Before you spearfish — this comes first
- Get proper instruction before you spearfish. A recognised freediving or spearfishing course teaches the things that actually kill people in this sport, and reading about them is not the same as being taught them.
- Dive with a buddy, and dive one up, one down — one diver in the water, one on the surface watching them, every dive. A breath-hold blackout is silent and gives no warning; a watching buddy is the difference between an incident and a fatality.
- Stay inside your own training and experience. Depth and breath-hold times are personal, they change day to day, and nobody else's numbers apply to you.
- Understand the conditions before you enter the water: visibility, current, swell, tide, boat traffic and your exit. Conditions decide whether a dive is reasonable far more often than skill does.
- Use equipment you have been shown how to use, check it before every session, and be visible — a marked float and line are how boat traffic knows you are there.
- Never extend a dive for a fish. No catch is worth pushing a breath-hold, and the urge to take one more second is exactly what a blackout feels like from the inside — which is to say, like nothing at all.
Why fish look bigger underwater
Explainer · Last reviewed 2026-08-25
A flat mask faceplate bends light at the water–air boundary, so everything looks larger and closer than it is — an error that runs in the direction most likely to make a diver shoot an undersized fish.
Safety
Fish welfare — Can cause injury needing treatment
Refraction through the mask and the water makes a fish appear noticeably larger and closer than it is. A diver who does not know this takes fish that are under the legal minimum size while believing they are comfortably over it, and does so consistently rather than occasionally.
What reduces it: Calibrate against something of known length — a marked shaft or a measured gun — and treat a fish that looks marginal as one that is under. Measure every fish taken rather than estimating, and adjust the internal reference rather than assuming experience corrects it.
Precautions that always apply here
- Never spearfish or freedive alone.
- Dive one up, one down: one diver down, one on the surface actively watching, every single dive.
A diving mask works by putting a pocket of air in front of the eye, because eyes cannot focus in water. That air pocket has a side effect, and it is not small.
Light passing from water into the air behind a flat faceplate is refracted, because the two media have different refractive indices. The result is a consistent perceptual distortion: objects appear LARGER and CLOSER than they are. The two errors are linked — they are the same refraction seen as size and as distance — and both run in the same direction.
The standard figures given for a flat faceplate in clear water are roughly a third larger and roughly a quarter closer. FishingHQ states the mechanism and the direction, and treats those figures as an approximate description of one specific case rather than as a correction a diver can apply, because the actual distortion varies with the faceplate geometry, the viewing angle, the distance to the object and the clarity of the water. A number that is right in a swimming pool and wrong on a reef will be used in both places.
WHY THE DIRECTION OF THE ERROR IS THE PROBLEM. Every size rule in fishing sets a MINIMUM. An error that makes fish look bigger is therefore an error that makes undersized fish look legal, and it does so to a diver who is not aware of being wrong. This is not a subtle statistical bias; it is a systematic perceptual distortion pushing consistently towards the outcome nobody wants.
IT ALSO INFLATES REPORTED SIZES. Fish estimated underwater and never measured are estimated through this distortion, which is one reason spearfishing size claims and rod-fishing size claims are not comparable, and one reason FishingHQ takes reported sizes from measured records rather than from accounts.
Distance and size are one error, not two
It is tempting to treat these as separate problems — things look bigger, and separately things look closer. They are the same refraction.
The practical consequence is that a diver cannot correct one by using the other. Judging size by comparing a fish to a shaft length at what feels like three metres does not help if the fish is actually four metres away and the shaft is being seen through the same distortion.
What DOES help is a reference at the same distance as the fish, in the same water, whose real size is known. A speargun of known length pointed at the fish, a float line with marked intervals, a section of reef whose scale the diver has established by touching it: these work because the distortion applies equally to the reference and to the target.
And there is one honest limit worth stating. Experienced divers become better at this, and their improvement is real. It is calibration by repetition rather than the removal of the distortion — which means it transfers poorly to unfamiliar water, unfamiliar species and unfamiliar visibility, all three of which change at once when somebody travels.
Scientific evidence
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
- NOAA Diving Manual: Diving for Science and Technology — National Oceanic and Atmospheric Administration, US Department of Commerce (Scientific institute)Relied on for: Refraction at a flat mask faceplate, and the resulting apparent-size and apparent-distance errors.States this directlyCitation checked — the work exists and is described correctly
What this means where a size rule applies
Size limits exist to let fish breed before they are taken, and in many species the large individuals are also the most productive spawners — so both ends of the size question have a conservation meaning.
