What Fish
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A practical guide to fish identification

What fish is that?

Saw a fish you can’t name? Here is every way to identify it, compared honestly with real data.

Get Seabook free Find your method

Which method fits you?

Four ways to identify a fish. Pick the one that matches where you are and how sure you need to be.

  1. Emperor angelfish framed by the Seabook photo scanner
    Our pick On a dive, a boat or a pier, even with no signal A fish ID app with offline mode, like Seabook Fish ID apps
  2. Serious study of one region A regional field guide Field guides
  3. A quick, casual check of a clear photo ChatGPT or Gemini AI chatbots
  4. 80‑100%
    Legal or scientific proof DNA barcoding in a lab Lab methods

How to identify a fish with any method

App, book or chatbot: identification always comes down to the same three moves.

  1. Scuba diver looking at a coral reef wall

    Spot a fish

    See something interesting underwater, on a line, or in a photo.

  2. Seabook search filtered by colour and category

    Snap or describe

    Take a photo for AI identification, or describe traits like colour, shape and habitat.

  3. Seabook photo identification result: Emperor Angelfish

    Get an identification

    Get a match with species details, photos and distribution. When accuracy is critical, check a second source.

The guide

How people identify fish, compared

Four approaches, from the phone in your pocket to a genetics lab. Each has trade-offs. Here is what you need to know about every one.

Field guides & reference books

Reliable and fully offline, but slow and only as good as the region they cover.

The original method. Dichotomous keys walk you through paired questions about morphology (body shape, fin placement, colour) until you reach a species name. FAO Species Catalogues and regional atlases like the Reef Fish Identification series by Humann & DeLoach are standards in the field.

Good for

  • Works 100% offline
  • Extremely reliable for its region
  • Serious, structured study

Watch out

  • Needs anatomical vocabulary
  • Takes minutes, not seconds
  • Regional, not global coverage
Cost $20-80 per book Species 500-1,500 per book
Book cover: Reef Coral Identification
Book cover: Reef Fish Identification
Book cover: Reef Fish Identification: Tropical Pacific
Book cover: Coastal Fish Identification

The Reef Fish Identification series by Humann & DeLoach

Fish identification apps

The fastest path from “what is that?” to an answer.

Dedicated apps combine AI photo recognition with structured species databases. Snap a photo, or filter by traits you remember: colour, body shape, habitat, region. Two are worth knowing.

  • 1,700+marine species in Seabook
  • 275groups: fish, invertebrates, corals
  • 10,000+species searchable by photo
  • 300+locations to search by
Our pick for divers & anglers
Seabook app icon

Seabook

Rated 4.7 on iOS and 4.5 on Android

  • Identify by photoPoint, shoot, get a species match
  • Smart filtersColour, pattern, shape, habitat, region
  • Species profilesPhotos, descriptions, distribution maps
  • Dive logDepth, duration, conditions, GPS
  • Sightings & collectionsPhotos, size notes, trips, dive-site map
  • Full offline modeThe whole library works without signal
Download on theApp Store Get it onGoogle Play

Free to start. Premium unlocks the full database for $20-$37. seabook.app

Seabook photo identification screen framing a nudibranch
Identify by photo
Seabook search results for sharks, filtered by colour and category
Filter by traits
Seabook species profile for the mandarinfish with size, depth, habitat and conservation status
Species profiles

iNaturalist, for citizen science

A general biodiversity platform, not fish-specific. Upload a photo, the AI suggests a species, then community experts verify it.

iNaturalist app icon

iNaturalist

Free on iOS, Android and Web. 300M+ observations, 372,000+ species

Good for

  • Completely free
  • Community-verified IDs
  • Contributing to science

Watch out

  • Requires internet
  • No dive logging
  • No fish-specific filters

App Store Google Play inaturalist.org

AI chatbots

A quick second opinion, not a primary identification tool.

Upload a photo and ask “what fish is this?” Multimodal models recognise colour patterns, body shapes and fin structures from their training data. For common species, the answer is often correct.

Good for

  • Free to try
  • Common species, clear photos
  • Any language

Watch out

  • No structured fish database
  • No trait search for unclear photos
  • No offline mode
  • Weaker on rare or regional species
Cost Free or $20 per month Dive log No
ChatGPT conversation identifying a fish from a photo
ChatGPT app icon

ChatGPT

Free, Plus $20/mo

App Store Google Play

Google Gemini conversation identifying a fish from a photo
Google Gemini app icon

Gemini

Free, Advanced $20/mo

App Store Google Play

Laboratory methods

The gold standard when a species must be proven. Not for everyday use.

When the stakes are highest (regulatory enforcement, forensic analysis, conservation research), scientists turn to the lab. It takes specialised equipment, trained personnel and lab access.

