Tuna are powerful, sleek, muscular, pelagic fish, with streamlined fins and scales, and torpedo-like bodies that propel them through the ocean at high speeds and for great distances. Tuna are a popular fish, consumed either cooked, as canned tuna and tuna steak, or raw, as sushi and sashimi. Tuna are vital elements in marine ecosystems, as both predator and prey, but many tuna fisheries are under threat or have collapsed from overfishing. As apex predators, tuna flesh contains high levels of mercury and organic pollutants. Eating canned tuna fished by purse seine may contribute to the death of dolphin calves.
When we open a can of tuna, we may think we are eating only one species, but there are 61 species of ‘tuna-like’ fish recognized by science, though only 15 species are considered ‘tuna’, with 8 species known as the ‘true tunas’.
Tuna are saltwater finfish belonging to the Thunnini sub-group of the mackerel family Scombridae, which is further divided into 5 genera.
The ‘true’ tunas
The ‘true tunas’ belong to the genus Thunnus, and are further classified into two subgenera:
Thunnus (Bluefin Tuna group) and Neothunnus (Yellowfin Tuna group).
There are 5 species in the Thunnus (Bluefin Tuna) group:
Thunnus alalunga (Albacore Tuna)
Thunnus maccoyii (Southern Bluefin Tuna)
Thunnus obesus (Bigeye Tuna)
Thunnus orientalis (Pacific Bluefin Tuna)
Thunnus thynnus (Atlantic Bluefin Tuna)
There are 3 species in the Neothunnus (Yellowfin Tuna) group:
Thunnus albacares (Yellowfin Tuna)
Thunnus atlanticus (Blackfin Tuna)
Thunnus tonggol (Longtail Tuna)
There are 7 additional species in the Thunnini sub-group:
Allothunnus fallai (Slender Tuna)
Auxis rochei (Bullet Tuna)
Auxis thazard (Frigate Tuna)
Euthynnus affinis (Mackerel Tuna)
Euthynnus alletteratus (Little Tunny)
Euthynnus lineatus (Black Skipjack Tuna)
Katsuwonus pelamis (Skipjack Tuna)
A highly prized fish
Tuna are a valuable commodity on global seafood markets. The seven species with the highest commercial importance are Albacore, Bigeye, Skipjack and Yellowfin, and the three Bluefin species – Atlantic, Pacific and Southern.
Depending on your geographical location, the most common species used in canned tuna are Albacore, Yellowfin and Skipjack. Bigeye tuna are used for tuna steaks or raw as sushi and sashimi, while Bluefin tuna are traditionally used raw for sushi and sashimi. Bluefin is not used for canned tuna because of its high market price.
Overfishing has depleted global fish stocks and pushed many fisheries to the point of collapse, outside biological limits, or potentially beyond recovery. Many tuna stocks are under pressure from overfishing.
All but three of the seven most commercially valuable species of tuna are at risk of extinction. The three species of tuna not yet threatened with extinction from overfishing are the most common species used in canned tuna – Albacore, Yellowfin and Skipjack.
The IUCN Red List flags Albacore and Yellowfin as ‘Near Threatened’ with ‘decreasing’ population trends. Only Skipjack is of ‘Least Concern’ with a ‘stable’ population.
Bluefin tuna fisheries collapse
Large tuna species are generally longer-lived, slower-growing, and reach reproductive maturity much later than smaller tuna species, so their populations take longer to recover, which makes them highly vulnerable to depletion from overfishing.
The three Bluefin tuna fisheries – Atlantic, Pacific and Southern – are at risk of collapse. The IUCN Red List classifies Atlantic Bluefin as ‘Endangered’, Pacific Bluefin as ‘Vulnerable’ and Southern Bluefin as ‘Critically Endangered’. All species have ‘decreasing’ population trends.
Given their IUCN status, it is environmentally irresponsible to consume Bluefin tuna.
The manager of IUCN’s Marine Biodiversity Unit, Dr. Kent Carpenter, has said the Southern Bluefin tuna fishery has already collapsed with little hope of recovery. The most recent stock assessment on Pacific Bluefin fisheries from scientists at the Pew Environment Group revealed populations have dropped 97.4% from overfishing.
Atlantic Bluefin tuna fisheries are at risk of collapse, but new research suggests their populations may be more resilient than previously thought. In a recent study by Richardson et al. (2016), scientists discovered a new breeding ground in the Atlantic Ocean where tuna spawn at a younger age and smaller size than normal.
In this article however, researchers and fisheries scientists have pointed out these single-study results are preliminary and must be applied with caution.
Tuna as marine apex predator
Research scientists from the Future of Marine Animal Populations (FMAP) and the Census of Marine Life (COML) programs have reported a significant decline in marine predatory fish populations world-wide.
Globally, 90% of large predatory fish, including cod, grouper, marlin, shark, swordfish and tuna, have already been removed from the ocean.
Loss of marine apex predators may cause marine food chains to collapse. Tuna perform multiple functions as both apex predator and prey for species above and below them on the food chain.
Tuna eat smaller schooling fish, including mackerel and herring, squid, eels, crustaceans and zooplankton (as juveniles). Tuna are eaten by larger fish, including other tuna, swordfish, marlin, sharks, rays and toothed whales (dolphins).
