Science

DEPARTMENT OF VERTEBRATES

Scientists from the South Australian Museumâs Mammal Section and Evolutionary Biology Unit and students from the University of Adelaideâs Molecular Biosciences and Environmental Biology Departments have recently solved the mystery of how many species of common dolphin are present in southern Australian waters. The study, funded in part by a grant from the Sea World Research and Rescue Foundation Inc, investigated the molecular make-up, and the shape and size of skulls and skeletons of common dolphins that died from various causes and were found along the southern coast of Australia.

Photo by E. Calder

Background

Common dolphins are highly mobile small cetaceans distributed widely in tropical and temperate seas. There is considerable size, shape and colour pattern variation between and within populations and as a result, scientists have described more than 20 species over the years.

Photo from the National Oceanic
and Atmospheric Administration,
National Marine Mammal Laboratory, Alaska.

Recent studies of northern hemisphere common dolphins suggest that there are only two species, the Long-beaked Common Dolphin (Delphinus capensis) and the Short-beaked Common Dolphin (D. delphis) and these are differentiated by skull and body proportions, body colour patterns and molecular genetic profiles.

Common dolphins, with various beak lengths, are found in southern Australian waters but the speciesâ identity had not been resolved, so a study, funded by SWRRFI, was carried out by Stephen Donnellan and Catherine Kemper at the South Australian Museum, and graduate students from the University of Adelaide Catherine Bell and Catherine White. It looked at the evolution of mitochondrial DNA sequences by comparing common dolphins from southern Australia with those from the eastern North Pacific and the Black Sea. It also studied the skulls and skeletons of southern Australian dolphins and compared them with common dolphins from the eastern North Pacific.

The Findings

Molecular analyses of 38 common dolphins (some with short and some with long beaks) from eastern and southern Australia showed that there were no obvious groups based on beak type but there was evidence of close relationships with the Short-beaked Common Dolphin from the northern hemisphere. Click here to see an evolutionary tree of common dolphin mitochondrial DNAs. More detailed analyses did not show any separation of common dolphin populations along the eastern and southern Australian coastlines.

For the morphological study, measurements, counts and features were recorded for 130 mature common dolphin skulls and skeletons from the southern coast of Australia, including Tasmania. Statistical analyses showed that there was no evidence for discrete groups and that the size and shape of the skulls was very variable. Dolphins from coasts near the edge of the continental shelf, and therefore deep water, tended to be larger than those from areas where the shelf was distant. The skulls of Australian common dolphins were similar to, but not entirely like, the Short-beaked Common Dolphin from the eastern North Pacific.

Extremes in shape variation in common dolphin skulls from southern Australia.
Scale bar = 10cm. Photos by Catherine Bell.

The study concluded that one species, the Short-beaked Common Dolphin Delphinus delphis Linnaeus, 1758, inhabits southern Australian waters and that this is variable in size.

Implications of our findings

Knowing how many species of marine mammals inhabit an area and how to identify them are critical parts of the conservation of marine ecosystems. For a number of years it was thought that two species of common dolphin might occur in Australian waters but our study has ended this uncertainty. Biologists, wildlife officers, fisheries managers, fishers and whale enthusiasts will now be confident in listing the Short-Beaked Common Dolphin as the only species of common dolphin present in the region. The study has also made an important contribution to the world-wide study of naming and describing whales and dolphins, mammals whose distributions usually span several continents. In the process, training has been provided for two student researchers in taxonomy, marine mammal biology, molecular systematics, molecular evolution and evolutionary biology.

For more information on common dolphins:
http://www.acsonline.org/factpack/common.htm

http://www.cetacea.org/common.htm

What the The Action Plan for Australian Cetaceans says about common dolphins:

http://www.ea.gov.au/coasts/species/cetaceans/actionplan/whaleap5l.html

Some facts about common dolphins in Australia

Scientific Name: Delphinus delphis Linnaeus

Common Names: Known in Australia as the Short-beaked Common Dolphin, but by many others names elsewhere.

