Compare fish
Pairs of fish that anglers genuinely confuse, and how to tell them apart. Every comparison rests on a recorded, evidenced relationship between the two — there is no page here for a pair FishingHQ cannot join.
Arctic char vs Brook trout
Both are char rather than trout, both carry pale spots on a dark ground and white-edged orange fins, and both are stocked and introduced well outside their native ranges — so the two now share water in places neither reached naturally. In spawning condition they converge further, because both turn orange-red underneath.
5 characters compared, 2 of them diagnostic on their own.
Arctic grayling vs Grayling
They are congeners that look almost identical: the same outsized sail dorsal, the same small mouth, the same silver-grey flank with scattered dark chevrons. Anglers who fish both continents routinely photograph one and caption it the other, and no anatomical character available in the field separates them cleanly.
4 characters compared, 1 of them diagnostic on their own.
Asp vs Ide
Two large silvery cyprinids sharing rivers across central and eastern Europe, similar in size, both taken on lures and both frequently reported as chub by anglers who meet neither often. The confusion runs in both directions and is resolved by the head rather than the body.
4 characters compared, 2 of them diagnostic on their own.
Atlantic bluefin tuna vs Yellowfin tuna
Both are large Thunnus, and a school-sized fish of either is a metallic torpedo with yellow finlets. The characters that separate them best are on the fins of adults, which is exactly what a small fish has not yet grown.
4 characters compared, 1 of them diagnostic on their own.
Atlantic bonito vs Narrow-barred Spanish mackerel
Both are fast, streamlined, striped or barred scombrids taken on trolled and spun lures, and both are widely called bonito or mackerel interchangeably in local usage. Where ranges approach one another in the eastern Atlantic and Mediterranean, catches are reported as each other regularly.
4 characters compared, 2 of them diagnostic on their own.
Atlantic cod vs Saithe
Two gadids taken over the same wrecks and reefs on the same tackle, frequently in the same drift, similar in outline and both carrying three dorsal fins and a barbel question. Small saithe and small cod are landed together and sorted afterwards across the whole of the north-east Atlantic.
4 characters compared, 2 of them diagnostic on their own.
Atlantic salmon vs Brown trout
The oldest identification problem in British and Irish angling, and it is genuinely hard rather than merely traditional. A sea trout and a fresh salmon are both bright silver, similarly proportioned, of similar size, running the same rivers at the same time of year. Colour and spotting help only until the fish has been in fresh water a while, and then both darken. The two also occasionally hybridise. Anglers on rivers where one species is protected and the other is not are required to tell them apart under exactly the conditions that make it difficult.
6 characters compared, 1 of them diagnostic on their own.
Atlantic salmon vs Chinook salmon
Both are called salmon, both are large silver anadromous fish, and both now occur outside their native ranges — Atlantic salmon are farmed and escape widely including in the Pacific, and chinook have been introduced to the southern hemisphere and to the Great Lakes. Anglers who know one well frequently meet the other for the first time in a place neither belongs, with no expectation that a second salmon is possible. They are not close relatives: one is a Salmo, the other an Oncorhynchus, and the differences are structural once you know where to look.
6 characters compared, 2 of them diagnostic on their own.
Australian bass vs Estuary perch
Two close congeners in the same south-eastern Australian estuaries, taken on the same lures at the same times of year, and called by each other's names constantly. They are similar enough that they were long treated as one species, they hybridise naturally where their ranges meet, and the colour difference most anglers rely on is a tendency rather than a rule.
6 characters compared, 1 of them diagnostic on their own.
Black crappie vs Bluegill
Both are deep-bodied, laterally compressed North American sunfish taken in the same swims on the same small baits, and juveniles of the two are close enough in outline to be counted together by anyone not looking closely.
4 characters compared, 2 of them diagnostic on their own.
Black drum vs Red drum
Two sciaenids sharing the same estuaries, flats and surf along the western Atlantic and Gulf, both taken on the same bottom baits, both capable of very large sizes, and both producing the drumming sound the family is named for. Juveniles overlap most.
4 characters compared, 2 of them diagnostic on their own.
Black drum vs Sheepshead
A juvenile black drum carries bold dark vertical bars over a pale flank, and so does a sheepshead at every size. They occupy the same bridges, pilings, jetties and oyster bars along the whole south-eastern and Gulf coast, both feed on shellfish, and both are caught on the same crab and shrimp baits by anglers fishing the same structure. The bars are what people see first and they are the character shared rather than the character that separates.
