Jacks of Cocos Island

Most, if not all, of the world’s best dive sites features several amazing marine wildlife that is usually in huge numbers. These massive population may come in a variety of form and shape depending on the environment it thrives.

In Cocos island, one of the massive fish population that you will see are Jacks. Collective referring to all fish species under the family Carangidae, Jacks are mainly composed of Pampano, Trevally, Scads and Rainbow Runners.

Major Species of Jacks in Cocos Island

African Pompano (Alectis ciliaris)

Other Name: Threadfin Trevally
Max. Length: 150 centimeters
Length at 1st Maturity: 76.7 centimeters
a value: 0.07860
b value: 2.579
Depth Range: 20 – 100 meters (60 – 300 feet)
Frequency: Abundant in the World’s tropical Oceans

An African pompano has a body that is deeply compressed and head that is steeply rounded. They are generally colored silver with a dash of metallic blue. But just in case you see this fish with five chevron-shaped lines along its body, it means that you are very lucky that you have just seen its juvenile version.

While appearing to have no scales, taking a closer look will tell you that they have. Their scales are just deeply embedded in their skin. Their body becomes elongated as it reaches sexual maturity which is pretty similar to the growth of human beings as it grows towards its teenage years.

If there is one thing that really stands out in identifying this fish, then it would be their highly elongated dorsal and anal fins. But it’s not the entire fins that protrudes where only the first 3 to 5 dorsal and anal spines becomes elongated.

Island Trevally (Carangoides orthogrammus)

Other Name: False Bluefin Trevally and Thicklip Trevally
Max. Length: 75 centimeters
Length at 1st Maturity: 41.5 centimeters
a value: 0.01750
b value: 2.994
Depth Range: 3 – 168 meters (10 – 555 feet)
Frequency: Abundant in the Indo-Pacific Ocean

This moderately large fish are often found in oceanic habitats which is perfectly represented in Cocos island. While diving, you may see them either in pairs or in small groups. They may also be seen in solitary where adult island trevallies usually does this in hunting small fish or crabs in the sand.

This silver to green-blue colored fish has a body that is highly compressed and a convex-shaped head. Similar to the African Pompano, their dorsal and anal fins are elongated but not as elongated when compared to the former species of jacks. If the highly elongated dorsal and anal fins are the distinguishing marks of an African Pompano, then the scattered yellow dots on the body are the main identifying mark of an island trevally.

Golden Trevally (Gnathanodon speciosus)

Other Name: Golden Jack
Max. Length: 120 centimeters
Length at 1st Maturity: 39 centimeters
a value: 0.07100
b value: 2.680
Depth Range: 1 – 80 meters (3 – 240 feet)
Frequency: Abundant in the Indo-Pacific Ocean

As their name suggest, the body color of this fish is gold in general. However, as the fish grows old, the golden coloration will be confined within the fins where their central body will now have a silver-grey color. But what is kept in its body, regardless of young or old, is the black vertical bars.

What is interesting with this fish is they take refuge in large animals (like sharks) when they are still in its juvenile stage. In Cocos island, you will typically see a juvenile golden trevally swimming along a tiger shark, whaleshark and other species of large shark. Usually you can see a small group of golden trevally swimming on the side, in front or under the belly of a large shark. They also ride along with other large marine animals like sea turtles and manta rays.

Mackerel Scad (Decapterus macarellus)

Max. Length: 46 centimeters
Length at 1st Maturity: 14.7 centimeters
a value: 0.00783
b value: 3.140
Depth Range: 20 – 200 meters (60 – 600 feet)
Frequency: Abundant in the World’s Tropical Oceans

Not all jacks look the same. In fact, the species that forms the group of Family Carangidae comes in many shapes and sizes. As far as the mackerel scad is concerned, they are not your typical jack that has a highly compressed body with elongated fins. Mackerel scads are more of the rounded and elongated type of fish. In fact, despite being classified as a small-sized fish, they grow too long that their longest recorded length was 46 centimeters.

This green-silver colored fish that has a light-yellow caudal tail is considered an excellent baitfish. This is in contrary to the general characteristics of Jacks where they usually chase other fish for food, instead of them being chased.

Rainbow Runner (Elagatis bipinnulata)

Other Name: Rainbow Yellowtail, Spanish Jack, Hawaiian Salmon
Max. Length: 180 centimeters
Length at 1st Maturity: 54.6 centimeters
a value: 0.02340
b value: 2.240
Depth Range: 1 – 150 meters (3 – 450 feet)
Frequency: Abundant in the World’s Tropical Oceans

Another species of jacks that is not typical of their body form is the rainbow runner. Instead of having a compressed body, they are elongated and sub-cylindrical in shape. Another difference could be the snout where instead of having a snub-nose, has a long and pointed snout. Their eyes are relatively small and most of their fins are short (except for the caudal fin) which is not typical of the long fins a Jack fish usually have.

Your typical interaction with a rainbow runner in Cocos island will either occur during your way down to a cleaning station where you can see them out in the blue or in your way up to the surface where the rays of the sun reflects the different body colors this fish have. As its name suggest, a rainbow runner is multi-colored where their upper body is colored blue-green and fades away to white as it goes down to the belly. They have several layers of horizontal stripes where you can clearly see the alternating blue and yellow stripes along its body.

Goes in School

Before we proceed, we would like to clarify that what we are talking about has nothing to do with wearing school uniforms, wearing identification cards, bringing books or doing homework as predicted in a popular American Animated Television series. What we mean is how this fish forms themselves in to groups called school. However, they do fall in line.

Often than not, jacks are seen swimming in groups. A small aggregation can consist of up to 50 individuals where they often form a “wall of fish”. A medium-sized school may count up to 100 individuals where they usually form a wall of passing-by fish. But their ultimate grouping, as exhibited by the bigeye trevally, may go over a hundred individuals of sexually matured jacks, wherein their famous formation is similar to the vortex of a tornado.

Reference

Fishbase: www.fishbase.org

IUCN Redlist of Threatened Species: www.iucnredlist.org

Wikipedia: en.wikipedia.org

Video courtesy from dylanBPK