Small and isolated dolphin populations are under threat

Burrunan DolphinsThe 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.

Populations of the Burrunan dolphin. Kate Charlton-Robb/AMMCF, Author provided
Click to enlarge

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. Read full story: The Conversation By Kate Charlton-Robb Conservation geneticist and dolphin researcher at Monash University

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