Threats to Wild Dolphins
Dolphins are one of the most beloved marine species in the world. For ages, they have been captivating people with their complex sociality and intriguing behavior. Unfortunately, wild dolphins are exposed to many threats that are difficult or impossible to overcome. While some threats may disturb and stress dolphins, others may cause dolphins to strand and sometimes perish. The term “strand” typically refers to when a live or dead marine animal is beached, but also describes any creature that cannot cope in its current situation and is left in a helpless position. If the threats are widespread or severe enough, large numbers of individuals may perish leading to a decline in a species’ abundance, which may lead to the species becoming threatened or endangered by extinction. While wild dolphins can strand due to natural causes, most threats that they encounter are actually anthropogenic in nature, meaning that human activity has caused the threat to arise. Such natural and anthropogenic threats include:
Dolphins in the wild are susceptible to a wide variety of diseases that may arise due to a single factor or combination of factors. Some diseases that we see in humans have also been documented in wild dolphin, including arthritis,1 cancer,2 and a neurological degenerative disease similar to Alzheimer’s.3 Additionally, exposure to pathogens, such as viruses, bacteria, fungi, and parasites, can lead to infectious disease and occasionally deadly outbreaks, called epizootics. For instance, over 1,600 bottlenose dolphins (Tursiops truncatus) stranded along the Atlantic coast of the U.S. from July 2013 to March 2015. Of those sampled for disease, 92% of the stranded dolphins tested positive for cetacean morbillivirus, and 25% had active Brucella species bacterial infections.4 Infectious agents such as these can be highly transmissible and cause mass mortalities to populations that have never encountered the pathogen or the certain strain of the pathogen. While diseases can infect wild dolphins naturally, often times the association of injury or stress from human-related impacts may lead to a reduced immune response, making them more susceptible to disease.
Other Natural Threats
Like other animals, dolphins may die from old age, severe storms, and predators, such as sharks or killer whales (Orcinus orca). They may also be affected by harmful algal blooms, or HABs. In Florida, “red tide” events are blooms of the algal species Karenia brevis, which produce a powerful neurotoxin called brevetoxin. Dolphins can breathe in brevetoxin fumes but primarily ingest the toxin through feeding on contaminated fish. Brevetoxin can disrupt neurological processes, often causing seizures, paralysis, and many times death in dolphins and other marine mammals.5 Although HABs are naturally occurring, the number and types of blooms are on the rise in recent decades, potentially due to rising sea surface temperatures and eutrophication, or excess nutrients, that is frequently due to runoff.
Florida waters discolored by a red tide harmful algal bloom, which produces a dangerous neurotoxin called brevetoxin.
Source: National Geographic, https://www.nationalgeographic.com/environment/2018/08/news-longest-red-tide-wildlife-deaths-marine-life-toxins/
Watercraft Strikes and Trauma
As often as we have seen dolphins playing alongside our research vessels, we have seen the results of what happens when dolphins, turtles, and manatees are hit by fast-moving boats in the shallow waters inside and outside of the marked channels. Injuries from being struck by watercraft typically includes propeller cuts and/or blunt force trauma from the hull of the boat. Propellers have the ability to slice through flesh as well as bone, sometimes dismembering struck individuals. Hull trauma can lead to broken or fractured bones, tissue damage, buoyancy issues, and death. The Marine Mammal Protection Act of 1972 prohibits people from harassing wild marine mammals, including following or chasing them in boats. This disruption of normal behavior can result in reduced life expectancy and reproductive rates, as dolphins will spend more time avoiding boats rather than resting or foraging.
