New 🐟 Mercury Safety Guide & Omega-3 Rankings Updated for 2026. View Omega-3 Rankings →

🌿 Sustainable Seafood ⚡ Omega-3 Rankings 🔬 Mercury Safety 🏆 World Records 🎣 Fishing Licenses
API Docs
Get API Key →
Fish Science Reference

Ichthyology Glossary

Every fish science term explained in plain English. From taxonomy and anatomy to ecology, conservation and water chemistry - with species examples throughout.

📖 308 terms 🏷️ 11 categories 🔤 24 letters

A 18 terms

Actinopterygian
A
A ray-finned fish; member of Actinopterygii, the class containing 99% of living fish...
Anatomy
Actinopterygii
A
The ray-finned fish - the vast majority of modern bony fish species.
Taxonomy
Adipose Fin
A
A small, fleshy, rayless fin located between the dorsal fin and the caudal fin in som...
Anatomy
Age and Growth
A
The study of how fish age is determined and how body size changes over time, fundamen...
Physiology
Age-at-Maturity
A
The age at which 50% of individuals in a fish population become sexually mature, a ke...
Fishery
Agonistic Behaviour
A
Any behaviour associated with conflict between conspecifics, including both aggressiv...
Behavior
Alevin
A
A salmon fry with yolk sac still attached, still within the gravel where it hatched.
Reproduction
Alkalinity
A
The buffering capacity of water against pH changes - measured as total titratable bas...
Water Science
Allopatric
A
Occurring in geographically separate areas; allopatric speciation is the most common...
Taxonomy
Ammonification
A
The bacterial breakdown of organic nitrogen (proteins, urea) into ammonia - the start...
Water Science
Amphidromous
A
A fish that migrates between salt and fresh water not primarily for reproduction - ma...
Ecology
Ampullae of Lorenzini
A
Electroreceptive sensory pores on the snouts of sharks, skates, and rays that detect...
Anatomy
Anadromous
A
Describes fish that hatch in freshwater, migrate to the sea to grow, then return to f...
Behavior
Anal Fin
A
The median fin on the ventral surface just posterior to the anus, providing stability...
Anatomy
Angling
A
Recreational fishing with rod and reel - as distinct from commercial or subsistence f...
Fishing
Apex Predator
A
A predator at the top of the food chain with no natural predators of its own; plays a...
Ecology
Aquaculture
A
The farming of aquatic organisms including fish, shellfish, and aquatic plants, under...
Fishery
ASC Certification
A
An independent eco-label for responsibly farmed seafood, covering environmental and s...
Fishery

B 20 terms

Backing
B
The reserve line beneath the main fly line on a fly reel - deployed only during long...
Fishing
Baitcaster
B
A revolving-spool fishing reel operated by casting weight - dominant in bass fishing...
Fishing
Ballyhoo
B
A small pelagic baitfish (Hemiramphidae) - the standard offshore trolling bait for tu...
Fishing
Barbel
B
Whisker-like tactile and chemosensory appendages around the mouth of certain fish, us...
Anatomy
Bathypelagic
B
The ocean zone from 1000–4000 m depth, in permanent darkness, where food is scarce an...
Ecology
Benthic
B
Relating to organisms that live on or in the substrate (bottom) of a body of water.
Ecology
Bight
B
A loop of line - the fundamental unit of knot tying.
Fishing
Binomial Nomenclature
B
The two-part scientific naming system for species, using a genus name and a specific...
Taxonomy
Biofilm
B
Microbial mats coating all submerged surfaces - the first colonisers of new aquariums...
Water Science
Bioindicator
B
A species or assemblage used to assess the health or quality of an environment, becau...
Ecology
Bioluminescence
B
The production and emission of light by living organisms through chemical reactions,...
Ecology
Biomass
B
The total mass of living organisms in a given area or volume; in fisheries, the total...
Ecology
Brackish Water
B
Water of intermediate salinity between freshwater and full seawater - typically 5-30...
Water Science
Branchiostegal Ray
B
Bony rays supporting the branchiostegal membrane (gill membrane) below the operculum,...
Anatomy
Broadcast Spawning
B
A reproductive strategy in which fish release eggs and sperm simultaneously into the...
Behavior
Broodstock
B
Mature fish selected and maintained for breeding purposes - the reproductive foundati...
Reproduction
Buccal Pumping
B
Active gill ventilation in which muscles alternately expand and compress the buccal a...
Behavior
Buzzbait
B
A topwater fishing lure with a propeller blade - creates surface commotion attractive...
Fishing
Bycatch
B
Non-target species caught incidentally during fishing operations.
Fishing
Bycatch Reduction Device
B
A modification to fishing gear designed to allow non-target species to escape from th...
Conservation

