Ichthyology Glossary

Actinopterygian

A

A ray-finned fish; member of Actinopterygii, the class containing 99% of living fish...

Anatomy

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

The newly-hatched salmonid larval stage, still attached to a yolk sac from which it d...

Physiology

Allopatric

A

Occurring in geographically separate areas; allopatric speciation is the most common...

Taxonomy

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

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

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

Binomial Nomenclature

B

The two-part scientific naming system for species, using a genus name and a specific...

Taxonomy

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

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 adult fish kept for breeding purposes in a hatchery or aquaculture facility to...

Fishery

Buccal Pumping

B

Active gill ventilation in which muscles alternately expand and compress the buccal a...

Behavior

Bycatch

B

Non-target species captured incidentally during fishing operations, often discarded d...

Fishery

Bycatch Reduction Device

B

A modification to fishing gear designed to allow non-target species to escape from th...

Conservation

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

Chromatophore

C

A pigment-containing cell in the skin responsible for fish coloration and rapid colou...

Anatomy

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

Condition Factor

C

A dimensionless index of the plumpness/wellbeing of a fish relative to its length: K...

Physiology

Coral Reef

C

A biologically diverse, shallow tropical marine habitat built by reef-forming corals,...

Ecology

Countershading

C

A camouflage pattern where the dorsal surface is dark and the ventral surface is pale...

Anatomy

CPUE

C

The amount of fish caught per unit of fishing effort, used as an index of stock abund...

Fishery

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

Cycloid Scale

C

A thin, smooth-edged fish scale with concentric growth rings but no cteni (teeth), fo...

Anatomy

Dead Zone

D

A hypoxic (low-oxygen) area of ocean or freshwater where most marine life cannot surv...

Ecology

Demersal

D

Relating to fish that live and feed on or near the seafloor.

Ecology

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

The overarching term for fish that migrate between freshwater and saltwater as part o...

Behavior

Diel Vertical Migration

D

The daily movement of aquatic organisms from deeper water at day to shallower water a...

Behavior

Diurnal

D

Active primarily during daylight hours; describes fish that rely on vision for feedin...

Behavior

Dorsal Fin

D

The fin located on the back (dorsal surface) of a fish, used primarily for stability.

Anatomy

Effective Population Size

E

The number of individuals in an ideal population that would undergo the same amount o...

Conservation

Elasmobranch

E

A cartilaginous fish of the subclass Elasmobranchii, including all sharks, skates, an...

Anatomy

Electric Organ

E

A specialised organ derived from muscle or nerve tissue that generates electric field...

Anatomy

Endemic

E

A species found exclusively in one particular geographic area and nowhere else in the...

Conservation

Epipelagic

E

The uppermost zone of the open ocean from the surface to 200 m depth, where photosynt...

Ecology

Estuary

E

A semi-enclosed coastal body of water where fresh and salt water mix, characterised b...

Ecology

Euphotic Zone

E

The sunlit upper layer of water where there is sufficient light for photosynthesis, t...

Ecology

Euryhaline

E

Capable of tolerating a wide range of salinities; characteristic of estuarine and dia...

Physiology

Eurythermal

E

Tolerant of a wide temperature range; characteristic of temperate and intertidal spec...

Physiology

Ex Situ Conservation

E

Conservation of species away from their natural habitat, such as in aquaria, gene ban...

Conservation

Family

F

A taxonomic rank above genus and below order, grouping related genera; fish family na...

Taxonomy

Feed Conversion Ratio

F

The weight of feed required to produce 1 kg of fish biomass; the primary measure of f...

Fishery

Filter Feeder

F

An organism that feeds by straining suspended matter (plankton, organic particles) fr...

Ecology

Fingerling

F

A juvenile fish roughly finger-sized (5–12 cm), past the fry stage but not yet adult;...

Fishery

Fish Aggregating Device

F

A man-made floating object that attracts fish and is used to concentrate them for mor...

Fishery

Fishing Mortality

F

The instantaneous rate at which fish are removed from a population by fishing, denote...

Fishery

Flagship Species

F

A charismatic species used as a symbol for a conservation campaign or protected area...

Conservation

Fry

F

Young fish that have absorbed their yolk sac and begun independent feeding; the stage...

