Definitions of marine ecological terms

This article provides a glossary of marine ecological terms used in the Coastal Wiki.

Many definitions are derived from the following references:

Nordberg et al. (2009), Andersen et al. (2013), IUCN (2021) and Blackart et al. (2006). Other definitions are based on the related Coastal Wiki articles and the references therein. Many similar definitions can be found in the Wikipedia, although not always for the specific coastal and marine context.

For terms related to physical coastal processes and coastal engineering, see Definitions of coastal terms.

The terms printed in blue are links to the corresponding definition.

A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z

Acclimation, Acclimatization

1. In experimental systems, allowing an organism to adjust to its environment prior to undertaking a study. 2. Phenotypic changes by an organism to stresses in the natural environment that result in the readjustment of the organism‘s tolerance levels

Adaptation

Initiatives and measures to reduce the vulnerability of natural and human systems to change in environmental conditions (in particular, actual or expected climate change effects).

Aerobic

Process in which O₂ is involved, e.g. aerobic respiration.

Angiosperms

Plants of the large subdivision Angiospermae that comprises those that have flowers and produce seeds enclosed within a carpel, including herbaceous plants, shrubs, grasses, and most trees.

Apex predator

Predator species at the top of the food chain.

Aphotic

Areas never reached by natural light.

Assemblage

1. An association of coexisting species, in space and time, with similar environmental tolerance, possibly trophic relationships, but not totally interdependent; 2. A collection of species inhabiting a given area, the interactions between the species, if any, being unspecified.

Asset

Refers to something of value and may be environmental, economic, social, recreational or a piece of built infrastructure.

Benthos

Organisms that live on or in the seabottom.

Bioavailability

Ratio of uptake clearance to the rate at which an organism encounters a given contaminant in an environmental medium (e.g., soil, sediment, water, food) being processed by the organism. Note: This is a measure of an organism’s extraction efficiency, via respiratory, dietary, and surface absorption processes, from the environmentally available (bioaccessible) portion of a material.

Biosphere

The total range of living beings and their environment that comprises the lithosphere (surface of the earth), the hydrosphere (earth waters) and the atmosphere.

Bivalve

A mollusc characterized by a shell in two parts joined by a hinge (oyster, cockle).

Calcification

A process by which the mineral calcium builds up in tissue, causing it to harden. Scleractinian corals produce aragonite (CaCO₃) skeletons via this process.
Calcinosis: Any pathological condition characterized by the deposition of calcium salts in tissues.

Cancritrophic

Having a diet specializing in crustacean prey.

Capsule

A dry fruit that when mature splits apart to release the seeds within.

Carbon sequestration

Carbon sequestration is a biochemical process by which atmospheric carbon is absorbed by living organisms, including trees, soil micro-organisms, and crops, and involving the storage of carbon in soils, with the potential to reduce atmospheric carbon dioxide levels. See: Blue carbon sequestration.

Cephalopod

A mollusc characterized by arms with tentacles and suckers.

Chemical Oxygen Demand (COD)

Measure of the amount of oxygen, divided by the volume of the system, required to oxidize the organic (and inorganic) matter present in the system. In practice, it is usually expressed in milligrams O₂ per litre.

Chemisorption

Sorption which results from chemical bond formation (strong interaction) between the sorbent and the sorbate. Note: Sorption can take place in a monolayer on a surface or internal to an absorbent.

Chlorinity

Proportion by mass of dissolved chloride ions in water. See Salinity.

Chloroplast

Cell organelle in which photosynthesis takes place.

Clade

A biological group of species that shares features inherited from a common ancestor.

Class

One of the taxonomic groups of organisms, containing related orders; related classes are grouped into phyla.

Classification

The ordering of organisms into groups on the basis of their relationships, which may be by similarity or common ancestry.

Climate variability

Climate variability refers to variations in the mean state and other statistics (such as standard deviations, the occurrence of extremes, etc.) of the climate on all temporal and spatial scales beyond that of individual weather events.

Climax community

Community of plants and animals in a steady state due to ecological succession resulting in a composition of the community best adapted to average conditions in the area. Climax species: Species which are stable and capable perpetuating themselves.

Clone

Organisms having identical genome.

Codend

The end of a trawl net that retains the catch and the part of the net where most size-selection takes place. Codend mesh sizes and structure are usually regulated and may be preceded by a sorting grid to reduce bycatch.