6 characters compared, 2 of them diagnostic on their own.
Blackhead seabream vs Yellowfin bream
Two Acanthopagrus of the western Pacific with the same deep silver-grey body, the same crushing dentition and the same habit of living in estuaries and around structure. Both are called simply 'bream' across their ranges, the ranges approach each other, and photographs of the two are difficult to tell apart unless somebody has thought to shoot the fins and the top of the head.
5 characters compared, 2 of them diagnostic on their own.
Blue catfish vs Flathead catfish
Two large North American catfish sharing the same big rivers and reservoirs, both taken on the same heavy bottom tackle, both growing to sizes where anglers report them by weight rather than by name. Small individuals are confused more often than large ones.
4 characters compared, 2 of them diagnostic on their own.
Bluefin trevally vs Giant trevally
Two large Caranx sharing reef, flat and surge-zone habitat across the whole Indo-Pacific, hitting the same lures in the same places and frequently in mixed groups. Adults are separable at a glance once you know what to look at; juveniles and sub-adults are not, because the character everyone relies on — the blue fins — is absent in young bluefin. Anglers also tend to judge by size, which fails whenever a young giant trevally and a mature bluefin are the same weight, which is often.
6 characters compared, 1 of them diagnostic on their own.
Bluegill vs Pumpkinseed
They share the same ponds, the same lily beds and frequently the same nesting colonies, they are the same shape, and both are caught by a child with a worm within ten minutes of starting. Both are small deep-bodied sunfish with a dark flap at the rear of the gill cover, and the flap is the character everybody reaches for without knowing what to look at on it. They also hybridise readily where both occur, which is common rather than rare in managed waters.
6 characters compared, 2 of them diagnostic on their own.
Bluegill vs Redear sunfish
Both are deep-bodied sunfish with a dark ear flap, both are stocked into the same southern and midwestern ponds, and they nest in the same shallow colonies within weeks of each other. Anglers call redear a shellcracker and bluegill a bream and then use both names loosely for whatever comes up. The two are stocked together deliberately in many managed fisheries, so a mixed catch is normal rather than exceptional.
6 characters compared, 1 of them diagnostic on their own.
Bonefish vs Atlantic tarpon
Rarely confused as adults and regularly confused as juveniles, which is the case this page is for. Small tarpon and bonefish share shallow tropical habitat, both are silver, both have large scales, and both pass through a transparent leptocephalus larval stage that surprises people who find one.
4 characters compared, 2 of them diagnostic on their own.
Bowfin vs Northern snakehead
This confusion has consequences beyond a misnamed photograph. Native bowfin are killed on sight in parts of North America by anglers who believe they have caught an invasive snakehead. Both are long, mottled, air-breathing predators of warm weedy water with long fins and a large mouth, and the resemblance is genuinely close at a glance.
4 characters compared, 2 of them diagnostic on their own.
Brook trout vs Bull trout
Both are char rather than trout, so both carry PALE spots on a DARK ground — the character that separates char from every true trout, and the character most anglers stop at. In the mountain rivers of western North America they occur in the same water, they are the same colours, and brook trout has frequently been stocked into bull trout country. A reader who has learned 'pale spots means char' has arrived at the hard part rather than finished.
8 characters compared, 1 of them diagnostic on their own.
Brook trout vs Lake trout
Both are Salvelinus — chars rather than trouts — with pale spots on a dark ground and no black spotting, which is the character that separates them from every true trout and therefore the one people stop at.
5 characters compared, 2 of them diagnostic on their own.
Brown trout vs Grayling
They share the same rivers, take the same flies, are caught by the same people on the same days, and are both salmonids carrying an adipose fin — which is the character most anglers use to sort salmonids from everything else and which therefore does not help here. On a fast grab-and-release in the water, a silvery grayling and a lightly-marked trout of the same size can be confused for the second it takes to unhook them, and a great many anglers have never held both.
6 characters compared, 2 of them diagnostic on their own.
Brown trout vs Rainbow trout
The commonest trout confusion in the world, because rainbows have been stocked into brown trout water on every continent and the two now share an enormous number of rivers and stillwaters. Both are spotted, adipose-finned salmonids of similar size and shape. The characters anglers reach for first — the pink stripe, the general colour — are exactly the ones that fade in fresh-run, sea-going or recently stocked fish, so the pair has to be settled on something more stable.
6 characters compared, 1 of them diagnostic on their own.
