I. BIOLOGY AND NATURAL HISTORY
All dolphins are toothed whales belonging to the sub-order, odontocetes, of the order cetacea (see section on humpback whales for more information). As a group, dolphins are often referred to as “small” cetaceans, even though some of them are quite large, attaining lengths of over 20 feet. In addition, although the terms dolphins and porpoises are often used interchangeably, they really refer to two different types of animals.
Porpoises belong to the family Phocoenidae. They are generally smaller and more robust species. Most attain about 5-7 feet in length. Porpoises have no distinct beak, or rostrum. Their foreheads slope almost uniformly to the tip of their snout, and their teeth are spade-like in shape. The family Phocoenidae is rather small, and consists of only six members. There are no porpoises found in Hawaiian waters.
Dolphins belong to the family Delphinidae. Dolphins possess a distinct beak. Their teeth are conical in shape. Most species of dolphins are larger than porpoises, with the males usually being larger than the females. The family Delphinidae is the largest and most diverse family of the cetacean order and includes 26 living species. Several species of dolphins are found in Hawaiian waters.
Below is the biological classification for the common dolphin:
Kingdom Anamalia Phylum Cordata (vertebrates) Class Mammalia Order Cetacea Sub-order Odontoceti Family Delphinidae Genus Delphinus Species delphis
In addition to being found around Hawaii, dolphins of some kind occupy virtually all oceans and major seas as well as some large river systems. Their distribution, however, is not random. Each species has become specialized to fit into a particular niche. A niche relates to all aspects of a species’ way of life, including not only its physical home but also its food, behavior, predators and physical environmental factors necessary for its survival. In short, the niche defines a species’ role within an ecosystem. For each dolphin species this role is unique.
In terms of their feeding habits, all dolphins are carnivores. Some feed exclusively on either fish or cephalopods (the class of marine invertebrates including squid, octopus and cuttlefish), while others have a more varied diet including fish, squid, crabs, shrimps and lobsters.
Like the other marine animals we have examined so far; the Hawaiian monk seal, the green sea turtle and the humpback whale, dolphins have become marvelously adapted to life in the sea. Anatomically, their bodies have become streamlined to move effectively in their aquatic environment. Their hind limbs have disappeared, their front limbs have developed into flippers, and their powerful tail provides their chief means of propulsion. While many claims have been made about the swimming speeds of dolphins, it appears that the rate of speed dolphins attain is closely related to their feeding habits. Coastal species that feed on slow moving prey rarely exceed speeds of 10 mph, and oceanic species that feed on fast-moving fish generally attain speeds of 15 mph, although bursts of speed up to 25 mph have been recorded. The way that dolphins are able to achieve such high speeds is by leaping from the water in a series of dives and spending as little time as possible under the water. This is known as “running”. Dolphins can attain greater speeds by riding the bow wave of a fast-moving vessel than they are able to on their own.
Another factor that increases dolphins’ swimming speed by reducing their drag in the water is the smooth skin they possess. Unlike most mammals, a dolphin’s skin is hairless, thick and lacks glands. It is also kept smooth by constantly being sloughed off and replaced. A bottlenose dolphin for example, replaces its outermost layer of skin every two hours. This is nine times the rate of human skin renewal. A drawback of their smoothness, however, is that their skin is easily scarred. Virtually all adult dolphins have an array of scars, notches and nicks that they acquired through interactions with companions, enemies or the environment. Scars on dolphins are so prevalent in fact that researchers often rely on them as a means of identifying individual animals. Like all other marine mammals, below the skin, dolphins have developed a thick layer of blubber to insulate them from heat loss.
The life cycle of dolphins is similar to that of other cetaceans. As mammals, dolphins bear live young and the mothers nurse them on milk and provide care. A dolphin calf is born tail-first with eyes open, senses alert and enough muscular coordination to follow its mother immediately. At birth, the mother helps her calf to the surface to get its first breath. While nursing lasts between one and a half to two years, the mother will remain with her calf for a period between three and eight years. There is some variation in the age at which sexual maturity is reached, the reproduction rate and the life expectancy among the different species of dolphins. Most species tend to bear one calf every other year or so during their reproductively active years and are believed to have an average life expectancy of about thirty years.
