In May 2013, a lone grey whale (Eschrichtius robustus) was spotted off the coast of Walvis Bay, Namibia – the first ever to be recorded south of the Equator.
Grey whales are known for their extensive migratory distances, sometimes traveling as far as 8,500 km between seasonal breeding and feeding grounds. Incredibly, this vagrant male was spotted approximately half-way around the world from the grey whale’s core distribution in the North Pacific. Whether this journey was intentional or accidental remains unclear, but researchers estimate that the vast distance covered by this individual from its natural range to Walvis Bay in the South Atlantic breaks any long-distance migration records previously recorded in marine vertebrates.
There are two possible grey whale populations in the North Pacific from which the Walvis Bay individual could have originated. The first being the eastern grey whale (EGW) stock and the second being the western grey whale stock (WGW). The EGW is also known as the Pacific Coast feeding group and migrates from wintering grounds in Baja, California up the west coast of North America to feeding grounds ranging from northern California to Southeast Alaska. This group has recently recovered to a healthy population size and been removed from the US list of endangered species.
The WGW stock, which largely inhabits the north-eastern coast of Sakhalin Island, as well as the southern Kamchatka Peninsula in Russia, has a far smaller population with an estimated size of approximately only 200 individuals. This population was heavily impacted by whaling during the early 19th century, when it was hunted to near extinction. The population numbers have yet to recover, and the population is currently listed as endangered. Historic breeding grounds were thought to have been around China and Japan – and individuals are sometimes spotted in these areas – however photo identification and telemetry data have shown that some of the WGW stock now winter in the eastern North Pacific. The population was recorded to reach its lowest population numbers around 2010, with only 26 actively breeding females. Given that it takes 11 years for an adult to reach maturity, it is perhaps unsurprising that numbers have remained low and are yet to recover.
So Why Was The Whale Here?
Marine biologists, Dr. Simon Elwen and Dr. Tess Gridley, directors of the Namibia Dolphin Project and research associates in the Department of Botany and Zoology at Stellenbosch University, were curious as to the whale’s presence in Walvis Bay and set out to gather as much information as possible during the whale’s two-month stay. This included taking biopsy samples for genetic analysis and photos of the whale for identification purposes – as individual grey whales have unique markings on their bodies the same way humans have unique fingerprints. So far, the markings have not produced a match to any individual on the photo-identification database.
The biopsy samples, however, were sent to Professor Rus Hoelzel who is an evolutionary biologist from Durham University and lead author of the paper published in Biology Letters on the whales’ population of origin. Prof. Hoelzel used a genomic technique known as shotgun sequencing to sequence the lone male’s DNA in order to determine his geographic origins. The bioinformatic steps were then carried out by co-author Fatih Sarigol. Conducting a bioinformatics study with such a limited sample size is very demanding in terms of genomic resources and bioinformatic processing and takes a considerable amount of time, which explains why this paper has been published so many years after the whale was sighted in Walvis Bay.
By looking at the degree of genomic similarity between different individuals, researchers can establish which population the lone whale belongs to, as each population has its own genomic signature. Based on this data, the researchers determined that the whale most likely belongs to the endangered WGW group, and has therefore traveled from the western north Pacific area.The authors suggest three possible routes of travel from the natal range for this wandering whale, with the most likely route being via the Arctic and covering a distance of around 27,000 km. This would be the longest recorded oceanic migration undertaken by a marine invertebrate, after the leatherback turtle (Dermochelys coriacea).
Another possible, and much shorter, route the whale could have traveled is southward from wintering grounds in the Baja California through the Indian ocean. Yet this journey is still a colossal 18,000km, which would still make this the world’s longest migration by ocean..
Scientists believe that the Arctic route is the most likely option, based on fossil records showing that grey whale populations have migrated several times using this route over the past 100,000 years. But these migrations are unusual; corresponding to periods when temperatures in the Arctic Ocean were relatively high, meaning the ice has receded enough to allow passage through. Given the recent increase in global temperatures, it is possible that this route has yet again opened up and allowed the whale to explore new areas of the ocean. Amazingly, the last time this migration route was used is thought to have been roughly 5000 years ago.
While the other routes are shorter, there is no evidence to suggest that grey whales have ever migrated along those paths. What’s particularly interesting is that grey whales are known to feed predominantly in shallow waters, possibly limiting the potential from traveling vast distances through the pelagic ocean.
Historic records have led scientists to believe that there was an Atlantic grey whale population that existed from at least the Pleistocene until about the 1740s, and it was hypothesized that this individual may be a surviving descendant of that population, but genomic data suggested that this was not the case.
Animal migrations are often driven by seasonal changes. In baleen whales (such as grey whales), which feed primarily on shrimp, migrations are driven by the seasonal variation of productivity cycles in the ocean. As conditions changed during the transition from the Pleistocene, ocean productivity became patchier and baleen whales experienced selection for large migration distances. While much about this male remains unknown, it is possible that these excursions may be another response to changing climatic conditions and result in changes in grey whale distribution.
More recently, there have been other recorded instances of grey whale sightings in the Atlantic which could have conservation implications if grey whales are capable of global dispersion potential and are responding to human-mediated change such as receding ice caps opening up new passageways for exploration. In addition, recording and understanding why these rare and extreme migrations occur could help predict how species will respond to global climate change.