Conservation issues vary between species. Generally speaking, those with wide distributions and non-specialized diets are resistant to conservation issues whilst those that have limited distributions or are endemic to specific regions and specialized diets are vulnerable to conservation issues. Dolphins are impacted by pollution (chemical, acoustic and physical), habitat degradation and loss, overfishing, bycatch, ship strikes, hunting, and tourism. It is worth noting that these pressures do not occur in isolation and that many may work synergistically to impact dolphin populations.
Chemical and pathogens: Pollutants known to affect dolphins include: organohalogens, metals and untreated fecal matter. Organohalogens are organic compounds containing at least one halogen bonded to carbon. They are important in various industries as solvents and intermediates and agriculture as pesticides. They are extremely persistent and prone to bioaccumulation. Organohalogens known to affect dolphins include DDT, PCBs, HCH, PBBs, PBDEs, PAHs and phenols. These lipid soluble chemicals are known to accumulate in dolphin blubber and, depending on the presence of each of these organohalogens in the environment, the blubber of any one individual may contain a cocktail of these toxins. When dolphins use their blubber reserves the stored organohalogens can be released – in the case of sexually reproductive female dolphins these harmful chemicals are transferred to their calves. The known effects of organohalogens on dolphins include endocrine impairment (the glands that produce hormones to control metabolism, growth, development, tissue, sex, reproduction, sleep and emotional state), reproductive impairment and a heightened vulnerability to pathogens such as the morbilli virus. While some well-known organohalogens such as DDT and PCBs are widely banned others are only now emerging threats and even banned organohalogens can be released into the environment when deposits are disturbed.
Metals include heavy metals such as zinc, copper, cadmium, lead, organotins (carbon tin compounds used as biocides such as TBT anti-fouling paint) and methyl mercury. They come from a variety of sources including road run-off, manufacturing processes and oil spills. Some of these heavy metals such as mercury are known to bioaccumulate and can be highly concentrated in dolphin skin. Heavy metals can be extremely toxic and cause organ damage to dolphins.
Pathogens can be introduced to the environment from untreated human sewage, farm run-off and fish farms. These include bacteria, viruses, and fungi – particularly in individuals compromised by other sources of pollution and stress. The diseases caused by these pathogens can affect the metabolism and impair reproduction as well as causing skin lesions.
Together pathogens and chemical pollutants can reduce individual and population fitness.
Physical: Physical pollution includes entanglement hazards and objects that may be ingested by dolphins.
Discarded or lost fishing gear are a major cause of entanglements and small cetaceans, such as dolphins, are at immediate risk of drowning if they become entangled. If the dolphins are able to surface despite entanglement, they then risk exhaustion and infection.
The effects of ingestion are less obvious. Research into microplastics in dolphins found microplastic particles in every individual studied but only in minute amounts. Microplastics are primarily fibers coming from clothes, netting, and toothbrushes though there are also fragments from packaging, nurdles, microbeads, tires, paint and bottles. While microplastics do not seem to accumulate in dolphin guts – either passing through or being regurgitated – their effect is unknown. As slightly raised numbers of microplastic particles have been found in dolphins which have died of disease and it is possible that microplastics act as a vector for chemical contaminants which render dolphins susceptible to pathogens.
Acoustic: Acoustic pollution is commonly referred to as noise pollution and comes from an array of sources including seismic surveys, sonar, oil and gas drilling platforms and boat/ship traffic. Dolphins use echolocation to navigate and vocalizations and non-vocal communications to conduct their highly complex social lives. As such, acoustic pollution from human sources can impact almost every facet of dolphin life. In addition to impairing reproduction, hunting, and parental care, we can assume that this type of disturbance will impact on the stress experienced by these animals. The role of some acoustic pollution, such as mid-frequency long range active sonar, in causing the mass strandings of other cetacean species suggests acoustic pollution may have further implications for dolphins.
Habitats can become degraded through both the introduction of pollutants and the development of areas for human use. Pollutants, both physical and chemical, can have the effects described above. Developments and human uses that degrade dolphin habitats include: damming rivers, deforestation (impact on rivers), converting estuaries and lagoons into industrial areas, hydroelectric and windfarm activities and seabed disturbance including dredging. River dolphins seem particularly vulnerable to habitat degradation.
