Dolphins prefer the Clarence, study finds

If you were a Northern Rivers bottlenose dolphin, the most popular place to live would be the Clarence River, according to new research.

Christine Fury, a researcher with Southern Cross University’s Whale Research Centre, has been studying local estuarine dolphin populations for three years and has uncovered some fascinating facts about our warm-blooded mammalian cousins.

The Clarence River, which runs through Yamba, Maclean and Grafton, is the number one dolphin housing option, while the Richmond River, which runs through Ballina, Wardell and Coraki, is the second most popular home-base choice, Christine found.

Although dolphins are widely studied in marine habitats, information on estuarine populations is very limited. Christine’s study, published today in the CSIRO journal Marine and Freshwater Research, provides the first published data on Indo-Pacific bottlenose dolphins in Australian estuaries.

Christine’s research has found that the Clarence River sustains a larger, predominantly resident dolphin community compared to the Richmond River, which supports a relatively small dolphin community with less permanent residents.

Christine estimates that about 71 dolphins utilise the Clarence River compared to about 34 dolphins in the Richmond River.

Differences in home-place ‘loyalty’ (or site fidelity) were observed between the estuaries with 60 percent and 37 percent of identified dolphins determined as residents, 26 percent and 21 percent as occasional visitors, and 14 percent and 42 percent as transients, for the Clarence River and Richmond River respectively.

Resident dolphins in both rivers roamed much further up the estuaries than transient or visiting dolphins, with mothers and their calves having the greatest range.

“The Clarence River is the most popular because it is the largest estuarine river system in NSW and therefore has a greater volume of water. It also has less urban and agricultural development. Both these factors mean the water quality is better,” Christine said.

“Dolphins are the top predators in the river systems, so the cleaner the water, the more fish in the river and the better the ability to sustain a bigger dolphin population.”

During her 2,000 hours on the rivers observing dolphins, Christine found that a dolphin’s favourite fish is mullet, or whiting as a second choice.

The mothers teach their calves how to catch fish, but it takes three to four years for the youngsters to become proficient at catching their own dinner and they are supplemented with their mother’s milk until that age.

Once weaned, the juveniles leave their mothers and hang out in mixed-sex pods, learning from each other and spending a lot of time in play.

As the males get a little older, they break off into pods of three or four, working collectively to catch fish and mate with females using an uncommon herding manoeuvre.

Mothers and their calves, and female pods, escape the more aggressive sexual attention of the males by entering the shallower waters of river tributaries, where the males generally do not follow as they prefer to remain in the deeper, main channels, where they can assert their dominance.

Also, the tributaries have smaller fish, which are easier for the calves to catch and eat. Fish are swallowed whole, head first, after first being either stunned by a tail flap or bitten. Dolphins will often flip a fish into the air and then catch it head-first so as to be able to swallow it properly.

Like humans, dolphins have distinct personalities. Research shows the more gregarious, inquisitive and curious dolphins prepared to stray furthest from mum have the best chance of long-term survival.

Diligent and informed management of future increased environmental disturbances will be needed to ensure the long-term survival of these dolphin populations, Christine said.

You can read the full research paper at www.csiro.au

Photo: Research shows that more inquisitive Bottlenose dolphins such as this one have better survival chances.