Kyoto, Japan -- The species Orcinus orca, generally known as orcas or killer whales, is made up of many genetically distinct populations called ecotypes. Each ecotype indicates an ecological specialization with its own ecological and phylogenetic characteristics. The North Pacific is home to three known ecotypes of killer whales, called the transient, resident, and offshore ecotypes. While transient ecotype killer whales are mammal-eating, the resident and offshore orcas are fish-eating.
Though killer whale populations in the eastern North Pacific near Vancouver Island and Alaska have been studied extensively, populations in the western North Pacific have been less studied, especially around Hokkaido, Japan's northernmost island. One of these sea areas, Shiretoko, is the best in Japan to spot killer whales and is even home to a UNESCO natural World Heritage site, yet information on its killer whale population has remained limited. Researchers from Kyoto University and collaborating institutions worked together with a shared determination to change that.
"Clarifying the ecological characteristics of killer whales is crucial for achieving coexistence with them, as they are deeply entwined with human activities such as tourism and fisheries in Hokkaido," says first and corresponding author Momoka Suzuki.
One previous study of this population suggested that orca around Hokkaido belong to both fish-eating and mammal-eating groups. However, the analysis was supported by haplotypes based on only partial regions of mitochondrial genomes, and was therefore not sufficient to differentiate between the resident and offshore ecotypes. This time, the team analyzed the whole mitochondrial genomes of killer whales sampled around Hokkaido.
The researchers obtained skin samples of 25 killer whales that had been biopsy sampled and stranded around the island, then compared the whole mitochondrial genome sequences of their samples with the known whole sequences of killer whales from around the North Pacific. The team's phylogenetic analysis revealed that the killer whales living around Hokkaido belong to the resident and transient ecotypes.
This study also helped to further establish the division between these two ecotypes of killer whales. In the eastern North Pacific, the genetic and behavioral differences of the resident and transient ecotypes have raised the possibility that they may be separate species, but currently they are still considered subspecies.
How we categorize animals into ecotypes, subspecies, and even species is constantly changing, but the data that prompts these decisions is crucial both for our coexistence with animals and the protection of vulnerable populations. In countries with advanced research on killer whales, conservation management strategies are often developed for each group based on genetic differences.
"Information on the ecological differences and conservation status of killer whales around Hokkaido remains insufficient," says Suzuki. "We will continue to collect ecological data using a wide range of methods, including behavioral observation and genetic analysis."
