Two common bottlenose dolphins swimming. (Image Source: Mila B./Openverse)

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The streamlined, grey bodies of bottlenose dolphins bobbing and sometimes leaping above the ocean’s surface are easily recognized by many worldwide: bottlenose dolphins can be found in coastal and offshore waters surrounding six of the seven continents.[i] Until recently, it was thought that two species of bottlenose dolphins inhabited our oceans. One of those species, Tursiops truncatus, was believed to be split into two ecotypes, an inshore ecotype and an offshore ecotype.[ii] In 2022, after a decade of research, a team of researchers discovered that the inshore ecotype inhabiting the United States’ East Coast waters is, in fact, a third species of bottlenose dolphin, Tursiops erebennus.[iii] The shifting scientific understanding of bottlenose dolphin populations may have implications for management and conservation of these marine mammals.

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Proper management of bottlenose dolphin populations is important because dolphins help preserve the balance of ecosystems by keeping prey populations in check and by being a food source for larger predators up the food chain.[iv] Moreover, bottlenose dolphins face a number of threats that make it imperative that they receive appropriate protection. First, interaction with fishing gear and the impacts of overfishing can harm dolphins. Bycatch causes thousands of dolphin deaths every year while overfishing reduces the abundance of their food sources.[v] High volume vessel traffic can also lead to collisions with dolphins and noise from engines can disrupt their behavior.[vi] Second, dolphins are facing habitat destruction, both from issues like oil spills that release contaminants and from climate change precipitating changes in their habitat and range.[vii] Third, human interactions with dolphins can lead dolphins to associate humans with food and increase their risk of being struck by a vessel or entangled in fishing gear.[viii]

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A common bottlenose dolphin (T. truncatus). (Image source: Elias Levy / Openverse)

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Bottlenose dolphins in U.S. waters are protected under the Marine Mammal Protection Act of 1972 (MMPA). The MMPA uses “stocks” as its unit to organize management efforts.[ix] A stock is “a group of marine mammals of the same species of smaller taxa in a common spatial arrangement, that interbreed when mature.”[x] Stocks can be designated as strategic or depleted if they meet certain criteria.[xi] Since 1995, the National Marine Fisheries Service and the Fish and Wildlife Service, federal agencies, have worked together to publish Guidelines for Assessing Marine Mammal Stocks (GAMMS) which set out the criteria needed for the establishment of a new stock. Currently, there are eleven key elements needed for stock designation, including information on morphology, genetics, and differences in habitat.[xii] Once designated, stocks must be reviewed at least every three years. The information from those reviews is published in Stock Assessment Reports which include details like current population trends, potential biological removal levels, and status of the stock.[xiii]

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There are more than sixty stocks of bottlenose dolphins in U.S. waters.[xiv] Off the coast of North Carolina, where the Learning from Whales Bass Connections team does its research, there are three MMPA-recognized stocks of bottlenose dolphins.[xv] The relevant stock designations have not been modified since the discovery of T. erebennus, but NOAA Fisheries has stated that it will “evaluate potential management implications” of the new study.[xvi] Although the current system may adequately protect these dolphin populations, the more information that is known about them, the more likely it is that they will be properly safeguarded. It is possible that T. erebennus faces distinct challenges and an updated MMPA stock designation could better protect the new species.

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In sum, dolphins’ important role in maintaining balance in marine ecosystems and our evolving understanding of their population structure make it essential that, when appropriate, policy and legal regimes consider and adapt to new scientific findings to ensure that these marine mammals are receiving the protection they require.

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References 

[i] See Common Bottlenose Dolphin, NOAA Fisheries, https://www.fisheries.noaa.gov/species/common-bottlenose-dolphin (last visited Dec. 1, 2023).

[ii] Leigh J. Klatsky, Randall S. Wells & Jay C. Sweeney, Offshore Bottlenose Dolphins (Tursiops Truncatus): Movement and Dive Behavior Near the Bermuda Pedestal, 88 J Mammal 59 (2007).

[iii] Bottlenose Dolphins Along the East Coast Proposed to be a Different Species, NOAA Fisheries (2022), https://www.fisheries.noaa.gov/feature-story/bottlenose-dolphins-along-east-coast-proposed-be-different-species.

[iv] Bottlenose Dolphins: Our Smart, Sociable Stars of The Sea, World Wildlife Fund, https://www.wwf.org.uk/learn/wildlife/dolphins (last visited Dec. 1, 2023).

[v] Id.

[vi] Id.

[vii] Id.; Common Bottlenose Dolphin, NOAA Fisheries, https://www.fisheries.noaa.gov/species/common-bottlenose-dolphin (last visited Dec. 1, 2023).

[viii] Common Bottlenose Dolphin, NOAA Fisheries, https://www.fisheries.noaa.gov/species/common-bottlenose-dolphin (last visited Dec. 1, 2023).

[ix] Rachel Hilt, The Evolution of Stock Definitions and Management Implications Under the U.S. Marine Mammal Protection Act, Apr. 28, 2023, at 5 (unpublished master’s thesis), https://hdl.handle.net/10161/27216.

[x] 16 U.S.C. § 1361.

[xi] 16 U.S.C. § 1362(1) & (19).

[xii] Rachel Hilt, The Evolution of Stock Definitions and Management Implications Under the U.S. Marine Mammal Protection Act, Apr. 28, 2023, at 14 (unpublished master’s thesis), https://hdl.handle.net/10161/27216.

[xiii] Marine Mammal Stock Assessments, NOAA Fisheries, https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-stock-assessments (last visited Dec. 1, 2023).

[xiv] Id.

[xv] The three stocks are the Northern North Carolina Estuarine Stock, the Southern North Carolina Estuarine System Stock, and the Western North Atlantic Southern Migratory Coastal Stock. Cite to the stock assessments.

[xvi] Bottlenose Dolphins Along the East Coast Proposed to be a Different Species, NOAA Fisheries (2022), https://www.fisheries.noaa.gov/feature-story/bottlenose-dolphins-along-east-coast-proposed-be-different-species.