A spearfisher's position is unusual: unlike a rod angler, they choose the fish. That is the discipline's strongest environmental argument, and it only holds if the choice is actually informed. A diver who cannot judge size reliably has not chosen; they have guessed with a weapon.
The workable rule follows from the direction of the error. Because fish look bigger than they are, a fish that looks MARGINAL against a size rule is very likely under it. Marginal means no. That is not caution beyond what the evidence supports — it is what the evidence supports.
It is also the same rule this family applies to species, arriving from the other direction: IF IDENTIFICATION IS UNCERTAIN, DO NOT TAKE THE FISH — and a fish whose size cannot be judged has not been identified as a legal one, whatever else is known about it.
Size rules themselves vary by country, by region, by protected area and by species, and they are revised. FishingHQ does not publish current limits and nothing here should be read as one. Check the current position with the fisheries or marine authority for the water you are diving before you get in, because it cannot be checked afterwards.
Authoritative guidance
Stated by a fisheries authority, scientific institute or recognised angling body. Authoritative, but summarising rather than demonstrating.
- CMAS technical and sporting standards for underwater activities — Confédération Mondiale des Activités Subaquatiques (Standards body)Relied on for: The existence of internationally maintained spearfishing standards, within which national size and species rules apply.Background and mechanism onlyCitation checked — the work exists and is described correctly
Where to go next
Where it matters
What these connections rest on (1)
Authoritative guidance — 1 connection
Stated by a fisheries authority, scientific institute or recognised angling body. Authoritative, but summarising rather than demonstrating.
Spearfishing
A membership assertion rather than a topical one: this record teaches the discipline or its equipment, so the discipline's mandatory safety posture applies to it.
- CMAS technical and sporting standards for underwater activities — Confédération Mondiale des Activités Subaquatiques (Standards body)Relied on for: Breath-hold spearfishing as a defined discipline with its own technical, equipment and safety standards, set out by the international underwater federation.States this directlyCitation checked — the work exists and is described correctly
- DAN guidance on freediving safety, hypoxia and shallow-water blackout — Divers Alert Network (Water-safety or diver-training authority)Relied on for: That the safety requirements of breath-hold diving apply across the discipline rather than to particular equipment or techniques within it.States this directlyPassage read and verified
Scientific evidence — 1 connection
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
Spearfishing
The perceptual distortion that bears directly on size-rule compliance.
- NOAA Diving Manual: Diving for Science and Technology — National Oceanic and Atmospheric Administration, US Department of Commerce (Scientific institute)Relied on for: Refraction at a flat mask faceplate and the resulting apparent-size and apparent-distance errors.States this directlyCitation checked — the work exists and is described correctly
Related reading
What these connections rest on (1)
Scientific evidence — 1 connection
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
Low-volume masks, and why volume matters
The equipment record and the optical consequence of the equipment.
- NOAA Diving Manual: Diving for Science and Technology — National Oceanic and Atmospheric Administration, US Department of Commerce (Scientific institute)Relied on for: Refraction at a flat mask faceplate and the resulting apparent-size and apparent-distance errors.States this directlyCitation checked — the work exists and is described correctly
Environment and conservation
What these connections rest on (1)
Scientific evidence — 1 connection
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
Fish stocks and sustainability
A perceptual error with a stock consequence, attached to the environmental record rather than left as prose.
- ICES Advice on fishing opportunities, catch and effort — International Council for the Exploration of the Sea (Scientific institute)Relied on for: Stock assessment and management advice concerning size at maturity and the removal of spawning adults.States this directlyCitation checked — the work exists and is described correctly
More explainer
Not related to this page — just the rest of the section, because this record connects to very little and a short page should still lead somewhere.
Check the rules before you fish
Fishing laws, bylaws, access rules, seasons, bait restrictions and permitted methods vary by location and can change. Check the current rules from the relevant authority and the water you intend to fish before fishing.
Check with:
- Current national or regional law for where you are fishing
- Current local bylaws
- The relevant fisheries authority
- The landowner, club or fishery whose water it is
- Any restrictions specific to the species or the method you intend to use
Sources for this page
- NOAA Diving Manual: Diving for Science and Technology — National Oceanic and Atmospheric Administration, US Department of Commerce (Scientific institute)Relied on for: The physics of diving as it affects vision: refraction, apparent size and apparent distance through a mask.States this directlyCitation checked — the work exists and is described correctly
How FishingHQ labels evidence is explained in the evidence policy.