Dolphins - The Oracles of the Sea
Evolution and Taxonomy Behaviour
Anatomy Human and Dolphin
Table of Contents Previous Page Next Page Bottom of Page

Contributions for this Page Discussion Forum

Go to:
  • Photo library
  • Diagram library
  • Media library

  • Threats to Dolphins

    1. Types of Threats
      1. Directed Takes
      2. Incidental Takes
      3. Habitat Degradation
      4. Pollution
      5. Disturbance
    2. Specific Cases
      1. Eastern Tropical Pacific Tuna Purse-Seine Fishery
        1. The Root of the Problem
        2. Dolphin Behavior in the Nets
        3. History of the Fishery
        4. Attempts to Reduce Kills
        5. New Concerns
      2. Black Sea Fishery
      3. Chilean Crab Bait Fishery

    As with a lot of other types of animals, the advancement of human civilization has started to threaten the survival of many types of small cetaceans. Two threats that profoundly affect small cetacean populations are direct taking, where the target of the hunt is the dolphin being caught, and incidental taking, where dolphins are caught or killed as a side effect of the method being used to catch other species. In addition, many species are affected by habitat degradation, pollution, and disturbance. River dolphins are especially vulnerable to these threats; the most endangered species of small cetacean is the Yangtze river dolphin or Baiji.

    Back to Outline Types of Threats
    Back to Outline Directed Takes

    In recent years, there has been a general increase in the directed taking of small cetaceans in lesser developed countries. These new and expanding fisheries require careful, but tactful monitoring. Many fisheries are for cetaceans in general rather than for specific types, so managing them on the basis of individual species is difficult. In Japanese waters, there is extensive directed take, mainly for human consumption. Some fisheries are for certain species of small cetacean, while others are generic. In South America, some of the resident species are hunted for crab bait, and many of those species are already vulnerable to incidental take. Sri Lanka has many generic small cetacean fisheries which might threaten some local populations.

    Back to Outline Incidental Takes

    Incidental takes most often occur when specimens get caught in large nets set to catch fish. Unable to escape or reach the surface, they drown and perish. Certain types of fishing gear, such as set nets, are particularly dangerous to small cetaceans. This problem may represent a greater threat to cetaceans than directed takes. For several populations, including the Baiji of the Yangtze River, the hump-backed and bottlenose dolphins off eastern South Africa, and the striped dolphins of the Mediterranean Sea, the mortality rate due to incidental taking is greater than the maximum amount that the population can sustain without significant decline. The effects of this type of threat on the dusky dolphins of the eastern South Pacific and the northern right whale dolphins of the central North Pacific is also a source of concern.

    A major concern is large-scale monofilament pelagic driftnetting. It causes overexploitation of the fish population, and marine mammals and seabirds often get entangled in them. Also, discarded nets, sometimes called "ghost nets," remain active for several years, during which they continue to entrap cetaceans. In a resolution made on 22 December 1989, the United Nations General Assembly agreed to end pelagic driftnetting by the end of June, 1992 unless and until procedures to reduce the problems of this type of fishery could be implemented. Some countries took steps to forbid any use of pelagic driftnets within 360 kilometers of their coasts.

    Back to Outline Habitat Degradation

    The encroachment of human civilization on natural habitats also represents a serious threat to the survival of some stocks. Coastal development may destroy fish nursuries, eliminating the food supply of the local cetaceans. Coastal mangrove swamps, found in tropical regions, are a very important habitat for some species of cetecean, especially the hump-backed dolphins of South Asia and West Africa. These areas are both fish nursuries and shelters for adult fish. Continued urban development and industrialization threatens these regions. The damming of rivers on such systems as the Ganges and the Amazon has a nasty effect on riverine populations. In addition to the ecological effect resulting from the change in the flow of water, riverine populations may be permanently separated. Individual stocks may not be viable in the long term and seasonal migrations may be disrupted.

    Back to Outline Pollution

    Pollutants enter the food chain near its bottom and build up in top predators, such as cetaceans. In addition, cetaceans may injest the wastes and debris from discarded fishing gear. Mineral exploration and exploitation are also a concern, but unless cetaceans are caught in survey explosions or oil spills, the danger is probably minimal. The effects of such toxins on cetaceans are not entirely clear. The death of cetaceans due to pollution is difficult to recognize, especially when it occurs in large numbers. In other mammals polychlorinated biphenyls (PCBs) adversely affect reproduction, and the same may hold true for cetaceans.

