Common Dolphin

COMMON DOLPHIN

Short-beaked common dolphins are frequent visitors to the Clyde and home to at least one resident animal called Kylie.

Classification
The common dolphin (Delphinus spp. ) is a widely distributed member of the dolphin family, Delphinidae and originally a bit of confusion  to how many different species there wereThe genus has now been split into the Short-beaked  (Delphinus delphis) and Long-beaked common dolphin (Delphinus capensis).  A third sub-species has been proposed,  the Very-long-beaked common dolphin (Delphinus tropicalis), but may be a regional variation of D.capensis. They are a member of the suborder Odontoceti  the toothed whales within the order Cetacea.

Morphology
When born individuals can be between 0.7 and a metre long and grow to between  1.5 – 2.2 metres in length. Males are usually a little longer than females and animals can weigh up to 200 Kg. They have a tall, falcate dorsal fin with large tapered pectoral fins.  Common dolphins are easily distinguished by a pale yellow thoracic and grey posterior hourglass body colouration that runs along the length of their body. Dorsal of the hourglass is black to dark grey in colour; the ventral surface is white. The beak is separated from the melon by a crease along which a black or dark grey stripe runs to the eye. The dorsal surface of the beak is grey, and both mandibles have a dark tip. From the lower mandible runs a thin black or dark grey stripe that joins to the pectoral fins. The dorsal fin is black to dark grey; adults often have a small lighter grey triangle within their dorsal. The pectoral fins and fluke are similar in colouration to the dorsal fin.

Behaviour
Short-beaked common dolphin groups generally consist of 30 or fewer individuals; thought to be closely related. However groups of hundreds or thousands have been seen gathered together, with possible segregations by age and sex. Recent research has suggested that D.delphis have a fluid fission-fusion social structure, similar to coastal Bottlenose dolphin (Tursiops truncatus). Common dolphin are known for their acrobatic breaching and boisterous behaviour, acoustically and visually detectable from miles away. They are fast swimmers, reach speeds in excess of 15 mph, and often come from distance to bow ride vessels.  Common dolphins have been seen in association with many species including Pilot whales, Risso’s, Grampus griseus, Lagenorhynchus dolphin species. A solitary common dolphin called ‘Kylie’ is known to associate  withHarbour Porpoises Phocoena phocoena in the Clyde.

Feeding 
Short-beaked common dolphins have between 41 – 54 pairs of teeth with a few more on their lower jaw which the use to grasp prey items to swallow whole. They generally  feed on epipelagic and mesopelagic squid or small schooling fish from the deep scattering layer. Stomach contents analysis in European waters on stranded or incidental fishing by-catch suggests that the main prey species of D.delphis are herring (Clupea harengus), mackerel (Scomber scombrus), sand eel (Ammodytes spp.), sprat (Sprattus sprattus), and long-finned squid (Loligo spp.).

Common dolphin time their foraging effort with the vertical movement of prey items during the night and dives to 200 metres have been recorded. Individuals will co-operate to herd fish in order to catch them more easily. During the daylight hours the animals spend most of their time resting and socialising before, in late afternoon, breaking up into smaller groups in anticipation of prey. They are often seen in association with diving gannets feeding on the same fish.

Vocalisations

Their high-pitched vocalisations can, at times, be heard by humans above the surface of the water.

Reproduction
The reproductive behaviour of the Short-beaked common dolphin seems to vary with its distribution. In tropical waters calving is thought to occur all year round. At higher latitudes it tends to peak in late spring and early summer with a 1-3 year calving interval depending on geogrpahical location. Gestattion last between ten and eleven months. Males in the black seee reach sexual maturity much younger from 3 years old compared to Eastern Pacific dolphins which become sexually mature t 7-11 years and thought to be related to animal density. Maximum age estimates also differ with maximum age estimates of 22 years in the Black Sea. Common dolphins with young calves have been observed during summer months in the Hebrides.

Distribution
The short-beaked common dolpohin can be found between approximately 60o North and 40o South. They are the most numerous delphinid species found in the offshore temperate and tropical waters of the Atlantic and Pacific Oceans. Short-beaked common dolphins can be found in the eastern Atlantic from southern Norway to Gabon in West Africa, including the Mediterranean and Black Seas; and from Newfoundland to Florida in the western Atlantic. In the eastern Pacific they can be found from southern Canada to central Chile; and in the western Pacific around New Caledonia, New Zealand, Tasmania, southern Japan, and Southeast Australia. Common dolphin population estimates in the Celtic Deep (UK) range between 23,000 and 249,000. Other estimates suggest 100,000 animals off the British Isles and France, tens of thousands in the Western Atlantic. Hundreds of thousands in the Northeast Pacific, and three populations estimated to total three million in the eastern tropical Pacific.

