Summary: To identify fresh fish in the market, look, in this order, at the eyes (bulging and clear), gills (bright red and moist), skin (shiny with firm scales), touch (firm, springs back), and smell (of the sea, never ammonia). If two of these five signs are missing, do not buy it. Always check the mandatory labeling: catch area, fresh or defrosted.
Why it matters to identify fresh fish
Buying bad fish has three real consequences: you lose money (fresh fish isn't cheap), you ruin the recipe (a bad product spoils the best dish), and, in the worst case, you risk food poisoning — histamine from improperly preserved oily fish causes symptoms often mistaken for allergies.
At Mercat del Ninot, we've been selecting fish every day since 1990. The reality is uncomfortable: not all fish sold as "fresh" truly is. Some has been caught 4-5 days ago, others have been frozen and defrosted, and a few are directly on the verge of spoiling. Learning the signs doesn't make you distrustful; it makes you a good buyer.
There are three common types of fraud in mediocre fishmongers:
- Defrosted frozen fish sold as fresh (it's illegal without labeling, but it happens).
- Several-day-old fish "refreshed" with ice, water, and favorable lighting.
- Species substitution: monkfish for grouper, grouper for pangasius, megrim for lemon sole. Oceana studies detect up to 30% fraud in Spanish fishmongers.
The 7 foolproof signs (in order of priority)
Not all signs carry equal weight. These are the seven clues we teach to those starting out in the market, ordered by reliability:
1. The eyes: the first glance
The eyes are the most visible and quickest indicator. In fresh fish:
- Bulging, slightly protruding from the face.
- Transparent cornea, not cloudy or whitish.
- Bright black pupil, well-defined.
In several-day-old fish: sunken, opaque, whitish, gray pupil. This sign fails with very large hake (eyes sink due to deep-sea pressure) and with fish that has been on ice for a long time (eyes become "moist"). That's why it's the first sign, not the only one.
2. The gills: the second level
Here begins the inspection most consumers don't do. Ask the fishmonger to lift the operculum (the side "lid"). Fresh gills are:
- Bright red or intense pinkish, almost bloody.
- Moist, with a natural slippery appearance.
- No brown mucus or unpleasant smell.
One of the oldest tricks: "refreshing" old gills with water or dye. Look if the red is uniform and deep or just superficial. If it looks "painted," be suspicious.
3. The skin and scales
The skin of fresh fish has a characteristic metallic sheen, as if freshly wet. The scales are firmly attached — if they fall off with just a touch, the fish is old. The mucus covering the skin should be transparent or slightly whitish, never yellowish or excessively sticky.
Mercat Tip: run your finger against the direction of the scales. If they come off, the fish is several days old.
4. The texture: the finger test
Gently press the flesh with your fingertip (without digging in). In fresh fish, the indentation disappears in 1-2 seconds. If it remains sunken, the muscle has lost turgor — the fish is several days old, and freshness is being lost.
5. The smell: sea yes, fish no
This is the definitive sign. Fresh fish smells of the sea, of algae, of iodine. It's a subtle, almost mineral smell. What we call "fishy smell" — strong, unpleasant, persistent — is trimethylamine (TMA) and ammonia, products of bacterial decomposition.
Bring your nose to 10-15 cm. If the smell is marine, it's fresh. If it's "aggressive" or smells of ammonia, don't buy.
6. Mandatory labeling
According to EU Regulation 1379/2013, every fish retail point must display:
- Commercial and scientific name of the species.
- Production method ("caught at sea," "caught in fresh water," or "farmed").
- Catch area (FAO area + sub-area) or country of rearing.
- Fishing gear (purse seine, longline, trawl, etc.).
- "Defrosted" if previously frozen.
If any label is missing, ask. If the seller avoids answering or improvises, it's a red flag. Serious fishmongers have the information visible because they know it.
7. The overall impression: the "ambience" of the counter
The last sign is comprehensive. A serious fishmonger has:
- Fish on abundant ice, not in stagnant water pools.
- Counter clean and without puddles.
- Visible rotation — fishmongers cutting and restocking.
