The many threats to dolphins and other cetaceans

By Milla de Villiers

There is NO nation that is totally innocent of harming dolphins. We do not see the suffering caused by many things which we are either unaware of or largely ignore. There is much that we all can do – we can all personally take responsibility for many things, and also lobby our respective governments to change destructive laws. Here a brief look at some of those threats.


dolphin_in_netDolphins and porpoises in particular are vulnerable to being caught and drowned in fishing nets, but larger whales are also prone. In excess of 300,000 cetaceans are estimated caught in fishing nets every year worldwide (Read et al 2003[1]). Although the majority of these are dolphins and porpoises, some large whales are vulnerable. The small and vulnerable North Atlantic Right Whale population in the north western Atlantic suffers several entanglements in nets every year. It is estimated that 72% of the current population have suffered entanglement at some time[2]



Most of the commercially viable fish stocks in the world are over-fished. Some are being driven to almost certain extinction, including the Blue-fin Tuna and several species of shark. However, it is not just the big predatory fish that are over-harvested; Anchovy and Sand Eels in European waters are among many species whose populations are in a poor state due to overfishing. These are important food sources for species further up the food chain, including seabirds and cetaceans, which are therefore put at risk.


For many species of whale and dolphin sound is the primary sense, and is used to navigate, find food and communicate in an environment where smell is useless and sight nearly so, and sound is greatly enhanced compared with in air.

Shipping noise

Until little over 100 years ago the oceans were almost unaffected by human related noise. Modern propeller driven ships and smaller vessels are very noisy and increasingly abundant in many parts of the world, especially in highly populated areas like around European, southeast Asian or USA coasts. Here shipping noise is constant and intense, and is considered likely to interupt communication and food-finding in cetaceans.

Military activity

Recent developments in anti-submarine measures that generate very loud short pulses of sound have been implicated in some mass-strandings of particularly, but not only, beaked whales. Some animals have suffered damage to brain and other tissue associated with hearing and sound transmission, as well as other organs. Meanwhile others have suffered ‘the bends’ (nitrogen bubbles in the bloodstream) as a result of overly rapid surfacing from great depth, presumably as a panic response to the noise. Less acute, but nonetheless serious, is the displacement of animals during these operations, forcing them to abandon important foraging habitats. Marinelife has noted during systematic surveys, periods following naval exercises in Biscay when cetaceans are absent from a large area around where the exercise took place, which possibly points to significant displacement levels.

Seismic survey

Oil, gas and other under-sea mineral exploration commonly uses seismic surveys (creating pulses of intense noise and recording its transmission through the rock strata) to find new mineral reserves. Cetaceans are known to be displaced by seismic survey activity, which can last for several weeks in a given area.

Construction noise

Offshore construction is rapidly increasing, particularly the construction of large wind farms, but also building of bridges and other structures, which require prolonged pile-driving and even underwater use of explosives. This can cause behavioural changes in marine mammals up to 10km distance.


Pollution comes in many forms and has many impacts on the marine environment and marinelife that live there. Industrial and domestic pollution enters the marine environment mainly through rivers. But direct pollution from shipping is also significant.


Nutrients from agriculture, sewerage and industrial processes enter the sea in huge amounts. This can lead to algal blooms sometimes known as red tides, some of which can cause death in fish, marine mammals, birds etc.. Increased nutrient loads can also lead to reduction in dissolved oxygen. In many developed areas of the world, legislation is controlling the amount of eg untreated sewerage entering the sea.

Chemicals and heavy metals

These arise from land based industrial, atmospheric or ship sourced pollution and enter the food chain. The are concentrated in marine mammals and can cause reductions in fertility, reduced immunity from disease etc. Ironically, recently, Iceland whale meat exported to Japan was rejected because of excessive heavy metal content.