C 34 terms

Calcium Reactor
C
Equipment that dissolves calcium carbonate media using CO2-acidified water - the reef...
Water Science
Carrying Capacity
C
The maximum population size that an environment can support indefinitely, given avail...
Ecology
Catadromous
C
Describes fish that live in freshwater but migrate to the sea to spawn, as in eels.
Behavior
Caudal Fin
C
The tail fin; the main propulsive organ in most fish.
Anatomy
Caudal Peduncle
C
The narrow, muscular region connecting the body of the fish to the caudal fin; the pr...
Anatomy
Ceratohyal
C
A specific gill arch bone commonly used in fish taxonomy and diagnostics.
Anatomy
Chartreuse
C
A yellow-green colour - one of the most productive lure colours in stained water.
Fishing
Chondrichthyes
C
The class of cartilaginous fish - sharks, rays, skates, and chimaeras.
Taxonomy
Chondrocranium
C
The cartilaginous braincase - the entire skull framework in cartilaginous fish.
Anatomy
Chromatophore
C
A pigment cell in fish skin that expands or contracts to change colour rapidly.
Anatomy
Chum
C
Ground or cut fish scattered in water to attract game fish to a fishing location.
Fishing
Chumming
C
Dispersing ground bait to attract fish - highly regulated in some jurisdictions.
Fishing
Cichlid
C
A diverse and often colorful family of freshwater fish (Cichlidae) native to Africa,...
Taxonomy
Circle Hook
C
A hook design with a curved point that reduces gut-hooking - preferred for catch-and-...
Fishing
CITES
C
The Convention on International Trade in Endangered Species of Wild Fauna and Flora;...
Conservation
Clade
C
A monophyletic group comprising an ancestor and all of its descendants, as shown on a...
Taxonomy
Clasper
C
The paired intromittent organs of male elasmobranchs (sharks and rays), formed from t...
Anatomy
Class
C
A major taxonomic rank above order; fish belong primarily to Actinopterygii (ray-finn...
Taxonomy
Cleaning Station
C
A fixed site on a reef where cleaner fish (or shrimp) remove parasites, dead tissue,...
Behavior
Coelacanth
C
A living fossil - a lobe-finned fish thought extinct until rediscovered in 1938 off S...
Taxonomy
Condition Factor
C
A dimensionless index of the plumpness/wellbeing of a fish relative to its length: K...
Physiology
Conductivity
C
The electrical conductivity of water - proportional to dissolved ion concentration an...
Water Science
Coral Reef
C
A biologically diverse, shallow tropical marine habitat built by reef-forming corals,...
Ecology
Countershading
C
A camouflage strategy where fish have dark backs and pale bellies to blend with water...
Behavior
CPUE
C
The amount of fish caught per unit of fishing effort, used as an index of stock abund...
Fishery
Crankbait
C
A hard-plastic diving lure with a plastic lip that causes it to wobble and dive - a b...
Fishing
Crepuscular
C
Active primarily at dawn and dusk (twilight); a transitional activity pattern between...
Behavior
Critically Endangered
C
An IUCN Red List category indicating a species faces an extremely high risk of extinc...
Conservation
Cryptic Species
C
Two or more morphologically similar but genetically distinct species that were previo...
Taxonomy
Ctenoid Scale
C
A fish scale with tiny tooth-like projections (ctenii) on the posterior margin, provi...
Anatomy
Ctenoid Scales
C
Scales with a comb-like edge, typical of perch and other advanced teleosts.
Anatomy
Cycling
C
The process of establishing beneficial bacteria populations in a new aquarium filter...
Water Science
Cycloid Scale
C
A thin, smooth-edged fish scale with concentric growth rings but no cteni (teeth), fo...
Anatomy
Cycloid Scales
C
Smooth-edged fish scales, typical of salmon and carp.
Anatomy

