Resource Guide Links:

language learning

dolphin perception

the bottlenose dolphin: natural history and ecology

getting to know humpback whales


tdi for kids




Threats to the Bottlenose Dolphin and Other Marine Mammals

Bottlenose dolphins and other marine mammals face a number of conservation threats due to anthropogenic, or human-induced, impacts on the marine environment. Marine mammals adapted to the aquatic environment when it was free from boats, pollution, noise, and human competitors for fish resources. As human beings have created boats that can travel to any part of the ocean, new challenges have developed that threaten the well being and even, existence of many marine mammal species. Some of the conservation threats to marine mammals include:

  • Habitat Degradation
  • Boat Traffic
  • Fishing Interactions
  • Yellowfin Tuna Fishery in the Eastern Tropical Pacific
  • Pollution, and
  • Direct Takes.

Laws must be created and strictly enforced to protect and conserve a diversity of marine mammal species. Two such laws or acts are the:

Marine Mammal Protection Act of 1972 (amendments of 1994) and the
Endangered Species Act of 1973.

Habitat Degradation

Human beings have exploited the resources of near shore and offshore ecosystems. Marine mammals utilize both of these environments for a variety of behaviors, including resting, foraging for prey, traveling, and socializing. Human use of these areas affects marine mammal behavior, distribution, and energetics and may cause short- (temporary) or long-term consequences for these individuals and species. Some examples of habitat degradation include areas that are disturbed by traffic from a large number of commercial and recreational vessels; pollution from sewage, toxins, and oil spills; and noise from boats, construction, dredging, oil and gas drilling, and explosions. Bottlenose dolphins and other marine mammals may avoid these areas of habitat degradation, possibly settling for less hospitable areas with fewer food resources.

Boat Traffic

As boat traffic in the oceans increases to keep up with today's society, so do the threats to marine mammals. Whale and dolphin watching vessels along with commercial and recreational fishing boats have the potential to present dangerous consequences to marine mammals. Poorly operated dolphin watching boats and irresponsible recreational boaters may approach dolphins too closely and too quickly in order to induce dramatic behaviors such as bow riding and breaching for paying customers. These boat activities can disrupt the behaviors of marine mammals and can scatter a group, which is especially harmful to females with young calves. According to the Marine Mammal Protection Act of 1972 (MMPA), these actions constitute harassment and are illegal; however, these illegal activities are rarely enforced. Although feeding is also forbidden under the MMPA, many boaters (including whale- and dolphin-watching vessels) feed wild marine mammals to entice them closer to boats. Marine mammals change their normal behavior patterns if fed by humans and may depend on people for food, instead of foraging for it themselves. Dolphins that approach boats are more susceptible to harm from fishing gear, engine propellers, poison, and susceptibility to disease from humans and pets. Dolphins can also bite and injure human beings that do not give them the food that they expect! The harmful effects of commercial and recreational fishing vessels are discussed in the next section.

Fishing Interactions

Both commercial and recreational fisheries threaten marine mammals. By-catch in commercial fishing nets occurs throughout the world's oceans. By-catch is the incidental capture of a species, such as the bottlenose dolphins, that is not the target of the fishermen. The MMPA and its amendments of 1994 were passed in large part to decrease the number of dolphin by-catch in the fishing industries. However, by-catch still occurs in both legal fishing industries such as gill nets and trawling as well as illegal drift nets that capture any marine life that swims or floats into a huge net that is towed behind a vessel! Fisheries also threaten marine mammals by fishing for the same food resources on which the animals depend. In many parts of the world, marine mammals are seen as competitors for dwindling fish resources and are poisoned and killed to maintain the fish resources for human consumption.

Yellowfin Tuna Fishery in the Eastern Tropical Pacific

In the late 1950s, San Diego tuna fishermen developed a new technology based upon purse-seine nets and a method known as "dolphin fishing." This fishing method relies on the phenomenon that some species of dolphins tend to school above yellowfin tuna in the Eastern Tropical Pacific (ETP), one of the most productive tuna fishing areas in the world; almost one quarter of the world's tuna catch comes from the ETP. Because of the close association between tuna and dolphins in the ETP, tuna boats can simply set nets around schools of dolphins, knowing tuna will be caught as well. Dolphins must frequently surface to breathe, making dolphins schools easy to spot on the surface. Tuna boats then use speedboats, helicopters and small explosives known as seal bombs to herd dolphins into purse seine nets which can be up to one mile in circumference. The dolphins become entangled in the nets along with the tuna, and die. Though data on dolphin mortality in purse seine nets was very poor prior to the passage of the MMPA, it has been estimated that mortality rates in the 1960s were as much as 250,000 per year, and that over 7 million dolphins have been killed through this fishery.

