Demersal

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Guest post: Lauren Flynn, New Mexico State University

Example of a demersal catch, painted in the 17th century. Nature morte de poissons au bord d’un ravage by Jan van Kessel the Elder, Source: Wikimedia Commons.

From the Latin verb demergere, meaning ‘to sink’, demersal describes fish that live on or just above the ocean or lake floor.  It can also refer to a commercial fishery for groundfish and shellfish, which typically uses gear that is weighted so it sinks to the bottom.  Demersal fish are frequently captured by a trawl net that drags along the ocean or lake floor.  As a result, demersal fisheries tend to be less discriminate than pelagic fishing methods such as long lines or gill nets that target fish using specific bait, specialized hooks, or mesh size.  Bycatch can be an issue for demersal fisheries, but trawl net modifications using Bycatch Reduction Devices (BRD) help to increase capture of target species and reduce unintended catch.

Examples of demersal fish species are cod, haddock, and flatfish such as turbot and halibut.  Popular shellfish include crab, shrimp, and lobster.

For comparison, see pelagic.

ITQ

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ITQ is an abbreviation for Individual Transferable Quota (image credit: Iceland Magazine).

An ITQ is an Individual Transferable Quota.  It is part of a quota system in which the managing agency divides up catch shares of the Total Allowable Catch (TAC) for the fishing season.  This essentially, creates a “stock market” like system for the fishery where ITQs can be bought and sold.

Setting up this type of management system is often contentious because some consider this process a privatization of public resources and others are concerned about the equity of the initial ITQ allocations.  Generally, the starting ITQs are based on recent records from the fishery, such as boat ownership or prior catch.  Once the system is in place, ITQs can be rented, bought, or sold.

While ITQs are considered effective in preventing “tragedy of the commons” collapses of fish stocks and have been credited with helping restore some fish populations, the approach remains controversial in some fisheries due to concerns with consolidation of ownership through market-based approaches.

TAC

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TAC is an abbreviation for Total Allowable Catch (image credit: mimiculture.com; icon design: Scott de Jonge from Flatiron).

TAC is Total Allowable Catch.  This is the upper limit in the amount of fish, either by weight or number, that a fishery is allocated in a given season or year.  TACs can apply to the targeted fish and/or bycatch, where once the limit is reached the fishery is closed for the remainder of the applicable time frame.  Often, the TAC is set based on an assessment of the stock status for the fishery, such as maximum sustainable yield (MSY).

In open-access or “derby” style fisheries, there is a free-for-all competition until the TAC is exhausted.  This can lead to risky or unsafe behavior, where vessels may continue to operate in hazardous conditions in order to harvest a larger proportion of the TAC.  In order to avoid accidents from such situations, some fisheries have opted for quota-based management systems where the TAC can be divided into catch shares, such as Individual Transferable Quotas (ITQs).

Quota

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Image credit: Fisheries New Zealand, mpi.govt.nz)

A quota is the allowable number or amount of fish that can be harvested in a commercial or recreational fishery.  This is a regulatory strategy to limit harvest.  They are set each fishing season based on allowance for sustainable harvest (e.g., see Maximum Sustainable Yield).

In certain commercial fisheries, quotas are essentially equivalent to a “share” in a fish stock.  These Individual Transferable Quotas (ITQs) can be bought and sold and entitle the owner to a given proportion of the Total Allowable Catch (TAC) for a fishing season.

Trap, pound, and fyke nets

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Trap net (image credit: Michigan Sea Grant).

A trap or pound net is a type of fishing gear with a complex organization of nets anchored to the substrate that “lead” or funnel fish into a final compartment (i.e., trap or pound) from which they cannot escape.  In the Laurentian Great Lakes, this type of fishing gear is called a trap net and in marine and estuarine systems, it is generally called a pound net.  A fyke (pronounced similar to “like”) is specific kind of trap net that uses a series of cone-shaped bags; it is typically a smaller size.

Fish harvested from this type of gear are often considered higher quality than those harvested from gill nets because fish can swim freely in the trap/ pound until the fishermen come to haul the nets.  Certain types of bycatch can also be more readily released unharmed from this gear than others.