Protected status: Since 1980, all whales and dolphins have been protected in Australian waters.

Appearance:
Common dolphins have a fine, streamlined body, with a long narrow ãbeakä and a sharp angle between the beak and the melon (forehead). The dorsal fin is tall and roughly triangular in shape, with a pointed tip. The flippers taper to a pointed tip and the tail flukes are concave with a distinctive median notch on the rear margin. There are 37 to 48 small, sharply pointed teeth in each row.

The colour pattern of the common dolphin is quite elaborate as far as cetaceans go. The back is dark grey to black from the top of the head to the tail dipping to a V on the sides below the dorsal fin. The flanks are light grey behind the dorsal fin and yellowish-tan forward of the dorsal fin, forming an hourglass pattern. The belly is white.

Photo by H. & A. Wapstra

The long, well-defined beak is generally black. The dorsal fin ranges in colour from all black to mostly pale grey with a dark border and often has a white patch. The flippers, often with a paler centre, and flukes are dark grey or black above.

Photo by E. Calder

There are large dark circles around the eyes connected by a dark line that runs across the head behind the beak and a dark stripe runs from the jaw to the flippers.

The colour pattern of juveniles is less distinct than that of adults but is nevertheless recognisably the common dolphin pattern.

Photo by L. McDiarmid

Identifying common dolphins in southern Australia:
Common dolphins are easily identified by their distinctive brown/yellow hourglass pattern along the side of the body, their very dark grey (almost black) colour on the upper parts of the body and by the dark stripe between the beak and the flipper. Several other species, the most common being the bottlenose dolphins, also occur in southern Australia.Ê Bottlenose dolphins are dark grey above and paler below, and lack the obvious patterns described above.

Photo by C. Kemper

Photo by M. Tarleton

Size:
Common dolphins in southern Australia may range up to 2.3 m in length and about 115 kg, although maximum size for individual adults can be a small as 1.8 m long and about 70 kg. Males tend to be slightly larger than females but this has not been tested statistically.

Lifespan:
The life span of the common dolphin in Australia is not known. However, elsewhere in the world they are known to live to at least 25 years.

Distribution:
The common dolphin maybe one of the most widely distributed species of cetacean. It is found world-wide in temperate, tropical and sub-tropical seas from 40^oN to 45^oS in the coastal waters of the Pacific, and north of 50^o in the Atlantic Ocean. Previously, common dolphins were thought to be entirely pelagic (of the open ocean) but recent information in southern Australia shows that they also occur in shallow, inshore waters. Studies on common dolphins outside Australia suggest that their distribution correlates with water temperature (10 to 28^¡C) and bottom topography such as seamounts, escarpments and undersea ridges.

Food:
From studies of a limited number of stomach contents of common dolphins from South Australia, fish and cephalopods appear to be the main food sources, with fish being predominant.Ê Carangids and clupeids are the most often represented fish families. The main cephalopod is the southern calamary (Sepioteuthis australis).

Common dolphins from elsewhere feed from the surface to at least 280 m depth, in the inshore and offshore environments. They sometimes take advantage of human fishing operations to get prey. There is some evidence of competitive interactions for food between spotted and spinner dolphins and tuna. Common dolphins have been observed in frenzied activity when feeding and it is possible that individuals co-operate to frighten fish in order to catch them. On one occasion a common dolphin was observed to throw a fish into the air four times before eating it.

Reproduction:
Almost nothing is known of the reproduction of common dolphins in Australia. In South Australia they become sexually mature at 1.7 to 2.0 m body length. In other parts of the world, mature common dolphins seem to go through seasonal cycles of sexual activity and these differ in duration and timing between populations. In the temperate eastern Pacific, calving occurs in both spring and autumn. Around the UK, juvenile common dolphins are first observed in June, suggesting that breeding takes place in early summer. In the Black Sea, calves are born mainly in summer and autumn, with births peaking in July. In Australian waters, the locations and timing of calving are not yet known.