    Back to Outline Disturbance

    Ironically, an important method of improving public awareness about cetaceans and promoting conservation measures may also be a serious threat to these creatures. In addition to normal ship traffic causing death due to collisions and propeller injuries, the increase in visitors that come for whale watching expeditions may disturb these animals. Studies show that if the visitors are careful and treat the resident cetaceans with propriety, the animals will have no difficulty acclimating. Commercial whale watching outfits are careful to ensure that visitors are tactful, but private individuals may not understand all of the necessary provisions.

    Back to Outline Specific Cases
    Back to Outline Eastern Tropical Pacific Tuna Purse-Seine Fishery

    Every year between 1959 and 1972, the tuna fisherman of the eastern tropical Pacific killed hundreds of thousands of dolphins, mostly spinners and spotted dolphins. In a report by the National Marine Fisheries Service's Southwest Fisheries Center made in 1979, the estimated total number of dolphins killed between 1959 and 1972 was 3,796,658. That figure is probably an underestimate, since it does not include individuals that were injured in the nets and died later. The level of incidental take is sufficient to pose a serious threat to the survival of some stocks.

    Back to Outline The Root of the Problem

    The basis of the problem is that the two main species involved associate in large numbers with yellowfin tuna. The nature of the association between the dolphins and the tuna is still a mystery, although there are some theories that try to explain it. It is possible that the relationship is symbiotic, as the tuna are most active during the day and the dolphins are most active during the night, so one group can rest while the other group feeds. A similar relationship is seen between spinners and spotters.

    Regardless of the reason, the fact that the three species school together allows the tuna fishermen to find schools of tuna very easily. In 1959, they discovered this relationship and began to exploit it. Fishermen look for a commotion on the horizon, indicating feeding seabirds, leaping dolphins, or both, and could indicate the presence of tuna Since spinners and spotters aggregate in the thousands, schools are not difficult to see even at a distance. When the tuna boat is in range, speedboats known as pongas are lowered and herd the school together. When the animals are compactly herded, the seiner lowers the net and surrounds the herd, trying to trap both dolphins and tuna. At first, the purse seine is about 1.6 km long and open, forming a wall. It is also open at the bottom, but herded individuals do not go under it to escape, probably because the water at a depth of 200 meters is too cold. The pongas continue to roar around the perimeter, making another barrier. When the dolphin school is completely surrounded, it is pursed, that is, it is closed at the bottom using a cable that passes through a series of rings. Every creature in the school is trapped. The net is then drawn onto the boat through a power block, the one invention that made this kind of fishing possible. Captured individuals are stacked methodically on the deck as the enclosure shrinks.

    Back to Outline Dolphin Behavior in the Nets

    Further compounding the problem is the fact that spinners and spotters tend to be timid, easily frightened, and fearful of objects. They depend on the presence of other dolphins and do not react well to new situations. As a result, they tend to react to the purse seines by panicing rather than by attempting to escape as they easily could given their acrobatic abilities. The passive behavior of these animals is described as a form of "capture myopathy" and often leads to death. Although normally a factor that aids their survival, in the case of purse seines, their timid nature is deadly.

    When not herded or chased, spinners and spotters swim slowly and spread over a long distance, often with between 20 and 30 body lengths between different individuals. When they are chased, they crowd together, leaving only 2 to 3 body lengths between them. They also move more quickly, making low leaps from the water, as it is the most efficient method of swimming. As the school is surrounded by the nets, various behavior patterns emerge. In a practice called milling, a large percentage of the school stays in one area, swimming and diving. Early in the set, there is some leaping, but that quickly subsides. In addition, some individuals start rafting, where a group hangs vertically in the water, head up. Individuals align themselves in layers, discrete bands oriented horizontally. Groups of four to five individuals make up each layer, and there may be up to four layers stacked vertically. After surfacing to breath, always return to their original position. Often some dolphins just passively sink slowly to the bottom of the nets, probably as a reaction to the stress from the boats, the noise, and the wake. This type of behavior is understandable considering that spinners and spotters are used to the open ocean where they have no barriers or confinement. Although spinners usually separate from the spotters in a net, they do exhibit similar behavior. However, spinners tend to move around more in the nets, and are located more in the periphery.