Cohort

In a stock, a group of fish generated during the same spawning season and born during the same time period.
Notes. For instance, the 1987 cohort would refer to fish that are age 0 in 1987, age 1 in 1988, and so on. In the tropics, where fish tend to be short-lived, cohorts may refer to shorter time intervals (e.g. spring cohort, autumn cohort, monthly cohorts). In cold and temperate areas, where fish are long-lived, a cohort corresponds usually to fish born during the same year (a year class).

Community

Assembly of populations of different species of living organisms, usually interdependent on and interacting with each other, within a specified location in space and time. In biological terms, a community is a group of interacting organisms sharing a common habitat.

Congener

In chemistry: One of two or more substances related to each other by origin, structure, or function;
In ecology: One of two or more species within the same genus;
In genetics: One of two or more organisms that have almost identical genomes.

Connectivity

The movement of organisms from place to place (e.g. among marine reserves) through dispersal or migration.

Containment

The restriction (by human hand) in area or range of a species that is spreading – possibly to become invasive – with intention to stop the spread to new areas.

Contaminants

Hazardous substances (pesticides, heavy metals, pharmaceuticals or persistent organic pollutants (POPs)) that cause harmful effects to the ecosystem when they end up in the marine environment.

Convergent evolution

A common trait in unrelated lineages.

Copepod

Minute marine or freshwater crustacean, usually having six pairs of limbs on the thorax; some are abundant in plankton and others are parasitic on fish, marine mammals, and macro-invertebrates.

Demersal

Occurring or living near or on the bottom of the ocean (opposite of pelagic). Notes: Cods, groupers, crabs, and lobsters are demersal species. The term usually refers to the living mode of the adult, i.e. demersal fish.

Deposit feeder

Animal that feeds on particles of matter in the soil or sediment, usually the top soil or sediment where it is filled with organic matter.
Feeding takes place either by ingesting soil or sediment or by trapping particles as they fall. Examples of organisms that are deposit feeders are earthworms, terebellids, and fiddler crabs.

Desiccation

1. A drying phenomenon. 2. Operation by which some elements are deprived of the moisture they contain.

Detrivorous

Refers to an organism that survives by eating detritus.

Diatom

Member of a major group of eukaryotic algae, with cells encased in a frustule, of widely diverse form, made of silica (hydrated silicon dioxide). Notes: Diatoms are unicellular; some form chains or simple colonies. Diatoms are among the most common types of phytoplankton. See Marine Plankton.

DIN

Dissolved inorganic nitrogen; the sum of nitrogen compounds (nitrate, nitrite and ammonium) that can be absorbed by plants.

DIP

Dissolved inorganic phosphorous; the chemical form in which phosphorous can be absorbed by plants.

Discards

The component of a catch returned to the sea, either dead or alive. Primarily made up of the bycatch but can include juveniles and damaged or unsuitable individuals of the target species.

Dissolved organic carbon (DOC)

Amount concentration of carbon found dissolved in water samples from aquatic systems, measured as total elemental carbon. Notes:

1. Operationally, DOC is defined as the organic matter that is able to pass through a defined filter (filters generally range in size between 0.7 and 0.22 μm). Conversely, particulate organic carbon (POC) in water is that carbon that is too large and is filtered out of a sample.
2. The DOC in marine and freshwater systems is part of the greatest cycled reservoir of organic matter on Earth and consists mostly of humic substances.
3. DOC is important in the transport and bioavailability of pollutants in aquatic systems.
4. Metals may form strong complexes with DOC, increasing metal solubility and concentration in water, while also reducing metal bioavailability.
5. Dissolved organic matter (DOM) is analogous to dissolved organic carbon, but refers to the entire organic pool dissolved in water.

Dissolved oxygen content (DOC)

Amount concentration of oxygen dissolved in water at a particular temperature and pressure.
Note: This can be a limiting factor on the growth of aquatic populations.

Dorsal

On the upper side of the body, opposite to ventral.

Dose–effect relationship

Association between dose and the resulting magnitude of a continuously graded change, either in an individual or in a population.

Driver

Any natural or human-induced factor that directly or indirectly causes a change in an ecosystem.

Dystrophic

Rich in organic matter, but low in nutrient content and unproductive.

Ecology

Branch of biology that studies the interactions between living organisms and all factors (including other organisms) in their environment. Such interactions encompass environmental factors that determine the distributions of living organisms.

Ecoregion

1. Any of a number of regions into which a continent, country, etc., can be divided according to their distinct environmental conditions and habitat types; 2. Large area of land or water with a characteristic, geographically distinct assemblage of natural communities and species comprising a recurring pattern of ecosystems associated with characteristic combinations of soil and landform.

