Project Title: Population structure of Long-finned pilot whales in Tasmania and social dynamics of mass strandings
Chief Investigators: Prof Scott Baker & Mr Marc Oremus
Long-finned pilot whales (Globicephala melas) are the most common species to strand along the coast of Australia and New Zealand. Mass strandings, often include more than a hundred individuals, most of which die unless rescued by human assistance. These strandings provide an important source of scientific information on this species otherwise hard to observe at sea. Yet, relatively little genetic research has been conducted on samples collected during these events. Here, we investigated the diversity of mitochondrial (mt) DNA control region sequences (345 bp) and microsattelite alleles (n = 16 loci) of 227 samples from five strandings around Tasmania between 2002 and 2006. In order to investigate regional population structure and social dynamic of mass stranding, we combined these results with previously analysed data from pilot whales mass strandings in New Zealand (n = 341).
Analysis of mtDNA showed strong differentiation between New Zealand and Tasmania (FST = 0.45), as well as between east and west Tasmania (mtDNA, FST = 0.32), indicating highly restricted female gene flow. Significant but much weaker differentiation was also found for the nuclear microsattelite loci (FST = 0.0084), suggesting some degree of male-mediated gene flow. Despite the evidence for male-mediated gene flow, we found no direct evidence for male-biased dispersal between the New Zealand and Tasmanian populations. These analyses suggest that mating occurs during occasional contact between social groups that otherwise show strong matrilineal social organisation and maternal fidelity to regional populations, the spatial boundaries of which are unknown.
We also investigated kinship of individual pilot whales in two mass strandings from Tasmania (Marion Bay 2006, n = 145; Ocean Beach 2006, n = 27), for which position and age/sex class of each stranded whale was mapped along the shore. In particular, we tested the assumption commonly made during rescue attempts that social bonds are maintained during these traumatic events. As with the previous finding of social disruption in mass strandings from New Zealand, various analyses failed to detect a correlation of kinship with the spatial distribution of stranded individuals from these two events. At Marion Bay, we found that whales sharing similar mtDNA haplotypes had stranded at different locations and different times. Even inferred mother-and-calf pairs were often found widely separated along the shore, being disrupted in space and time. This pattern was confirmed at Ocean Beach, although the results were less obvious due to fewer whales involved and absence of diversity in mtDNA.
In conclusion, the strong differentiation of mtDNA between strandings from New Zealand and Tasmania, as well as between the east and west coasts of Tasmania, provides new evidence that each of these populations represent independent genetic management units. The analysis of kinship within Tasmanian mass strandings confirms previous analysis from New Zealand, suggesting that social disruption is common during these tragic events. This disruption of kinship bonds could help to explain the behavioural distress of stranded individuals and the tendency of many whales to re-strand after being re-floated. These results have implications for management and animal welfare of mass strandings, particularly efforts to unite mother-calf pairs during rescue attempts. Our understanding of population structure and social organisation of long-finned pilot whales would be improved by collection of genetic samples from social groups at sea, as well as by satellite tagging of rescued individuals and diet analysis from stomach contents or by stable isotopes.