Tunas and Mackerels: The Road Runners of Cocos Island

Photo courtesy from wsknow com

If you hear the words “Road Runner”, two things may come up in to your mind. One is a fast-running bird and another pertains to a popular cartoon character who is always chased by a coyote. But how do tunas and mackerels relate to road runners? They have no similarity except for one thing: speed.

With a swift motion reaching 40 to 50 miles per hour (65 – 80 kph), tunas and mackerels are one of the fast swimming fish species that you can find in Cocos island. This is due to their streamlined body that is composed of two types of muscles. If you were able to witness the slicing of a whole tuna or mackerel, you will notice that their meat muscles are colored red and white. Located mostly on the sides of their body, red muscles are used for swimming in a normal cruising speed. But when tunas and mackerels need to swim fast, like when in pursuit of a prey, they use a combination of both the red and white muscle where they can able to sustain it for a prolonged period.

Major Species of Tunas and Mackerels in Cocos Island

Wahoo Mackerel (Acanthocybium solandri)

Photo courtesy from www.sportfishingmag.com

Other names: Ono and Peto
Max. Length: 250 centimeters
Length at 1st Maturity: 78.8 centimeters
a value: 0.00440
b value: 3.082
Depth Range: 0 – 20 meters (0 – 60 feet)
Frequency: Abundant in the Indo-Pacific Ocean

Photo courtesy from Fishes of Australia

The Wahoo is one of the accomplished predators in Cocos island where they can easily chase any fish. Their anatomical features include a highly elongated body that is filled with mostly red muscles easily giving them a burst of speed of up to 50 miles per hour (80 kph). This is also the reason why they are mostly caught using towed baited hook where they have become a prized game fish for sports fishermen.

During your underwater adventures in Cocos island, you will usually see a solitary wahoo patrolling the open seas. Initially, you will see them in a distance where they come close after a while. Having a closer look and with the dark blue background of the open sea, you can clearly see their blue-green colored body with a dash of silver on the side with blue vertical bars. Aside from speed, one of their assets in being an efficient predator is their large triangular mouth that is equipped with finely serrated and compressed teeth. This will tell you that a simple grab to its prey means game over.

Striped Bonito (Sarda orientalis)

Photo courtesy from Eastern Cape Scuba Diving

Other name: Mackerel Bonito
Max. Length: 102 centimeters
Length at 1st Maturity: 54.5 centimeters
a value: 0.02170
b value: 2.870
Depth Range: 1 – 167 meters (3 – 550 feet)
Frequency: Abundant in the World’s Tropical Ocean

Photo courtesy from Fishes of Australia

Striped bonitos are a schooling fish where they prefer to inhabit coastal waters and offshore islands. Although they can grow up to a meter in length, the ones you will see in Cocos island are mid-sized specimens usually measuring 50 centimeters.

During fine weather condition in Cocos island where the sun is radiantly shining, you can observe the approaching school of striped bonito. They appear from the open sea and traverse along the ledges of the reef where you are usually situated. As the school of striped bonito comes closer to the reef, sunlight is being reflected creating a sparkle of shining silver. And as their school pass by closest to your position, you will notice that they have a relatively large mouth and their body has narrow oblique stripes that runs behind the head up to the tail.

Chub Mackerel (Scomber japonicus)

Photo courtesy from Neotropical Fish

Max. Length: 64 centimeters
Length at 1st Maturity: 22 centimeters
a value: 0.02820
b value: 2.810
Depth Range: 0 – 300 meters (0 – 1,000 feet)
Frequency: Abundant in the Indo-Pacific Ocean

Photo courtesy from Randall JE

The chub mackerels that you will find in Cocos island are the smallest of all the member species in the tuna and mackerel family of Scombridae. But this doesn’t mean that their existence is being jeopardize due to size inferiority. What lacks in size is being compensated by the numbers. Yes, chub mackerels often form a thick school that may comprise over a thousand individuals.

This is the species that you often see in a fish market. This is due to the fact that they are easily caught in commercial fishing activities.  Speaking of which, if you were able to get on board a commercial fishing vessel, you will notice that the school of chub mackerels registers a circular dot over the sonar screen. While the electronic monitor has a flat lay-out, this does not mean that what you will see underwater is also flat. In fact, when you go diving in Cocos island and encounter the school of chub mackerels, you will notice that actual scenario and the one that you saw in the sonar screen has a close resemblance where their school is usually in a circular formation.

