Tuna

People have valued tuna as a favourite food source for centuries, but most of us know very little about this diverse and agile creature, found in all the world's oceans.

Here, you can learn about the principal types of tuna, gain an understanding of its place in the food chain, and discover how these extremely interesting and valuable fish are brought to our tables through environmentally-friendly, renewable harvesting.

A TUNA’S LIFE

Tuna are among the most beautiful of the sea’s creatures. These fish, with their delicate colours and wonderfully streamlined bodies, have evolved to what appears to be the limit of hydrodynamic refinement. When tuna swim rapidly, their fins retract into grooves, and even their eyes form a smooth surface with the rest of the head.

Tuna live in the open sea rather than near the shore, and stay for the most part in the upper layer of water, called the mixed layer. The mixed layer is warmed by the sun and air, stirred by the wind and waves, and is a rich environment for tuna to live in. Where the oceans are warm, tuna are there. The Skipjack tuna, for example, is confined for the most part, in water at 25 degrees celsius (78 degrees Fahrenheit) or warmer. Tuna travel a lot. All of these fishes are nomads, and move across entire oceans. By scientific tagging, Albacore tuna have been tracked over 8500 km from California to Japan at the rate of 25 km per day. Tuna never rest; they must always be moving. Their demand for oxygen requires moving one body length per second in order to get enough oxygen. Tuna get their oxygen from water, not from the air. They swim with their mouth open, which shoots a jet of water over their gills with which they extract oxygen from the water. Because of this system, they must remain swimming and can never stand still. This constant physical action also creates an enormous demand for energy, and a need to eat large quantities of food. A typical Tuna may eat one-quarter his own weight in food in one day.

Tuna are not cold-blooded as most fish, but have a system whereby they maintain their temperature a few degrees warmer than the water in which they find themselves. As warm blood returns to the gills to get more oxygen, it passes close to cooler blood coming from the gills. As they pass, the warmth is transferred to the cooler oxygen-rich blood.

All creatures have some sort of camouflage against their natural predators, and tuna have theirs. From deep in the water, looking up, the light-coloured belly of tuna blends with the surface light and makes the fish hard to discern. Looking down into the water, you don't see the tuna, because their back is blue, and blends with the deep blue of the sea .

Tuna begin life when a female lays her eggs (in fish, this is called spawning) in the open water. (While tuna and tuna-like fishes are at home almost anywhere in the world, it is interesting that they return to the same general locality for spawning.) The eggs are fertilized by males who release their sperm in the water where the eggs have been laid. Very small larvae hatch from the eggs usually within 24 hours. One large female may lay as many as 6,000,000 eggs in a single spawning. The number of eggs laid is related to size of the fish; a smaller female may yield only a million eggs. The eggs can be considered tiny: they measure one mm (0.04 inches) in diameter at spawning, including the droplet of oil surrounding the egg which makes them buoyant. From hatching to full growth, some of these species increase their size by one billion times.

If these fish lay so many eggs, why are the oceans not clogged with them? Actually, out of the millions of eggs spawned by tuna only about two, on average, grow to maturity from a single female. What happened to all those eggs and larvae and juveniles and young fish? They were eaten by bigger fish, even by their own kind. They also become food for many other kinds of swimmers in the sea as well as sea birds. Those who escape the perils of being eaten, and grow into adults, will find themselves at the top of the food chain, with few other swimmers to threaten them.

The daily menu for tuna includes fishes, crustaceans and mollusks. The Skipjack tuna is not picky about its diet, and neither are its many tuna cousins. They will eat whatever is available.

The more marine biologists have learned about tuna, the greater the mysteries surrounding these remarkable fish. Some of the unresolved questions are: How do the tuna navigate over thousands of miles in the trackless wilderness of the open ocean? To what extent are they homoeothermic – that is, able to maintain a roughly-constant temperature above that of the water in which they live? Why do the female tuna lay such enormous quantities of eggs? How do adult tuna, living at times in the desert waters of the high seas, obtain the quantities of food they need to swim continuously? As time continues forward we can only hope researches will be able to find the answers about these nomads, the tuna.

Tuna Species

Albacore (Thunnus alalunga)

Albacore is the only kind of tuna allowed to be called "white meat tuna" in the Canadian market. Albacore, usually range in size between 4.5 kg (10 lbs) to 23 kg (50 lbs). These fish are continually on the move throughout their lives. Albacore usually migrate across the Pacific Ocean from Japan and up the coast of North America. Albacore, however, do not swim in tight schools like other tuna species. This usually means commercial fishing boats need to work harder and longer to fill their freezer full of fish. Usually, commercial fishing boats need to be several hundred miles from shore to catch this long-finned tuna.