Oceans around the world are experiencing elevated levels of water contaminants, including persistent organic pollutants (POPs), crude oil, and heavy metals. While oil and heavy metals can enter the marine environment naturally as well as through human pollution, POPs typically originate from pesticides, industrial chemicals, and by-products of industrial processes.6 Many of these contaminants are considered endocrine-disrupting chemicals, which interfere with hormones and normal bodily processes. This has the potential to create decreased immune response, greater risk of cancer, altered development, and decreased fertility and reproductive success. For instance, polychlorinated biphenyls (PCB) are a concerning class of POPs, which have been associated with an increased risk of stillbirths and calf mortalities in bottlenose dolphin populations located on the U.S. southeast coast.7 In another case, coupled with low prey availability, extremely high PCB levels in southern resident killer whales have contributed to the population’s low reproductive success in recent decades.8
Killer whales and other dolphins are considered high level predators of the marine food webs, meaning that they have very few or no predators that feed on them. Unfortunately, top predators often experience higher levels of contaminants than any other organism in the food web because each time they feed on a contaminated prey item, a certain percentage of the consumed contaminant is stored in the predator’s body; this process is called bioaccumulation. This applies to all levels of the food web, so contaminant levels biomagnify as they are passed to each predator.
Aside from chemical contaminants, plastic and other solid debris items have been known to negatively affect wild dolphins and many other sea creatures. Marine debris interactions typically involve animals ingesting the debris or becoming entangled in it. When debris becomes ingested, it has the potential to block the passage of intestinal contents, as well as puncture or tear the digestive tract. Dolphins can also become entangled in discarded ropes, plastics, or abandoned or lost fishing lines and nets, which are sometimes referred to as “ghost gear.” Entanglement can prevent dolphins from surfacing to breathe, causing them to drown. It can also become wrapped and constricted around the body, fins, or flukes, which then often cuts into and becomes embedded in the skin leading to infection or even amputation. Dragging entangled gear may also create more drag during swimming, causing animals to expend energy faster, which may affect feeding reproduction, and immune status. Both debris ingestion and entanglement may be life-threatening to dolphins but not always. However, it is still important to report any entangled dolphins as soon as possible to marine animal rescue facilities and organizations.
In the last century, fishery technology has dramatically increased our ability to catch fish and other seafood. In just the Pacific Ocean alone, the amount of fish caught rose from 2 million tons in 1948 to over 50 million tons in 1992.9 Unfortunately, while technologies have drastically increased fishery landings, many fishing practices are biologically unsustainable and detrimental to the ocean systems. In other words, humans are removing fish from the oceans at a greater rate than fish populations can grow, potentially creating many lasting effects on marine ecosystems. Since many large fish stocks have been depleted, fisheries have started targeting smaller fish more and more. However, many of these smaller fish are food sources for dolphins, creating competition. For instance, one study found that commercial fisheries in the Pacific Ocean target about 50% of prey items sought out by dolphins and porpoises.9 This has resulted in fishermen acting aggressively toward dolphins because they believe the dolphins are stealing their catch and profit. While many of these claims are anecdotal and unfounded, there are documented instances of dolphins and other marine mammals removing fish from gear.10
Another fishery-related threat to dolphins is bycatch, which is when the dolphins themselves are accidently caught in active fishing gear. Bycatch is not to be confused with entanglement in ghost gear, which is abandoned and unsupervised. Dolphin bycatch is typically associated with longlines, trawl nets, and gillnets. Once captured by this gear, dolphins are often restricted from surfacing and drown. They can also experience physical trauma from struggling in the gear or being hauled to the boats.
Striped dolphin (Stenella coeruleoalba) caught in a gillnet.
