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 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 these gentle giants were capable of trans-oceanic migrations... until now... So how far do basking sharks travel? Where do they go? And why?
Slow and Steady
Basking sharks (Cetorhinus maximus) are the second largest shark alive today, reaching almost eight metres in length. Their mouth alone can be one metre across! Yet, there is nothing to fear. They are not voracious predators, instead cruising through the water with their mouths open in order "filter feed" on microscopic zooplankton. Whilst they favour temperate coastal waters, basking sharks can also be found in the Artic and open ocean; from Newfoundland to Florida, southern Brazil to Argentina and from Iceland and Norway to Senegal, and the Mediterranean. It was thought that these sharks generally stick around the coasts and whilst we knew they migrate seasonally, scientists couldn't be certain where they went (Compagno, 1984).
However, studies employing advanced electronic tags has changed everything we thought we knew about the movement ecology of basking sharks. Scientists tagging basking sharks off the coast of Donegal, Ireland, using a satellite transmitters (aka SPOT tags) were amazed when they realised one shark was located 993 days after the original tagging, off the coast of Massachusetts in the USA! (Johnston et al, 2019).
After she was tagged, this shark moved north, towards the Hebrides and was last located off the coast of Barra, Scotland, before she disappeared and reappeared in America (Johnston et al, 2019).
As these tags do not record the exact location of the shark continuously, but instead transmit the location via satellite periodically when the tag is near to the surface, we cannot say exactly the path she took. But if this shark swam in an absolutely straight line (known as "straight line displacement"), 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! (Johnston et al, 2019).
In a 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 more recent study has reported such similar findings, it seems very likely that basking sharks do indeed undertake trans-Atlantic migrations, somewhat commonly (Gore et al, 2008; Johnston et al, 2019).
Interestingly, 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; Johnston et al, 2019).
Therefore, scientists wonder if 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, it is also is possible that basking sharks migrate 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!
We're All Connected
These finding are very 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).
"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
It has been estimated that only a few thousand basking sharks may still exist in the wild and they are now flagged 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.
Compagno LJV (1984) Sharks of the World. An Annotated and Illustrated Catalogue of Shark Species Known to Date. Part 1. Hexanchiformes to Lamniformes. Food and Agriculture Organization of the United Nations Species Catalogue, Volume 4, Rome, 125-249. Access online.
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.