[2] This trend is likely to continue due to the pressures of human population growth on the environment, the growing popularity of seafood, and the commercialization of deep-sea fishing as operations expand into waters which were previously protected.

In commercial fishing practices, cetaceans are captured as bycatch but then retained because of their value as food or bait.

[3] The mean annual bycatch in the U.S. alone from 1990 to 1999 was 6,215 marine mammals, with dolphins and porpoises being the primary cetaceans caught in gillnets.

[3] Based on global bycatch observation of U.S. fisheries, an estimated 653,365 marine mammals, comprising 307,753 cetaceans and 345,611 pinnipeds were caught from 1990 to 1994.

The following cetaceans are at high risk for entanglement in gillnets: The Atlantic humpback dolphin (Sousa teuszii) is endemic to West Africa.

[13] Baleen whales (Mysticeti) are often taken in gill-nets and in fisheries that use vertical lines to mark traps and pots.

Surveys have shown that bycatch remains a concern in that area today, and it is unknown whether the population is declining.

[13] Improvements in fishing technology, public awareness, and a large-scale survey of the impact of bycatch on Commerson's dolphin populations are essential.

The La Plata or Franciscana dolphin (Pontoporia blainvillei) is the most threatened small cetacean in the southwest Atlantic Ocean due to bycatch.

To rectify this situation, more surveys are needed as well as political commitment, awareness campaigns, and bycatch mitigation techniques.

[17] A study by Caswell et al.[18] in the western North Atlantic combined the mean annual rate of increase of the harbour porpoise with the uncertainty of incidental mortality and population size.

It was found that the incidental mortality exceeds critical values, thus showing bycatch is a significant threat to the harbour porpoise.

Based on a survey in 2001, fewer than 70 Irrawaddy dolphins (Orcaella brevirostris) remain in the upper region of the Malampaya Sound in the Philippines and only 69 individuals in the Mekong River.

[15] This may be due, in part, to the construction of the Three Gorges Dam, which covers a significant amount of the dolphin's natural habitat.

Endemic to the upper Gulf of California in Mexico, they are killed in both gillnets and trawl nets from commercial fishing and poaching.

[24] As of 2024, there are less than 8 individuals left in the Gulf of California, making the species functionally extinct similar to the baiji.

Acoustic deterrent devices, or pingers, have reduced the number of cetaceans caught in gill nets.

Harbour porpoises have been effectively excluded from bottom-set gill nets during many experiments for instance in the Gulf of Maine,[26] along the Olympic Peninsula,[27] in the Bay of Fundy,[28] and in the North Sea.

There has been a recent re-evaluation of the potential benefit of pingers,[30] and their use in other fisheries has expaneded due to their growing success.

An experiment on the California drift gill net fishery demonstrated how acoustic pingers reduce marine mammal bycatch.

[31] However, the widespread use of pingers along coastlines effectively excludes cetaceans such as porpoises from their prime habitat and resources.

[32] Cetaceans, extremely sensitive to noise, are effectively driven from their preferred coastal habitats by the use of these acoustic devices.

This situation is recognized as range contraction, which can be a result of climate change, anthropogenic activity, or population decline.

Conservation efforts should be directed to areas where marine mammal bycatch is high but where no infrastructure exists to assess the impact.

[3] As another potential mitigation measure, temporary closure of fisheries during the short periods of the year when cetaceans are migrating through the area would significantly decrease bycatch.

[37] Another mitigation measure is to use observers on fishing vessels to spot cetaceans in the water in order to avoid bycatch.

However, the only fishery in the world where independent scientific observers certify whether a dolphin has been harmed is the Eastern Tropical Pacific, home to the AIDCP Treaty program.

For all other tuna fisheries of the world, the efficacy of onboard observer certification has come under increasing scrutiny as such programs have proven indefensible or unmanageable: [39] In an interview with Radio Australia last year, Mark Palmer of EII confirmed that it is mostly the case that EII monitors do not go on board of the vessels, and their organization does not have the kind of resources to put observers on the "many thousands" of ships that are out there catching tuna.

[40] Additionally, environmental groups have criticized Earth Island Institute's support of U.S. policies that do not require independent, on-board observation and instead only rely on self-certification by fishing captains.

Even if there is a requirement of independent observers, the lack of uniformity in tracing and verifying certifications in different countries implies that non-certified products can still become certified if they are taken to a different port.