Bullet Tuna (Auxis rochei)

Photo courtesy from fishesofaustralia.net.au

Max. Length: 50 centimeters
Length at 1st Maturity: 24.1 centimeters
a value: 0.01870
b value: 2.907
Depth Range: 10 – 100 meters (30 – 300  feet)
Frequency: Abundant in the World’s tropical Ocean

Photo courtesy from Fishes of Australia

As the name suggest, this medium-sized fish is a fast-swimming species that can reach speed of up to 45 miles per hour (70 kph). While their speed is nothing compared to the speed of a fired live bullet that can reach 2,500 feet per second, the speed of bullet tunas can easily evade the dragging effects of water and can be sustained in long distances and over a period of time.

If we are to be insistent in comparing bullet tunas to a bullet of a gun, then might as well compare them with a shotgun. Bullet tunas travel in groups of up to 100 individuals which has a close resemblance to the number of pellets in a shotgun. In a normal situation while not chasing prey, you can see them out in the open sea in mid-water swimming in a relax mode. This gives you the opportunity to chase their school and see how relax they are while swimming calmly. Getting close to their school allows you to see their body structure where you will notice that they have a purple to blue colored body with a white belly. Just make sure that while you are interacting with bullet tunas, make sure that you constantly monitor your air and that you need to ascend once you reach the low on air mark which is set at 500 psi or 50 bars.

Black Skipjack Tuna (Euthynnus lineatus)

Photo courtesy from ZipcodeZoo

Max. Length: 84 centimeters
Length at 1st Maturity: 45.9 centimeters
a value: 0.02420
b value: 3.018
Depth Range: 0 – 40 meters (0 – 120 feet)
Frequency: Abundant in the Eastern Pacific Ocean

Photo courtesy from Shorefishes

While having a blunt and bulging body, this medium-sized fish has a muscular system that gives them the endurance to migrate around the waters of the Eastern Pacific Ocean. They seldom visit the shore waters of the mainland and prefers to thrive in an offshore oceanic ecosystem.

Although they occupy the bottom spot of the speed list of tunas and mackerels, black skipjack tunas are still capable of producing speeds of up to 40 miles per hour (65 kph). While they go behind other tunas and mackerels in terms of speed in chasing food, they don’t allow themselves to go hungry. In fact, they are classified as an opportunistic species where they trail behind the feeding migration patterns of other tunas and mackerels ending up in having their own share of the pie.

Skipjack Tuna (Katsuwonus pelamis)

Photo courtesy from Fishes of Australia

Max. Length: 110 centimeters
Length at 1st Maturity: 42.3 centimeters
a value: 0.02257
b value: 2.965
Depth Range: 0 – 260 meters (0 – 860 feet)
Frequency: Abundant in the World’s Tropical Ocean

Photo courtesy from UniProt

A skipjack tuna has a close resemblance to a black skipjack tuna. They usually go together in a tightly pack school where both of this opportunistic species trail behind fast-swimming tunas following a food trail. While both of these species are found in a single group, identifying and differentiating them is not really that difficult.

First, skipjack tunas are slightly bigger than a black skipjack tuna where they can attain a maximum length of 110 centimeters. But since size varies according to age, we suggest you look at the body where differentiating the two species can easily be attained. Look at the black horizontal lines that traverses across its body. A black skipjack tuna has horizontal lines that are located on the upper portion of its body while a skipjack tuna is concentrated in the lower portion.

Yellowfin Tuna (Thunnus albacares)

Photo courtesy from Tunalux

Max. Length: 239 centimeters
Length at 1st Maturity: 94 centimeters
a value: 0.07350
b value: 2.690
Depth Range: 1 – 250 meters (3 – 750 feet)
Frequency: Threatened; found in the World’s tropical Oceans

Photo courtesy from Fishes of Australia

If there is a contest for tunas and mackerels in terms of size and speed, then the yellowfin tuna would easily get the silver medal. Their huge and robust body is equipped with sprinting muscles that allows them to engage in a migration pathway around the world. This is the reason why you don’t see them whole year round in Cocos island.