Burbot vs Ling
They are the same family, and the resemblance is not superficial: long body, two dorsal fins with a very long second, a long anal fin beneath, and a single prominent barbel on the chin. The confusion is made worse by the words. Burbot is called ling across much of North America, alongside eelpout and lawyer, so an angler reading about one can easily be reading about the other, and photographs are captioned across the two in both directions.
6 characters compared, 2 of them diagnostic on their own.
California halibut vs Summer flounder
Two large-mouthed Paralichthys on two coasts of the same country, both called 'flounder' by some anglers and 'halibut' by others, both with toothy jaws and mottled sandy backs, and both sharing the awkward habit of turning up with their eyes on either side. Photographs of the two circulate interchangeably online, and a reader who has learned one from a picture will recognise the other and name it wrong.
5 characters compared, 2 of them diagnostic on their own.
Catla vs Rohu
The two most important carps of South Asian fresh water are farmed together, stocked together, sold side by side on the same slab and caught out of the same river in the same session. Both are large, deep-bodied, big-scaled silver cyprinids, and a reader who has learned 'Indian major carp' as one thing has to unlearn it before either name means anything. Photographs of the two are routinely captioned as each other, including in the candidate queue this comparison came out of.
6 characters compared, 2 of them diagnostic on their own.
Chain pickerel vs Northern pike
Both are Esox: long cylindrical body, duck-billed snout, sharp teeth, and dorsal and anal fins set far back near the tail. Across the north-eastern United States and eastern Canada they share water, they take the same lures out of the same weed, and both are called jack locally. A small pike and a large chain pickerel are the same length, the same green, and hooked on the same cast.
6 characters compared, 2 of them diagnostic on their own.
Channel catfish vs Blue catfish
Both are large North American catfish sharing a deeply forked tail, and a young blue catfish is often spotted in the way a channel catfish is, which removes the character most people rely on. Colour is unreliable in both: either can appear pale, slate or dark depending on water.
4 characters compared, 1 of them diagnostic on their own.
Chinook salmon vs Rainbow trout
Both are Oncorhynchus, both occur in the same Pacific rivers and in the Great Lakes, and the sea-run form of the rainbow — the steelhead — is a large, bright silver, tail-spotted anadromous fish that looks a great deal like a salmon and behaves like one. Add in that juvenile chinook and resident rainbows share the same streams, and the pair produces genuine errors at both ends of the size range. The characters that separate them are small and specific rather than obvious.
6 characters compared, 2 of them diagnostic on their own.
Chinook salmon vs Sockeye salmon
Two Pacific salmon running the same rivers at overlapping times, both bright silver at sea, both turning dark and red-toned on the spawning grounds. Anglers meeting Pacific salmon for the first time frequently sort them by size and by how red they are, and both of those fail: a small chinook and a large sockeye overlap, and both species colour up. The reliable separations are the presence or absence of spotting and a feature inside the gill cover that most people never think to look at.
6 characters compared, 2 of them diagnostic on their own.
Chinook salmon vs Coho salmon
Both are Pacific salmon of similar shape and colour that run the same rivers at overlapping times, and a bright fish fresh from salt water of either species is silver with a dark back and spotted tail. Small chinook and large coho overlap substantially in size, so size settles nothing.
5 characters compared, 1 of them diagnostic on their own.
Chub vs Ide
The genuine identification problem of continental European coarse fishing, and the one that catches British anglers abroad. Both are deep silvery cyprinids of similar size with reddish lower fins, they share rivers throughout the overlap, and they hybridise — so intermediate fish exist and defeat every character below.
5 characters compared, 2 of them diagnostic on their own.
Chum salmon vs Pink salmon
Both run in autumn, both are the smaller Pacific salmon, and in the river they are separated at a glance by the pink's hump. The problem is in salt water and in the first hours of fresh, where both are bright silver fish of similar build and the spawning characters everybody knows have not developed yet. The two also hybridise where they overlap, and the hybrids are fertile, so an intermediate fish is a genuine possibility.
6 characters compared, 1 of them diagnostic on their own.
Chum salmon vs Sockeye salmon
This is the hardest pair in the genus for a fish straight off the tide, because both are essentially unspotted. Everybody knows the spawning states — a scarlet sockeye with a green head is unmistakable, and so is a barred chum with dog teeth — and neither helps at the moment of capture in salt water, when both are plain bright silver fish of overlapping size in overlapping runs.
6 characters compared, 2 of them diagnostic on their own.