Predatory species at the top of the marine food chain, including tuna, contain higher concentrations of heavy metals than other species lower on the marine food chain.
Mercury is a fat-soluble heavy metal that converts to methylmercury when absorbed by seafood, and increases up the food chain due to bio-accumulation, when larger fish eat fish that have eaten smaller fish.
Mercury is a neurotoxin that damages the kidneys, brain and central nervous system. High levels of mercury are toxic to human beings. The World Health Organization recommends a daily tolerable intake of 2µg total mercury per kilogram of body weight.
Canned tuna (albacore) contains on average 0.353ppm of mercury. If the daily tolerable intake of mercury at 2µg/kg is equivalent to 0.002ppm, consuming an average serve of canned albacore tuna weekly exceeds the daily tolerable limit.
It is necessary to be mindful when consuming canned tuna because of the high levels of heavy metal contamination, and although the species used for canned tuna are currently not threatened with extinction, it is important to ensure all tuna comes from sustainable sources.
Death by purse seine
I haven’t eaten canned tuna in a while because I don’t want to be responsible for playing a part in their declining stocks (not to mention the mercury issue). I knew my choices as a consumer could either exacerbate the problem or help solve it.
If you eat canned tuna fished by purse seine, you may be indirectly and unknowingly contributing to the death of dolphin adults and calves.
I am horrified to think my choices as a consumer could be killing dolphins. I won’t be eating canned tuna fished by purse seine again.
Dolphin calves are able to stay close to their mothers’ side when young, not because they are strong swimmers, but because their proximity reduces the forces they would need to generate themselves in order to swim independently.
Mother dolphin does all the work and baby gets a free ride, with no physical contact.
This process is called drafting, and it happens due to a combination of hydrodynamics and a scientific principle known as the Bernoulli Effect. The movement of water around a mother-calf pair pulls the calf towards its mother’s side.
In Chapter 12: Dolphin Snatchers (pp. 89-98) of Watching Giants: The Secret Life of Whales, zoologist and journalist Elin Kelsey documents the high incidence of ‘mother-calf separation’ as an unintended consequence of fishing Thunnus albacares (Yellowfin Tuna) by purse seine.
Kelsey explains how tuna were originally caught using the pole-and-line technique, but purse seine nets eventually replaced this traditional fishing method. Purse seine nets were historically ‘set’ on dolphin pods, because tuna tend to school beneath dolphins.
Kelsey cites data from a research paper published in 2007 where 75-95% of lactating dolphin mothers killed in purse seine nets was not killed with their calf.
During the chase and capture process characteristic of purse seine fishing, mother and calf pairs may become separated and the calf is lost.
The frightening and stressful speed and terror of a purse seine chase severs the energetic connection between dolphin mother and calf.
A calf has little chance of survival once separated from its mother.
Kelsey’s book was published in 2009 and cites research conducted nearly a decade ago, so it may seem that the incidence of mother-calf separation is no longer an issue.
The tuna fishing industry has undergone major legislative and operational changes since the 1980’s.
It is now illegal to intentionally set purse seine nets on dolphin pods, but it is completely legal for fishermen to catch dolphins with the tuna and then release them.
Even though dolphin deaths have declined significantly, marine scientists are still researching ‘mother-calf separation’ in the tuna fishing industry. The most recent evidence I found was on the NOAA fisheries website dated December 24, 2014 (although the link has now been removed).
Is canned tuna really dolphin-safe?
It is estimated over 6 million dolphins have been killed in the Eastern Tropical Pacific purse seine tuna fishery since the late 1950’s. Dolphin bycatch has been reduced by more than 99%, yet around 1000 dolphins are still killed every year.
Canned tuna fished by purse seine will never be truly dolphin-safe. Dolphin bycatch can never be reduced to zero, simply due to the unique ecological relationship that exists between tuna and dolphins. Where the dolphins are, will be the tuna…and vice versa – although the exact nature of the dynamics remains unknown.
International limits do exist on the maximum number of dolphins permitted to be killed in purse seine nets, but the reality is that dolphins still die for canned tuna.
What does ‘dolphin-safe’ mean?
The dolphin-safe canned tuna label doesn’t mean dolphin deaths have been completely eliminated, only that they have been reduced. The labelling also doesn’t consider other marine creatures killed by tuna fishing.
Besides purse seine nets, commercial tuna fisheries utilize fish aggregation devices (FADs), anchored or free-floating devices that attract tuna to the nets, but also attract other marine species, including sharks and sea turtles. Bycatch in tuna fisheries is not limited to dolphins, but we rarely hear about the other animals killed for canned tuna.
Should we eat canned tuna?
Consuming tuna is a personal choice. There are three issues to consider – environmental impact, toxicity and dolphin deaths. We can make informed choices as consumers in line with our ethics and implement those choices in the most sustainable way.
I’m concerned about mercury levels, and as a marine conservationist, I know how seafood gets to our tables. I feel a deep affinity with cetaceans and am concerned about the impact tuna consumption has on dolphins.
Eating canned tuna fished by the pole-and-line method, without the use of fish aggregation devices (FADs) seems to be the most ethical choice to make.