FISH TECHNOLOGY: TYPES OF FISH
Diagram
Two oily fish, two different lives
Salmon
An oily fish that is born in fresh water, lives in the sea, and returns to fresh water to spawn. Its flesh is pink or orange.
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Tuna
An oily fish that lives its whole life in the open sea. It swims fast and far, and its flesh is dark red.
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Both oily
Both are fatty fish with omega-3 fatty acids, and both are among the four fish Greenberg follows in Four Fish.
Figure 1: Salmon migrates between river and sea; tuna lives in the open ocean. Drawn in our own style from Paul Greenberg, Four Fish, which follows salmon and tuna in depth.
Introduction
Salmon and tuna are the two most popular oily fish, but they are different species with different lives, different flesh and different uses. Salmon is born in fresh water, grows in the sea, and returns to fresh water to spawn, while tuna lives its whole life in the open sea, swimming fast and far. Greenberg’s Four Fish follows both species in depth: salmon, sea bass, cod and tuna are the four fish that dominate our menus.
The core difference is the life cycle: salmon is anadromous, moving between river and sea, while tuna is a true ocean fish that never enters fresh water.
Quick comparison
Table 1: Salmon compared with Tuna
| Aspect | Salmon | Tuna |
|---|---|---|
| Life cycle | Born in fresh water, lives in the sea, returns to fresh water to spawn | Lives its whole life in the open sea |
| Flesh colour | Pink or orange | Dark red |
| Why the colour | Pigments from the crustaceans it eats colour the flesh | Dark red muscle, rich in myoglobin for fast, sustained swimming |
| Size | Medium: most salmon are well under a metre long | Large: tuna grow much bigger than salmon |
| Swimming | Strong swimmer, migrates up rivers | Very fast, long distance ocean swimmer |
| Fat | Oily flesh, rich in omega-3 fatty acids | Oily flesh, rich in omega-3 fatty acids |
| Mercury | Lower, because salmon are smaller and shorter lived | Higher, because tuna are larger, longer lived predators that build up more mercury |
| Farming | Most salmon eaten today is farmed; salmon was the first large scale farmed fish | Tuna farming is limited: most tuna is wild caught |
| Typical dishes | Grilled fillets, smoked salmon, salmon sushi | Seared steaks, canned tuna, tuna sushi and sashimi |
| Eaten raw | Yes, as sushi and as smoked salmon | Yes, as sushi and sashimi, where the red flesh is prized |
Scientific explanation
Definition
Salmon is an anadromous fish: it hatches in fresh water, migrates to the sea to grow, and returns to fresh water to spawn. Tuna is a pelagic ocean fish: it lives in the open sea, swims fast over long distances, and never enters fresh water. Greenberg covers both species in depth in Four Fish, and notes that salmon was the first large scale domestication project for the fish we eat.
Both are fatty fish, rich in omega-3 fatty acids, which is why they are grouped together on the fish counter. The differences are in the life cycle, the flesh and the size, not in the fat group.
Working principle
Salmon flesh is pink or orange because of pigments in the crustaceans it eats: the pigments are stored in the flesh. Farmed salmon are fed a feed that includes the same kind of pigment, so their flesh is coloured too. Tuna flesh is dark red because tuna muscle holds a lot of myoglobin, the pigment that stores oxygen for fast, sustained swimming.
Mercury builds up up the food chain: a large, long lived predator such as tuna eats many smaller fish over a long life, so it gathers more mercury than a smaller, shorter lived fish such as salmon. That is why tuna is the fish most often named in mercury advice, a point Greenberg investigates in Four Fish and Hightower documents in Diagnosis: Mercury.
Apparatus
The books used for this article name no single standard apparatus for this comparison: the two fish are told apart by their species, their flesh colour and their cut. In a laboratory, mercury is measured by atomic absorption or mass spectrometry, but no such instrument is part of the everyday definition.
Measurement procedure
There is no standard measurement procedure for this comparison in the books used here. In practice the fish is identified by its label, its flesh colour and its cut: pink or orange flesh in a fillet is salmon, and dark red flesh in a steak or a can is tuna.
Factors affecting readings
The diet sets the colour of salmon flesh: wild salmon eating crustaceans are deeply coloured, and farmed salmon are coloured by their feed. The size and age set the mercury in tuna: bigger, older tuna carry more. The season sets the fat in both, because both are fatter before spawning.
Formulas and calculations
The books used for this article give no formula for this comparison, so none is given here. The comparison is qualitative: it rests on definitions, materials and method, not on a calculation.
Industrial applications
- Salmon is used for grilled fillets and baked salmon, where the pink flesh is the point.
- Salmon is used for smoked salmon, where the oily flesh takes the smoke well.
- Salmon is used for sushi, where the pink flesh is prized.
- Tuna is used for seared steaks, where the dark red flesh is cooked quickly and left pink inside.
- Tuna is used for canned tuna, the most common way tuna is eaten.
- Tuna is used for sushi and sashimi, where the red flesh is eaten raw.
Advantages and limitations
- Salmon, advantage: rich in omega-3 fatty acids, widely farmed so supply is steady, and lower in mercury than tuna.
- Salmon, limitation: farmed salmon depends on feed and farming, and its colour comes from the feed as well as from nature.
- Tuna, advantage: firm, meaty flesh that suits steaks and raw dishes, and rich in omega-3 fatty acids.
- Tuna, limitation: higher in mercury than salmon, and mostly wild caught, so supply and price move with the catch.
FAQ
Why is salmon pink and tuna red?
Salmon flesh is pink or orange from pigments in the crustaceans it eats, while tuna flesh is dark red from myoglobin, the pigment that stores oxygen in its fast swimming muscle.
Which has more mercury, salmon or tuna?
Tuna has more: it is larger and longer lived, and it eats many smaller fish, so mercury builds up in it. Salmon is smaller and shorter lived, so it carries less.
Is most salmon farmed?
Yes: Greenberg notes that there are many more farmed salmon in the world than wild salmon, and salmon was the first large scale domestication project for the fish we eat.
Can both be eaten raw?
Yes: both are eaten raw as sushi and sashimi. Only fish handled for raw eating should be used, and it must be kept very cold.
Why is canned tuna so common but canned salmon less so?
Because tuna is caught in huge numbers in the open sea and cans well, while salmon is mostly sold fresh, frozen or smoked. Canned salmon exists, but canned tuna is the bigger trade.
References and further reading
- Paul Greenberg, Four Fish: The Future of the Last Wild Food, Penguin, 2010 (Google Books page read)
- Jane Marie Hightower, Diagnosis: Mercury: Money, Politics, and Poison, Island Press, 2013