As anyone who has had the opportunity to watch dolphins perform in a show can attest, dolphins have an impressive ability to learn and imitate behaviors, often for what appears the sheer pleasure of doing so. This observation, together with their large brain size, has led to numerous studies of dolphin intelligence. Dolphins’ brains are about the size of our own. Size alone, however, is not always a reliable indicator of intelligence. Elephants, for example have brains four times the size of humans, but we do not consider them to be four times as smart! Scientists believe a more accurate factor in determining a species’ level of intelligence is the ratio of brain weight to spinal chord weight. In fishes, the brain weighs less than the spinal chord; in cats the ratio is 5:1; in apes, 8:1; in humans, 50:1; and in bottlenose dolphins, about 40:1, suggesting that dolphins have a level of intelligence comparable with humans.
Another characteristic used to determine the level of intelligence is the amount of folding in the cerebral cortex, the portion of the mammalian brain associated with thinking and reasoning. A cerebral cortex which is more deeply folded has a greater surface area available for thinking. Some species of dolphins have brains that are more deeply folded than human brains, although the cortex itself is not as thick. The level of folding in dolphin brains again suggests that they have a level of intelligence comparable to ours.
When discussing the intelligence of dolphins, or other species for that matter, it is important to realize that the environment in which they live is often very different from our own. We must be careful not to place our standards of intelligence on other species, or assume that they “think” the same way we do. Dolphins may require completely different types of mental abilities for survival in their watery home. Sound and light, for instance, travel very differently in water than they do in air. Earthtrust’s Project Delphis actively explores dolphin cognition from its underwater laboratory in Hawaii. [Transfer to Earthtrust’s Project Delphis Web Page]
The speed of sound in water is roughly four times greater than it is in air. In addition, sound waves are able to bend around corners and pass through objects and can be detected at any time of the day or night. On the other hand, water is much worse for vision as vision depends on the presence of light, and the sea is generally dark and shadowy except for regions near the surface. Although dolphins are believed to have fairly good eyesight, their visibility is often limited by their dark and murky environment. Not surprisingly, dolphins and whales have come up with an efficient way to combat this problem. They tend to rely chiefly on their sense of hearing to understand the world around them, much as humans rely on a combination of sight, sound and smell.
Dolphins and many species of toothed whales use their sense of hearing in a very sophisticated behavior known as echolocation. Echolocation is a process where a dolphin emits a steady series of split-second “clicks” through its blowhole. The “clicks” are pulses of ultrasonic sound (sounds repeated as rapidly as 800 times/second) produced in a dolphin’s nasal passages and focused in a large, lens-shaped organ in the forehead known as the melon. The melon concentrates the sound pulses into a directional beam. When the outgoing sound waves or “clicks” bounce off objects in their path, a portion of the signal is reflected back to the dolphin. The bony lower jaw of the dolphin receives the incoming sound waves and transmits them to the inner ear where they are converted into nerve impulses and then transmitted to the brain.
Through echolocation, a dolphin is able to determine the distance of a target on a continuing basis by measuring the time between emitting the clicks and their return. Dolphins regulate their rate of click production to allow the returning “echo” to be heard between outgoing clicks. Using this amazing skill, a dolphin can create an acoustic picture of its surroundings and can determine the size, shape, direction of movement and distance of objects in the water. This permits dolphins to hunt prey over a greater range than the limits of visibility allow.
Dolphins are characteristically very social creatures and often depend on social interaction for the purposes of hunting prey, defense and reproduction. Unlike humpbacks and other species of baleen whales, dolphins tend to form long-lasting groups that range in size from a few animals (2-40) called pods, to larger groups of up to several hundred members, known as schools or herds. Such groups may consist of more than one species, with little competition for resources, as each species occupies a different niche. Spotted dolphins and spinner dolphins are found living in such an association. Scientists believe this is possible because spotted dolphins tend to feed on larger species living near the surface, while spinner dolphins tend to feed at night on smaller species found in deeper waters.
Dolphins feed mainly on schools of prey. As a result, most species have developed communal and cooperative hunting practices, as searching for food as a group is more efficient than searching individually. There are some exceptions to this, such as the river dolphins that feed on individual prey on the river bottom. They are often found living alone or in very small groups. Most dolphins move in groups that are wider than they are long. This allows them to scan as large an area as possible using echolocation. The size of the groups may be determined by the number of dolphins able to be sustained by the school of prey.
Many dolphin groups appear to have rigid hierarchies of power with a few individuals considered dominant. Large groups are often mixed in terms of age and sex, but smaller groups are generally one of three types: 1) a nuclear group, comprised of a single adult male and female; 2) a nursery group, consisting of a number of females and young; and 3) a bachelor group comprised of adult and younger males. Regardless of the type, all groups of dolphins seem to have well-developed skills in cooperating and working together as a team whether it be for the purpose of finding food, mates or caring for their young.