D 17 terms

Dead Zone
D
A hypoxic (low-oxygen) area of ocean or freshwater where most marine life cannot surv...
Ecology
Deep Sand Bed
D
A layered marine aquarium substrate of 4-6+ inches that hosts anaerobic bacteria for...
Water Science
Deionization
D
Ion exchange resins that further purify RO water - producing near-zero TDS output for...
Water Science
Demersal
D
Relating to fish that live and feed on or near the seafloor.
Ecology
Denitrification
D
The anaerobic conversion of nitrate to nitrogen gas by specialised bacteria - the fin...
Water Science
Density Dependence
D
Population regulation in which vital rates (birth, death, growth) change in response...
Ecology
Dentary
D
The main tooth-bearing bone of the lower jaw in bony fishes; equivalent to the mandib...
Anatomy
Detritivore
D
An organism that feeds on dead organic matter (detritus) and the microorganisms decom...
Ecology
Diadromous
D
A fish that migrates between salt and fresh water - the umbrella term including anadr...
Ecology
Diel Vertical Migration
D
The daily movement of aquatic organisms from deeper water at day to shallower water a...
Behavior
Discard
D
Fish returned to the sea (dead or alive) after being caught - often due to regulation...
Fishing
Diurnal
D
Active primarily during daylight hours; describes fish that rely on vision for feedin...
Behavior
DOC (Dissolved Organic Carbon)
D
The total dissolved organic material in aquarium water - the primary target of protei...
Water Science
Dorsal Fin
D
The fin located on the back (dorsal surface) of a fish, used primarily for stability.
Anatomy
Downrigger
D
A trolling device using a heavy weight to sink lures to precise depths - essential fo...
Fishing
Dredge
D
A commercial fishing gear dragged across the sea floor - primarily for scallops, oyst...
Fishing
Dry Fly
D
A fly designed to float on the water surface - imitates emerged insects on trout stre...
Fishing

M 20 terms

Mangrove
M
Tropical coastal forests of salt-tolerant trees growing in intertidal zones, providin...
Ecology
Mariculture
M
Aquaculture conducted in marine environments, either in enclosed coastal areas or ope...
Fishery
Marine Protected Area
M
A clearly defined geographic space in the ocean that is regulated to achieve long-ter...
Conservation
Match the Hatch
M
Fly fishing philosophy - selecting fly patterns that mimic the specific insects fish...
Fishing
Maxillary Bone
M
One of the bones forming the upper jaw of fishes; in advanced teleosts it functions t...
Anatomy
Maximum Sustainable Yield
M
The largest catch that can be taken indefinitely without depleting a fish population.
Fishing
Mend
M
The upstream flip of fly line to remove drag and let the fly drift naturally - critic...
Fishing
Meroplankton
M
Organisms that are planktonic only for part of their life cycle - including most fish...
Ecology
Mesopelagic
M
The ocean zone from 200–1000 m depth, the "twilight zone," characterised by dim light...
Ecology
Metamorphosis
M
Dramatic developmental transformation from larva to juvenile - especially pronounced...
Reproduction
Microhabitat
M
The small-scale physical features within a broader habitat that a particular organism...
Ecology
Milling
M
The loose circular movement of a fish shoal, distinct from directional schooling.
Behaviour
Milt
M
The seminal fluid of male fish - milky white, expressed during spawning, eaten as shi...
Reproduction
Mimicry
M
The resemblance of one organism to another species or object, conferring an adaptive...
Behavior
Mini-Cycle
M
A brief re-establishment of the nitrogen cycle after disturbance (filter replacement,...
Water Science
Minimum Viable Population
M
The smallest population of a species that can persist in a region for a specified tim...
Conservation
Monotypic
M
A genus containing only one species, or a family containing only one genus.
Taxonomy
Mouthbrooding
M
A reproductive strategy in which one parent (usually the male) incubates eggs and/or...
Behavior
MSC Certification
M
An independent eco-label awarded to wild-capture fisheries that are scientifically as...
Fishery
Myomere
M
A single block of muscle tissue in a fish, arranged in a W-shaped chevron pattern alo...
Anatomy

