrich 1985).
Feeding
Feeding appears to be the primary activity for white-sided dolphins
in the Stellwagen Bank area. Prey items include herring, Clupea harengus,
short-finned squid, Illex illecebrocus, silver hake, Merluccius
bilinearis, and sand lance, Ammodytes (spp.) with the latter
being the principal food item in this area (Schevill 1956, Katona et
al. 1978, CeTAP 1982, Mayo 1982). Observers have documented that white-sided
dolphins commonly associate with humpback or finback whales while feeding
(Katona et al. 1978, Belt 1987), although Belt (1987) found that white-sided
dolphins associated with these other species whether or not food was
present.
Reproduction
Mother and calf pairs are frequently observed in the Stellwagen region.
Information from stranded animals suggests that calving mainly occurs
during June and July after a gestation period of approximately 11 months
(Sergeant et al. 1980). Nursing is believed to continue for about 18
months and calves are thought to stay with the breeding herd until 2
years of age (Sergeant et al. 1980).
Group
Size and Composition
In the Stellwagen region, Atlantic white-sided dolphins have been observed
in pod sizes ranging from 15 to 300 animals (Mayo 1982). CeTAP (1982)
reported that the average group size consisted of approximately 50 animals.
In the southern Gulf of Maine, Belt (1987) found that group size varied
between the months of May and September. Between the spring and late
summer, sightings of small groups gradually decreased whereas during
the fall such sightings became more frequent. Little has been reported
regarding the social structure or group composition of Atlantic white-sided
dolphins.
Pilot
Whales
General
Description and Status
Pilot whales, Globicephala melaena, are easily recognized by
their large bulbous heads and jet black coloration. Males can reach
lengths of up to 20 feet however, lengths of approximately 13 feet are
more common (Katona et al. 1993). Pilot whales were hunted off the shores
of Cape Cod up until the 1950's and still periodically strand in large
numbers (McFee 1990). Reliable population estimates for the western
North Atlantic are not available.
Distribution
and Abundance
In the western North Atlantic, pilot whales range from Greenland to
North Carolina (Klinowska 1991). Historical records from strandings
and local hunts suggest that pilot whales were once abundant in the
Cape Cod region during the summer and autumn, however, in recent times,
the majority of sightings now occur in the fall (McFee 1990, Center
for Coastal Studies unpublished data). Historically, the eastern coastline
of Cape Cod Bay has been the site for mass strandings of pilot whales
(Goodyear et al. 1987). Such strandings usually occur during turbulent
weather however, there have been times when strandings have happened
in calm weather. Although no one reason is known for certain, there
have been several suggestions put forth as to why these whales strand.
Most theories include reasons which may interfere with the animals ability
to properly ecolocate such as: turbulent weather which may cloud the
water near shore with sand causing possible "blinding" of
the animals both visually and acoustically; parasites in the inner ear
or possibly local magnetic anomalies. Another theory suggests that,
based on their gregarious nature, pilot whales will respond to a grounded
animal that is emitting distress calls and remain with that individual,
risking stranding themselves. Efforts to restabilize stranded animals
and release them right from the beach have proven successful (Goodyear
et al. 1987).
Feeding
In the Cape Cod region, squid (which may include both Illex and
Loligo spp.) is believed to be the main prey item for this species
(McFee 1990, Katona et al. 1993). Pilot whales are reported to eat fish
and invertebrates as alternate food items as well (Katona et al. 1977).
Reproduction
Mother and calf pairs are observed in the Stellwagen Bank area. Most
information regarding the reproduction of pilot whales comes from stranded
animals or whaling data. Data from the Faroe Islands drive fishery suggest
that mating occurs throughout the year with a peak in summer and that
the gestation period is approximately 12 months (Martin and Rothery
1993). Presently, no breeding or calving grounds have been established
for this species. Calving is thought to occur approximately every 3
years and calves are believed to nurse for nearly 2 years (Sergeant
1962), although Martin and Rothery (1993) reported a calving interval
of a little more than 5 years. Females are thought to reach sexual maturity
around 6 years of age whereas males appear not to mature sexually before
the age of 12 (Sergeant 1962). Little is currently known about the reproductive
habits of pilot whales, however, data from the Faroe Islands indicate
that neither sex disperses from its natal pod and that males rarely
mate within the pods in which they were caught (Amos et al. 1993). This
suggests that pilot whales either mate when two or more pods meet or
when sexually mature males travel to other pods to mate with receptive
females (Amos et al. 1993).
