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    Toothed Whales

     
         
     

    The different species within the suborder Odontoceti share at least one characteristic in common; they all have teeth of some kind inside their jaws. This very diverse suborder of mammals includes over sixty species that differ widely in size, body design and behavior. From giants like the sperm whale to the diminutive harbor porpoise, the Odontoceti have radiated throughout the marine (and riverine) habitats of the world yet share a common ancestry. 

    Differing from baleen whales in how they obtain food, toothed whales are designed for catching one food item at a time. Most have a battery of similarly shaped teeth to grip slippery fish, squid or other invertebrates. There are a few toothed whales that have lost functional teeth completely as adults, like the beaked whales and others that have highly modified teeth for purposes other than feeding, like the narwhal. 

    Many toothed whales are highly social animals, moving around in groups called pods. Different species and different populations within a species may vary in how these pods are organized. Some pods may be stable relationships between individuals over long periods of time or, may represent seasonal associations surrounding feeding or reproduction. 

    It is believed that many if not all, toothed whales have the ability of echolocation, for navigation and finding food. The main tools of echolocation include a good sense of hearing, a means of producing sound and a way of directing that sound to the surrounding environment. 

    diagram of harbor porpoise head showing skull, nasal  
    passages in yellow, melon in red and inner ear in 
    orange. red arrows: sounds leaving; 
    orange arrows: sounds returning

    The exact manner and production of echolocation is still poorly understood but, it is believed that air, passing out of the lungs and through a series of convoluted air sacs along the nasal passage produce vibrations, or audible clicks. These clicks may be directed out forward of the whale by a flexible, oil-filled organ called the melon (creating the distinctive forehead profile of many toothed whales). These sound waves travel out into the environment bouncing off of anything that is denser than the surrounding water. These returning sound waves may travel along the nerve and oil-filled channel of the hollow, lower jaw to the inner ear. These signals can be processed as a changing image of the environment: moving schools of fish, other whales, predators, etc. Despite the common darkness of marine environments, toothed whales have a means of “seeing.” 

    Just how this ability differs between species is still unknown but may represent the overall diversity of the suborder. Stellwagen Bank gives us a glimpse of this variation. Sightings of different species vary from year to year and day to day and may demonstrate cyclical changes on the Bank or the wanderings of toothed whales searching for food. Some of these whales are regular visitors while others are seen only rarely. 

     

    Species descriptions:

    The following descriptions include species that have been seen on

    Stellwagen Bank;only three of the species are found with any regularity.

     

    Sperm whales, Physeter macrocephalus, are the largest of the toothed whales, growing up to 18m (60 feet) in length. They are animals of deep water, usually off the continental slope, well away from the relatively shallow waters of Stellwagen Bank. Sightings in our area are extremely rare, usually amounting to a stranding of lone individuals along our beaches. 

     

    Beluga, Delphinapterus leucas, have been sighted in our area on occasion. Individuals from the St. Lawrence, Canada, population may follow cold water currents south (as far south as Long Island, NY). In 1971, a medium-sized, white whale with no dorsal fin was spotted inside the Cape Cod Canal. Only a wandering beluga could fit that description. 

     

    Orca, Orcinus orca, are the largest species of the dolphin family. Up to 9m (30 feet) these massive black and white predators use Stellwagen Bank and surrounding waters only rarely. Over the years most sightings of orca in our area have occurred in August and September, perhaps tied to the end of the northward run of bluefin tuna. Different social groups of these whales may specialize on different prey items in the Gulf of Maine, including herring, cod and other whales, like minkes and humpbacks. Almost nothing is known about these populations: where they come from, general movements, social structure, etc. Sightings of orca are sporadic at best; many years may pass between sightings. 