Physiology

Ganoid Scale

G

Heavy, rhomboid, non-overlapping scales with a layer of ganoine (enamel-like material...

Anatomy

Genetic Erosion

G

The loss of genetic diversity within a species or population, reducing its ability to...

Conservation

Genus

G

A taxonomic rank above species and below family, grouping closely related species tha...

Taxonomy

Ghost Fishing

G

The continued trapping and killing of fish and other marine life by lost or abandoned...

Fishery

Gill

G

The primary respiratory organs of fish, extracting dissolved oxygen from water as it...

Anatomy

Gill Raker

G

Comb-like projections on the gill arches used to filter plankton or prevent food item...

Physiology

Gillnet

G

A wall of netting set vertically in the water in which fish become entangled by their...

Fishery

Gonopodium

G

A modified anal fin in male live-bearing fishes used as an intromittent organ for int...

Anatomy

Halocline

H

A layer where salinity changes rapidly with depth, creating a density gradient that r...

Ecology

Hatchery

H

A facility where fish eggs are artificially incubated and larvae are reared through e...

Fishery

Heterocercal Tail

H

An asymmetrical tail in which the vertebral column curves upward into the enlarged up...

Anatomy

Holotype

H

The single specimen designated as the name-bearing type of a species when it is first...

Taxonomy

Homocercal Tail

H

An externally symmetrical tail found in most modern bony fishes; the vertebral column...

Anatomy

Hypoxia

H

Condition of low dissolved oxygen in water (below 2 mg/L), causing fish kills and beh...

Ecology

Ichthyofauna

I

The fish species assemblage of a particular geographic area; the totality of fish spe...

Taxonomy

Ichthyology

I

The scientific study of fish, including their biology, taxonomy, behaviour, and distr...

Taxonomy

Indeterminate Growth

I

Growth that continues throughout an organism's entire life, though at a progressively...

Physiology

Invasive Species

I

A non-native species that establishes and spreads in a new location, causing ecologic...

Conservation

Iridophore

I

A light-reflecting cell containing crystalline guanine platelets that produce iridesc...

Anatomy

Isthmus

I

The narrow fleshy area on the ventral surface of the head between the gill openings;...

Anatomy

IUCN Red List

I

The world's most comprehensive inventory of the global conservation status of species...

Conservation

IUU Fishing

I

Fishing activity that breaks national or international laws, is not reported to autho...

Fishery

Lure (Illicium)

I

A modified dorsal fin ray on the snout of anglerfishes that acts as a bioluminescent...

Anatomy

Junior Synonym

J

A scientific name that was validly published after the currently accepted name for th...

Taxonomy

Keel

K

A lateral or ventral ridge on the caudal peduncle of fast-swimming fishes that stabil...

Anatomy

Kelp Forest

K

A dense underwater forest of giant brown algae (kelp) found in cold, nutrient-rich co...

Ecology

Keystone Species

K

A species that has a disproportionately large effect on its ecosystem relative to its...

Ecology

Kype

K

The hooked lower jaw that develops in male salmonids during the spawning season, used...

Anatomy

Lateral Line

L

A sensory organ running along each side of the body that detects low-frequency vibrat...

Anatomy

Littoral Zone

L

The coastal zone extending from the high tide mark to the edge of the continental she...

Ecology

Longline

L

A fishing gear consisting of a long mainline with many branch lines (snoods) bearing...

Fishery

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

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 average catch that can theoretically be taken from a stock indefinitely u...

Fishery

Mesopelagic

M

The ocean zone from 200–1000 m depth, the "twilight zone," characterised by dim light...

Ecology

Microhabitat

M

The small-scale physical features within a broader habitat that a particular organism...

Ecology

Mimicry

M

The resemblance of one organism to another species or object, conferring an adaptive...

Behavior

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

Nape

N

The dorsal surface of the head just behind the skull and anterior to the dorsal fin o...

Anatomy

Natural Mortality

N

The rate at which fish die from causes other than fishing, including predation, disea...

Fishery

Nocturnal

N

Active primarily at night; describes fish that feed, spawn, or migrate during darknes...

Behavior

Nomen Dubium

N

A scientific name of uncertain or doubtful application because its type specimen is l...

Taxonomy

Nuptial Tubercle

N

Small, hard, keratinised bumps that develop on the head, fins, and body of male cypri...