Group
Size and Composition
Pilot whales have been found traveling alone as well as in groups containing
several hundred animals (Katona et al. 1993). CeTAP (1982) reported
an average group size of 20 animals. Data from the Faroe Islands have
shown that all age and sex classes are found within a pod, however,
there are usually more adult females than males (Amos et al. 1993).
Amos and others (1993) also reported that pod members are related and
form very cohesive groups.
Harbor
Porpoises
General
Description and Status
Harbor porpoises, Phocoena phocoena, are the smallest cetaceans
observed in the Stellwagen Bank area. They reach a maximum length of
6 feet. Harbor porpoises are distinguishable by their small size and
triangular shaped dorsal fin. They rarely show much of their body when
they surface and lack a visible spout. The exhalations of harbor porpoises
sound like soft puffing and has given them the nickname of "puffing
pig" (Katona et al. 1993). The abundance estimate for the Gulf
of Maine and Bay of Fundy region (based on 1991/1992 National Marine
Fisheries surveys) is 47,200 animals (see Palka 1994). However, it is
believed that approximately 1,200 harbor porpoise are killed annually
by entanglement in fishing gear, thus the National Marine Fisheries
Service has proposed listing the harbor porpoise as "threatened"
in United States waters (Katona et al. 1993).
Distribution
and Abundance
Harbor porpoises occur from Greenland to Cape Hatteras and it is generally
believed that the porpoises found in the Gulf of Maine belong to a single
population which ranges from the Bay of Fundy southward to Cape May,
New Jersey (Polacheck 1989). Harbor porpoises are most common in the
Stellwagen Bank and Cape Cod Bay region during the spring (CeTAP 1982,
Mayo 1982). According to Mayo (1982), this species is most abundant
in the area between April and May with numbers decreasing by June although,
in some years, there have been sightings recorded in the summer and
fall (Center for Coastal Studies unpublished data). During the summer,
harbor porpoises are most abundant in the northern Gulf of Maine and
are largely absent from the southwestern part, including the Stellwagen
Bank area (CeTAP 1982). In the autumn, most are thought to move southward
along the coast with some harbor porpoises possibly migrating further
offshore (CeTAP 1982, Read et al. 1993a.). The winter distribution of
harbor porpoises is poorly understood although scattered sightings have
been reported throughout the Gulf of Maine as well as around the Cape
Cod area (CeTAP 1982).
Feeding
CeTAP (1982) found only a few sightings of surface feeding by harbor
porpoises. Read and others (1993b) examined the stomach contents of
harbor porpoises killed incidentally in Gulf of Maine fisheries and
found two fishes dominated: herring, Clupea harengus, and silver
hake, Merluccius bilinearis. Squid, Illex spp., as well
as pearlsides, Maurolicus muelleri, and lanternfish, Ceratoscopelus
maderensis, were also found.
Reproduction
In the Gulf of Maine, calves have been sighted from Nantucket northward
to the Bay of Fundy with the majority of calf sightings occurring in
the north during the summer (CeTAP 1982). Data from harbor porpoises
incidentally taken in Bay of Fundy fisheries indicate that harbor porpoises
calve in mid-May after approximately 10 to 11 months of gestation (Read
1990a). Sexual maturity is thought to occur around 3 years of age and
females probably calve annually (Read 1990b).
Group
Size and Composition
In the Stellwagen region, Mayo (1982) reported harbor porpoises in group
sizes ranging from 2 to 30 individuals. In the Bay of Fundy, groups
of 8 to 12 animals have been observed in the fall (Gaskin 1982). Large
groups of harbor porpoises have rarely been reported in the western
North Atlantic, however, Hoek (1992) reported observing a large aggregation
of approximately 800 harbor porpoises spread out over a 2 kilometer
area in the Jacques Cartier Passage, Gulf of St. Lawrence, Canada. The
porpoises occurred singly or in small, cohesive pods containing 3 to
10 animals. They were actively diving throughout the area for nearly
2 hours and then seemed to enter a resting period (remaining motionless
at the surface) which they maintained for approximately 2 more hours
before finally dispersing. The social structure and group composition
of this species remains largely unknown.
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