Overfishing is essentially interspecific competition for fish stocks between humans and dolphins with the humans easily outcompeting their cetacean rivals. This depletion of fish stocks drives conflict between dolphins and fishermen. With fewer available fish the dolphins in some areas, such as around Cyprus, have begun to actively target fishing nets as a source of food. Raiding the nets damages small artisanal fisheries but also places dolphins at an increased risk of entanglement in these static gears. Indeed, acoustic pingers designed to warn dolphins of the presence of nets may in these cases simply attract them by broadcasting the position of potential food. The damage of nets and loss of revenue means that fishermen have to compensate by increasing the number of nets they set which costs more and requires larger catches thereby further depleting fish stocks.
Bycatch is when non-target species are taken by fishing gear. Historically tuna fishermen found that targeting dolphin pods with seine nets allowed them to catch the large schools of tuna that swam further beneath the surface – a practice that is estimated to have killed up to 7 million dolphins. It wasn’t until a public boycott spearheaded by the Earth Island Institute and Sea Shepherd that this practice was curtailed. While we have mentioned that interspecific competition from overfishing increases the likelihood of entanglement in static nets, the bigger threat to dolphin species is to be caught in long line and trawl gear. Long lines consist of a main line which may be miles long with hundreds or thousands of baited branch lines hanging down – they can be used to catch demersal or pelagic fish. Trawled gear refers to the variety of nets towed by vessels to catch fish and includes. Some reports place 86% of toothed whale species at risk of bycatch, indeed, over a 20-year period 53% of common dolphins stranded in England and Wales were found to have perished as bycatch in nets.
Ship strikes are any instance in which an animal such as a dolphin is hit by any part of a ship. While propellers can leave deep open wounds, non-penetrative blunt force injuries can still kill dolphins. The severity of injuries and the speed of vessels are directly linked with velocities exceeding 13 knots drastically increasing the likelihood of a fatal outcome. Busy shipping lanes can be dangerous to larger cetaceans, but dolphins are primarily at risk from smaller craft in high traffic areas such as marinas, harbors and inlets. Dolphin curiosity can pose a problem here with dolphins approaching and inspecting vessels and their propellers.
Dolphins are widely hunted as both a source of food and a means to remove interspecific competition for fish resources. Dolphin hunts occur in Turkey, Peru, Sri Lanka, Japan and the Faroe Islands among others – with catches of dolphins and small whales around the world being estimated at up to 100,000 a year. Dolphin hunts are thought to be underreported but they are thought to far exceed the number known to be taken in commercial and aboriginal hunts. The deficiency of data makes it difficult to assess the impact of this particular threat though some believe it may be second only to bycatch as a cause of death for dolphins.
This is compounded to some degree by the focus on a few high-profile hunts such as the Faroese Grindadráp and Japanese Taiji dolphin drives. The Taiji dolphin hunts are in part supported by the captive industry which relies on such methods to secure new animals.
While foreign denunciation of such practices is vocal it is the growing pressure within countries and communities where hunting occurs, which will ultimately lead to change.
Tourism, specifically the rise of ecotourism and dolphin watching presents a non-lethal disturbance that can, in some cases, negatively impact populations of dolphins. The impact of disturbance varies with species and population. Generally speaking, small, closed populations of dolphin in areas where prey items may be limited are more susceptible to long-term damage from disturbance while large, open populations of dolphin without food restrictions are quite resilient. Different vessels offer different levels of disturbance with dolphins becoming aware of moderately-sized ships up to 3 km away and smaller craft as near as 800 m. The presence of nearby vessels can disrupt natural behavior including hunting, feeding, sleep, play, reproduction, and socialization with a long-term reduction in population fitness for the small, closed populations. As little as a 10% chance of being disturbed can affect these small populations while the larger, open populations can tolerate anywhere from a 25-50% of disturbance before behavior is impacted. Boat avoidance, increased demands on respiration, increased stress levels and alertness alongside decreased foraging rates are all known effects but repeated disturbances can also drive dolphin populations out of important habitats used for foraging and reproduction.