Ecosystem integrity

The continuity and full character of a complex system, including its ability to perform all the essential functions throughout its geographic setting; the integrity concept within a managed system implies maintaining key components and processes throughout time.

Ecosystem restoration

Recovery of the structure, function and processes of the original ecosystem.

Ecosystem structure

The individuals and communities of plants and animals of which an ecosystem is composed, their age and spatial distribution, and the non-living natural resources present.

Ecotone

Area of gradual transition between two or more ecosystems.

Edaphic

Pertaining to the soil.

Emergent properties

New properties emerging with upward steps in hierarchical systems, such as ecological communities or ecosystems, that cannot be predicted solely from our understanding of the system’s parts or components.
Note: Such properties arise during the self-organization of complex systems and are the product of the evolution of these systems.

Endogenous

Produced within or caused by factors within an organism (antonym: exogenous).

Endotherm

“Warm-blooded organism” that regulates its body temperature to be (almost) constant.

Environmental indicator

A parameter or value derived from general parameters that describes in a synthesized manner the pressures, condition, responses and/or trends of environmental and socio-environmental ecological phenomena, which meaning is broader than the properties directly associated to the parameter‘s value. See for example Sustainability indicators.

Environmental risk assessment

Estimate of the probability that harm will result from a defined exposure to a substance in an environmental medium. The assessment refers to the specific species and conditions involved.

Environmental status

The overall state of the environment in marine waters, taking into account the structure, function and processes of the constituent marine ecosystems together with natural physiographic, geographic, biological, geological and climatic factors, as well as physical, acoustic and chemical conditions, including those resulting from human activities inside or outside the area concerned.

Environmental valuation

Estimate about the magnitude or quality of the natural environment (air, water, soil) or investigation about the effects that a certain function or activity has on another function or activity.

Epibenthic

1.The area just above and including the seabed. 2. Living on or near the bed of an aquatic system, normally on sediment.

Epipelagic

The upper part of the oceanic zone beyond the continental and insular shelves, from the surface to about 200m.

Equity

Equal access to opportunities, often through the development of basic capacities, implying elimination of barriers hindering economic, political opportunities and the access to education and basic services, so that the people (women and men of all ages, conditions and positions) may be able to enjoy such opportunities and benefit from them.

Eradication

The complete removal of all living representatives of a species that is becoming (or is likely to become) invasive in a specified area or country.

Euryhaline

Organisms able to tolerate a wide range of salinity.

Evolution

1. Transformation of animals, plants, and other living organisms into different forms by the accumulation of changes over successive generations. 2. Transmutation of species. 3. Origination or transformation of an organism, organ, physiological process, biological molecule, etc., by a series of changes.

Evapotranspiration

The sum of water loss from both plants and soil measured over a specific area.

Existence value

The value that individuals place on knowing that a resource exists, even if they never use that resource (also sometimes known as conservation value or passive use value).

Externality

The positive or negative consequence of an economic activity that is experienced by unrelated third parties, that is not reflected in the price of the goods or services being produced and for which no compensation is paid or received.

Extinction

An irreversible process whereby a species or distinct biological population forever ceases to exist.

Finfish

Vertebrate and cartilanginous fishery species, not including crustaceans, cephalopds, or other mollusks.

Fishing Gear

See Gear

Functional group of species

An ecologically relevant group of species representing a predominant ecological role. Examples: offshore surface-feeding birds, demersal fish, etc. Often applied to particular groups of mobile species: birds, reptiles, marine mammals, fish and cephalopods.

Gamete

Gametes are an organism's reproductive cells. A gamete is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually.

Gastropods

A large class of snails and other mollusks (e.g. abalone, Queen conch, cones, conches, periwinkles, whelks, limpets) that typically possess a coiled dorsal shell and a ventral creeping foot. Gastropods are found in very diverse habitats, such as estuaries, mudflats, the rocky intertidal, the sandy subtidal, the abyssal depths of the oceans, including the hydrothermal vents, and numerous other ecological niches, including parasitic ones.

Gear

The equipment used for fishing (e.g. gillnet, hand line, harpoon, haul seine, long line, bottom and midwater trawls, purse seine, rod-and-reel, pots and traps). Each of these gears can have multiple configurations.