II. A CLOSER LOOK AT NAI’`A
The Hawaiian name for dolphin is nai’`a , and refers to all species of dolphins found in Hawaiian waters. Four species of dolphins are regularly found in the waters around Hawaii. They are the Pacific bottlenose dolphin, the Rough-toothed dolphin, the Spotted dolphin and the Spinner dolphin. Other dolphin species have been known to pass through the islands but are relatively rare. Let’s take a closer look at each of these species.
Pacific bottlenose dolphin (Tursiops truncatus)
Known for their inshore habits, playfulness around vessels and star performances at oceanariums, bottlenose dolphins are probably the most popular of all cetacean species.
Adults range in size from seven to eleven feet in length and weigh between 600 and 850 pounds. Their backs are medium gray, their sides are lighter gray and their bellies are white or pink. Offshore animals are darker in color than those found inshore and sometimes appear to be less interested in swimming along with boats.
A few thousand bottlenose dolphins are believed to inhabit the waters around Hawaii, usually living in groups of two to fifteen individuals. Most of these groups are permanent residents of certain coastlines and harbors, and are therefore easy to spot.
Rough-toothed dolphins get their name from the “wrinkled” surface of their teeth, which have many fine vertical grooves running from the gumline to the tip. It is difficult to identify this species from a boat on the basis of this characteristic, but there are several other external features that should prove helpful.
Rough-toothed dolphins are dark gray to purplish black in color with pink and white spots and streaks covering their bodies. This unusual speckling led early researchers to refer to it as the “polka dot” or “calico” dolphin. Although their body size and proportion is much like bottlenose dolphins, rough-toothed dolphins can easily be distinguished by their longer and more pointed snouts that are distinctly white underneath and at the tip.
Rough-toothed dolphins are fairly common in Hawaii, but they are found mostly in offshore waters of 6,000 feet or more, so they are usually only seen by deep-sea fishermen or pleasure boaters. They tend to travel in small groups comprised of three to four individuals, but there can be several groups in one area.
Spotted dolphins are smaller than either bottlenose or rough-toothed dolphins, measuring between six and seven feet in length, and weighing about 240 pounds with males being larger than females.
Their coloration is dark gray on the back and lighter gray below, with a characteristic dark “cape” extending from the forehead to the dorsal fin. Spotting, if present, consists of light spots on their darker areas or, dark spots on their lighter areas, and is more prevalent in older individuals than in juveniles. This species can also be identified by the slender white-tipped snouts they possess.
Spotted dolphins are extremely social creatures and form herds of a few dozen to more than a thousand. Around Hawaii, they are commonly sighted in the channels between the islands.
Hawaiian Spinner Dolphins-Photo
Spinner dolphin (Stenella longirostris)
Spinner dolphins are the smallest of Hawaii’s common dolphins. They are generally between five and six feet in length and weigh 130 to 200 pounds. Hawaii has its own subspecies that is easy to recognize by its distinctive “three-tone” color pattern which consists of a sharply defined dark gray “cape” on their backs, a stripe of lighter gray on their sides and a white or pink belly.
This species gets its name from its spectacular habit of leaping high into the air and spinning several times on their tails before falling back into the water. Researchers are not sure why the dolphins spin, but most people who have had the opportunity to watch the dolphins don’t seem to mind, and find it a real treat.
Around Hawaii, spinner dolphins congregate at night in large herds in the deep channels between the islands to feed. During the day, they break up into smaller groups and come near shore to rest and play. One of the places where they can commonly be seen is in Kealake’akua Bay on the island of Hawaii.
While none of the dolphins found in Hawaiian waters are considered threatened or endangered, species found in other parts of the world are not as lucky. Some species of dolphins such as the vacquita off Mexico, the harbor porpoise found throughout the northern hemisphere and several of the river dolphins of Asia and South America, are not as lucky. Some are in real danger of extinction.
III. FACTORS AFFECTING DOLPHIN POPULATIONS:
A. The Tuna/Dolphin Issue
For reasons not completely understood by scientists, dolphins and yellowfin tuna are often found swimming together, with dolphins closer to the surface and the tuna beneath them. It is thought that the tuna may be trying to take advantage of the dolphin’s superior food-finding talents. Fishermen have long known of the association between the two species, and traditionally have used the presence of dolphins as a lead to finding schools of tuna. In the days when tuna were caught by the pole and line, this presented no problems as dolphins were rarely involved or injured. This changed dramatically in the 1950’s when a fishing technique known as purse-seining was introduced.