    Back to Outline History of the Fishery

    Before the fishermen discovered the dolphin-tuna aggregations, they used a method of giving the tuna bait until they were in a feeding frenzy, at which point they would bite anything, including unbaited hooks. By 1960, however, the new purse seine method had largely replaced the old method. In 1966, 62% of the tuna caught in the Pacific were in association with dolphins.

    In the early years of the purse seine fishery, everything in the net was hauled aboard. The valuable tuna was kept, while the dolphins and other fish were tossed overboard. The tuna fishermen did not publicize this slaughter, although the information did eventually leak. The problem was noticed by William F. Perrin, who was working on a Ph.D. dissertation on Stenella at UCLA. Scientists quickly began a study of the dolphins from this fishery. Much information was collected, and the species involved went from little known to some of the most intensively studied.

    Based on an extrapolation form the early irregular data supplied by fishermen, some estimates were made on the annual kill rate. In 1959, before the entire fleet had switched to using purse-seine, up to 100,000 dolphins were killed. By 1960, when the entire fleet had switched, an estimated 500,000 individuals were killed. Before the examination started, no records were kept on the proportions of species involved. Later studies showed that the species most involved was the spotter, then the spinner, and then the common dolphin. Several additional species were involved in small quantities.

    Back to Outline Attempts to Reduce Kills

    As a by-product of the tuna-dolphin problem, the Dedicated Vessel Program was started. Under this program, a tuna boat performs its regular duties with a team of biologists on board. These scientists monitor the dolphin kills and the behavior of the dolphins in the nets. As a result of these observations, much more care is taken by the tuna fishermen. On one tuna boat in 1977, only 11 dolphins died in a total of 20 sets. On the Queen Mary in 1978, out of 17 sets, only three dolphins perished. In addition, it was seen that some dolphins were already familiar with the nets. They stopped swimming and knew how to escape the net using the methods developed by the tuna fishermen to lower the casualty rate.

    Very soon after the purse-seine fishery developed, various techniques were put into use to protect the dolphins. These methods were used not necessarily to help the dolphins, but to reduce the labor of removing the dead dolphins from the deck, as well as to avoid depleting the entire dolphin population, lest there be no more dolphins to use. These strategies mostly revolved around releasing the dolphins from the nets before they were killed. In one method, known as "backing down," when one half of the net had been hauled in, the vessel was put in reverse, and the net was drawn into a finger-shaped configuration. At the far end of the finger, the water pressure against the webbing cased the net to sink. The dolphins literally have the net pulled out from under them. Although this technique worked to some degree, thousands still died every year. In addition, tuna sometimes escaped during this maneuver, and some captains were more interested in preserving the tuna then in preventing the slaughter of the dolphins. Fine meshed panels like the Medina panel and the super-apron were also developed to prevent entanglement and facilitate the escape of the dolphins.

    By the 1970s, the United States government was under a great deal of pressure from humane and conservation groups and began monitoring the situation. When the American public noticed the problem, their outcry helped to cause the passage of the Marine Mammal Protection Act of 1972. The tuna industry was given a two year grace period to reduce dolphin kills. They did not abide by this, and only after a long drawn-out legal battle was any action taken. The techniques developed to release the trapped dolphins were probably not strictly enforced throughout the international tuna fleet. In addition, some of the reduction in the reported kill may have been because ships previously under United States registration changed their flag to avoid strict US regulations.

    Despite the problems in enforcing the legislation, the kill rate was reduced significantly during the 1970s. Whereas in 1973, a total of 114,087 dolphins were killed, by 1978, only 21,805 were killed. The most important factor was the pressure on the fishermen and the techniques to release trapped dolphins. In October of 1980, a quota of 20,500 dolphins was set for all species by the United States government. The previous year's kills had not been that high, and it appeared that the problem had vanished.

