Assessment of maternal migration for hector's and maui dolphins

There are two sub-species of hector's dolphin, Cephalorhynchus hectori (South Island hector's dolphin) and C. hectori maui (maui dolphin). These dolphins are endemic to the relatively shallow waters of New Zealand (within four nautical miles of land). They can be commonly recognised by their small size, black rounded dorsal fin and grey bodies with black and white markings around the snout. Various populations of the South Island hectors dolphin exist scattered around the east, west and southern coast of the South Island, while the only population of maui dolphins can be found on the west coast of the North Island.

South Island hector's dolphin. Photo by Tewhaipounamu, flickr.

The South Island hectors dolphin is considered to be endangered with an estimated population size of 7270 individuals. The maui dolphin is thought to be critically endangered with an estimated population size of 111 individuals. Both sub-species populations are in decline, mainly due to the effect of entanglement in gill nets.

It was generally thought that there was minimal migration between various South Island hector's dolphin and maui dolphin populations; however it was important to assess whether these populations were genetically isolated. The study "Geographic isolation of hector's dolphin popilations, described by mitochondrial DNA sequences" carried out by F B Pichler, S M Dawson, E Slooten and C S Baker, had the aim of using this information to identify the appropriate population units for management.

Samples were taken from a total of 34 individuals which had been found beachcast or accidentally caught in gillnets. Of the 34 individuals, 20 were from the east coast of the South Island, 12 from the west coast of South Island, and 2 from the North Island (at the time maui dolphin had not yet been recognised as a separate sub-species). A section of mitochondrial DNA (mtDNA) known as the control region was isolated and sequenced. The control region was used because it is known to be highly variable in other Cetacean species, and is therefore a good measure of the isolation of populations.

The mtDNA sequencing showed 13 polymorphic sites, with 11 distinct mtDNA haplotypes in the control region. This mtDNA control region showed a 0.28%-1.67% variation between different populations of South Island hector's and maui dolphins. Finally there was a 0.659-0.929% variation within local populations of South Island hector's and maui dolphins. The results also identified three distinct clades, one on the east coast of the South Island, one from the west coast of the South Island, and a third clade found in the maui population from the North Island.

From these results a number of conclusions can be drawn. The different haplotypes found in each population, distinct clades and higher variation between populations than within populations all indicate a lack of maternal migration between populations. As there is no known geographical barrier to inhibit migration, it is thought that this is due to ecological preferences and a strong sense of philopatry (when an individual returns to its birth place to breed). This level of isolation between populations is unusual for Cetacean species. However, this method only gives an indication of maternal migration, as mtDNA is only passed down maternal lineages. To get an idea of the true level of isolation further investigations should be conducted into nucleic DNA and male movement patterns; however it is thought that male migration between populations is also minimal.

There are a number of implications that come from these conclusions. If this genetic isolation continues, individual populations will continue to become genetically dissimilar from one another. Another implication is due to the small size of isolated populations and lack of gene flow, it is likely that the each population's gene pool will decrease and genetic variation will get lower and lower. This could potentially create an inbreeding depression and effect their survival. This isolation also means that if a local extinction occurs, populations will be very slow to recover as recruitment from outside females is low.

These results also quantified these populations as independent stock, defined by demographic criteria (migration between populations is less than reproduction or natural mortality within a population). The significant divergence between allele frequencies of mtDNA also qualified the sub-species to be considered independent genetic management units. From this, the recommendation was made for separate conservation plans for east and west coast South Island populations. They then went on to recommend maui dolphin be considered a separate management unit and since the study, maui dolphin have been recognised as a separate subspecies.