In purse-seine fishing, schools of dolphins are first spotted, indicating to fishermen that schools of tuna are most likely swimming beneath them. The dolphin schools are then chased by small high-speed boats or even helicopters that accompany the fishing boats. When the dolphins begin to tire, the fishermen encircle the school with huge nylon nets that are up to one mile long and 325 feet deep. When both the dolphins and the tuna have been completely surrounded, the bottom of the net is pulled closed, much like a drawstring purse, hence the name purse-seining. Purse-seining has proven to be an extremely effective method of catching fish. Entire schools of tuna are able to be scooped up without a single fish escaping. Unfortunately, many dolphins are also killed in the process, as they become entangled in the nets and drown, or are crushed as the nets are pursed and hauled in. Every year, approximately 20,000 dolphins are believed to die in purse-seine nets. The method of fishing is especially predominant in the Eastern Tropical Pacific Ocean, a region which extends from southern California to northern Chile.
Not all dolphins are affected by purse-seining; only certain species associate with tuna in the Eastern Tropical Pacific. Tuna do not appear to associate with dolphins in other areas of the world. In addition, not all tuna in the Eastern Tropical Pacific is caught using purse-seines. The primary species of dolphins that are affected include spotted, spinner and common dolphins. Purse-seining is believed to have dramatically reduced populations of these species.
Driftnets and gill nets (see section on humpback whales) are other fishing technologies responsible for killing hundreds of dolphins each year. More dolphins are killed each year by being caught incidentally in these fishing nets than by any other means. The use of driftnets on the high seas was banned in 1993 through an international driftnet moratorium, but gill nets remain legal, and continue to pose a threat to coastal species. Earthtrust’s DriftNetwork program has for many years been leading the attack on this destructive fishing technology. [Transfer to Earthtrust’s DriftNetwork Web Page]
B. Hunting/Subsistence Use
Thousands of dolphins and small whales throughout the world continue to be hunted for food, oil, fertilizer and other products.
In Chile, some of the world’s most endangered dolphins, the black dolphins, are hunted to provide bait for crab pots for the king crab industry. Catches of several thousand have been reported in recent years. Turkey has greatly reduced dolphin populations in the Black Sea to produce oil and chicken feed. In the Faroe Islands and parts of Asia, a type of fishing known as “drive fishing” has been practiced for centuries by fishermen who view the fish-eating marine mammals as competition for their catch. The fishermen use boats and large nets to “drive” schools of dolphins into shallow bays and harbors where they are killed, often brutally. The meat is often sold for human or animal consumption, or made into fertilizer. Japan is believed to be the largest consumer of dolphin.
In addition, several countries continue to allowing hunting of dolphins for aboriginal or subsistence purposes. Subsistence hunting takes place in the Solomon Islands of the Pacific, in Greenland and in Arctic parts of Canada and the United States. The dolphins killed are to be used for food, oil or other traditional uses and cannot legally be sold. Unfortunately, this has proven difficult to regulate and many subsistence hunters have bowed to commercial incentives. Some of the species involved in subsistence hunting are among those most endangered such as the narwhal, beluga whale and harbor porpoise.
Ancient Hawaiians occasionally harvested dolphins by herding them to the beach. They were apparently used as a food source as they were one of the foods listed forbidden to women under the kapu system.
C. Habitat Degradation
What may turn out to pose the greatest threat to dolphins (and other marine animals for that matter) is our contamination of the marine environment- their home. Pesticides, PCBs, heavy metals, plastic particles, radioisotopes and other industrial wastes are released into our oceans, bays and rivers in unknown amounts daily. Many of these pollutants do not break down in the environment, or take a very long time to do so.
Through a process known as biomagnification these toxins are absorbed and accumulate in organisms in lower levels of the marine food chain. In feeding on these lower level organisms, animals in the second trophic level receive a higher dose of the toxins and accumulate even higher concentrations in their tissues. Animals such as dolphins that are near the top of the marine food chain, receive even higher doses as they feed on animals of the second and third trophic levels. Dolphins living in regions where toxins are found tend to accumulate high concentrations in their blubber. The toxins, which are resistant to chemical breakdown are thought to lower the animal’s reproductive success and weaken their immune systems, making them more susceptible to disease.
For species with a wide geographical range, environmental degradation may not be as critical, but those species that have a limited or localized distribution may become quite vulnerable. The populations of several of the river dolphins have been drastically reduced by a combination of man-made factors including pollution, damming, increases in boat traffic and shoreline development.
