How do basking sharks reproduce?

One of the biggest mysteries with basking sharks is reproduction – where and how do they breed? Where are their pupping grounds? There are many unanswered questions but we’re now seeing small pieces of the puzzle come together.

In 2015 we recorded the first footage of a torus in Scotland, and then saw it again in the Summer of 2020. Torus is a bit like shark speed dating, with individual males and females associating with most other members of the group. The sharks begin by swimming alongside each other in a tight knit circle from the surface spiralling down to the sea floor in a 3D pattern. Other filter feeders such as manta rays can sometimes be seen feeding in this ‘vortex’ formation, however the sharks in a torus have their mouths closed and seem to show some unique behaviours such as head nodding and males turning darker in colour. Now nobody has ever seen basking sharks mating, but we have a pretty good idea that this torus is part of the courtship display – possibly a form of mate selection. We also see the occasional female with scaring on the pectoral fins which reflects recent mating, as seen in other shark species. 

There are very few juvenile shark sightings in the Hebrides, leading to the conclusion that this area is not a significant pupping area. However we have had a few sightings over the years, and were in the water with one of the first juveniles when a lucky passenger Stuart Holmes managed to capture a picture of it. Although its unlikely that they pup here, we do think basking sharks breed in the area due to the courtship and breaching behaviours seen commonly in the Summer. In Summer 2022 we had another juvenile basking shark sighting from the boat and it was observed to be feeding. However, due to its erratic movements and choppy surface conditions we decided to watch it from the boat instead.

Juveniles are easily distinguished from adults, firstly by their size as they are much smaller. It is understood that juveniles are around 1.6-1.8m at birth, from records of a Norwegian fisherman pulling a large female shark aboard, only for her to give birth to 1 live and 5 stillborn sharks. The average size of adult sharks we see is 5-6m, and the largest are around 8-9m. The maximum size is thought to be 10-12m. However, it’s their nose (or rostrum) that is the most unique feature. Instead of the large bulbous nose of the adult, they have a small, upturned nose with a thin tip. If you can imagine a cross between a Mr whippy ice cream and a basking shark nose – then you’ll get an idea of the shape! There is no definitive answer to why they have this feature as a baby shark, some suggest benefits around initial feeding strategies, others around development inside mum besides the other pups.

So as it stands the question of ‘how do basking sharks reproduce’ still remains unanswered, but hopefully we see a breakthrough in the coming years as it may provide an extra boost in conservation and protection for our favourite fish.