For my PhD, I am carrying out year-round surveys of Indo-Pacific bottlenose dolphins in Bunbury, Western Australia. Last year we expanded our study area by adding on two offshore transects that extend 10km from shore to the 20m depth contour (view transect lines here).
We hypothesised that in winter adult males would move offshore, helping to explain their larger home ranges. As winter approached we were looking forward to what we would find.
June was the first month of our winter season and right there and then we came across the first adult male group, which just happened to be a male alliance. “Rocky” and “Triton” were in 15.3m depth of water at 18°C and they were travelling further offshore. The Leeuwin current is a warm water current which flows southwards along the coast and is strongest in the winter time. The current flows offshore from Bunbury and this warm water and the associated prey species could be reasons to why dolphins venture into the deeper waters. Whilst offshore, we documented several feeding events, including dolphins tossing octopus and snacking on bait fish (see picture above).
In total for the winter season, we completed nine offshore transects where we encountered dolphins on four occasions. We followed dolphins 10km offshore in >23m depth of water. Group sizes ranged from three to thirty and amazingly in each sighting there was at least one male alliance. This is interesting, as it is helping to explain the ranging patterns of adult males. Overall, nine male alliances were sighted, including a re-sighting of Rocky and Triton three times.
Rocky and Triton have been in an alliance since at least 2007 and during the winter they were found escorting an adult female dolphin named “Cracker”. Cracker was first sighted in the middle of three male alliances in July and then again two weeks later in the middle of the same three alliances. Perhaps these males had formed a ‘super alliance’ like they do in Shark Bay. Cracker is the mother of “Gouda” and unfortunately the calf was not seen during this time. We have been suspecting that Gouda could be the dolphin that was found floating dead in Koombana Bay. Volunteers from the Bunbury Dolphin Discovery Centre helped to retrieve the carcass and a necropsy by MUCRU vets was performed. However, the cause of death is not confirmed due to the high level of decomposition.
Another necropsy that MUCRU vets carried out was on an adult male dolphin named “Fold”. Fold was in a male alliance with “Fuji” and “Star”. By the state of the body it had not been dead for long, so we drove it straight to Murdoch University for a necropsy. It was determined that Fold (2.52m long) died due to severe blunt force trauma; perhaps from being rammed by another dolphin or a vessel. Now that Fold has passed and Fuji has also disappeared, the last alliance member, Star, may find new alliance partners. We will be keeping a close eye on any new associations with Star.
Although we have witnessed two deaths we have also recorded several winter births! Births in winter are unusual as the water temperature is lower than the peak birthing seasons (Summer-Autumn), reducing the calf’s chance of survival. We will be keeping an eye on these calves, including “Lumpy’s” baby “Limpet” and “Iona’s” baby “Tiree”.
During the winter we were on the water for over 130 hours (with 64 dolphin group sightings). I would like to thank my assistants who braved the cold, wet winter days. A special thank you to my current interns Hannah from Germany and Charlotte from France, who are here for three months and have helped out substantially; especially with going through the thousands of photographs we have of dolphins dorsal fins.
The research for the South West Marine Research Program is made possible through the funding partners, including, the Bunbury Dolphin Discovery Centre, Bemax Cable Sands, BHP Billiton Worsley Alumina, Bunbury Dolphin Discovery Centre, Bunbury Port Authority, City of Bunbury, Cristal Global, Department of Environment and Conservation, Iluka, Millard Marine, Naturaliste Charters, Newmont Boddington Gold, South West Development Commission and WAPRES.