Dolphin live in a world of sound. Dolphins use sound for communication with other dolphins, navigation, detecting threats, and foraging, or hunting prey. Unfortunately, human activity has made the oceans unnaturally noisy, which can have negative effects on dolphin health and behavior. Seismic airguns used for locating pockets of fossil fuels in the Earth’s crust, sonar from both military and civilian use, coastal construction, and the propellers of commercial shipping fleets, and other watercraft all contribute to noise pollution underwater.11 Sound travels much faster and farther through water than it does in air, meaning that these anthropogenic noises can disturb dolphins and other marine life even when they are great distances away from the sound source. For instance, one study found that seismic blasts from airguns could be detectable at distances of up to 4,000 km away.12
Noise pollution can potentially cause temporary or permanent hearing loss, changes in hearing development, and masking of echolocation and communication signals critical to survival. Noise pollution may also cause dolphins to vacate the area, cease foraging, or rapidly surface or dive down. In some documented cases, there have been mass strandings of beaked whales coinciding with naval sonar activities nearby.13 As the effects of anthropogenic noise are sometimes difficult to observe and quantify, there is still much that scientists do not know but are actively studying. However, the effects that noise pollution has on dolphins and other marine mammals depends on a number of factors, including the sound pressure level, sound frequency, duration of the sound, novelty of the sound, physical and behavioral state of the animal, sound sensitivity of the species, and environmental features.13
While climate change is well-known for affecting the habitat and lives of polar bears and pinnipeds, it also affects dolphins in the wild. As sea and air temperatures are rising, polar ice is melting and sea levels are rising. This degrades the habitat of certain dolphin species and affects their local food source. For instance, one study revealed significant declines in the female reproductive rates of resident Indo-Pacific bottlenose dolphins (Tursiops aduncus) in Shark Bay, Australia, following a marine heatwave that reduced local prey populations and habitat-building organisms.14 With reduced prey, the nutrition from the limited food sources available were likely not enough to sustain both mothers and calves, resulting in failed pregnancies and calf mortality. Other dolphin species have been known to shift their geographical range according to temperature and prey changes in distribution.15 Dolphin species that are restricted to specific habitats, such as freshwater river dolphins, may be particularly vulnerable to temperature changes as they are unable to relocate.16
What Can You Do?
Together we can help make a difference in the lives and survival rates of dolphins in the wild. Anyone can have a part to play in marine conservation! Whether it be reducing your use of single-use plastics, such as grocery bags and straws, properly disposing of fishing line and chemicals, or spreading awareness to others makes a world of difference to dolphins and other sea creatures. Additionally, you can show your support for dolphin-friendly products, legislation protecting marine life and habitats, and facilities that promote marine conservation and stranding response. Be sure to respect conservation laws, such as the Marine Mammal Protection Act, and immediately report any wild dolphin or other marine animal that you believe may be injured, entangled, or in distress to NOAA, FWC, or other marine animal response organization in your area ((877) – WHALE – HELP).
It isn’t too late for us to make a positive impact on the health and well-being of our oceans and marine life. Even though it will take a lot of hard work and dedication, we can do great things if we work together. By saving our oceans and wildlife, we are helping to save the planet for future generations. If you’d like to learn more about how you can help our marine conservation efforts, explore our website for additional information.
- Davison, N.J., Barnett, J.E.F., Perrett, L.L., Dawson, C.E., Perkins, M.W., Deaville, R.C., and Jepson, P.D. 2013. Meningoencephalitis and arthritis associated with Brucella ceti in a short-beaked common dolphin (Delphinus delphis). Journal of Wildlife Diseases, 49: 632-636. Available at: https://www.jwildlifedis.org/doi/pdf/10.7589/2012-06-165