Plastics are abundant polluters of the marine environment. Plastic sheeting, bags etc are commonly seen in shipping lanes and are mistaken by cetaceans, turtles and seabirds for food, and ingestion is often fatal. The impact is most famously to be seen in Laysan Albatrosses living in the Pacific Ocean that feed in the area known as the Pacific Gyre where huge volumes of plastic debris are concentrated by currents. Thousands die each year from plastic ingestion. However, seabirds in all oceans are at risk. A study of dead Fulmars in the North Sea found plastics in 98% of stomachs[1]. Balloons are also implicated. Plastic pollution also occurs as very fine granules, either direct from shore-based industrial emissions or from breakdown of plastics in the ocean. Granules often mimic plankton and are therefore eaten by plankton feeding marinelife. The plastics eventually break down and then release chemicals into the marine environment.

Oil pollution

Oil pollution from oil tanker wrecks, oil installation blow-outs and from illegal discharges at sea of fuel tanks is a well known and acute pollution source that kills a lot of the marinelife in the vacinity of the incident. The numbers of animals affected depends on many factors; proximity to breeding or wintering concentrations, size of spill, type of oil, sea currents and weather conditions. However the impact is probably longer and wider than immediately apparent as oil pollution will disperse, and break down and enter the foodchain. Pollution incidents continue with little evidence of success in reducing their frequency or impact, even though pollution monitoring has increased, and many incidents are missed.

Alien species

The rate at which species are being redistributed around the globe is increasing dramatically, often carried in seawater used as ballast by pan-global shipping – to balance the ship whilst transiting with little or no cargo. Taken on in one port and pump it out in another ocean. The seawater often includes fish, and crab larvae, algaes etc., some of which survive, and may thrive in their new home and can imbalance the ecosystem, either by predating on rare species or consuming their food source.

Shipping emissions

Most shipping uses low-grade bunker fuel oil, which has a sulphur content 2,000 times that of car diesel. The seas are a major sulphur sink (absorber), but the increased rates may lead to increasing acidification. Shipping emissions pollution is concentrated in the high shipping traffic areas, which include inshore European waters. Shipping is also considered to contribute 3.5-4% of all climate change emissions. However, research suggests that the same pollution helps form clouds which reflect solar heat back to space, so helping cool the planet. There are increasing moves to limit sulphur emissions from shipping, led by the UN.



Whale and dolphin collisions with ships and pleasure craft are a potentially significant cause of mortality. The seas are busier now than ever before, and the size and speed of commercial shipping is increasing. Cetaceans, and other marinelife are not evolved to be able to adapt to these changes. The scale of the problem is very hard to assess as most large ship collisions go unreported and often unnoticed. Where shipping is concentrated on busy routes that pass through important marine mammal habitat it is inevitable that the risk of collision increases. In the case of rare species any deaths through collision are highly significant, and when the species range is restricted to areas with high shipping traffic, eg in the case of the North Atlantic Right Whale with a global population of less than 400 animals, the problem could be critical. More than 50% of the known deaths of North Atlantic Right Whales between 1970 and 2006 involved shipping collisions[1]. The small isolated Fin Whale population in the north western Mediterranean is also vulnerable. Concern has been expressed by ASCOBANS[2] about the proliferation of high speed ferries (traveling in excess of 30 knots) and the risk of collision with small as well as larger cetaceans in the English Channel in particular, but also in other high traffic areas. Fast ferries have limited manoeuvrability and the mammals have less time to react to their presence hence collision risk is increased. Small pleasure craft abound in coastal waters, and collisions are inevitable. Inshore dolphins often show injuries from high speed small propellers. The numbers of fatalities are impossible to quantify. There are even several documented cases of whale watching vessels fatally injuring whales! Species include Minke, Fin and Humpback Whales[3].


[2] ASCOBANS the Agreement on Small Cetaceans of the Baltic and North Sea. An agreement between governments on the conservation of small cetaceans, in an area that is now extended to include western European Atlantic


The summaries were all found here:

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