Why dolphins, primarily spotted (Stenella attenuata), spinner (Stenella longirostris) and common dolphins (Delphinus delphis), and less frequently striped (Stenella coeruleoalba), roughtoothed (Steno bredanensis), bottlenose (Tursiops truncatus) and Fraser's dolphin (Lagenodelphis hosei), school with tuna is not precisely known. It seems as though the tuna follow the dolphins, not vice versa, as dolphin fishing works because the air-breathing dolphins can be corralled into the net.

One of the main predicaments in this issue is the fact that purse seine dolphin fishing is by far the easiest and most productive ways to catch tuna. When dolphin fishing was first devised in the 1950s, catches up to 250 tons of tuna per set were not uncommon. Even by the 1980s, tuna boats were still bringing in an average of 18 tons per set. Dolphin fishing has larger average catches than other methods of purse seine fishing and tends to catch larger and more sexually mature tuna than other methods. Economically, dolphin fishing is not only the most productive method, but also is the least harmful to the tuna population as it catches sexually mature fish.

Dolphin bycatch in the yellowfin tuna fishing industry was already an issue when the MMPA was passed in 1972; in fact, the MMPA specifically ordered incidental dolphin kills associated with tuna fishing to be reduced to "insignificant levels approaching zero." The tuna industry was granted a two year grace period to develop new techniques safe for dolphins, but none were forthcoming. Federal courts enforced a decreasing quota system to reduce kills throughout the 1970s and early 1980s. A 1981 Amendment to the MMPA asserted that "[the goal of zero mortality] shall be satisfied in the case of the incidental taking of marine mammals in the course of purse-seine fishing for yellowfin tuna by a continuation of the application of the best marine mammal safety techniques and equipment that are economically and technologically practicable."

In 1984, more MMPA Amendments were passed to in regards to the dolphin/tuna issue. The phrase "insignificant levels approaching zero" was redefined as a quota of "20,500." The Department of Commerce was required to ban imports of purse-seine-caught tuna from foreign fishing fleets without dolphin kill rates comparable to the US fleet's by 1991 and from countries where governmental dolphin protection program had not yet been instituted.

By 1988, the incidental dolphin kill rate of the US tuna fleet had dropped, but foreign tuna boats still killed about four times as many dolphins as US tuna fishermen. It was at this time that the reauthorization hearings of MMPA were held. The testimony of an American named Sam LaBudde was crucial to these proceedings: LaBudde had worked as a cook on a Panamanian tuna boat and provided graphic footage of dolphins being killed. These proceedings resulted not only in the reauthorization of the MMPA itself, but new 1988 amendments required US boats to have a special panel made of fine mesh netting, called a Medina panel, in the rear of the net to help facilitate the release of dolphins from the nets. 1984 amendments were also clarified, directing foreign fleets to prove kill rates of no more than twice the US rate of 20,500 for the year 1989, and no more than 1.25 times the US rate in 1990.

Also in 1988, environmental groups, frustrated by the fact that dolphins were still being killed in tuna nets at all, launched a nationwide consumer boycott of the three major tuna processors in the US: Heinz's Starkist Tuna, Ralston Purina's Chicken of the Sea and Pillsbury's Bumble Bee Tuna. Together, these three companies controlled 70% of US tuna market. In 1990, after two years of concerted efforts by environmental groups, all three tuna processors agreed voluntarily to accept only "dolphin-safe" tuna, meaning tuna that was not caught by purse seine dolphin fishing or drift nets.

Also in 1990, the Dolphin Protection Consumer Information Act was passed. This act mandated standards for labeling tuna "dolphin-safe," following the same guidelines the tuna companies had: no tuna caught by setting a purse seine net around dolphins could be labeled "dolphin safe." Finally, the International Dolphin Conservation Act was passed in 1992. This provided for a five year moratorium on purse seine net "dolphin" fishing beginning in 1994. The US fishing fleets were prohibited from chasing, capturing and setting of nets on dolphins at all.