     

    Long-finned pilot whales, Globicephala melaena, are seasonal residents of our area but, like most other toothed whales, their abundance from year to year depends upon the presence of their favored prey. These whales are sexually dimorphic in size and, to some extent, shape. Males tend to be larger than females, growing up to 6m (20 feet), weighing up to 2.5 mt (3 tons) and develop a more pronounced ‘pothead’ and more rounded dorsal fin. They are all black (hence the common name ‘blackfish’) except for a light anchor patch on the belly between the flippers. Some may have faint gray markings behind the eyes or behind the dorsal fin. 

     

    long-finned pilot whale; note the large, low dorsal fin; round head; long, curved flippers and black skin

     

    Throughout much of the year, pilot whales concentrate along the continental slope feeding on squid. In turn, the squid concentrate around the rich upwellings along the slope, feeding on schooling fish like herring and mackerel. As the schooling fish migrate inshore during the late summer and fall, so do the squid and pilot whales. Adult female pilot whales may direct the tight knit pods, numbering from less than a dozen to over a hundred, to the changeable feeding grounds. In some populations, pilot whale calves may remain in their maternal pods. To reduce inbreeding, many pods may form massive herds, especially in early summer. Gestation lasts 16 months and calves may nurse for more than a year and a half. The calving interval may be over three years and life span is estimated at about 45 years. 

    Sightings of pilot whales on the Bank can occur throughout the year with a peak in fall. As pilot whales head inshore to hunt they can be sighted from land. Such sightings often preclude a mass stranding where entire pods come ashore. Records show that such strandings have occurred throughout history but reactions toward these strandings have changed. Until the 1920’s Cape Cod communities would actively herd pilot whales toward shore or take advantage of strandings for meat and oil. Large scale human efforts today work at returning the whales to sea or reducing the amount of suffering. 

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    White-beaked dolphin, Lagenorhynchus albirostris, are closely related to the white-sided dolphin and, like them, are found only in the North Atlantic. White-beaks tend to be longer, heftier animals, uniformly gray with a swath of light gray or white across the flanks and tail stock. The belly and, sometimes the beak, are white. 

     

    Little is known about this species as sightings and strandings are quite rare on this side of the Atlantic. Pods tend to be smaller in number than the white-sides and have been seen moving in echelon formation: side by side as a front. Their diet seems to be more tied to squid than fish and sightings may be correlated to the abundance of these invertebrates. 

     

    Atlantic white-sided dolphin, Lagenorhynchus acutus, are a colorful cetacean endemic to the North Atlantic, between northern Europe and Cape Cod. Males grow up to 2.5 meters (8 feet) and may weigh up to 225kg (500 pounds), females tend to be slightly smaller. These are relatively stocky animals, built for the open ocean. The rostrum, or beak, is short; the flippers are small, curved and pointed; the dorsal fin is large and curved back; the tailstock is laterally compressed, forming a keel. The complex pigmentation pattern includes the dark gray of the back and flippers, light gray along the sides and white belly. A swath of white runs across the flanks; a patch of tan or yellow runs the length of the tailstock. 

     

    a white-sided dolphin hunting silver hake (whiting); from the surface, look for the swaths of white, gray and yellow and the tall, curved dorsal fin

     

    Prior to the late 1970’s, white-sides were relatively uncommon in our area and white-beaked dolphin, common. Both of these related species have a somewhat varied diet but differ in their preferences: white-beakeds tend to favor squid while white-sides favor small, schooling fish. About two decades ago, sand lance populations exploded on Stellwagen and sightings of white-beakeds became rare while white-sides increased.

     

    Pod structure seems to be based upon closely related females, accompanied by calves of all ages and a few unrelated males. These highly mobile groups, constantly on the move, seek out prime feeding grounds that change quickly over time. In other words, these dolphin are not permanent residents of the Bank. They range widely throughout the Gulf of Maine and are sighted where food, such as herring and sand lance, are most abundant. Gathering at good feeding sites, many pods may form herds numbering in the hundreds. These superpods are unstable and break apart quickly as the local food supply dwindles. Pods may also join other species of whales during feeding, such as humpbacks, finbacks and pilot whales. 