Skipjack Tuna (Katsuwonus pelamis)

Skipjack is the smallest tuna canned. Its average size is 3 kg (6 lbs) to 5.5 kg (12 lbs). Although small, it accounts for the largest share of tuna caught and eaten by people around the world. In fact the majority of Canadian canned tuna is packed with Skipjack. Skipjack live in warmer water temperatures which means they can be found around the world in the central belt of water. Purse seining and pole & line are the most common methods of catching this tuna.

Yellowfin Tuna (Thunnus albacares)

Yellowfin gets its name from the bright yellow colouration of its fins and finlets. Its body reflects the colour of most tuna, being metallic dark blue on the back, changing to yellow and silver on the belly. In the past, Yellowfin was the most important commercial catch, but because of improved dolphin safe catching methods, it's second to the Skipjack. Yellowfin tuna is still the largest of the tunas canned. This tuna can range in size between 13.5 kg (30 lbs) all the way up to 181 kg (400 lbs)!

FISHING FOR TUNA

COMMERCIAL TUNA CATCHING METHOD

PURSE SEINING is a method used to catch tuna. A seine or net is drawn out from the fishing boat and around the school of tuna. Weights carry one edge of the seine deep into the water. When the fish are surrounded, the bottom of the seine is drawn together or pursed, so that the fish are trapped and hauled aboard the boat. This method accounts for about 80 percent of the commercial catch of Light Meat Tuna.

LONG-LINING also accounts for about 80 percent of the world's Albacore tuna harvest. A very long line (as much as 130 km or 81 m) supported by floats and marked with flags is set out from a fishing boat. Branch lines attached to the long line are sunk with baited hooks to depths of 55 to 150 meters. It may take as many as 20 hours to set a line, and longer to retrieve it with its catch.

POLE AND LINE or TROLLING are other means used to catch tuna commercially. Live bait (anchovies or sardines) are dumped from the fishing boat to bring tuna into a feeding frenzy. This is called "chumming". Fishermen on the boat drop lines with barbless hooks into the school of fish, and bring them aboard as they are hooked.

HOW TUNA ARE PROCESSED

FROM CATCH OF THE DAY TO CANNING

Tuna is delivered to the cannery either directly from the fishing vessel or from freezer ships delivering from foreign fishing companies. The majority of the tuna delivered is frozen, and to maintain quality, is moved promptly from the vessel into cold storage facilities at the cannery. During the unloading, the fish is "sized" which keeps fish of the same size and weight together. The purpose of this step is to provide additional quality assurance during thawing and precooking.

The processing starts with tuna being moved from the freezer to thawing tanks. These are large water tanks where the tuna is uniformly thawed prior to the initial cleaning of the tuna. The thawed tuna is then loaded onto metal racks, which are wheeled into large steam pressure-cooking chambers, called retorts. Tuna is baked for a prescribed time and temperature, depending upon the size of the fish. This steam baking removes the natural tuna oil which is too strong in flavour to remain in the fish and prepares the tuna for the easy removal of skin and bones.

After baking, the racks of tuna are moved to a temperature-controlled room for cooling. Once the tuna is cooled it is sent to the cleaning tables where the separation of edible meat from skin & bones begins. There are two stations used for preparing Albacore tuna, one where the skin and bones are removed and a separate one for an additional inspection and cleaning. This is an "extra" Cloverleaf process for our premium quality canned tuna. The cleaned loins are then moved to the canning process. Cans are automatically filled with tuna and move in a single line from the filling machine towards the vacuum sealer. Next the cans of tuna automatically receive the appropriate measure of salt, vegetable broth, water or oil depending on the style of pack. Lids are then automatically clinched on the top of the can before entering the vacuum sealer, where the air is withdrawn and the lid hermetically sealed. The cans proceed through a water bath for cleaning off oil residue and then are placed back into retort baskets. After the prescribed cook time and temperature the sterilized canned tuna is removed from the retort and moved to a cooling area. When the cans have cooled they are labeled.

The day following packing, quality control personnel cut open representative samples from the previous days pack and grade them for the following: vacuum, appearance, smell, texture, style of pack, cleanliness, and flavour. The results of the cuttings determine if the product is marketable as Cloverleaf. If the product receives Quality Assurance approval it is sent to the warehouse for shipment to the marketplace.

Throughout the entire processing, quality control measures are exercised to maintain the standards that result in a "Cloverleaf" quality product, a product that is superior to, or competitive with any premium brand for consistency in appearance, cleanliness, smell and flavour. Cloverleaf Solid White Albacore Tuna is considered by the trade, consumers and competitors, to be the standard of quality for white meat tuna and as a result has enjoyed the Number 1 market position for many years.