In case you are just in time for their arrival in Cocos island, then we suggest you stay in the thermocline zone where yellowfin tunas prefer to travel along this zone where cold and warm water meet. And for you to be sure that you are interacting with a yellowfin tuna, do not forget to look for the highly elongated dorsal and anal fin which is their primary identification mark.

Bigeye Tuna (Thunnus obesus)

Photo courtesy from www.horta.uac.pt

Max. Length: 250 centimeters
Length at 1st Maturity: 97.4 centimeters
a value: 0.03210
b value: 2.810
Depth Range: 0 – 250 meters (0 – 750 feet)
Frequency: Threatened; found in the World’s tropical Oceans

Photo courtesy from culture.teldap.tw

Continuing with the competition as previously mentioned, the bigeye tuna will take the first place position. They are the biggest and the fastest tuna species you will find in Cocos island. If only bluefin tuna exist in Cocos island, then they would slide down to 2nd place. Unfortunately we were not able to find any records proving that bluefin tunas thrive the waters of Cocos island.

Similar to a yellowfin tuna, bigeye tunas are large-sized fish which can attain a maximum size of 2.5 meters. Their muscular system is quite efficient in lifting their heavy weight body while traveling around the world’s tropical water following an annual migration pattern.

Preferred Meat for Sushi and Sashimi

Photo courtesy from regimea.com

In case you have been to a Japanese restaurant, you will never miss being served with the usual Specialty of the House. Prepared either as raw meat sashimi or added with rice as sushi, mackerels especially tunas are the preference for Japanese Master Chefs. This is due to the fact that their meat contains a certain amount of fat. But not the bad fat. Instead, it has the good cholesterol called Omega 3 which is considered good for the heart. The higher the fat content, the higher the price where a pound of tuna meat can fetch up to twenty dollars.

But it’s not all good nutrition that you can get out of eating fresh tuna. You should take note, especially if you do not have a palate for Japanese dish, that raw fish meat may contain bacteria and other harmful marine toxins. In the case of tunas and mackerels, they are known to possess ciguatera. When ingested in huge quantity, this marine toxin can be harmful to humans and can lead to poisoning.

Your Usual Interaction with Tunas and Mackerels in Cocos Island

Photo courtesy from www.choice.com

Definitely your interaction with tunas and mackerels will not be in the dining room’s plate of your liveaboard vessel. Of course, we know that you want to see them underwater alive and swimming in all its glory.

If one of your dive objectives in Cocos island is to see tunas and mackerels, then you need to drift further out in the open sea. Do not expect to interact with tunas and mackerels in the reef as they are not a reef-associated species. In most instances, you will interact with tunas and mackerels towards the end of your dive where you usually drift out off the reef while riding along mild to moderate current in the open sea. And the best time to see them is during the arrival of small migrating sardines where this apex fish predators follow their trail in close proximity and do a shallow water feast called the baitball.

They’re not a Game fish in Cocos Island

Photo courtesy from Overton’s

While tunas and mackerels are considered a highly-priced game fish in other countries, they are not here in Cocos island. Tunas, mackerels and other species of fish, as well as all marine wildlife in Cocos island are protected by law by virtue of the Cocos Island National Marine Park Policy and taking them out of their habitat is prohibited.

But our major concern for tunas and mackerels is their worldwide population. Studies show that tuna and mackerel populations have steadily declined over the years. This is due to the fact that their huge market demand has led the commercial fishing industry to exploit this species by all means.

We are not saying that we stop eating tunas, mackerels and other species of fish. What we just want to appeal to everybody is to influence our commercial fishermen to do their fishing routine after this valuable marine resources have already spawned and accomplished their cycle of sexual reproduction.

In this way, all of us will be benefited, regardless if you are a diver, a fisherman or simply just a person who loves to eat fish. And ultimately, this will prevent our children’s grandchildren to know the existence of these species only through books and on the internet without seeing them alive in its wild environment like the ones you will see in Cocos island.

Reference

Fishbase: www.fishbase.org

Encyclopedia of Life: www.eol.org

The Nature Conservancy: www.nature.org

Video courtesy from Maria Tsourlini

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