- Bossart, G.D., Ghim, S., Rehtanz, M., Goldstein, J., et al. 2005. Orogenital Neoplasia in Atlantic Bottlenose Dolphins (Tursiops truncatus). Aquatic Mammals, 31: 473-480. Available at: https://search.proquest.com/openview/15270d0eb8aa254597ba9a9bfb8e4a25/1?pq-origsite=gscholar&cbl=38594
- Di Guardo, G. 2018. Alzheimer’s disease, cellular prion protein, and dolphins. Alzheimer’s & Dementia Letter, 14: 259-260. Available at: https://www.alzheimersanddementia.com/article/S1552-5260(17)33873-6/abstract
- NOAA. 2014. 2013-2014 Bottlenose Dolphin Unusual Mortality Event in the Mid-Atlantic. Available at: https://www.fisheries.noaa.gov/national/marine-life-distress/2013-2015-bottlenose-dolphin-unusual-mortality-event-mid-atlantic
- Twiner, M.J., Fire, S., Schwacke, L., Davidson, L., Wang, Z., et al. 2011. Concurrent Exposure of Bottlenose Dolphins (Tursiops truncatus) to Multiple Algal Toxins in Sarasota Bay, Florida, USA. PLoS ONE, 6: e17394. Available at: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0017394
- World Health Organization. “Persistant Organic Pollutants (POPs)”. Available at: https://www.who.int/foodsafety/areas_work/chemical-risks/pops/en/
- Schwacke, L.H., Voit, E.O., Hansen, L.J., Wells, R.S., Mitchum, G.B., Hohn, A.A., et al. 2002. Probabilistic risk assessment of reproductive effects of polychlorinated biphenyls on bottlenose dolphins (Tursiops truncatus) from the southeast United States coast. Environmental Toxicology and Chemistry, 21: 2752-2764. Available at: https://setac.onlinelibrary.wiley.com/doi/pdf/10.1002/etc.5620211232
- Wasser, S.K., Lundin, J.I., Ayres, K., Seely, E., Giles, D., and Balcomb, K. 2017. Population growth is limited by nutritional impacts on pregnancy success in endangered Southern Resident Killer whales (Orcinus orca). PLos ONE, 12: e0179824. Available at: https://doi.org/10.1371/journal. pone.0179824
- Trites, A.W., Christensen, V., and Pauly, D. 1997. Competition Between Fisheries and Marine Mammals for Prey and Primary Production in the Pacific Ocean. Journal of the Northwest Atlantic Fishery Science, 22: 173-187. Available at: http://www.marinemammal.org/wp-content/pdfs/Trites_etal1997-competition.pdf
- Purves, M.G., Agnew, D.J., Balguerías, E., Moreno, C.A., and Watkins, B. 2004. Killer whale (Orcinus orca) and sperm whale (Physeter macrocephalus) interactions with longline vessels in the Patagonian toothfish fishery at South Georgia, South Atlantic. CCAMLR Science, 11: 111-126. Available at: https://www.ccamlr.org/en/publications/science_journal/ccamlr-science-volume-11/ccamlr-science-volume-11111-126
- Perry, C. 1998. A review of the impact of anthropogenic noise on cetaceans. Paper presented to the Scientific Committee at the 50th Meeting of the International Whaling Commission, 27 April –9 May 1998 Oman. SC50/E9. Available at: https://www.researchgate.net/publication/228587909_A_review_of_the_impact_of_anthropogenic_noise_on_cetaceans
- Weilgart, L. 2013. A review of the impacts of seismic airgun surveys on marine life. Submitted to the CBD Expert Workshop on Underwater Noise and its Impacts on Marine and Coastal Biodiversity, 25-27 February 2014, London, UK. Available at: https://www.cbd.int/doc/meetings/mar/mcbem-2014-01/other/mcbem-2014-01-submission-seismic-airgun-en.pdf
- Nowacek, D.P., Thorne, L.H., Johnston, D.W., and Tyack, P.L. 2007. Responses of cetaceans to anthropogenic noise. Mammal Review, 37: 81-115. Available at: https://cpb-us-e1.wpmucdn.com/you.stonybrook.edu/dist/6/1408/files/2015/12/Lesley-Thorne-Nowacek-et-al.-2007-1l5lg6r.pdf
- Wild, S., Krützen, M., Rankin, R.W., Hoppitt, W.J.E., Gerber, L., and Allen, S.J. 2019. Long-term decline in survival and reproduction of dolphins following a marine heatwave. Current Biology, 29: R239-R240. Available at: https://www.cell.com/current-biology/fulltext/S0960-9822(19)30217-9?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0960982219302179%3Fshowall%3Dtrue
- Simmonds, M.P., and Isaac, S.J. 2007. The impacts of climate change on marine mammals: early signs of significant problems. Oryx, 41: 19-26. Available at: http://www.cetus.ucsd.edu/sio133/PDF/Simmonds&Isaac.pdf
- Simmonds, M.P., and Eliott, W.J. 2009. Climate change and cetaceans: concerns and recent developments. Journal of the Marine Biological Association of the United Kingdom, 89: 203-210. Available at: https://www.researchgate.net/publication/228483586_Cetaceans_and_Climate_Change-Assessing_the_Risks