Genetic adaptation

Result of random genetic variation due to mutation and (or) to changes in allele frequencies, causing variation in the survival and reproductive success of individuals and hence of groups of organisms, with the consequence that those best adjusted to their environment flourish. Note: This process underlies the concept of natural selection leading to Darwinian evolution.
Genetic drift: Evolutionary process of change in the allele frequencies in a population due to random changes in the frequency by which different alleles are transferred to the next generation.
Note: In small populations, genetic drift may result in extinction of some alleles leading to evolutionary change over time.

Genes

Elements in all living things that carry hereditary characteristics, which, when expressed, make each individual different from all others.

Hadal Zone

Deep bottom area or portion of submerged geoform at depths >7,000 meters.

Halophytic plants

Salt-tolerant vegetation.

Hazardous substances

Substances or groups of substances being toxic, persistent and liable to bioaccumulation, and other substances or groups of substances which give rise to an equivalent level of concern. Hazardous substances are either naturally occurring substances, such as heavy metals, or intentionally or unintentionally formed anthropogenic compounds.

Hermaphrodite

Organism having both male and female characteristics.

Heterotrophic

Obtaining organic food by eating other organisms or their excreta. Heterotroph: An organism that cannot make its own food, and which eats other organisms or complex organic substances that are produced by other organisms.

Homeostasis

Totality of processes occurring in an open system or a closed system, especially a living organism, enabling it to regulate its internal environment to maintain stable, constant conditions or the outcome of these processes.

Hybridization

Process of combining different varieties or species of organisms to create a hybrid.

Hypersaline

Term used to characterize the waters with a salinity above 40 ppm, derived from salts from inner land.

Hypoxia

1. Abnormally low dioxygen content or tension. 2. Deficiency of dioxygen in the inspired air, in blood, or in tissues.

Imposex

Pseudo-hermaphroditic condition in female gastropods (snails) manifested by the development (imposition) of male characteristics such as a penis or vas deferens. Note: Quantitation of imposex in the dog whelk (Nucella lapillus) is used to monitor pollution by the antifouling agent tributyl tin oxide (TBTO) in marine environments.

Indicator

1. In biology: an organism, species, or community whose presence shows the presence of defined environmental conditions. Examples: Abundance, yield, and age/weight ratios are indicators of population production; a low cholinesterase level is an indicator of exposure to cholinesterase-inhibiting pesticides.
2. In chemistry: a substance that shows a visible change, usually of color, at a defined point in a chemical reaction.
3. A device that indicates the result of a measurement, e.g., a pressure gauge or a moveable scale.

Indicator species

Species whose presence shows the occurrence of defined environmental conditions.

Indigenous

Native to a given region or ecosystem. Notes: (1) The term is applied to a native species to distinguish it from species introduced as a result of human activity; (2) "Indigenous" is not synonymous with "endemic". In ecology, endemic means exclusively native to the biota of a specific place, whereas an indigenous species may occur in two or more different habitats.

Infauna

Animals living in the sediment of aquatic systems but not on the surface.

Invasive species

An introduced species that out-competes native species for space and resources.

Leachate

Water or waste-water that has percolated through a column of soil or solid waste in the environment, carrying with it substances dissolved from the soil or waste.

Lenthic

Body of continental waters that are stagnant, settled, or have very little movement.

Lenticels

The breathing pores in the outer bark of woody plants.

Lichen

Close association between a photosynthetic algae (which produces its food through solar energy) and a fungus that settles on rocky surfaces.

Macrophyte

Aquatic plant large enough to be seen easily with the naked eye (as distinct from phytoplankton and small algae).

Meiosis

Process of "reductive" cell division, occurring in the production of gametes, by means of which each daughter nucleus receives half the number of chromosomes characteristic of the somatic cells of the species.

Mesocosm

Enclosed and essentially self-sufficient (but not necessarily isolated) experimental environment or ecosystem that is on a larger scale than a laboratory microcosm. Note: A mesocosm is normally used outdoors or, in some manner, incorporated intimately with the ecosystem that it is designed to reflect. Macrocosm: Large multi-species test system.

Mesohaline

Term used to characterize waters with a salinity from 5 to 18 ppm, deriving from ocean salts.

Mineralization

1. Conversion of organic substances to inorganic derivatives (often by microbial decomposition, see Nutrient conversion in the marine environment). 2. The hydrothermal deposition of metals in the formation of ore bodies.

Mycorrhiza

A mutually beneficial (symbiotic) association between a plant root and a fungus that enhances the ability of the root to absorb water and nutrients.

Native Species

A species that has not been introduced. Native is similar to indigenous but usually refers to a broader region.