Many studies have looked at their distribution in relation to oceanographic features such as bottom topography, and sea surface temperature (SST). D.delphis distribution has often been linked with areas of upwelling along continental drop-offs and underwater banks. It has been suggested that Common dolphin use offshore ridges as migration channels. Very little is known about their movements, although offshore migrations have been reported during the autumn and winter months in the California Bight, North-west Bay of Plenty and Irish/ Celtic Sea.

Sea surface temperature (SST) studies have presented mixed results. Dohl et .al (1986) reported Common dolphin movements related to SST. As SST rose in late spring/ early summer they observed an increase in animal sightings, no animals were sighted in waters cooler than 14oC. More recent studies suggest that although D.delphis prefer warmer surface waters, it is not the driving force behind their movements. They both suggest that the availability and movement of prey are more likely to be the primary factor influencing Common dolphin distribution. This theory is backed up by commercial fishermen in Neumann’s New Zealand study areaas, like the dolphins, they too move further offshore in pursuit of fish.

Where the Celtic and Irish Sea meet an oceanographic front is known to seasonally occur during spring and summer. This is the boundary where tidal mixed, nutrient rich, water from the Irish Sea meets thermally stratified water from the Celtic Sea, creating a thermal front. The stratified water on the Celtic Sea side holds planktonic organisms at the surface of the water, where sunlight enables them to photosynthesise. While tidal water from the Irish Sea provides the plankton with nutrients, enabling the front to persist. Thermal fronts are usually associated with areas of high primary productivity, capable of supporting top-level predators, such as dolphins. The Celtic Sea front may be one of the most important areas in British waters for predators such as the Short-beaked common dolphin. This makes it a good area to encounter D.delphis.

Conservation
The global population of D.delphis is considered by the IUCN2 to be ‘lower risk, least concern’ meaning it is under little threat of extinction (CITES, 2004 & IUCN, 2003). However in 2003 the Mediterranean subpopulation of Short- beaked common dolphin was placed on the IUCN red list as ‘endangered’ due to a 50 percent reduction in numbers over the last 30-45 years; reported to be because of over fishing and habitat degradation (IUCN, 2003).

In Scotland cetaceans are protected by the Nature Conservation (Scotland) Act 2004 and protected by section nine of the Wildlife and Countryside Act 1981, and Wildlife Order 1985 in the UK. These acts prohibit the intentional killing, injuring of an animal, and destruction of its habitat. All cetaceans are also listed on Annex IV of the EC Habitats Directive, and Annex A of the EU Council Regulation 338/97 prohibiting commercial trade in the species. Despite this legislation the future of D.delphis in British waters is still uncertain. There are four main threats affecting the population; interaction with commercial fisheries, boat activity, pollution, and over exploitation of marine resources leading to changes in the ecosystem.

Each year vast numbers of D.delphis are caught in the fishing nets of industrial trawlers around the Celtic Sea and English Channel.  593 Common dolphins were reported dead off the Portuguese coast between 1975 and 1998 a total of which 44% were reported to be as a result of fisheries bycatch and interaction. French drift-net fisheries reported 400 Common dolphin accidentally caught in their nets between 1992 and ’93. In 1995 a small UK drift net fleet operating in the Bay of Biscay reported a total bycatch of 60 Common dolphin. Between 1990 and 1995 a total of 138 Common dolphin washed up along the British coast line. Post mortems took place and concluded that at least 62% were as a result of fisheries interaction. These are potentially sizable chunks of a Common dolphin population with no accurate estimation of size.

The English Channel and Irish Sea are some of the most intensively used stretches of water in the world. Ships can potentially cause physical harm to cetaceans via collisions, and indirectly by underwater noise pollution resulting in auditory damage at close range. If prolonged this can affect a dolphins’ ability to navigate, communicate with conspecifics and find prey. Seismic surveys of the sea bed have recently taken place in the southern Irish and Celtic Seas in pursuit of oil and gas. No relationship was found between the presences of D.delphis in the area of seismic surveying. However Short-beaked common dolphin have been found to significantly increase both the frequency and rate of their vocalisations during a survey.

Pollution of the marine environment with man-made pollutants, like PCB’s and DDT, are hidden dangers affecting cetaceans. As top-level predators, dolphins consume a large variety of man-made industrial pollutants through their prey. These contaminates bind with fats and accumulate in their blubber layer. Organochlorines have been shown to cause suppression of the immune system and reduce reproductive potential.