In addition, many dolphin deaths have been attributed to swallowing man-made objects including balls, nets and pieces of plastic.
D. Public Awareness
Each year thousands of people around the world are introduced to dolphins and the problems they currently face through their visits to aquariums, zoos and marine parks. Education can be a very powerful tool as learning about a particular animal is often the first step towards becoming active in its conservation. As noted dolphin researcher Ken Norris states, “Our knowledge of these dolphins is their protection. You cannot kill what you come to love.”
Public education is proving effective in some places where dolphins had traditionally been considered to be undesirable predators or competition to fishermen. Changes in public sentiment towards dolphins and other marine mammals have occurred as a result of people learning more about them. Once public sentiment changes, political sentiment is often soon to follow, bringing about the establishment new laws and regulations needed to protect species.
Such was the case with the consumer boycott on tuna initiated in 1989 by several conservation organizations in the United States. By teaching the public about the number of dolphin deaths associated with the use of driftnets and purse-seines, the conservation groups were able to convince consumers to refrain from buying canned tuna that was caught using either of these fishing methods. This in turn, put economic pressure on the tuna companies to refrain from catching tuna using driftnets or purse-seines, and led to the development of the many “dolphin-safe” labels you see on tuna cans in the supermarkets today. By displaying a “dolphin-safe” label on their can, the tuna companies are helping consumers exercise their choice for dolphin protection. Earthtrust’s Flipper Seal of Approval program is an innovative program for licensing dolphin-safe tuna firms. [Transfer to Earthtrust’s Flipper Seal of Approval Web Page.]
In addition, the United States government passed the Dolphin Protection Consumer Information Act (DPCIA) in 1990 which sets firm criteria tuna must meet before being able to be labeled “dolphin-safe,” and in 1992, the International Dolphin Conservation Act, (IDCA) which forces a rapid phase-out of purse-seining on dolphins by all countries through the threat of United States trade embargoes and trade sanctions against those countries that do not abide.
IV. PROTECTIVE MEASURES
A. Legal Protection within the United States
In the United States dolphins are protected under the Marine Mammal Protection Act (MMPA) which prohibits the hunting, harming or harassing of any marine mammal by any person under legal jurisdiction of the United States (see Appendix 2 for more detailed information on and MMPA) and under the International Dolphin Conservation Act of 1992.
The International Dolphin Conservation Act (IDCA) is a federal law passed by Congress in 1992 that is designed to bring about a rapid phase-out of all purse-seines set on schools of dolphins. The way the Act is designed to work is by providing countries that are currently purse-seining on dolphins with economic incentives to stop. Once the Act goes into effect in 1994, any country that continues to set purse-seines on schools of dolphins in order to catch their tuna will have trade embargoes placed on them by the United States. This would mean that the United States would refuse to buy any seafood products from these countries, until they are able to reliably prove they have stopped purse-seining. The Act also establishes the United States as an official “dolphin-safe zone,” making it illegal for any person to sell, purchase, offer for sale, transport, or ship tuna or tuna products that are not considered dolphin-safe, and provides three million dollars to fund research on alternative fishing technologies. The IDCA is truly a landmark in legislation and is important to the conservation of dolphins and other marine life.
A limited number of dolphin species that are listed on the federal endangered species list also receive protection under the Endangered Species Act (again, see Appendix 2 for more detailed information).
B. International Legal Protection
International protection of dolphin species considered to be endangered or threatened is provided under CITES, the Convention on International Trade of Endangered Species of Wild Fauna and Flora (see Appendix 2 for more information on CITES).
SUGGESTED ACTIVITIES FOR TEACHERS
– Lead a discussion about niches. Ask each student to define their own particular “niche.”
– Much of a dolphin’s life revolves around finding food. As a class, make a list of all the adaptations (anatomical, behavioral, physiological) dolphins have undergone to successfully gather all the food they need.
– Four species of dolphins are commonly found in Hawaiian waters. Ask each student to draw a picture of their favorite “Hawaiian” dolphin and write a brief essay explaining why they like that dolphin the most.
– Several methods of catching fish have been determined to be harmful to dolphins (ie., purse-seining, driftnetting). Knowing this, why do you think some countries continue to use these fishing techniques? What would YOU do to try and convince them to stop?
– Discuss the effects of biomagnification in the marine food chain. What effects could this potentially have on humans?
– Lead a discussion about subsistence hunting of dolphins and other marine mammals. Ask students to give reasons why they are in support of subsistence hunting or why they are against it.