Deep dive into...Common dolphins

Common dolphins (Delphinus delphis) are often spotted in large groups and are highly sociable. 

Slender in size, they’re easily identifiable because of their colour patterns – dark grey/black above and white underneath. They’ve a distinctive hourglass pattern on their sides, with a yellow-cream area starting behind their beaks.  

Common dolphins also have dark patches around their eyes and their dorsal fins are tall and triangular, curving slightly backwards. Typically, they weigh up to 140kg and have a length of 2.7 metres. Their average lifespan is 35 years.  

As their name suggests, they are one of the most common cetacean species in the world with an estimated population of six million. They are listed as ‘Least Concern’ on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species. However, the Mediterranean population is listed as ‘Endangered’ and the Black Sea population is ‘Vulnerable’.  

At one stage, common dolphins were categorised into short-beaked and long-beaked types but in 2015, it was determined that all populations should be more logically grouped under one species called Delphinus delphis, with four recognised subspecies of common dolphins throughout the species’ range.  

These are the common dolphin, Eastern North Pacific long-beaked common dolphin, Black Sea common dolphin and the Indo-Pacific common dolphin.  

Some differences between short-beaked and long-beaked common dolphins are that the former is relatively heavier, has a larger dorsal fin and flippers  and also has a more rounded melon that meets the beak at a sharp angle (making it short-beaked as opposed to long-beaked).  

Common dolphins are usually found offshore but do sometimes come closer to shore to feed. They often form superpods – huge groups made up of hundreds or thousands of animals – when they’re out at sea.  

The species often approaches boats and ride the bow and stern waves of the vessels. They’re also well known for their energetic leaps out of the water.  

Very fast in the water, they regularly reach speeds of up to 55-60 km per hour. Common dolphins are known to be a highly vocal species, producing a range of clicks, whistles and pulses when they are communicating.  

Check out this video of a superpod of common dolphins in Monterey Bay, California:

They’re found in warm temperate and tropical waters around the world, both in deep ocean waters and offshore regions. They’re less likely to be spotted near to the shore.  

Common dolphins’ prey varies depending on their habitat – some populations feed in productive offshore regions (they concentrate on fish and squid species) but common dolphins living in continental shelf or coastal areas usually feed on schooling fish such as sardines, herring, anchovies and mackerel.  

They often work together as a group to herd their prey into a ‘bait-ball’ and are even known to team up with tuna when both species are preying on schooling fish.  

Like other cetaceans, common dolphins can become entangled in fishing gear which goes on to cause injury, fatigue, comprised feeding and sometimes even death.  

In several regions, common dolphins’ association with tuna brings them into contact with tuna fisheries and puts them at risk of bycatch (particularly in the eastern tropical Pacific Ocean).  

It’s estimated that up to 10,000 common dolphins are killed in the Bay of Biscay each year due to fishing gear entanglement. Recently, a French court made a decision to establish no-fishing zones to protect these dolphins. Find out more in this Environmental Investigation Agency article.  

Common dolphins are at risk of vessel strikes throughout their range but the threat is much higher in areas with busy ship traffic.

Climate change and pollution are a threat to all whales and dolphins because of the loss of habitat as waters become warmer.  
 
Plastics and micro plastics, along with chemical pollutants, entering into the water system are a serious threat to all creatures in our ocean.

Common dolphins, like other cetaceans, use noise to communicate and to locate prey. Increased noise pollution from vessels and other human activity interferes with this ability.

In 2013, an undercover investigation found that up to 15,000 dolphins a year (including common dolphins) were being hunted by Peruvian fishermen, the catch mostly being used as shark-bait. There have also been directed hunts for common dolphins in Japan.  

For many years, Turkish and Russian fishermen caught common dolphins in the Black Sea for meat and oil but this has since ceased.  

Attempts to take common dolphins from the wild and keep them in marine parks have usually resulted in death. While they are sociable and active in the wild, common dolphins are actually very shy. When kept in captivity, they are prone to illness, shock and/or depression, all of which can lead to premature death.  

As with other dolphin species, common dolphins are sometimes the prey of orcas and large sharks. Researchers also believe they may be preyed upon by false killer whales or pilot whales in some areas.