Sightings of leatherback turtles (Dermochelys coriacea) off the UK coastline are ordinarily a very rare occurrence however, in the past fortnight, no less than 8 observations have been made. This has resulted in the Marine Conservation Society (MCS) to ask members of the public to log any sightings of this animal.
The interest surrounding the relatively high number of sightings off the UK coastline (particularly the Irish Sea) is related to leatherbacks being listed as a critically endangered species on the IUCN red list.
Leatherbacks are the largest of 7 extant sea turtle species, commonly reaching lengths of around 1 – 2m. They are globally distributed and specialise in feeding on gelatinous plankton. However, within the past two decades leatherback populations worldwide have been decimated, mainly by longline fishing and litter discarding. Plastics in particular are notoriously mistaken as jellyfish by leatherbacks leading to high incidences of drowning related mortality. It has been estimated that populations of leatherbacks in the Pacific Ocean have declined by as much as 95% in the last 20 years.
Increased levels of protection for leatherbacks at their nesting sites in the Caribbean have been successful yet this is hampered by insufficient knowledge of their biology at sea. However, within areas of the North Atlantic such as the Irish Sea, they have become a common sighting.
This is largely attributed to the abundance of jellyfish found in these regions. Jellyfish numbers on a global scale have risen exponentially which, according to the MCS is related to the general rise in sea temperatures, nutrient run off from coastal pollution and in the Irish Sea in particular, the virtual collapse in Herring as a result of overfishing. Remarkably, the rise in temperatures may explain the distribution of both jellyfish into more northern latitudes and subsequently the increased numbers of leatherbacks.
Increases in jellyfish abundance not only benefits leatherbacks through food availability but also by building up their fat reserves which allows them to nest more frequently. Reductions in shark abundances due to the shark finning industry and by – catch may also contribute to increased leatherback populations through decreased predation.
Although leatherbacks are still critically endangered, parts of world such as the North Atlantic are showing increases. The hope is that the highly migratory leatherbacks will continue to increase in number within the North Atlantic and be observed around the UK coastline feeding on the ubiquitous jellyfish populations.