Anatomy

Operculum

O

The bony gill cover in bony fishes, consisting of four bones that protect and seal th...

Anatomy

Order

O

A taxonomic rank above family and below class, grouping related families of organisms...

Taxonomy

Osmoconformer

O

An organism whose body fluid osmolality matches the external environment, avoiding th...

Physiology

Osmoregulation

O

The physiological process by which fish maintain the balance of water and salts in th...

Physiology

Otolith

O

Calcium carbonate concretions in the inner ear of fish, used for balance and hearing;...

Anatomy

Overfishing

O

Harvesting fish at a rate higher than the population can replace itself, causing stoc...

Fishery

Parr

P

The freshwater juvenile stage of a salmonid, characterised by dark vertical "parr mar...

Physiology

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 borne on modified gill arch bones in the pharynx (throat) of many teleosts, use...

Anatomy

Photoperiod

P

Day length; the ratio of light to dark hours in a 24-hour cycle, which regulates repr...

Physiology

Piscivore

P

An organism that feeds primarily on fish; fish-eating predators.

Ecology

Placoid Scale

P

The tooth-like scales of sharks and rays, composed of dentine and enamel — homologous...

Anatomy

Planktivore

P

An organism that feeds on plankton (phytoplankton and/or zooplankton); includes many...

Ecology

Population Viability Analysis

P

A quantitative modelling approach that estimates the probability of a population beco...

Conservation

Potamodromous

P

Describes freshwater fish that migrate entirely within freshwater systems, such as be...

Behavior

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

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

Recruitment

R

The process by which young fish survive long enough to join the harvestable (or detec...

Fishery

Red Muscle

R

Slow-twitch, highly oxidative muscle tissue rich in myoglobin, used for sustained cru...

Anatomy

Regional Endothermy

R

The ability of certain large, active fish to maintain specific body regions (muscles,...

Physiology

Rete Mirabile

R

A network of intertwined capillaries that acts as a countercurrent heat or gas exchan...

Anatomy

Rheotaxis

R

The behavioural response of fish to water current; positive rheotaxis is facing into...

Behavior

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

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

Sequential Hermaphroditism

S

A reproductive strategy in which an individual fish changes sex once during its lifet...

Behavior

Shoaling

S

A loosely aggregated social group of fish of the same species that remain in proximit...

Behavior

Smolt

S

The silvery, seawater-ready juvenile stage of an anadromous salmonid, physiologically...

Physiology

Smoltification

S

The complex physiological and morphological transformation that prepares anadromous s...

Physiology

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

Standing Stock

S

The total biomass of a population or community present at a given point in time.

Ecology

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 process of estimating the current size, productivity, and status of a fish popula...

Fishery

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

Teleost

T

A member of Teleostei, the largest and most diverse group of vertebrates, encompassin...

Anatomy

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

Total Allowable Catch

T

The maximum quantity of a fish species that may legally be caught from a defined stoc...

Fishery

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

Indirect ecological effects that ripple through a food web when a top predator is add...

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

Type Locality

T

The specific geographic location from which the type specimen (holotype) of a species...

Taxonomy

Upwelling

U

The wind-driven rise of cold, nutrient-rich water from depth to the surface, creating...

Ecology

Urohyal

U

A median unpaired bone at the front of the throat connecting hyoid elements; helps op...

Anatomy

Virtual Population Analysis

V

A backward-looking cohort analysis method used in stock assessment to reconstruct his...

Fishery

Vitellogenesis

V

The process of yolk protein (vitellogenin) synthesis in the liver and deposition into...

Physiology

Vomerine Teeth

V

Teeth borne on the vomer bone in the roof of the mouth, used in identifying many fish...

Anatomy

Von Bertalanffy Growth Function

V

The standard mathematical model describing indeterminate fish growth: L(t) = L∞(1 − e...

Physiology

Weberian Apparatus

W

A chain of small bones (ossicles) in cyprinids and catfishes that transmit vibrations...

Anatomy

White Muscle

W

Fast-twitch, anaerobic muscle that makes up the bulk of the body in most fish, used f...

Anatomy

Year Class

Y

All fish born in the same calendar year, tracked as a group through the population; s...

Fishery

Year Class Strength

Y

The relative number of surviving recruits produced by a given year's spawning; highly...

Ecology