  • 80‑100%accuracy, the standard for legal and scientific verification

  • DNA barcoding

    Reads the CO1 gene

  • SNP analysis

    Compares genetic markers

  • Scale morphology

    Examines scale structure

  • Otolith analysis

    Studies the fish ear stone

Side by side

Compare your options

Real features, real data. Seabook is our pick for divers, snorkellers and anglers. Every other column is here so you can judge for yourself.

Fish identification methods compared by features, cost and capabilities
Feature BooksField guides RecommendedSeabook Citizen scienceiNaturalist AI chatbotsChatGPT / Gemini
FocusRegional speciesMarine speciesAll biodiversityGeneral AI
Species database500-1,500 per book1,700+ marine372,000+ (all life)No database
ID by photoManual lookupYes: 10,000+ speciesYes: AI + communityYes: Multimodal AI
Search by filtersDichotomous keysYes: Colour, shape, habitat, regionYes: Location, taxaNatural language
Search by locationRegional onlyYes: 300+ locationsYes: Global, map-basedNo
Dive logNoYes: Depth, duration, GPSNoNo
Species collectionsNoYes: Custom / by regionYes: Projects & listsNo
SightingsNoYes: Photos, notes, sizeYes: 300M+ observationsNo
EncyclopediaYes: Printed entriesYes: Full species profilesCommunity notesGeneral knowledge
MultilingualPer-language editionsYes: 6 languagesYes: Web onlyYes: All languages
Offline modeYes: It’s a bookYes: Full encyclopediaNoNo
PlatformsPrintiOS, AndroidiOS, Android, WebiOS, Android, Web
Cost$20-80 per bookFree / $20-$37FreeFree / $20 per month
Best forSerious studyDivers, snorkellers, anglersCitizen scientistsQuick casual checks

Lab methods (DNA barcoding, SNP, scale and otolith analysis) are left out of the table: they reach 80-100% accuracy but aren’t a consumer tool. Prices in USD.

Questions, answered

Short, direct answers. The detail is in the method breakdown above.

What is the best way to identify a fish?

The most common methods are: (1) fish identification apps like Seabook, which offer AI photo ID and trait-based search across 1,700+ marine species with offline support; (2) citizen science platforms like iNaturalist, with community verification; (3) general AI chatbots like ChatGPT and Gemini; (4) traditional field guides and dichotomous keys; and (5) laboratory methods like DNA barcoding for scientific-grade accuracy.

What is the best fish identification app?

Seabook is a dedicated fish identification app covering 1,700+ marine species across 275 groups, including fish, invertebrates and corals. It offers AI photo identification with 10,000+ species searchable by photo, trait-based search by colour, shape, habitat and region, integrated dive logging, a sighting journal and full offline mode. It is rated 4.7 on iOS and 4.5 on Android. Free to start, with premium options from $20 to $37.

Can ChatGPT identify fish from a photo?

Yes. ChatGPT and Google Gemini can identify fish from photos using their multimodal AI. They work well for common species, but accuracy varies for rare or region-specific fish. They lack the structured species database, offline mode and fish-specific search filters that dedicated apps like Seabook provide.

Is there a free fish identification app?

Yes. Seabook has a free tier that lets you explore species and try photo identification; premium plans from $20 to $37 add full database access, offline mode and advanced features. iNaturalist is completely free, but it is a general biodiversity platform, not fish-specific.

Can I identify fish offline?

Yes. Seabook is one of the few apps with full offline mode: the entire species encyclopedia is available without an internet connection, which matters for divers, anglers and snorkellers in remote locations. Printed field guides also work offline by nature. iNaturalist, ChatGPT and Gemini need a connection.

Are field guides still worth using?

For serious study of one region, yes. Field guides and dichotomous keys, such as the FAO Species Catalogues and the Reef Fish Identification series by Humann & DeLoach, are extremely reliable for the regions they cover and work 100% offline. They cost $20-80 per book, cover 500-1,500 species each, and identification takes minutes rather than seconds.

How accurate are laboratory methods like DNA barcoding?

Laboratory methods (DNA barcoding using the CO1 gene, SNP analysis, scale morphology and otolith analysis) reach 80-100% accuracy. They require specialised equipment, trained personnel and lab access, so they are used for regulatory enforcement, forensics and conservation research rather than everyday identification.

Start identifying fish today

Try Seabook on your next dive, snorkel or fishing trip. The species library works offline, so you’re covered in the most remote spots.

Download on theApp Store Get it onGoogle Play

Free to start. Premium $20-$37. Works offline.

Seabook location guide for Indonesia with the species most often seen there