Going the Distance
Updated: Apr 27, 2021
For many years, scientists have been in awe of the incredible, long-distance migrations undertaken by sharks. Many shark species migrate seasonally, either to follow abundant food sources, find a mate or pass the changing seasons in less extreme conditions. For example, basking sharks (Cetorhinus maximus) have been known to seasonally migrate along the continental shelf of Europe, to over-winter in deep water beyond the shelf-edge. However, we didn't think that basking sharks underwent trans-Atlantic migrations... until now...
In a recent study, scientists tagged basking sharks off the coast of Donegal, Ireland, using a satellite transmitters (aka SPOT tags). These devices are pinned to the dorsal fin of the shark and can transmit the sharks location via satellite. This allows scientists to trace their movements on a day to day basis, and if the tag stays attached for a long period, then they can track seasonal migrations too.
Remarkably, one sharks was located 993 days after the original tagging, off the coast of Massachusetts in the USA! After she was tagged, this basking shark moved north, towards the Hebridean Islands and was last located off the coast of Barra, Scotland, before she disappeared and reappeared in America (Johnston et al, 2019). If the shark swam in an absolutely straight line (known as "straight line displacement", as the tag only recorded the starting and stopping point, not the details in-between), this meant that she must have moved at least 4632 km! All the way across the Atlantic ocean! This would take you at least 7 hours on an airplane!
In one previous study, basking sharks were found to make trans-Atlantic migrations, from the Isle of Man to Newfoundland. However, as we had so few records of this phenomena, we could not be sure that this was a genuine pattern, or just a one-off (Gore et al, 2008). However, as this new study has reported such similar findings, it seems very likely that basking sharks do indeed undertake trans-Atlantic migrations, somewhat commonly (Johnstone et al, 2019).
However, the scientists noted that the migration patterns of individual basking sharks were very different; one shark migrated over to America whilst the other tagged shark remained within European waters. They suggested this pattern was probably due to the different sizes of the sharks; with the larger, sexually mature shark making the long migration and the smaller staying closer to home. This suggests that there may be a change in movement ecology as these sharks mature (Gore et al, 2008).
Therefore, the researchers suggested that mature basking sharks might migrate for reproduction. Indeed courtship behaviour has been observed in both Nova Scotia and Britain, so it is possible that they mate in both locations (Gore et al, 2008).
On the other hand, they also suggested it is possible that basking sharks migrated in order to find food. The basking sharks' preferred prey species of zooplankton (Calanus finmarchicus) is abundant off the Labrador coast, where one of the tagged sharks visited. Whatsmore, satellite imagery has shown that the area was especially "productive" (when high levels of organic matter and/or energy is accumulated during a particular time) because of upwelling events during the period the sharks were there. This suggests that the sharks were targeting this area specifically for its dining opportunities (Gore et al, 2008)... haven't we all done that? Sometimes it is worth making a long journey for really good food!
These finding are very important information because they tell us that populations of basking sharks in America and Europe are connected to some degree. This could mean that these populations breed together, which has implications in "population genetics" (the study of how different populations of a species interbreed and how genetically diverse the species is as a whole). In order to maintain a healthy population, genetic diversity is very important. So having movement between populations (mixing genetic material amongst a higher number of individuals and shaking up the genetic diversity) increases a species' resilience in the face of global climate change and extinction risk (Gore et al, 2008).
It has been estimated that only a few thousand basking sharks may still exist in the wild and they are now classified as 'endangered' by the IUCN (2020). If conservation measures for this species are to be successful we must understand more about their population genetics and identify their critical habitats. We must also all work together, through international collaboration, to ensure that basking sharks are protected throughout their whole range.
"Despite protective legislation, the numbers [of basking sharks] in the northeast Atlantic may show only limited recovery if mature adults are exposed to exploitation in other oceanic regions."
- Gore et al, 2008
Johnston E, Mayo P, Savetsky E & Houghton J (2019). Serendipitous re-sighting of a basking shark Cetorhinus maximus reveals inter-annual connectivity between American and European coastal hotspots. Journal of Fish Biology, 95, 1-5. Access online.