The Burrunan dolphin, Tursiops australis, has only recently been discovered but is already under threat due to its small and isolated populations.
Our team of researchers from the Australian Marine Mammal Conservation Foundation (AMMCF), Museum Victoria and Monash University, have investigated the population genetic structure of the Burrunan dolphin from the two only known resident populations: one from Victoria’s Port Phillip Bay and the Gippsland Lakes, and from other locations across coastal Victoria and Tasmania.
Our latest results, published in Conservation Genetics, found that the Port Phillip Bay and Gippsland Lakes populations were genetically very distinct, with little genetic mixing.
Who’s who of the Burrunan dolphins
Dolphins are quite cryptic species, only spending a short amount of time at the surface. It is therefore very difficult to track the population structure and levels of mixing between populations, especially from those that are hundreds of kilometres away.
By using the dolphins’ DNA we can investigate their population size and structure, identify who is who, and who is contributing genes to the next generation. We can also use the DNA to analyse the levels of migration and genetic distinctiveness of these populations.
Using two regions of mitochondrial DNA and ten micro-satellite (genetic) markers from more than 160 dolphin samples, we were able to assess for the first time the actual population structure of the Burrunan dolphin.
Mitochondrial DNA is maternally inherited, with the haplotype (genetic signature) sequences acting like surnames that are passed on by the mothers to their calves. These allowed us to genetically trace lineages within and between populations or regions.
There are no physical barriers to prevent dolphins moving between populations; both Port Phillip Bay and the Gippsland Lakes have open access to Bass Strait.
With this in mind, we hypothesised that the Port Phillip and Gippsland Lakes populations should be more similar and the Tasmanian population would differ, given the potential barrier of Bass Strait. But what we found was quite unexpected.
An unexpected result
Our results suggest the Gippsland Lakes population is more genetically similar to those in Tasmania, while the Port Phillip population is more isolated.
Each haplotype from a Tasmanian Burrunan dolphin matched a Gippsland Lakes dolphin, indicating gene flow between these two regions, while the Port Phillip haplotypes were distinct.
This genetic differentiation was also supported by the micro-satellite data, nuclear DNA inherited from the mother and father, and was also suggestive of limited gene flow perhaps by the males in the populations.
In comparison to other well-known dolphin populations such as Shark Bay in Western Australia (with an estimated dolphin population of more than 2,000), populations of the Burrunan dolphin are incredibly small.
Based on the dolphin’s DNA, the effective population size – comprised only of individuals that are contributing genes to the next generation – is estimated to be less than 100 dolphins in each region.
The Burrunan populations also showed a lack of genetic diversity in comparison to other dolphin populations world-wide. A lack of genetic diversity may leave populations less able to adapt to environmental change and places them at a higher risk of extinction.
This is a major concern, as a small effective population size indicates the population may be at risk of losing further genetic diversity, and the loss of breeding animals would be detrimental to an already small population.
A species under threat
Conservation action is required to protect this species, which is now listed as “threatened” under Victoria’s Flora and Fauna Guarantee Act.
The very small population size, lack of genetic diversity and the isolation of these populations mean the Burrunan dolphins are especially vulnerable.
The Burrunan dolphins are susceptible to numerous threats, including commercial and recreation fisheries, tourism, anthropogenic contaminants, shipping, gas and oil mining exploration and environmental change. These effects could impact on the future of not only these resident populations, but on the entire species.
Our research is continuing to explore the resident populations and to investigate other locations across coastal Victoria and out into Bass Strait. We aim to further assess the migration levels, what potential barriers to gene flow exist and identify regions of significance for these dolphins.
For a species that has existed right under our noses, living so close to a major capital city such as Melbourne, we still know very little about the Burrunan dolphin.
Our ongoing research will aim to address the data deficiencies of the species and will use the results to further protect and conserve these unique and iconic dolphins.