Common bream vs Silver bream
A young common bream is silver, deep-bodied and about the size of an adult silver bream, and the two share the same slow lowland water, the same soft bottom and the same swim. Almost every match angler in England has weighed both in the same net without separating them. The confusion is genuinely hard rather than careless, and it is made worse by a third fish: roach hybridise with both, and a hybrid is intermediate in exactly the characters people use.
6 characters compared, 2 of them diagnostic on their own.
Common carp vs Grass carp
Both are large cyprinids stocked worldwide, frequently into the same waters, and both are reported simply as carp by anglers who catch one unexpectedly. Size and general silhouette overlap, and both reach weights where the fish is photographed rather than examined.
4 characters compared, 2 of them diagnostic on their own.
Common carp vs Crucian carp
Both are deep, bronze, big-scaled coarse fish in the same still waters, both answer to 'carp', and a small common carp and a large crucian genuinely overlap in size and shape. Worse, the fish crucians are most often confused with is not the common carp at all but the Prussian carp, which shares the crucian's most obvious character. So a reader who separates crucian from common has not finished; they have only started.
5 characters compared, 2 of them diagnostic on their own.
Common dentex vs Common pandora
Two Mediterranean sparids of similar build taken over the same mixed ground, both silver with a coloured cast, both spotted on the upper flank, and both sold under names that shade into each other on a market slab. A juvenile dentex of a few hundred grammes is the same size and shape as an adult pandora, which is precisely the size at which the identification matters most, because one of the two is vulnerable and the other is not.
7 characters compared, 2 of them diagnostic on their own.
Common limpet vs Common periwinkle
The two most abundant gatherable molluscs on a British rocky shore, sitting on the same rock at the same tide, both small, both grey-brown, both encrusted, and both routinely lumped together as 'winkles' by people who have only ever seen them from above at walking height. Anyone gathering either needs to know which is which, because they are prepared differently and regulated differently.
5 characters compared, 2 of them diagnostic on their own.
Common periwinkle vs Common whelk
Winkle and whelk are used interchangeably by a great many people, they appear beside each other on the same seafood stall boiled and shelled where no identification is possible at all, and a large periwinkle and a small whelk are both mid-sized brown-grey spiral shells. The confusion is mostly one of language, and it is compounded by a third animal — the dog whelk of the rocky shore — which is neither of these and is the one most often picked up believing it is.
6 characters compared, 2 of them diagnostic on their own.
Common sole vs European plaice
Both are right-eyed flatfish of the same sandy ground, taken on the same baits, and both come over the gunwale as a brown oval. In a mixed catch on a dark beach, outline is all most people have to go on.
4 characters compared, 2 of them diagnostic on their own.
Conger eel vs European eel
Both are long, snake-bodied, apparently scaleless, dark above and pale below, both feed at night by smell, and both turn up on harbour walls and in estuaries where fresh and salt water meet. A small conger of half a metre from a pier at night and a large silver eel dropping downriver in autumn are the same shape in the same torchlight, and both are called eel by everybody who catches one.
7 characters compared, 2 of them diagnostic on their own.
Cutthroat trout vs Rainbow trout
They hybridise. That is the honest answer, and it is why this pair is harder than any other on this list: in much of the cutthroat's range the fish in front of you may be genetically intermediate, and no field character will resolve it.
4 characters compared, 1 of them diagnostic on their own.
Dusky flathead vs Sand flathead
Two flathead over the same Australian sand, both taken on the same drift and the same lure, both flat-headed and sandy-mottled with the eyes on top. Small dusky flathead and adult sand flathead overlap squarely in size, and because both change tone to match whatever they have been lying on, the colour a reader instinctively reaches for is the least useful thing about either.
5 characters compared, 2 of them diagnostic on their own.
Dusky grouper vs Gag grouper
Two large, mottled grey-brown groupers of similar size and habit, both holding on structure, both changing colour and pattern with mood and depth, and both routinely photographed in circumstances that flatten every distinguishing feature. Divers and anglers moving between the eastern Atlantic and the western Atlantic meet one having learned the other, and 'grouper' does an enormous amount of unhelpful work in both languages. Colour is the character people use and it is the least reliable one this pair has.
6 characters compared, 2 of them diagnostic on their own.
European perch vs Yellow perch
Two Perca species on two continents that look, at a glance, like the same fish: deep-bodied, vertically barred, spiny-dorsalled and orange-finned. Anglers who fish both, and readers of material written for the other continent, mix them constantly.
4 characters compared, 2 of them diagnostic on their own.