P 24 terms

Parr
P
A juvenile salmon still in freshwater, marked by dark vertical bars - the last stage...
Reproduction
Pectoral Fin
P
The paired fins located behind the head on either side of the body, homologous to for...
Anatomy
Pelagic
P
Relating to the open water column of the sea or a lake, away from the bottom and shor...
Ecology
Pelvic Fin
P
Paired ventral fins homologous to the hindlimbs of tetrapods, used for stabilisation...
Anatomy
Pharyngeal Teeth
P
Teeth deep in the throat of many bony fish, used for grinding food.
Anatomy
Phenotype
P
The observable characteristics of an organism, resulting from genotype interacting wi...
Taxonomy
Phosphate Cycle
P
The uptake, recycling, and export of phosphorus in aquatic systems - the main non-nit...
Water Science
Photoperiod
P
The duration of daily light exposure, often triggering seasonal reproduction and migr...
Ecology
Piscivore
P
A fish-eating predator - the primary strategy of apex aquatic predators.
Ecology
Placoderm
P
An extinct class of armored jawed fish that dominated Devonian seas.
Taxonomy
Placoid Scale
P
The tooth-like scales of sharks and rays, composed of dentine and enamel — homologous...
Anatomy
Placoid Scales
P
The tooth-like scales of sharks and rays - dermal denticles that reduce drag.
Anatomy
Planktivore
P
An organism that feeds on plankton (phytoplankton and/or zooplankton); includes many...
Ecology
Point
P
The end of a piece of underwater structure extending from shore - a productive fishin...
Fishing
Population Viability Analysis
P
A quantitative modelling approach that estimates the probability of a population beco...
Conservation
Potamodromous
P
A fish that migrates entirely within freshwater - such as many trout populations movi...
Ecology
Precautionary Principle
P
The management approach of setting fishery limits below scientifically-modelled susta...
Conservation
Precocity
P
Reaching sexual maturity earlier than typical for the species - common in cultured po...
Reproduction
Premaxilla
P
The paired bones forming the front of the upper jaw in teleosts; in advanced species...
Anatomy
Preopercle
P
The L-shaped bone forming the anterior and ventral borders of the opercular series, o...
Anatomy
Protandry
P
Sequential hermaphroditism in which the organism begins life as male and later change...
Behavior
Protogyny
P
Sequential hermaphroditism in which the organism begins life as a female and later ch...
Behavior
Purse Seine
P
A large fishing net set in a circle around a school of fish, then pursed (closed) at...
Fishery
Pyloric Caeca
P
Finger-like blind pouches at the junction of the stomach and intestine in teleosts, w...
Anatomy

R 15 terms

Ram Ventilation
R
A mode of gill ventilation in which the fish swims continuously with its mouth open,...
Behavior
Range Contraction
R
The reduction in the geographic range of a species due to habitat loss, overfishing,...
Conservation
RAS
R
Recirculating Aquaculture System - a closed-loop indoor fish farming method with cont...
Water Science
Recruitment
R
The addition of new fish to a population - typically measuring survival of juveniles...
Ecology
Red Muscle
R
Slow-twitch, highly oxidative muscle tissue rich in myoglobin, used for sustained cru...
Anatomy
Redd
R
A salmonid spawning nest - a depression dug in gravel where eggs are deposited and co...
Reproduction
Redox Potential
R
The oxidation-reduction potential (ORP) of water - measured in millivolts, indicates...
Water Science
Regional Endothermy
R
The ability of certain large, active fish to maintain specific body regions (muscles,...
Physiology
Rete Mirabile
R
A specialized countercurrent network of blood vessels used for heat retention, gas ex...
Anatomy
Reverse Osmosis
R
A pressure-driven water purification process that removes 99%+ of dissolved contamina...
Water Science
Rheotaxis
R
The reflexive orientation of a fish to face into water current.
Behavior
Rip
R
A boundary between two different water bodies - colour, temperature, or current - con...
Fishing
Roe
R
Fish eggs - both a reproductive stage and a food commodity (caviar, ikura).
Reproduction
Root Effect
R
A fish-specific property of haemoglobin in which a drop in pH reduces both the oxygen...
Physiology
Rostrum
R
The projecting snout or beak of a fish; in billfishes a greatly elongated upper jaw u...
Anatomy