Based on studies elsewhere, pregnancy lasts for about 9 to 11 months and the young are believed to suckle for about 6 months. Calves eat solid food from 2 to 3 months. Mature females have a calf every 1 to 3 years.

Behaviour:
Common dolphins are very gregarious. Some aggregations observed in Australian waters number thousands, or even 100 000 individuals. They are very acrobatic and ride the bow waves of boats and large whales. They have been seen with other species of dolphin, including bottlenose, as well as larger cetaceans (fin, humpback, blue, southern right whales). Common dolphins are believed to travel great distances although no studies have been conducted in Australian waters to verify this.

Altruistic behaviours, such as females assisting others with birth, and baby-sitting behaviour, have been observed in common dolphins, both in captivity and in the wild.

Sounds
Common dolphins use echolocation as well as audible sounds to communicate, navigate and hunt prey, much like other members of the dolphin family. You can listen to common dolphin sounds at http://neptune.atlantis-intl.com/dolphins/sounds.html

Population size:
Population estimates for the northern hemisphere indicate declines in some areas, particularly the eastern tropical Pacific where purse-seine netting results in many incidental deaths. No attempt has been made to estimate numbers in Australian waters but they are considered common based on the number of strandings and sightings.

Threats:Ê
Live strandings of single and multiple animals are fairly common along the Australian coast. One mass stranding of 34 animals has been reported for Victoria and several (up to 109 individuals) for Tasmania. Many single common dolphins are found dead along the southern Australian coast and human influences are suspected in a number of these. Causes of mortality are often not established but have included entanglement in fishing gear and aquaculture nets, intentional killing by humans (usually shooting), presumed stillbirths, severe lungworm infection, haemorrhagic septicaemia in the lungs, pneumonia, congested lungs, and bone lesions associated with hepatitis. Predators include the Killer Whale. An unfortunate stranded animal in Western Australia had a stingray barb embedded in its body.

Photo by C. Halstead

Common dolphins have been used as bait in craypots by some Australian fishers and one such illegal case in South Australia resulted in conviction for the offenders. In the past, some animals were captured for display in oceanaria. The true extent of intentional and unintentional human interactions is not known because many cases go unreported.

Potential threats include entanglement in drift-nets set outside Australian territorial waters and in lost or discarded netting, pollution (including increasing amounts of plastic debris at sea, oil spills and dumping of industrial wastes into waterways and the sea) leading to bio-accumulation of toxic substances in body tissues. Levels of cadmium have been shown to be quite high in some South Australian common dolphins but the effects on the animalsâ health are not known.

In other parts of the speciesâ range, threats are more varied and immediate and possibly more serious. In the past 20 years, hundreds of thousands of common dolphins have accidentally died, along with Spinner and Pantropical Spotted Dolphins, in purse seine nets used during tuna fishing operations in the eastern tropical Pacific. Common dolphins also may be caught accidentally in other fishing gear, such as midwater trawls. Turkish and Russian fishermen used to catch large numbers of common dolphins in the Black Sea for meat (to be used for fish meal) and oil. The fishery stopped after the common dolphin numbers became severely depleted (and still is); there are several reports suggesting that the Turkish fishery may have resumed. Many common dolphins are taken in a Japanese small cetacean fishery and directly caught in the Mediterranean. Some common dolphins may be taken in Peru for human consumption.

Reporting strandings or sightings:

To report a stranded or injured whale or dolphin, ring Fishwatch 1800 065 522 or your local National Parks and Wildlife SA Office.Ê To provide information regarding strandings, fill in the bold fields on the attached form (Adobe Acrobat - 59kb) and fax or post it to the South Australian Museum.

If youâd like to report any sightings of live whales or dolphins to the South Australian Museumâs Mammal researchers, you could use the WHALE AND DOLPHIN REPORT FORM. (Adobe Acrobat - 118kb)