Do basking sharks sleep?

Do basking sharks sleep!? On our recent social post, a lot of people suggested the answer was that the shark was sleeping. Look below and see what you think?

So firstly, the answer to the question is that basking sharks do not sleep per se.  They cannot just stop swimming as they have to keep water flowing through their gills to absorb oxygen e.g to breathe, this is called obligate ram ventilation. However they will go through periods of ‘rest’ where they are less active and will allow them to recover. During these periods they will be moving around so they would appear to be ‘active’ rather than ‘sleeping’ however due to energy dynamics it is likely that they will have their mouth closed during this period to be saving as much energy as possible.

You may not know that some sharks can actually sit on the bottom and rest, local species such as small spotted catsharks and spurdogs are often found resting on the bottom not moving. These sharks have a different part of their body called spiracles which they can draw water in and  pump it through their gills. This is called buccal pumping and is like a manual way to breath rather than on auto!

So back to our basking shark video, with the limited movement it would be an easy thing to guess that the shark is sleeping. However it’s mouth is open and gills clearly flared so it’s feeding – remember we were also watching the shark for quite a while and this is a small isolated video clip.

With basking sharks you need to remember is that due to their feeding strategy and diet, everything is based on efficiency. So what is happening is that the shark is sitting with the current, using it’s large fin to help it move but not using much energy to swim. Watch again and look at the limited tail movements.

In this mode they can use the least amount of energy possible for the most amount of food input. We have observed this in a couple of different situations, usually we see this when plankton conditions are variable and they are using the current to help them burn as little energy to gain food. In abundant plankton conditions they will swim into the current, swimming hard as they are able to counter the energy use with lot’s of food intake. You can imagine this situation like mowing a lawn, going with and against the current, working against the tide then drifting back with it. Basking sharks can initially seem to only just move around and feed however there is a lot more going on that you initially think!