Neritic

Refers to the part of the oceans covering the continental and insular shelves, from the intertidal to 200m.

No-take zone

A Marine Protected Area that is completely (or seasonally) free of all extractive or non-extractive human uses that have an impact.

Nursery

That part of a fish’s or animal’s habitat where the young develop and grow.

Option value

A component of Total Economic Value: the premium placed on maintaining environmental or natural resources for future possible uses, over and above the direct or indirect value of these uses.

Otolith

The ear bone of a fish. Otoliths have rings on them like the rings on a tree stump, and are used to find the age of the fish and its growth rate.

Oxygen radical

Highly reactive oxygen molecules that have lost an electron and thus stabilise themselves by 'stealing' an electron from a nearby molecule. Their high reactivity means they can cause cell damage

Parthenogenesis

Growth and development of an embryo or seed without male fertilization. Note 1: Occurs in lower plants, invertebrate species (water fleas, aphids, some bees, and parasitic wasps), vertebrates (some reptiles, fish, and, very rarely, birds and sharks). Note 2: Also used to describe reproduction in self-fertilizing hermaphroditic species.

Pathogen

An organism which causes a disease within another organism.

Persistent inorganic pollutant (PIP)

Inorganic substance that is stable in the environment, is liable to long-range transport, may bioaccumulate in human and animal tissue, and may have significant impacts on human health and the environment. Note 1: Examples are arsenides, fluorides, cadmium salts, and lead salts. Note 2: Some inorganic chemicals, like crocidolite asbestos, are persistent in almost all circumstances, but others, like metal sulfides, are persistent only in unreactive environments; sulfides can generate hydrogen sulfide in a reducing environment or sulfates and sulfuric acid in oxidizing environments. As with organic substances, persistence is often a function of environmental properties.

Persistent organic pollutant (POP)

Organic chemical that is stable in the environment, is liable to long-range transport, may bioaccumulate in human and animal tissue, and may have significant impacts on human health and the environment. Examples are tetrachlorodibenzodioxin (dioxin), PCBs, and DDT.

Photic Zone

The surface water layer where there is sufficient light for photosynthesis to occur.

Pinnipeds

Of or belonging to the Pinnipedia, a suborder of carnivorous aquatic mammals that includes the seals, walruses, and similar animals having finlike flippers used for locomotion.

Pneumatophores

Aerial roots of a mangrove that typically rises from the soil into the air above the low tide level, thereby allowing the plant to obtain oxygen directly from the air (―breathing roots).

Predator-Prey Relationship

The interaction between a species (predator) that eats another species (prey). The stages of each species’ life cycle and the degree of interaction are important factors.

Pressure

The mechanism (physical, chemical or biological) through which a human activity has a direct or indirect adverse effect on any part of the ecosystem, e.g. physical disturbance to the seabed.

Primary producer

Organism capable of using the energy derived from light or a chemical substance in order to manufacture energy-rich organic compounds.
Primary productivity: A measurement of plant production that is the start of the food chain. Much primary productivity in marine or aquatic systems is made up of phytoplankton, which are one-celled algae that float freely in the water.

Propagule

1. Portion of a plant, fungus, etc., that is capable, when detached, of giving rise to a new individual by asexual reproduction (e.g., a cutting, leaf bud, bulbil, seed, or spore). 2. Less commonly, any of the products of asexual reproduction in certain invertebrates.

Pycnocline

The transitional zone in the water column between layers of two densities.
Isopycnals: Lines of constant density.
Notes:

1. A water body is said to be stratified if the thickness of the pycnocline is much smaller than the water depth (sharp density gradients = squeezed isopycnals).
2. Pycnoclines are generally formed by salinity or temperature differences between the upper and lower water layers and create effective barriers to transport across layers.
3. In some cases, high suspended sediment concentrations also contribute to pycnocline formation.

Race

A distinct variety within a species or subspecies.

Recovery

Restoration of natural processes and genetic, demographic, or ecological parameters of a population or species. It also refers to its past local abundance, structure and dynamics, to resume its ecological and evolutionary role, and the consequent improvement regarding habitat quality.

Rehabilitation (of ecosystems)

Re-establishment of part of the productivity, structure, function and processes of the original ecosystem.

Restoration (of ecosystems)

All of the key ecological processes and functions are re-established and all of the original biodiversity is re-established.

Riparian

Pertaining to the border or the banks of a body of water, such as an estuary.

