FISH TECHNOLOGY: TYPES OF FISH
Diagram
Where fish store their fat
Fatty fish
Store fat right through the flesh. The flesh is oily, rich and darker, and it carries omega-3 fatty acids.
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Lean fish
Store fat mainly in the liver, not in the flesh. The flesh is white, mild and low in fat.
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The test
Cut the flesh: if it looks oily and rich, it is a fatty fish; if it looks white and dry, it is a lean fish.
Figure 1: Fatty fish carry oil in the flesh; lean fish carry it in the liver. Drawn in our own style from A. S. Ninawe et al., Fish Nutrition And Its Relevance To Human Health, CRC, 2020, and the omega-3 discussion in Paul Greenberg, Four Fish and The Omega Principle.
Introduction
Fatty fish and lean fish are the two groups of fish by fat, and the difference is where the fish stores its fat. Fatty fish store fat right through their flesh, so the flesh is oily and rich, while lean fish store fat mainly in the liver, so the flesh stays white and low in fat. Ninawe et al. cover fish nutrition and its relevance to human health, and Greenberg covers omega-3 fatty acids and the fish that carry them.
The core difference is the store: in a fatty fish the flesh itself is the fat store, and in a lean fish the liver is the fat store and the flesh stays lean.
Quick comparison
Table 1: Fatty fish compared with Lean fish
| Aspect | Fatty fish | Lean fish |
|---|---|---|
| Where fat is stored | Through the flesh | Mainly in the liver, not in the flesh |
| Flesh colour | Darker: pink, orange or grey, and oily looking | White and pale |
| Fat in the flesh | High: the flesh is oily | Low: the flesh is lean and dry |
| Omega-3 fatty acids | Rich in omega-3 fatty acids, EPA and DHA | Low in omega-3 fatty acids in the flesh |
| Flavour | Rich, strong, oily flavour | Mild, delicate flavour |
| Texture | Moist and soft, even when cooked | Firm and flaky, and it dries out if overcooked |
| Typical examples | Salmon, tuna, mackerel, sardines, herring | Cod, haddock, tilapia, catfish |
| Cooking | Suits grilling, baking and smoking; the oil keeps it moist | Suits frying, steaming and baking in sauce; needs care not to dry out |
| Use of the liver | The liver is not the main product | The liver is rich in oil, and cod liver oil is a known product |
| Shelf life | Spoils faster, because the oil in the flesh goes rancid | Keeps a little better chilled, because there is less oil to go rancid |
Scientific explanation
Definition
Fatty fish are fish that store fat in their flesh, so the flesh is oily and rich in omega-3 fatty acids. Salmon, tuna, mackerel, sardines and herring are the usual examples. Lean fish are fish that store fat mainly in the liver, so the flesh stays white and low in fat: cod, haddock, tilapia and catfish are the usual examples.
The books used here treat the groups by their fat store and their nutrition. The distinction is about the flesh, not the whole fish: a lean fish can still have a fatty liver, and cod liver oil is the proof of that.
Working principle
Fish store fat where their life needs it. Fatty fish are usually active swimmers that need energy through the body, so they store fat in the muscle, and that fat carries the omega-3 fatty acids EPA and DHA. Lean fish are usually slower bottom dwellers that store their fat in the liver, and their muscle stays lean and white.
That is why the two groups cook differently: the oil in fatty fish keeps the flesh moist under heat, while lean fish flesh has little oil to protect it, so it dries out fast if it is overcooked.
Apparatus
The books used for this article name no single standard apparatus for this comparison: the groups are told apart by the look and feel of the flesh. In a laboratory, fat content is measured by extracting the fat from the flesh and weighing it, and omega-3 fatty acids by chromatography, 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 flesh is judged by eye and by eating: oily, rich flesh means a fatty fish, and white, dry flesh means a lean fish. Where a number is needed, the fat is extracted from a sample of flesh and weighed.
Factors affecting readings
The season matters: fish are fatter before spawning and leaner after it. The diet matters too: what the fish eats changes its fat. The species matters most of all, because the fat store is set by the species, not by the season alone.
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.
The books used for this article give no fat percentage table for these groups, so no numbers are given here: the comparison rests on where the fat is stored, not on a cut off value.
Industrial applications
- Fatty fish is used for grilled salmon, smoked salmon and tuna steaks, where the rich flesh is the point.
- Fatty fish is used for canned sardines and canned tuna, where the oil keeps the fish moist.
- Fatty fish is used as a source of omega-3 fatty acids in the diet.
- Lean fish is used for fish and chips, fish fingers and battered fillets, where a mild white flesh is wanted.
- Lean fish is used for dried and salted fish, where the low fat keeps well.
- Cod liver oil is made from the liver of lean fish, where the fat is stored.
Advantages and limitations
- Fatty fish, advantage: rich in omega-3 fatty acids and stays moist when cooked.
- Fatty fish, limitation: spoils faster, because the oil in the flesh goes rancid.
- Lean fish, advantage: mild and lean, so it fits many dishes and keeps a little better chilled.
- Lean fish, limitation: dries out quickly if overcooked, and gives little omega-3 from the flesh.
FAQ
Is salmon a fatty fish?
Yes: salmon stores fat through its flesh, which is why it is pink, oily and rich. That is also why it stays moist when grilled.
Is cod a lean fish?
Yes: cod stores its fat in the liver, not in the flesh, so cod flesh is white and lean. Cod liver oil comes from that liver.
Do fatty fish have more omega-3 than lean fish?
Yes, in the flesh: fatty fish carry EPA and DHA through their flesh, while lean fish carry little omega-3 in the flesh. The lean fish’s omega-3 sits in its liver instead.
Which is better for health, fatty or lean fish?
Both have a place: fatty fish give omega-3 fatty acids, and lean fish give lean protein with little fat. The books used here describe the nutrition of both without ranking one above the other for all people.
Why does fatty fish spoil faster?
Because the oil in the flesh reacts with air and goes rancid, which gives the stale fishy smell. Lean fish has less oil in the flesh, so it goes rancid more slowly, though it still spoils by bacteria.
References and further reading
- A. S. Ninawe et al., Fish Nutrition And Its Relevance To Human Health, CRC, 2020
- Paul Greenberg, Four Fish: The Future of the Last Wild Food, Penguin, 2010 (Google Books page read)