With the US tuna market now "dolphin-safe," the rest of the world began to take notice. At an international meeting of the Interamerican Tropical Tuna Commission (IATTC) in 1992, the "La Jolla Agreement" established the International Dolphin Conservation Program (IDCP). This represented the first time a fisheries organization recognized the need to deal with the issue of marine mammal deaths in a fishery. The IDCP called for a decrease in dolphin deaths, with a goal of under 5000 deaths by the year 2000. The program includes 100% observer coverage, captain and crew training in dolphin release techniques, data collection on dolphin biology and bycatch, and payment of funds by the tuna fishermen to support the observer program. Only those vessels which comply with these regulations are allocated portions of the annual limit, and are the only vessels allowed to set nets on dolphins.

In October of 1995, meetings were held between US and government officials from Belize, Colombia, Costa Rica, Ecuador, France, Honduras, Mexico, Panama, Spain, Vanuatu and Venezuela, and representatives from a number of environmental groups such as Greenpeace, the World Wildlife Fund, the Center for Marine Conservation, the National Wildlife Federation and the Environmental Defense Fund. The result of these meetings was the Panama Declaration, which would make the La Jolla Agreement binding under international law. Furthermore, the US would lift the embargoes on foreign tuna caught in purse seine nets and would change the definition of "dolphin safe" as stipulated in the Dolphin Information Protection Act of 1992 to mean that no dolphin mortality was observed when the tuna was caught. The IDCP would become legally binding in the US and Mexico and would set elimination of dolphin mortality in fishing program as a goal.

In the summer of 1997, the International Dolphin Conservation Act, legislation (known as HR 408 and S39) implementing the Panama Declaration into law, was passed by both the House and Senate and was signed by President Clinton on August 18th, 1997, becoming Public Law No. 105-42. The consequences of this new law is that the US tuna fishermen will again be able to catch tuna using the purse seine dolphin fishing method and the tuna embargoes on nations using this method will be lifted. A quota of 5000 dolphins killed per year was set, with no provisions set in place for future quota reduction.

Before the new "dolphin-safe" label (meaning there was no observed dolphin mortality) could go into effect, studies were conducted by the National Marine Fisheries Service to investigate the impact of purse seine fishing on dolphins. These studies found that the impacted species of dolphin (including two species listed as "depleted" under the MMPA, the Eastern spinner dolphin and the Northeastern offshore spotted dolphin) were not recovering as expected. Additionally, it was shown that the stress induced by the dolphin fishing method was likely to have a population level effect (via stress-induced changes in immune system and reproductive system function, as well as severe muscle damage (capture myopathy) from the chase itself. However, in April of 1999, Secretary of Commerce William Daley issued a rule implementing the change in the dolphin safe-label, claiming that NFMS was unable to prove that the fishery was causing in a significant adverse impact on the depleted dolphin populations. This ruling was challenged in a California court in Brower vs. Daley, where Judge Thelton Henderson blocked the labeling change from going into effect on April 11, 2000. This ruling is currently being appealed; however, for now, the former and stricter standards of "dolphin-safe" remain in effect.

Environmental groups have divided sharply on the International Dolphin Conservation Act and its effects on dolphin mortality. Supporters of the new law include Greenpeace and the Center for Marine Conservation; these groups feel that only international cooperation can eliminate dolphin deaths through the tuna fishing industry worldwide. Although the US tuna fishing fleet was prohibited from setting nets in dolphins in 1994, as a result many US boats repatriated to countries that allowed dolphin fishing of tuna. Furthermore, the primary motivation behind the La Jolla Agreement for the signing nations was to re-enter the lucrative US tuna market. The environmental groups backing the new law feared that if the US did not accept the La Jolla Agreement, the nations involved would no longer have an incentive to pursue dolphin-safe tuna fishing at all. The La Jolla Agreement has been successful in lowering dolphin deaths in the ETP, reducing that figure by over 90% since 1990, to 2657 deaths in 1996. If the US market had remained closed, the La Jolla Agreement may have been abandoned completely and worldwide, more dolphins would be killed.

However, environmental groups including the Earth Island Institute, the Sierra Club and Friends of the Earth were staunchly opposed to the new law. The dolphin fishing technique causes severe stress upon the surrounded dolphins and can have serious consequences; many dolphins may still die unobserved later. Furthermore, nighttime sets, murky water or unreliable observers may also result in a falsely labeled "dolphin-safe" catch. Finally, the currently lowered number of annual dolphin deaths may not be due to the La Jolla Agreement, but to the fact that fewer fishermen are pursuing tuna at all. As foreign fishermen are currently unable to sell their tuna in the US tuna market, fewer boats may be fishing for tuna in the ETP since they have nowhere to sell it.