    Back to Outline New Concerns

    New concerns have cropped up, though. The Porpoise Stocks Workship in La Jolla analyzed the earlier data and found that the estimates on population were seriously flawed. The actual number of spotters in the eastern tropical Pacific was much lower than previously assumed. Earlier estimates were based on the estimates of school size given by the observers and crew members on tuna boats, while estimates in 1979 were based on aerial observations. The 1979 report gave an estimated offshore population of 34-55% of the 1959 level. This is below the optimal sustainable pouplation, where the population level leads to maximum net productivity. The same data analyzed in 1973 gave a population of 92-95% of the 1959 level, which is in the OSP range. The population in 1959 was probably more like three million than the four million originally assumed, and as more than four million dolphins were killed between 1959 and 1978. Although some did reproduce, the population was probably at a critical stage.

    The government could declare the spotters depleted, if the population has declined much over several years, and if the decline continues it will be eligable for the Endagered Species Conservation Act of 1969, or if the population is below the optimum carrying capacity for the species or stock within the environment. If it does so, no taking will be allowed at all. Tuna fishermen complain that they will be shut down entirely if this happens. They state that they have cooperated with environmentalists and reduced kills significantly. They threaten to change the flags of their ships to countries without provisions if the government declares the species depleted. Doing this would place the tuna fishermen outside of the jurisdiction of the laws of the United States and would free them to kill as many dolphins as they wish. As a result, conservationists are sometimes on the side of the tuna fishermen.

    Back to Outline Black Sea Fishery

    Between 1870 and 1983, in the Black Sea, there existed a fishery for three species of cetacean: the bottlenose dolphin, the harbor porpoise, and the common dolphin. The method of catch was similar to that of the eastern tropical Pacific tuna purse seine fishery, but the dolphins are the prime target instead of the tuna. When herding the dolphins, fishermen clang together cobblestones, causing the dolphins to panic and run into the nets. When the fishery started operation, harpoons were used, but that method was later replaced by purse seining. The main products of the fishery were food and oil.

    Four countries were involved in this fishery: Turkey, Romania, Bulgaria, and the USSR. An estimated total of 200,000 dolphins were taken each year, a shocking 20% of the population. The effects of depletion became evident after the Second World War, when the USSR could not reach the same rates of catch as in the 1930s. Between 1931 and 1941, the USSR took 110,000-130,000 dolphins per year, but in later years, it only averaged 75,000 per year. The Turkish fishery's destruction of the breeding stock and the young during breeding season was blamed for this. In 1966, the fishery operations in all countries except Turkey stopped, due to a collapse of the population. The Turkish fishery is not well documented, since many individuals were killed from small boats using rifles and shot guns, leading to a high loss rate.

    The Turkish fishery was banned in 1983, however there is much concern that it might reopen when a reassessment of the stock is made. The fishing industry is putting forth much pressure to end the ban. Although the European Community bans the import of cetacean products of commerical uses, Japan is an open market for the products of this fishery, and it is already a destination of many of the products of current Turkish fisheries. The Berne Convention includes an exemption clause, allowing species listed under Appendix II to be taken to prevent serious damage to fisheries, as long as the survival of the species is not threatened. The fishery might reopen under the provisions of this clause.

    Back to Outline Chilean Crab Bait Fishery

    The waters of the Strait of Magellan are an important source of crab meat, and the fishery there is expanding. Unfortunately, the fishermen who need bait to catch these crabs have turned to the local dolphin population. Under Chilean law, the slaughter of dolphins for crab bait is illegal, but because the region is so remote, the laws are difficult to enforce. As of the early 1990s, only four of the 26 companies operating in the Magellans provided crab bait to the fishermen, and in most cases, the amount supplied wasn't nearly adequate. The Commerson's and black dolphins are known to be involved in this operation, as well as seals, sealions, penguins, guancos, and other wildlife. There are very few estimates of the level of take, but the dolphins killed may number in the thousands. It is feared that the dolphins are being killed too rapidly to be replenished naturally.

    Table of Contents Previous Page Next Page Top of Page


    Ellis, Richard. Dolphins and Porpoises. New York: Alfred & Knopf, Inc., 1982.

    Klinowska, Margaret. Dolphins, Porpoises, and Whales of the World: The IUCN Red Data Book. Gland, Switzerland: World Conservation Union, 1991.

    Evolution and Taxonomy Behaviour Anatomy Human and Dolphin
    © 1998 Thinkquest Team 17963 <17963@advanced.orgREMOTE>: Bradford Hovinen, Onno Faber, Vincent Goh
    Modified: 30 August 1998, Created: 30 June 1998
    Thinkquest 98 banner