Why fish size figures disagree
Two sources giving a fish as 22 cm and 40 cm may both be right and may not disagree at all. Four separate distinctions are routinely collapsed in aquarium writing — measurement convention, maximum against typical, wild against captive, and what kind of document the figure came from — and untangling them resolves most apparent conflicts and leaves a small number of real ones.
Short answer
Because they are usually measuring different things. Standard length excludes the tail fin and total length includes it; a maximum is the largest specimen anybody recorded and a typical size is what a well-kept adult reaches; and a taxonomic checklist records the biggest fish its compiler examined, which is not the same as the biggest the species gets.
Standard length and total length
Standard length runs from the snout to the base of the tail fin. Total length runs from the snout to the tip of it. For a fish with a short tail the difference is ten per cent; for one with a long trailing caudal it can be a third of the animal.
Museum and taxonomic sources overwhelmingly publish standard length, because it is reproducible on a preserved specimen whose fins may be damaged. Angling records and aquarium literature overwhelmingly publish total length, because it is what you get when you lay a fish against a ruler.
So a museum figure and a shop figure for the same species will differ, systematically, in the same direction, without either being wrong. AquariumHQ records the measurement type on every size value where the source states one, and records that the source did not state one where it did not — because '40 cm' of an unstated kind is a different fact from '40 cm TL'.
Maximum, typical, and the gap between them
A maximum recorded size is one extreme specimen. A typical adult size is what a well-kept fish reaches. The gap is usually large and occasionally decisive.
The goldfish is the clean example. USGS NAS gives a maximum of 410 mm SL and a typical 120–220 mm SL in the same sentence, and separately reports an angling record of 59 cm TL. Those three numbers span a factor of five and every one of them is correct about a different question.
Fishes of Australia publishes both, routinely and explicitly: 'To 7 cm SL, commonly to 4 cm' for the Pacific blue-eye. Almost no aquarium source does this, and its absence is why 'maximum size' has quietly become the number the trade quotes for everything.
| Figure | What it answers | Where it comes from |
|---|---|---|
| 41 cm SL | The largest specimen recorded | Field guide, standard length |
| 12–22 cm SL | What one usually is | Same field guide, same page |
| 59 cm TL | The largest ever caught on a line | Angling record body |
| 20–35 cm | What a pond fish reaches in practice | AquariumHQ's own position |
A checklist is not a survey
This is the distinction that produced the sharpest disagreement AquariumHQ found this sprint. USGS NAS gives the jaguar cichlid as 22 cm SL, crediting a taxonomic checklist. AquariumHQ publishes 30–40 cm. That is roughly a factor of two and it is not a standard-versus-total-length artefact.
The likeliest reconciliation is that a checklist records the largest specimen its compiler examined — a museum collection, assembled for taxonomy rather than for size records — while the aquarium figure describes a heavily farmed fish that has been seen much larger. That is a reasonable story and it has not been established, so both figures are published and the disagreement is left open rather than resolved by picking the convenient one.
Two sources agreeing may be one source twice
USGS NAS gives the jack dempsey as 25 cm SL and credits Kullander (2003). It gives the convict cichlid as 10 cm SL and credits Kullander (2003). It gives the jaguar cichlid as 22 cm SL and credits Kullander (2003). Those figures also stand behind other databases' entries for the same fish.
Two databases agreeing on a number is not two measurements agreeing. It can be one measurement reported twice, and from the outside the two look identical. AquariumHQ records the upstream reference a source credits, wherever it credits one, and refuses to count two values as independent when they name the same work.
What to do with a size figure you have just found
Four questions, in order. They resolve most of it.
- Is it standard length or total length? If the source does not say, treat the figure as approximate rather than assuming.
- Is it a maximum or a typical? A maximum is not a planning figure and is quoted as though it were constantly.
- Is it wild or captive? Farmed stock of several species runs consistently smaller than wild; a few run larger.
- What kind of document is it? A taxonomic checklist, an angling record and a care sheet are answering three different questions with the same unit.