    New calves are most commonly seen in May, June and July. Females may start calving at age five after an eleven-month gestation and calve every 2.5 years thereafter. Calves nurse for up to 18 months. Female calves may remain in their natal group for life while males migrate out. 

     

    Migration is still poorly understood and may be characterized as inshore for winter, offshore for summer. In early fall (August) a few scattered pods may be sighted becoming more common through late fall and winter. By mid-April most pods move off, perhaps to more offshore and northern feeding grounds. Mass strandings are most common in fall and spring. 

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    Common dolphin, Delphinus delphis, are a more offshore species, preferring the warmer, deeper waters south and east of Georges Bank. A few live individuals have been sighted over the years in our area, especially during the summer months. More often, stranded individuals may come ashore during the winter. 

     

    Bottlenose dolphin, Tursiops truncatus, are large, robust dolphin found in cool water habitats further to the south of our area. At least two forms of bottlenose exist: the larger offshore populations and the smaller, more familiar inshore populations. It is not clear whether the few live sightings of individuals in our area are of the offshore or inshore forms. Both forms occasionally strand along our coasts. 

     

    Risso’s dolphin, Grampus griseus, are animals of warmer, deeper waters to the south of our area. They are believed to be squid hunters and the few sightings of live individuals may represent strays during warm water episodes or during northward movements of their favored prey. A few individuals have been found stranded on Cape Cod beaches. 

     

    Harbor porpoise, Phocoena phocoena, are among the smallest cetaceans in the world reaching 1.7m (6 feet) and 63kg (140 pounds) in weight.  Their short, stout design is built for diving. The dorsal fin is relatively low; a shallow triangle. Flippers are small and rounded. The dorsal surface tends to be dark, brownish-gray lightening toward the belly. They often have a dark line running from the mouth to the base of each flipper and a light blaze on the sides, forward of the dorsal fin. Their teeth, like all true porpoise, are spoon-, or spade-shaped (unlike the sharp, peg-like teeth of dolphin). They range through the Northern Hemisphere of the Atlantic and Pacific Oceans

     

    These are truly coastal animals and are only rarely found crossing Stellwagen. More often they are spotted around harbors while heading out for whale-watches or research cruises. They tend to be shy, inconspicuous animals that are difficult to spot. Despite their size and more coastal affinities, harbor porpoise are prodigious divers, reaching down to 230m (760 feet) in search of prey. Like most marine mammals, porpoise are opportunistic feeders, taking advantage of whatever is locally abundant. Yet the distribution and movements of porpoise in the Gulf of Maine seems to be intimately tied to the annual movements of different species of herring. As herring move toward spawning rivers in spring and early summer, harbor porpoise are likely to follow. As the young herring head out to sea so do the porpoise (sightings of porpoise in the Gulf of Maine are very rare during the winter months; many may head to offshore waters or to the south, as far as the 

     

    a harbor porpoise feeding on herring - 
    note the low, triangular dorsal fin; small, rounded flippers 
    and lack of a prominant beak or rostrum

     

    Females reach sexual maturity at about the age of three and may calve each year thereafter. Unlike many toothed whales, females are, on average, larger than males. As small cetaceans, the marine environment especially challenges harbor porpoise: small animals lose heat more quickly. Consequently, harbor porpoise may consume more than thirty times their own weight of food in a year and their life span is relatively short (less than 20 years). 

     

    As coastal animals tied to a relatively restricted diet, harbor porpoise populations are susceptible to a variety of human disturbances. Some of the highest concentrations of industrial pollutants have been found in tissue samples of porpoise, including large loads of PCB’s and heavy metals. Entanglements in gillnets pose a serious threat to the population throughout the Gulf of Maine. In 1993, the National Marine Fisheries Service proposed to nominate the species as threatened under the Endangered Species Act, but has not yet acted on the proposal. 

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