Tuna




A yellowfin tuna cruises a kelp paddy on the hunt for baitfish. Divers can encounter tuna in deep, open water, especially near seamounts and pinnacles.


Perhaps while diving you have seen tuna — stout, bullet-shaped creatures zipping by in the hazy distance. Maybe they even came in close for one pass but not close enough for a good still or video. You might have spotted them in Tubbataha in the Philippines, at the Revillagigedos off Baja California or near some other offshore island, pinnacle or seamount at the edge of deep water. If you are lucky, you might have caught a glimpse of the giant of the family, the Atlantic bluefin, while diving with blue or basking sharks in the cold green waters of Maine or Scotland.

When you come across a tuna underwater, you are witnessing one of the wolves of the sea — an open-ocean predator that, because of its physiology, cannot ever stop swimming. The ephemeral tuna disappears as fast as it appears, and seeing one in the wild can really make an impact on you.


Tuna are warm-blooded, open-sea hunters that must stay in motion their entire lives.
The family Scombridae includes mackerel, bonito and the 14 species of true tuna, which include albacore, bluefin, bigeye, yellowfin and skipjack. Tuna range in size from small skipjacks and bonito to the massive 800- to 1,500-pound Atlantic bluefin. Fishermen call smaller tuna "footballs" because of their oblong bodies. They are built for speed: compact and muscular with lunate (crescent-shaped) tails. One of tunas' main predators, the mako shark (sometimes called the bonito shark), is similarly built — it would have a very hard time catching tuna otherwise.

While most fish can remain motionless and respire by opening and closing their mouths to pass water through their gills, tuna do not have the ability to do that. Larger tunas are warm-blooded and have "mega-gills," which may be 10 times the size of cold-blooded fishes' gills. This adaptation requires consistent passage of a very large volume of water through the gills. The warm blood comes from vascular heat exchangers that maintain a temperature in selected body regions that is higher than the ambient water temperature. This physiological feature gives tuna a speed advantage over their prey and warms the cold water that passes through the gills during cruising. Tuna can also use vascular activity to dissipate heat to the surrounding water to prevent overheating during feeding activity.


Divers film bluefin tuna in a grow-out pen in the Mediterranean.
My personal history with tuna began when I started taking underwater photos about 35 years ago. At the time there were few images of tuna in the wild. One day in the early 1980s after I had been diving on a drifting kelp paddy off the California coast, I watched as a school of 60-pound Pacific bluefin tuna arose from the depths to feed on a school of baitfish that was swimming below me. The speed and ferocity of the attack impressed me, and I began dreaming about capturing tuna on film. Although I had photographed some hooked tuna for fishing magazines, getting photos in the wild seemed like a long shot.

At first the best photo opportunities I could get were in grow-out pens of Pacific bluefin tuna destined for the Japanese sushi market. I got myself into the nets early in the pen-raised era and could for the first time get really close to the fish. In 2007 I was working on the Disney film Oceans and got to do an Atlantic bluefin tuna photo shoot in the Mediterranean. For the first time I saw what a 1,000-pound tuna looks like underwater. Nothing can prepare you for the experience of swimming among such massive giants. The huge bluefin seemed like Volkswagen buses swimming by me.

Yellowfin, albacore and skipjack have long been targeted for canned tuna, but in recent years the bluefin has become the real "cash cow." There are three species of bluefin tuna: the southern bluefin tuna (Thunnus maccoyii) from the Southern Hemisphere, the Atlantic bluefin tuna (Thunnus thynnus) and the Pacific bluefin tuna (Thunnus orientalis). All three species are captured in the wild and raised in grow-out pens for later harvesting.


The wolves of the sea, tuna travel in schools and attack baitfish with speed and brutal efficiency.


Japan consumes about 80 percent of commercially distributed bluefin tuna, and prices have undergone massive hikes in the past decade. A single 489-pound bluefin tuna fetched $1.76 million in 2013. These bluefin tuna are the same fish that were popularly called "horse mackerel" and were used for cat food and fertilizer in the 1950s and 1960s. Because of the price increase, fishing pressure has increased exponentially, especially on Atlantic bluefin tuna in their spawning grounds in the Mediterranean.

The environmental impact of this demand has been such that today the Atlantic bluefin is listed as endangered on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species. Recent research shows that Pacific bluefin tuna stocks are at 2.6 percent of the prefished biomass levels. These same studies warn that the existing take rates for Pacific bluefin are not sustainable. Bluefin conservation is the subject of much controversy between commercial interests and the scientific community.

Yellowfin, albacore and skipjack have long been targeted for canned tuna, but in recent years the bluefin has become the real "cash cow." There are three species of bluefin tuna: the southern bluefin tuna (Thunnus maccoyii) from the Southern Hemisphere, the Atlantic bluefin tuna (Thunnus thynnus) and the Pacific bluefin tuna (Thunnus orientalis). All three species are captured in the wild and raised in grow-out pens for later harvesting.


Seabirds feeding opportunistically on baitfish at the surface is one the best
indicators of tuna predation in the wild.
Japan consumes about 80 percent of commercially distributed bluefin tuna, and prices have undergone massive hikes in the past decade. A single 489-pound bluefin tuna fetched $1.76 million in 2013. These bluefin tuna are the same fish that were popularly called "horse mackerel" and were used for cat food and fertilizer in the 1950s and 1960s. Because of the price increase, fishing pressure has increased exponentially, especially on Atlantic bluefin tuna in their spawning grounds in the Mediterranean.

The environmental impact of this demand has been such that today the Atlantic bluefin is listed as endangered on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species. Recent research shows that Pacific bluefin tuna stocks are at 2.6 percent of the prefished biomass levels. These same studies warn that the existing take rates for Pacific bluefin are not sustainable. Bluefin conservation is the subject of much controversy between commercial interests and the scientific community.

As with sharks, billfish and the world's other great pelagic fishes, it will be interesting to see how we manage tuna populations in coming years. These beautiful, specialized fish help maintain the ecological balance in tropical and temperate seas of the world. We need them for food, but we also need healthy populations in the wild for the benefit of future generations.

© Alert Diver — Q1 Winter 2018