Bigeye Trevally – the Tornado Maker

Photo courtesy from Canvas Printing

Bigeye Trevally (Caranx sexfasciatus)

Common Name: Jacks
Max. Length: 120 centimeters
Length at 1^st Maturity: 48.4 centimeters
Max. Weight: 18 kilograms
a value: 0.04930
b value: 2.797
Depth Range: 1 – 145 meters (3 – 480 feet)
Frequency: Common especially in the Indo-Pacific Ocean

If Cocos island is known for a large congregation of big fish like the hammerhead sharks, then your diving experience in this offshore island will never be complete without seeing the huge school of bigeye trevelly or commonly known as Jacks.

Photo courtesy from CSIRO

You can easily identify this large marine fish with its compressed and long body where its dorsal portion is more convex compared to the ventral part or the belly portion. Their bodies are completely covered with scales especially in the breast side. When the dorsal fins are erected, you can see that there are two distinct dorsal fins each having at least 8 dorsal spines. Its caudal fin is strongly forked while the pectoral fins are slightly falcate.

Bigeye trevally thrives from coastal to oceanic waters usually near a coral reef ecosystem. Scientist has considered them an indicator species where their presence indicates that the surrounding reef is in healthy condition with good water quality as it is the main description of its habitat.

Why do they tend to Circle or make a Tornado formation?

Your experience with bigeye trevally in Cocos island will often amaze you where you can see them in huge quantity that is tightly compact in a circular formation. Yes, this is true especially when you are doing a day dive. But it is a different scenario during night where they shift from school in to a solitary mode and go hunting for smaller fish.

Photo courtesy from Michael Patrick ONeill

The reason behind their circular formation is to seek refuge in numbers. A school of bigeye trevally can consist anywhere between a few hundred to over a thousand individuals. When predators are around during the day, their compact formation will confuse the predator in focusing a single target giving them a hard time having a meal. So for predators, its better to look for other fish species that can easily be converted in to a meal.

Challenge – If you were a predator like sharks and you are given 10 seconds to choose just one bigeye trevally as your meal for the day: which bigeye trevally would you choose from the picture just above within this subheading?

. . . . . Are you having a hard time? So does sharks and other predators.

Focus leads to Hypnosis

Photo courtesy from
en.eldoradobeachresort.com

One characteristic of a school formed by a congregation of bigeye trevally is that it moves and spins very slowly. Although their formation has been compared to as an underwater tornado, the main difference is that real tornadoes moves fast while the tornado being described here is moving in slow motion.

With this, if you try to focus your sight on the huge vortex of fish, we are quite sure you will be hypnotized with its slow movement where you will realize that your breathing will be more calm and relax. This is especially true if there is ample sunlight penetration where the light can reflect to its silver skin giving you this flash of radiant light.

As we end this article, we just hope you were not hypnotized by the pictures we presented and the contents of this article. So watch out for more amazing articles about the marine animals of Cocos Island.

Reference

Fishbase: www.fishbase.org

Wikipedia: www.wikipedia.org

IUCN Redlist of Threatened Species: www.iucnredlist.org

Video courtesy from Costa Rica

About Tristan Paylado
Tristan is a marine biologist and a CMAS 2-Star Dive Instructor
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