- General smell of the sea, not strong.
- Seller who answers without hesitation and allows inspection.
Summary table: what to look for, what to avoid
| Sign | Fresh fish | Discard |
|---|---|---|
| Eyes | Bulging, clear, black pupil | Sunken, opaque, whitish |
| Gills | Bright red, moist, no mucus | Gray, brown, dry |
| Skin | Shiny, firm scales | Dull, loose scales, sticky |
| Flesh | Firm, quickly springs back | Soft, remains sunken when pressed |
| Smell | Of the sea, algae, subtle | Ammonia, strong, repulsive |
| Labeling | Complete: species, area, gear | Missing info or "improvised" |
| Counter | Ice, clean, rotation | Stagnant water, chaos, strong smell |
Source: Mercat del Ninot operational experience + AECOSAN (Food safety recommendations for fish).
How to inspect fish at the market step by step
This is the procedure we teach new customers at Ninot. It takes 30 seconds and prevents most mistakes:
- Look at the counter from a distance before approaching. Fish well-arranged on abundant ice, with spacing, homogeneous color.
- Approach the fish you are interested in. Examine eyes and skin without touching.
- Ask the fishmonger to show you the piece. Good seller: picks it up without problem, opens the operculum, shows you gills and belly.
- Check texture and smell. Accept the piece only if all five signs (eyes, gills, skin, touch, smell) are correct.
- Read the label. Species, catch area, fresh or defrosted.
- Ask for it to be cut at the moment if it's a fillet or slice. Never pre-cut hours before.
- Keep the receipt. If something goes wrong, it's your right to claim.
Common frauds and how to detect them
NGO Oceana, in studies from 2018 and 2020, detected that between 8% and 30% of fish sold in Spain is of a different species than labeled. The most common frauds:
Species substitution
- Grouper for pangasius or Nile perch (true grouper exceeds €25/kg; pangasius €5-7/kg).
- Monkfish for Asian farmed monkfish (inferior quality, same name).
- Cod for ling (similar but very inferior — more on this in our ling vs. cod guide).
- Bluefin tuna for skipjack or bonito in processed preparations.
Defrosted sold as fresh
Signs that betray "fresh" fish that was frozen:
- Skin excessively shiny and wet (defrosting water).
- Larger than normal puddle under the piece.
- Flesh whitish and soft, releases liquid when pressed.
- Eyes too flat (freezing flattens them).
"Refreshed" with additives
Some low-profile fishmongers use polyphosphates (E-450, E-451, E-452) to make fish retain water and appear juicier. It's legal with a label, but often omitted. Hint: the meat releases a lot of liquid when cooked and weighs less than indicated on the receipt.
Common consumer myths and mistakes
Myth 1: "If it has a lot of ice, it's fresh"
Abundant ice is necessary, but it doesn't guarantee freshness — 4-day-old fish on ice is still 4-day-old fish. Look at the fish's signs, not just the counter.
Myth 2: "Expensive fish = fresh fish"
Price reflects species, scarcity, and season. An Almadraba bluefin tuna at €80/kg may not be fresh if it's been unsold for 3 days. Always inspect.
Myth 3: "Port fish is always the best"
It depends on the port and time of day. Fish markets primarily sell to professionals at auction. The fish that reaches the port fishmonger goes through the same channels as the fish at your market. The critical factors are the cold chain and rotation, not proximity to the boat.
Myth 4: "If it's packaged in a modified atmosphere, it stays fresher longer"
Modified atmosphere packaging (MAP) extends shelf life, but it doesn't turn 3-day-old fish into hours-old fish. Always check the packaging date, not just the expiration date.
When to be suspicious and when to return fish
Return the fish if:
- It smells of ammonia or "sour" when taken out of the packaging.
- The flesh is soft, mushy, or changes color in the air.
- It releases more than an inch of water in the refrigerator within 4-6 hours.
- The packaging date does not match what was said at the point of sale.
- The whole fish has sunken eyes and gray gills (it should not have been sold).
Your right: return or exchange with a receipt according to Royal Legislative Decree 1/2007. If the fishmonger refuses, file a complaint at your municipal OMIC (Consumer Information Office).