For an example of how this fishing gear is used, please visit “Know Your Nets” from Michigan Sea Grant.

Gill net

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Gill net (image credit: Michigan Sea Grant).

A gill net is a simple type of fishing gear that is staked to a substrate to create a wall of mesh.  Gill nets are often made of a material, such as monofilament nylon, which is practically invisible to fish attempting to swim by.  As the name implies, fish are often caught by the gills because these nets are designed for the head of a fish to fit through but not its body.

Management strategies to reduce bycatch in gill nets include regulating the mesh size so that the net openings are large enough, for example, for undersized fish to swim through unharmed but for legal-sized fish to be harvested.

For an example of how this fishing gear is used, please visit “Know Your Nets” from Michigan Sea Grant.

Tubercles

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Breeding tubercles (photo credit: G. Engman and P. Cooney).

Tubercles are skin nodules made of keratin, the same materials as hair, hooves, and fingernails.  They are present on species representing at least 15 families of fishes, including even pet goldfish.  In many species, tubercles are found only on males during the breeding season and are shed shortly there after.  They are often called breeding tubercles for that reason.  The main functions for tubercles include species recognition, fighting and defense of spawning territory or nests, and stimulation of breeding females.

For more information, please see “Breeding tubercles and contact organs in fishes: their occurrence, structure, and significance” (Wiley and Collette 1970) and the related Fisheries Blog post: “Top 10 Weirdest Things Found on a Fish’s Head.”

BRD

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A shrimp net with a mesh-panel, bycatch reduction device (BRD) and grill-type, turtle excluder device (TED; photo credit: NOAA).

BRD is an acronym for bycatch reduction device.  Bycatch is non-target catch within any fishery.

For certain fisheries, these modifications to the gear (i.e., BRDs) can help lower the amount of bycatch.  For example, a turtle excluder device (TED) is a specific type of BRD designed to help turtles escape trawl nets.  Especially in cases where fisheries have bycatch quotas, the use of BRDs can often allow fisheries to operate longer, and more profitably, because the quotas are not exceeded quickly.

BRDs are generally designed with the cooperation of the fishery – they can be continually modified and improved to help ensure their effectiveness without compromising the productivity of the fishery – which is in the best interests of the industry and conservation, alike.

Population dynamics

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Fish population dynamics (image credit: Katherine Nicholson, slideplayer.com)

Fish population dynamics is the study of change in a fish population over time.  Fundamentally, a population size after some time interval equates to the population size before that interval plus births (i.e., recruitment) and immigration and minus mortality and emigration.  Many fish populations follow a logistic pattern of density-dependent growth.  Beginning with a population size where space and food are not limiting, the population grows rapidly in an exponential pattern; at a certain population density, population growth slows and stabilizes at a given carrying capacity.

Density-dependent factors governing fish population dynamics include competition, predation, disease, and parasitism.  Fish population dynamics can also be driven by density-independent abiotic factors such as temperature, dissolved oxygen, and water chemistry.

In fisheries, these basic population dynamics principles can be applied to harvest strategies to estimate maximum sustainable yield (MSY) with the goal to prevent overfishing.

Exclusive Economic Zone (EEZ)

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Exclusive economic zone of the United States (image credit: NOAA).

An Exclusive Economic Zone (EEZ), as defined by the United Nations Convention on the Law of the Sea, is a 200 nautical mile jurisdiction granted to coastal nations for “sovereign rights for the purpose of exploring and exploiting, conserving and managing the natural resources, whether living or non-living, of the waters superjacent to the seabed and of the seabed and its subsoil, and with regard to other activities for the economic exploitation and exploration of the zone.”  Within an EEZ, a country claims exclusive rights to manage fisheries and prevent overfishing.  While the outer 200 mile boundary determines a country’s sovereign limits, within the U.S., most coastal states claim an inner three nautical mile zone from the shore as state waters (some places claim nine nautical miles).  For fisheries, such as defined under the Magnuson-Stevens Fishery Conservation and Management Act, management in the inner zone belongs to the states and the remainder of the EEZ is under purview of the federal government (e.g., the National Oceanic and Atmospheric Administration).