S 31 terms

Sarcopterygii
S
The lobe-finned fish - includes lungfish and the ancestral group that gave rise to al...
Taxonomy
Schooling
S
A highly coordinated group of fish swimming together in the same direction with synch...
Behavior
Scute
S
A bony, shield-like plate or modified scale that provides protection or reduces drag...
Anatomy
Seagrass Bed
S
Dense underwater meadows of flowering plants in shallow coastal marine waters, servin...
Ecology
Selective Breeding
S
The practice of choosing parent fish with desired traits to breed together - basis fo...
Reproduction
Semelparous
S
A species that reproduces only once in its lifetime, often dying immediately after -...
Reproduction
Sequential Hermaphroditism
S
A reproductive strategy in which an individual fish changes sex once during its lifet...
Behavior
Sex Ratio
S
The proportion of males to females in a population - often manipulated in aquaculture...
Reproduction
Shoaling
S
A loosely aggregated social group of fish of the same species that remain in proximit...
Behavior
Sinker
S
A weight added to a fishing line - variants include split shot, egg sinker, pyramid,...
Fishing
Smolt
S
A salmon in the physiological process of transitioning from freshwater to saltwater t...
Reproduction
Smoltification
S
The physiological transformation of a young salmon (parr) into a saltwater-tolerant s...
Reproduction
Sneaker Male
S
A small, non-territorial male that darts in to fertilize eggs during another male's c...
Reproduction
Spawning
S
The act of releasing eggs and sperm - either simultaneous (broadcast) or sequential (...
Reproduction
Spawning Aggregation
S
A large gathering of fish at a specific site and time of year for the purpose of repr...
Behavior
Spawning Stock Biomass
S
The total weight of sexually mature fish in a stock; the primary metric of reproducti...
Fishery
Species
S
The basic unit of biological classification; a group of organisms that can interbreed...
Taxonomy
Specific Growth Rate
S
The percentage increase in body weight per day; the standard measure of fish growth p...
Fishery
Spinnerbait
S
A bladed lure that spins as it moves through water - the flash and vibration mimic ba...
Fishing
Splanchnocranium
S
The visceral skull - the jaws, gill arches, and associated bones.
Anatomy
Squid Jig
S
A weighted lure with prongs designed to catch squid - especially popular in the Medit...
Fishing
Standard Length
S
A fish measurement from the tip of the snout to the caudal peduncle - excluding the t...
Anatomy
Standing Stock
S
The total biomass of a population or community present at a given point in time.
Ecology
Startle Response
S
The C-shaped body bend of a fish reacting to sudden threat - one of the fastest neura...
Behaviour
Steelhead
S
The anadromous (sea-run) form of rainbow trout (Oncorhynchus mykiss).
Taxonomy
Stenohaline
S
Restricted to a narrow salinity range; unable to tolerate significant changes in envi...
Physiology
Stenothermal
S
Restricted to a narrow temperature range; intolerant of significant temperature fluct...
Physiology
Stock Assessment
S
The scientific evaluation of a fish population's status - the foundation of quota dec...
Conservation
Supraoccipital Crest
S
A bony ridge on the top of the skull that provides attachment for the epaxial muscles...
Anatomy
Swim Bladder
S
A gas-filled internal organ that provides neutral buoyancy, allowing fish to maintain...
Anatomy
Sympatric
S
Occurring in the same geographic area or region; sympatric species occupy overlapping...
Taxonomy

T 17 terms

Tandem Fly
T
Two flies fished on the same leader - increases hookup rates and pattern-testing effi...
Fishing
TDS
T
Total Dissolved Solids - the concentration of all dissolved minerals and salts in wat...
Water Science
Teleost
T
A member of Teleostei, the largest and most diverse group of vertebrates, encompassin...
Anatomy
Terminal Tackle
T
Everything on your line beyond the main line - hooks, sinkers, swivels, leaders.
Fishing
Territoriality
T
The defence of a defined area against intruders, typically of the same species, to se...
Behavior
Thermocline
T
A thin layer in a body of water where temperature decreases rapidly with depth, actin...
Ecology
Tippet
T
The finest section of a fly leader, tied directly to the fly - matched to fly size an...
Fishing
Total Allowable Catch
T
The annual quota set by fishery managers for a species or stock - TAC in fisheries sh...
Conservation
Total Dissolved Solids
T
The sum of all dissolved substances in water - a quick proxy for water purity, measur...
Water Science
Total Length
T
A fish measurement from the tip of the snout to the tip of the extended caudal fin.
Anatomy
Trap
T
A fixed or free-floating passive fishing device into which fish or crustaceans enter...
Fishery
Trawling
T
A fishing method using a cone-shaped net (trawl) dragged through the water column or...
Fishery
Trophic Cascade
T
A cascading effect on ecosystem structure when top predators are removed or added.
Ecology
Trophic Downgrading
T
The phenomenon of declining mean trophic level in ecosystems caused by preferential r...
Conservation
Trophic Level
T
A measure of a species' position in the food chain, where 1 = primary producers and 4...
Ecology
Trotline
T
A stationary long line with multiple baited hooks tied off between two points - tradi...
Fishing
Type Locality
T
The specific geographic location from which the type specimen (holotype) of a species...
Taxonomy
🔍

No terms match your search. Try a different keyword.