Common Dolphin Stranding at Die Plaat

June 30, 2015 by dyertrust

Meredith Thornton
Research Co-ordinator

On the 21st of June we received a call about a dead dolphin stranded at 2nd beach, along Die Plaat. We promptly got all our stranding gear together measuring tape, knives, clipboards, data sheets and sampling equipment, and as soon as the volunteers returned from their shark cage diving trip we all hopped into the International Marine Volunteers (IMV) Samil and headed down to the coast.

After about half an hour of bumping and swaying along the alternately sandy and rocky road we excitedly arrived at the stranded animal. However, the surf was big, the tide was coming in and the sun was setting quickly, so we opted to load the dolphin onto the truck and take it back to the International Marine Volunteer Center for processing there instead. We arrived home cold, but happy, because it was such a privilege to spend some time in the beautiful Walker Bay Nature Reserve such a stunning locality!

Dolphin strandingWhen the next day dawned, we donned our oldest clothes and gloves and got down to measuring, dissecting and collecting samples. Our International Marine Volunteers were given an impromptu lesson on dolphin anatomy, both inside and outso now they know how to tell the sex of a dolphin and they understand more about the differences between sharks and cetaceans.

This specimen turned out to be a male common dolphin, 2.3m long. The rostrum (beak) and lower jaw were broken, but it wasnt possible to say whether this occurred pre- or post-mortem. When we cut into the carcass and examined the lungs, they were spongy and full of air this is a sure sign that the animal suffocated under the surface. Dolphins are conscious breathers which mean that they have to think about when to breathe (unlike humans). This animal would have thus held its breath because it was trapped underwater and could not surface to breathe. It is not unusual for this to occur because common dolphins are known to form part of the bycatch of our purse seine fishery.

Dolphin Stranding 2Common dolphins are fast swimming animals and occur in large schools, following pelagic fish species (anchovy and sardines) up and down our coast. These are the dolphins that do all the hard work during the sardine run, rounding the fish up into dense bait balls in order for the whole school to feed, but then along come seals and sharks to help themselves as well, and even Brydes whales too they can engulf the whole bait ball in one big mouthfuland then the dolphins have to start all over again!

Common dolphins are also crazy about bow-riding vessels and can swim up to 20-30 km/h! If you listen closely then you will hear them whistling while they porpoise alongside the vessel. They have a beautiful hourglass marking on their flanks, the front half of which is yellowish and the rest is pale grey colour.

Dolphin Stranding 2We collected and froze several samples for further studies: a skin sample for genetics, a cross section of the testis for reproductive analysis, the stomach for diet analysis and the skull for the Mammal Research Institute of the University of Pretoria. They will process the samples and place the material in the Marine Mammal Collection at the South African Natural History Museum in Cape Town.

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