Sustainable Seafood 10. Tuna and Swordfish
In which the E@L features the fastest fish in the sea, from Charlie to sushi
Almost everybody in my demographic is familiar with Charlie the Tuna, and the phrase “Sorry, Charlie”. Tuna is one of the most popular types of fish in the world. Canned tuna is such a household staple in the United States that most people just call it “tunafish” as if it was all one word. Tunafish salad. Tunafish sandwich. Tuna melt. In fact, there are many species of tuna, including albacore, bigeye, yellowfin, bluefin, skipjack, and others. Most tuna consumed in the United States is imported; we only catch about 21,000 t per year, but import 103,000 t.
Tuna are some of the fastest swimmers in the ocean, capable of reaching 50 mph. They swim continuously, and have evolved a torpedo shaped body, called fusiform, that allows them to achieve such high speeds. Swimming fast requires large amounts of oxygen, so their bodies produce large amounts of hemoglobin. The also have the ability to warm their bodies slightly by exchanging heat with seawater, sort of a fishy heat pump. This tissue forms the “dark meat” along the outside of the body.
Tuna are apex predators, preying on other fish and squid, and are themselves preyed on by sharks and large billfish like marlin and swordfish. They live mainly in pelagic (surface) waters in subtropical oceans, and are highly migratory, ranging over thousands of miles. They live in large schools of thousands of fish that may be up to 20 miles across. Tuna have been known and eaten by humans for thousands of years, and show up in cave art from 9200 BCE.
Tuna Fisheries
Because tuna don’t reside within the boundaries of any single country, and are caught in the open ocean, high seas fisheries are managed by international organizations such as the International Commission on Conservation of Atlantic Tuna (ICCAT). Fisheries that occur in the United States are managed by NOAA.
Tuna are caught by multiple methods. One of the most sustainable is hook & line fishing (pole-caught), because it produces the least bycatch of other species. Tuna that travel in large schools are often caught by purse seine, a large net that is spread around the school and drawn together. This is the most efficient method, but may also catch other species. Some of these can be released by tipping the edge of the net. Some tuna are caught by drift gillnets, suspended from the ocean surface, that may be hundreds of miles long. These catch many other species of fish as well as turtles, birds, and marine mammals, so are illegal within the US EEZ. Tuna are also caught using longlines, a weighted line that may be 10-100 km in length with, 200-2000 baited hooks. These can also produce high bycatch if they are set too shallow, with catches of bluefin tuna, marlin, sea turtles, albatrosses, and sharks. Ensuring that tuna are caught responsibly, with little bycatch, is the goal of the Dolphin-Safe Tuna Tracking and Verification Program, managed by NOAA.
A common method for attracting tuna is the use of Fish Aggregating Devices (FADs). These are floating structures, usually made of wood or bamboo, sometimes with attached nets or lines. Set in the open ocean, they not only attract tuna, but also turtles, sharks, and dolphins, so also contribute to high bycatch. Annually, over 100,000 are used, and 65% of seine-caught tuna are captured using FADs. International law requires that all FADs in the open ocean have trackers on them, but many do not.
Tuna Species
In the boxes below, I summarize each of the major tuna species caught for human consumption. Albacore and skipjack tuna are the smallest species, and usually used for canned tuna, with Albacore labeled as “White” tuna, and skipjack as “Light” tuna. Prices can vary widely among canned tuna depending on the species and method of catch. Most expensive (>$4.00 /can) is Skipjack or Albacore labeled as “Wild” Pole-caught. MSC-Certified White Albacore is next, at about $3.00 per can. Chunk/Solid White Albacore sells in the $2.00 range, and cheapest is “Chunk Light” which is mostly skipjack tuna, at about $1.00 per can. Yellowfin and Bigeye tuna may be canned or sold as Ahi. Bluefin tuna is the largest and most valuable species and is prized for its use as sushi or sashimi, where it is called maguro, but the fattiest, richest meat is called toro.
Swordfish
Though they look similar, tuna and swordfish are not closely related. Swordfish are members of the family Carangidae, which includes jacks and mahi-mahi. They primarily consist of one species Xiphias gladius, that occurs in tropical parts of all oceans. Atlantic swordfish spawn in the Caribbean sea, and swim as far north as Newfoundland.
Swordfish are the fastest fish in the ocean, capable of swimming speeds up to 50 mph. They are pelagic, living in the surface layers of the open ocean. As adults, they can weigh up to 1000 lbs, but most are in the range of 50-200 lbs. They are apex predators that consume squid and other fish, and as adults, have no predators of their own. Like tuna, international swordfish stocks are managed by ICCAT, and those in US waters are managed by NOAA. Stocks in the Pacific Ocean are healthy, those in the North Atlantic are not threatened, but those in the South Atlantic are overfished. Swordfish are caught using Hook and line (“pole-caught”), harpoons, and longlines. The commercial fishery lands 5.3 million lbs annually, worth about $23 million. A recreational fishery lands about 4 million lbs. The product is typically sold as filets or steaks.
Nutrient Content of Tuna and Swordfish
A 3-ounce serving of skipjack tuna, as it comes from the can, is highly nutritious. It has some of the highest protein concentration of any protein source. Calories are low, and fats (total, saturated, and cholesterol) are in the lowest 5%. Omega-3 fatty acids are also low for the same reason. Bluefin tuna has mid-level quantities of calories, protein and saturated fat. Both may have high levels of mercury. You can safely consume up to three 5-oz cans of skipjack or “light” tuna per week but should not eat more than one can of albacore tuna. Children should eat less, and pregnant women should avoid tuna altogether. Swordfish have medium levels of most nutrients including omega-3 fatty acids, but may also have high levels of mercury.
Fast Fish Summary
Canned tuna is one of the most popular, convenient, and healthy foods available, and should be a regular part of your diet. It provides a high concentration of protein with low fat content. Tuna steaks, as ahi, or sushi (maguro, toro), and swordfish are delicious main meals but may have high mercury content, so should be eaten in moderation. When I eat in a restaurant, I’d much rather order seared ahi than steak, because tuna has much lower saturated fat and higher omega-3 fatty acids than beef, and a much lower environmental impact.
Bon apetit! Itadakimasu!
Writing about nature is not easy. It requires preparation, hard work, and sometimes sweat to observe nature, and time, thought, and effort to describe it. Although this post is free, becoming a paid subscriber will help me continue to share my thoughts, and encourage future postings.
This issue of Ecologist at Large is available to all readers. However, if you would like to support my work with a one-off contribution, click “Buy me a coffee” below.
Thanks for pointing out that ahi has lower environmental impact than beef. I’ve never been sure how they compare.