European sea bass vs Japanese sea bass
Both are called sea bass in English, both are slim silver estuary and coastal predators with two separate dorsal fins and a spiny first, both are hunted with lures around structure and current at night, and both are farmed and sold under the same word on the same menu. Japanese lure-fishing technique has been widely adopted in Europe, so the two fish now appear in the same articles, and photographs are captioned across them routinely.
6 characters compared, 1 of them diagnostic on their own.
Flathead grey mullet vs Thick-lipped grey mullet
Grey mullet are the classic 'they all look the same' group, and these two overlap in harbours, estuaries and along the same coasts. Both are silver-grey with faint longitudinal stripes, two widely separated dorsal fins, a blunt head and a small mouth. Anglers commonly do not attempt the identification at all and simply record 'mullet'. The characters that separate them are on the lip and around the eye, and they take a close look at a fish in hand rather than a glance at one in the water.
6 characters compared, 2 of them diagnostic on their own.
Gag grouper vs Atlantic goliath grouper
At size nobody confuses them — a 200 kg goliath is not a gag. The confusion is at the small end, where a juvenile goliath of a few kilos on inshore structure is a mottled brown grouper among other mottled brown groupers, and where an angler who has only ever seen photographs of enormous fish does not expect one. Both use wrecks, ledges and artificial reef in the same water off Florida and the Gulf, and both come to the same baits.
6 characters compared, 2 of them diagnostic on their own.
Gilthead bream vs Common dentex
Two large Sparidae of the same Mediterranean and eastern Atlantic ground, both deep-bodied, both silver-grey, and both taken on the same baits over the same rough bottom.
4 characters compared, 2 of them diagnostic on their own.
Grass carp vs Silver carp
Both are large barbel-less Asian cyprinids introduced worldwide, both turn up in the same rivers and lakes where stocking or escape has put them, and neither looks like the common carp most anglers expect. Somebody who has caught neither before has one mental slot marked big silver carp-shaped fish with no barbels, and the two fall into it together. Their management implications are quite different, which is why the slot needs splitting.
6 characters compared, 3 of them diagnostic on their own.
Greater amberjack vs Yellowtail kingfish
Two large Seriola taken over the same deep structure on the same jigs and live baits, similar in shape and colouring, and called kingfish, amberjack or yellowtail interchangeably depending on the country. Both carry a dark band through the eye and a yellow tail.
4 characters compared, 1 of them diagnostic on their own.
Haddock vs Atlantic cod
Both are three-dorsal gadoids of the same waters, taken on the same ground and often in the same drift, and a small cod and a large haddock are close enough in size and outline to give somebody pause on a moving deck.
4 characters compared, 2 of them diagnostic on their own.
King mackerel vs Narrow-barred Spanish mackerel
Two large Scomberomorus of very similar shape, size and habit, both fast pelagic predators taken by trolling and casting over reef and structure, and both known locally as forms of 'Spanish mackerel' or 'kingfish' in ways that vary by country and contradict each other. They do not naturally overlap in range, which is what makes the confusion a travelling angler's problem: somebody who knows one arrives where the other lives and applies the wrong name, the wrong expectations and sometimes the wrong food-safety assumption.
6 characters compared, 2 of them diagnostic on their own.
King mackerel vs Wahoo
They share water, prey, tackle and technique across the whole tropical and subtropical Atlantic, they come to the same trolled baits and lures at the same speeds, and both are long silver fish with finlets and razor teeth. A big king mackerel and a modest wahoo are close enough in length and weight that the deck answer is frequently a guess. A wahoo also loses the vertical barring that makes it obvious within minutes of coming aboard.
6 characters compared, 2 of them diagnostic on their own.
Mangrove jack vs Red emperor
Two red Lutjanus of the same Indo-Pacific reef, taken on the same baits and jigs from the same ground, and both known to anglers by a colour rather than by a character. A large mangrove jack from offshore reef is bright red rather than the dark maroon of an estuary fish, and a large red emperor has lost almost all of the banding that makes a young one unmistakable — so the two converge with size, which is the wrong direction for an easy identification.
6 characters compared, 1 of them diagnostic on their own.
Meagre vs Dusky kob
Congeners in Argyrosomus that look extremely similar and are conflated constantly in international angling writing, largely because each is the big estuary drum of its own region — meagre in Europe and West Africa, dusky kob in southern Africa and Australia — and articles about one are read as being about the other.
4 characters compared, 1 of them diagnostic on their own.
Muskellunge vs Northern pike
Two large esocids sharing lakes and rivers across much of North America, similar in shape, similar in behaviour, taken on the same lures, and producing fertile-looking hybrids — tiger muskellunge — that are intermediate in nearly every character.