The practical result of this legislation is that tuna caught by encircling dolphins may soon be appearing on US shelves; Mexico and Ecuador currently have "affirmative findings," meaning the embargoes on incoming tuna from those countries has been lifted. Tuna that does not bear the "dolphin-safe" label was caught during a set where dolphins were observed dead or mortally injured. For now, tuna bearing the "dolphin-safe" label still means it was not caught in conjunction with dolphins. However, pending appeals, it could soon mean that the tuna could still have been caught by setting nets on dolphins, but no dolphins were seen killed or mortally injured during the set. Despite these issues, the US Tuna Federation, which represents the interests of the US tuna industry, including Bumblebee, Starkist and Chicken of the Sea, has reported that all US canned tuna processors intend to keep their previous policy of only purchasing tuna that was not caught by setting nets on dolphins.


Pollution occurs throughout the oceans due to human sewage, oil spills, toxic leaks and dumping, and noise. Noise pollution is a rising concern among marine mammalogists today. Marine mammals adapted to oceans that were void of human produced sounds. Today, the oceans are being bombarded with sounds from transportation vessels, dredging and construction, oil and gas drilling, seismic exploration, explosions, oceans and geologic studies such as ATOC or Acoustic Thermometry of Ocean Climate, and sonar such as LFA sonar or Low-Frequency Active sonar. Loud sounds have the potential to cause ear damage or destruction while low-frequency sounds can affect communication, prey and predator detection, and navigation. In fact, marine mammals depend on sound for almost every daily behavior! Although most of the consequences of the sounds mentioned above are still unknown, the consequences of some of these noises on marine mammals may be devastating. Research is necessary and is now being conducted by a number of researchers to determine the effects of high- and low-frequency as well as loud sounds within the marine environment on marine mammals. Further information on LFA sonar can be found at: or

Direct Takes

Although it has been illegal to take marine mammals in the United States since the MMPA of 1972, marine mammals are still killed in some parts of the world for meat, oil, and leather. In several countries around the world such as Turkey, Peru, Sri Lanka, and Japan, dolphins are being killed for human consumption and to decrease the competition for fish resources.

Marine Mammal Protection Act of 1972 (amendments of 1994) (MMPA)

The MMPA, the primary federal legislation designed to protect marine mammals, was originally passed in 1972 due in part to concern of dolphin and other marine mammal by-catch in commercial fisheries. The MMPA prohibits a "take" of a marine mammal, which is anything that may harm, harass, or kill a marine mammal. In the amendments of 1994, two types of fishery threats to marine mammals were identified: Category I activities which are those that frequently kill or seriously injure marine mammals and Category II fisheries which are those that occasionally kill or injure marine mammals. In the Atlantic Ocean, Gulf of Mexico, and United States' Caribbean waters, bottlenose dolphins were among the species listed as potentially injured or killed in three of four Category I fisheries and three of six Category II fisheries. The MMPA is enforced by the National Marine Fisheries Service (NMFS), which is part of the Department of Commerce and the National Oceanic and Atmospheric Administration (NOAA). Information regarding the MMPA regulations can be found at: or at NOAA's web page:

Endangered Species Act of 1973 (ESA)

The ESA is designed to protect species that are endangered to the point that they are being threatened to near extinction. The National Marine Fisheries Service (NMFS) is responsible for marine species that are protected by the ESA. All decisions that determine which animals should be considered threatened or endangered are based on scientific and commercial data, rather than on the economic needs of human beings. The species that are considered endangered are those that are in imminent danger of extinction in all of their significant habitats. A threatened species is one that is likely to become endangered in the foreseeable future. A recovery plan is designed for all endangered and threatened species to aid in their conservation and recovery. Marine mammal species within the United States' waters that are endangered include the blue, bowhead, fin, humpback, Northern right, sei, and sperm whales, along with Caribbean and Hawaiian monk seals. Domestic threatened marine mammals consist of the Steller sea lion. Other protected species in international waters include the Chinese and Indus River dolphins, the gray whales of the Western North Pacific population, the Gulf of California harbor porpoise, the Southern right whale, the Mediterranean monk seal, and the Ringed Seal. More information on these species can be found at:

More information on the ESA can be found on NMFS's web page at: or a full version of the act can be seen at:

Back to Top

Dolphin Programs | Whale Programs | Education Programs | Our Research | Resource Guide

Copyright © 2002, The Dolphin Institute