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The meaning of pectoral fin position part I

The meaning of pectoral fin position part I

Fundamentals of pectoral fin position – a guestpost by Pascal Gospodinov

When we see sharks, we are impressed, amazed, overwhelmed and during first encounters we never have the time or mindset to really observe behavior details. But with time and after several encounters, we will still be extremely amazed to dive with sharks, but we will begin to see different things that we couldn’t realize due to first astonishment.

This is gonna be a little introduction in the meaning of pectoral fin position. It will help you to know and to understand when a shark will turn and in which direction. The comprehension about it can help to feel more than comfortable during shark diving, it can help to understand a situation or it can be a great „tool“ to make the picture you want. All in all it is always good to know when and if a shark is gonna turn or not. We are not going to discuss WHY a shark is turning, this can have a multitude of reasons, but when and how you can recognize it.

As the title already shows it have something to do with the pectoral fins (breast fins), the pectoral fins can be defined as ruder and the tail fin as propeller. So the shark steers with his pectoral fins.

Knowing that it becomes very easy. If a shark wants to turn to his right he simply pushes his right pectoral fin down (see picture 1), if he wants to turn left he pushes the left fin down. The lowering of a pectoral fin increases the lateral surface and creates a speed burst, so than its all about physics.

Picture 1: Oceanic Whitetip turning to his right – He is lowering his right pectoral fin

In a result of lateral surface increase and speed burst on one side the shark is turning.

So depending on the inclination of the pectoral fin the bow that the shark is gonna describe is smaller or bigger. If the shark pushes his fin completely down (90° from starting position) the bow will be small, if the angle is smaller the bow will respectively be bigger. How fast the shark is veering depends on how quick he is lowering his fin and how fast he is (due to tail fin frequency). If he wants to veer quickly the shark will have to increase his speed, quick speed burst, 2-3 or more rapid tail beats.

So if we see a shark lowering one of his pectoral fin we know now that he will veer in the respective direction. As discussed it can happen quickly or slowly depending on his speed and the bow can be narrow or wide depending on the angle of the pectoral fin. Beside being used as ruder the pectoral fins can have other purposes!

When sharks are cruising the pectoral fins are used as kind of wings, to stabilize and to counteract the slightly negative buoyancy (downforce). This is the case especially when the shark don’t use his tail fin for a short while, than it really looks like the sharks is flying and the pectoral fins works as wings, because of their morphology (slightly concave). The water stream has a longer way on the upper side of the fin due to the structure and so the water is accelerated on the upper side and kind of „sucks“ the shark upwards.

If sharks want to stop cruising because an object is suddenly in their way, they try to use the pectoral fins as brakes… Sharks are awfully bad in putting on brakes! To brake they put up the pectoral fins (see picture 2) that will slow them down but sharks are not really able to stop like bony fishes do and if you are the suddenly appearing object and they don’t have the time to turn anymore they will crush into you.

Don´t panic because the shark is probably more afraid to have crush into you than you are.

Picture 2: Tiger shark trying to „brake“