Human accelerated climate change has a number of implications for dolphins including shifts in distribution, shifts in food webs, increases in algal blooms and the loss of important habitats.
Changing water temperatures are likely to lead to the contraction of the distribution of cold-water species into increasingly smaller areas with a corresponding decrease in the available food sources. While warmer water species are likely to increase their geographic ranges but increasingly face interspecific competition through overlapping distributions. Additionally, it is thought that some important prey may shift in their depth distributions – moving down to cooler, deeper waters out of the diving range of some dolphin species and increasing the competition for remaining food sources. Increased water temperatures may also trigger earlier migration patterns with longer residencies of transient populations in given areas and increased breeding in response to a longer warm season. Alongside heightened interspecific competition and possible shifts or collapses in marine communities (due to the effect of increased water temperatures on other species) it seems unlikely that increased breeding and population growth could be readily supported by their new environment.
Furthermore, some mass mortality events have already been linked to toxic algal blooms in low latitude regions as a result of higher water temperatures – increasingly warmer waters are likely to spread these events further and widen their impact.
What can you do?
Pollution: You can reduce physical pollution by following the 6 Rs for waste, particularly plastic, these are: Rethink, Refuse, Reduce, Reuse, Repair and Recycle. Rethink your options – use alternatives to plastic products and particularly single use plastic products. Refuse – politely decline things you do not need such as disposable straws and utensils. Reuse – try to reuse items as much as it is practical and safe to do so. Repair – instead of discarding damaged items try to repair them if it is safe to do so. Recycle – when you have to discard items try to ensure they are correctly recycled where possible. A key note here is that we know fibers from clothing can be an important source of microplastics, if you are able to switch to natural fibers then you can reduce the amount of microplastic fibers entering the environment.
With regards to chemical pollution and pathogens from untreated sewage, look to raising the issue with elected officials and vote for political leaders and parties which will issue production bans and waste treatment.
Overfishing: You can help prevent overfishing in different ways. First and foremost, you can reduce demand and support sustainable fisheries management – you may choose to pursue vegan seafood alternatives, switch to less pressured fish species or shop for sustainably resourced fish.
Secondly, you can support fisheries management efforts – in particular the establishment and maintenance of Marine Protected Areas and, more specifically, Highly Protected Marine Areas.
Bycatch: You can ask elected officials to support incentives and legislation to increase the use of bycatch reduction devices and changes to fishing practices that reduce the threat posed to dolphins by fishing trawls and long-lining.
Habitat degradation and loss: You can help to raise awareness of the impact of habitat loss on species such as dolphins, elect people who will protect environments and reduce your use of products that fuel habitat degradation such as deforestation in the Amazon which impacts the Botos dolphin.
Dolphin Hunts: Ultimately dolphin hunts are most susceptible to internal pressures from the communities they serve. This makes education the key to regulating and reducing dolphin hunting activities. You could use social media platforms to share and raise awareness about these practices or consider supporting the various charities which work with communities to reduce dolphin hunts or those charities which campaign to raise awareness of them.
Ship strikes: If you are a boat user then please make sure you educate yourself on the best practices for boating in and around cetaceans. If you are not a boat user then look for charter companies and boats that do follow best policy.
Tourism: When looking for dolphin watching charters ask them how they minimize disturbances to the dolphin populations they visit and ensure they follow best practices for approaching and navigating around dolphins.
Climate change: Look for ways that you can reduce your carbon footprint (transport, power usage, diet, family planning) and elect governments that commit to tackling climate change.
MarineBio would like to offer its thanks to Katrin Lohrengel of the Sea Watch Foundation for her help with this page, the Sea Watch Foundation (https://www.seawatchfoundation.org.uk/) for materials which were key in producing the sections on Conservation and to Chloe Robinson for kindly providing access to her paper, ‘Don’t Hold Your Breath: Limited DNA Capture Using Non-Invasive Blow Sampling for Small Cetaceans’ by Robinson and Nuuttila, which was helpful in understanding the function and biology of blowholes.
Send us your questions or leave them as comments below about this topic.
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