4 characters compared, 2 of them diagnostic on their own.
Northern red snapper vs Yellowtail snapper
Both are reef snappers of the Gulf and the western Atlantic, both are caught on the same trips and sold under overlapping market names, and both carry yellow somewhere. A reader who has learned 'red snapper' as the name for any reddish snapper on a Florida boat will apply it to a yellowtail, and the fillets are sold as different things at different prices, so the confusion has money in it as well as biology.
5 characters compared, 2 of them diagnostic on their own.
Northern snakehead vs Striped snakehead
Both are Channa: long cylindrical fish with dorsal and anal fins running most of the body, flattened plate-scaled heads and rounded tails. To anybody meeting the family for the first time they are the same animal at different sizes. Their ranges meet in Asia and both have been introduced outside them, so a fish out of an unfamiliar water can genuinely be either. Neither carries the bold banding or the caudal eyespot that make the giant and great snakeheads easy.
6 characters compared, 1 of them diagnostic on their own.
Permit vs Florida pompano
Congeners sharing shape, colour and water. A small permit and a large Florida pompano overlap closely in size, and both are deep silver fish with a blunt head and a yellow wash on the belly. Anglers on Florida and Caribbean beaches catch both and mix them up regularly.
4 characters compared, 1 of them diagnostic on their own.
Pouting vs Whiting
The standing confusion of European winter beach fishing. Both are small gadoids caught on the same beaches on the same rigs on the same night, both are silver-flanked with three dorsal fins, and both are frequently identified by torchlight in the rain by somebody who wants to get the fish off the hook and the bait back in the water. Pouting loses its barring within minutes of capture, which removes the character most anglers would use.
6 characters compared, 2 of them diagnostic on their own.
Rudd vs Roach
The defining identification problem of European coarse fishing. The two share water, size, shape and a silver-and-red colour scheme, they hybridise freely, and the resulting intermediates defeat any single character. Colour is the thing most people rely on and the thing that varies most with water clarity and season.
5 characters compared, 2 of them diagnostic on their own.
Saithe vs Pollack
Two Pollachius species caught over the same marks, on the same tactics, often in the same session. Both are elongate three-dorsal-finned gadoids of similar size and both are called 'pollock' by somebody, somewhere — a naming collision that outlives every attempt to fix it.
5 characters compared, 2 of them diagnostic on their own.
Shanny vs Rock goby
They are not alike. They are in the same net. Both are small, brown, mottled, blunt-headed bottom fish that come out of the same British rock pool at the same size, and for anybody who has just turned a stone that is the entire basis of the confusion — proximity rather than resemblance. It is made worse by the fact that both change colour with the ground and with mood, so the two things a beginner reaches for first, colour and size, are exactly the two that do not work. The good news is that the real answer is easier than either: this is one of the very few identifications in British fishing that is genuinely binary, and once seen it is never forgotten.
6 characters compared, 2 of them diagnostic on their own.
Spotted bass vs Smallmouth bass
Both are bronze-flanked black bass of moving water with a jaw that stops short of the eye, which is the character most anglers use to rule out largemouth — and having ruled that out, many stop looking.
4 characters compared, 1 of them diagnostic on their own.
Spotted seatrout vs Weakfish
Congeners of the same American Atlantic estuaries, both called sea trout by somebody, both silver with a purple sheen and a pair of prominent canine teeth. They are caught on the same lures in the same water, and anglers routinely report one as the other.
4 characters compared, 1 of them diagnostic on their own.
Striped bass vs White bass
Same genus, same silver flank with dark horizontal stripes, same two separate dorsal fins, and in the reservoirs and large rivers of the United States they are caught on the same cast out of the same surface-feeding school. A small striped bass and a large white bass are the same length. And the two hybridise, deliberately in stocking programmes and naturally where they meet, so a third animal is in the boat with them.
6 characters compared, 1 of them diagnostic on their own.
Turbot vs Brill
Two left-eyed flatfish taken over the same sand and gravel ground, frequently in the same drift, and similar enough in outline that they are routinely landed together and sorted afterwards. Both are highly valued, and in most European waters they carry different size limits.
4 characters compared, 2 of them diagnostic on their own.
Why these pairs and not others: 87 species make 3,741 possible pairings, and generating them all would produce thousands of pages that say two fish are different without knowing how. A comparison is built only where the catalogue already carries an evidenced relationship between the two species, and only where anglers actually hesitate.