Risk

Risk is the probability that a situation will produce harm under specified conditions. It is a combination of two factors: the probability that an adverse event will occur; and the consequences of the adverse event. Risk encompasses impacts on human and natural systems, and arises from exposure and hazard. Hazard is determined by whether a particular situation or event has the potential to cause harmful effects.

Salinity

Mass of dissolved salts in seawater, brackish water, brine, or other saline solution divided by the mass of the solution.

Saprobic

Living in or being an environment rich in organic matter but lacking oxygen.

Seaweed (Macroalgae)

Multicellular algae large enough to be seen by the human eye.

Sessile

Attached to the substrate.

Seston

Minute living organisms and particles of non-living matter that float in water and contribute to turbidity.

Shellfish

Shellfish include both mollusks, such as clams, and crustaceans, such as lobsters.

Sorption

Process whereby a solute becomes physically or chemically associated with a sorbent regardless of the mechanism (absorption, adsorption, chemisorption).

Spawning ground

A place where fish leave their eggs for fertilization.

Speciation

1. Distribution of an element amongst defined chemical species in a system. 2. The evolutionary formation of new biological species, usually by the division of a single species into two or more genetically distinct ones.

Species richness

Total number of species in an ecosystem.

Stress

Any condition that results in reduced growth of an organism or that prevents an organism from realizing its “genetic potential”.

Stock

A group of individuals in a species which occupy a well-defined geographical range independent of other stocks of the same species. A stock is often regarded as an entity for management and assessment purposes or from the point of view of actual or potential utilisation.

Stolon

Similar to a rhizome, but exists above ground, sprouting from an existing stem.

Subsoil

The soil beneath the topsoil - compacted, with little or no organic material.

Substrate

The material making up the base upon which an organism lives or to which it is attached.
Notes: The substrate is not necessarily the seafloor; it can be any biotic or abiotic material. For example, encrusting algae that live on a rock can be substrate for another animal that lives on top of the algae.

Succession

Orderly sequential progression of changes in community composition that occurs during development of new populations in any area, from initial colonization to the attainment of the climax typical of a particular geographic area.

Succulent

A plant adapted to arid conditions and characterized by fleshy water-storing tissues that act as water reservoirs.

Suspension feeder (filter feeder}

Animal that feeds by straining suspended matter and food particles from water, typically by passing the water over a specialized structure, such as the baleen of baleen whales. Note 1: Some animals that use this method of feeding are clams, barnacles, krill, mysids, sponges, whale sharks, and flamingoes. Note 2: Other types of feeder are deposit feeder, fluid feeder, and food-mass feeder.

Tetrapods

Vertebrates with four legs (or other appendages).

Threshold/tipping point

1. A point or level at which ecosystems change, sometimes irreversibly, to a significantly different state, seriously affecting their capacity to deliver certain ecosystem services. 2. A point or level at which new properties emerge in an ecological, economic, or other system, invalidating predictions based on mathematical relationships that apply at lower levels. See Resilience and resistance and Ecological thresholds and regime shifts.
Notes. For example, species diversity of a landscape may decline steadily with increasing habitat degradation to a certain point, and then fall sharply after a critical threshold of degradation is reached. Human behavior, especially at group levels, sometimes exhibits threshold effects. Thresholds at which irreversible changes occur are especially of concern to decision-makers, see Thresholds and Marine Policies.

Tragedy of the Commons

The overuse of a resource resulting from a lack of assigned and enforceable property rights. See The tragedy of the commons: Is the Newfoundland's cod crisis a good example? and The Tragedy of the Commons - The Tuna Example.

Trawling (trawl netting)

A fishing method utilising a towed net consisting of a cone or funnel shaped net body, closed by a codend and extended at the openings by wings. Can be used on the bottom (demersal trawl) or in midwater (pelagic trawl). This technique is used extensively in the harvest of pollock, cod, and other flatfish in North Pacific and New England fisheries. It includes bottom- and midwater fishing activities.

Trophic level, trophic position

1. Position in a food chain, assessed by the number of energy-transfer steps to reach that level. Plant producers constitute the lowest level, followed by herbivores and a series of carnivores at the higher levels. 2. Group of organisms eating resources from a similar level in the energy cycle.

Valuation

The process of estimating a value for a particular good or service in a certain context in monetary terms.

Vulnerable

The degree to which a system is susceptible to adverse effects of hazards and threats, including climate change, climate variability and extremes. Vulnerability is a function of the character, magnitude and rate of change and variation to which a system is exposed, its sensitivity and its adaptive capacity.