Professional tips from Mercat del Ninot
Three decades of inspecting fish have taught us these little tricks that are rarely shared:
The tail trick
Hold the whole fish by the head and let the body hang freely. If the tail stays horizontal or slightly droops, it's fresh. If it hangs loose and bends, it's several days old — the muscles have lost firmness.
The belly test
Press the belly between the operculum and the pelvic fin. It should be firm, without soft protrusions. A bloated or soft belly indicates internal fermentation — spoiled fish.
Checking the anus
Yes, you have to check it. In fresh fish, the anal opening is closed, retracted, and spotless. If it's open, dilated, or has discharge, the fish is days old or has begun to decompose.
The smell of the cut
If you order a fillet, ask to smell the fresh cut area. It should smell like the sea. If it smells "off" right after being cut, there's an internal problem (parasites, onset of decomposition).
The key day
On Mondays, many market fishmongers sell fish from Friday — there are no deliveries on Sunday. If you want the best fish, buy Tuesday through Saturday. Exception: fishmongers with direct daily supply from the fish market (like at Ninot).
Fresh vs. frozen: when does one make more sense than the other?
Intuition says "fresh is always better." The reality is more nuanced:
| Use | Best option | Why |
|---|---|---|
| Sashimi or raw | Frozen at -20 °C for 24h | Mandatory by law (anisakis). Fresh raw = risk. |
| Oily fish (sardine, anchovy) | Fresh from the day | Loses quality quickly. Fresh same day > frozen. |
| White fish (hake, sea bass) | Indistinguishable if quality is good | IQF frozen can match "3-day old" fresh. |
| Tuna, bonito | IQF frozen on board | Better than "fresh" after 4-5 days in a truck. |
| Frying (battered, tempura) | Frozen | Cheaper, same result when fried. |
| Raw shellfish | Fresh and alive | Live clams, mussels, oysters: only fresh. |
IQF = Individually Quick Frozen, a rapid freezing technique that minimizes ice crystals.
More on when each is appropriate in our fresh vs. frozen cod guide.
How signals vary by type of fish
Oily fish (sardine, anchovy, mackerel)
More perishable. Key signs: intense metallic shine, firm belly, no aggressive odor. Fresh anchovies have a straight, firm body, not bent.
White fish (hake, sea bass, sea bream, monkfish)
More resistant. Bulging eyes, firm flesh. Fresh hake has almost transparent, pearlescent flesh. If it's opaque and whitish, it's days old.
Flat fish (sole, John Dory, turbot)
Check the dark side (more visible signs than the white side). Clear mucus, no brown spots. Body stiff if you lift it — if it bends loosely, it's not fresh.
Shellfish and cephalopods
- Fresh octopus: bright eyes, firm suction cups, ink present. (Much octopus is bought already cooked — freshness refers to the cooked product and usually indicates a date).
- Squid/cuttlefish: iridescent skin, clear eyes, no ammoniacal odor.
- Mussels, clams: should be closed. For open ones: tap gently; if they don't close, discard them. Clean sea smell.
- Shrimp and prawns: head firm and attached to the body, without black spots (= melanosis). Translucent flesh, not opaque.
Conclusion: your mental checklist before buying
Identifying fresh fish isn't a secret art — it's a 30-second routine:
- Eyes clear and bulging.
- Gills red and moist.
- Skin shiny with firm scales.
- Flesh that springs back when pressed.
- Smell of the sea, never ammonia.
- Label complete and truthful.
- Counter with ice, clean, rotated stock.
If three of these seven fail, turn around. No recipe can salvage bad fish. And if you want to skip the inspection altogether, one option is to buy already processed and verified products — such as quality controlled desalted cod, Cantabrian "00" anchovies, or already cooked Galician octopus — where we handle the quality chain from the source.
Related articles
- How to desalt cod quickly and properly at home
- Ling vs. cod: how to tell them apart
- Fresh or frozen cod: what really matters
- Fresh vs. salted vs. desalted cod
- How to cook octopus: times and tips
