Author: abby

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).

Maxium sustainable yield (MSY)

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Optimizing fishing effort (Diagram credit: http://ib.bioninja.com.au).

Maximum sustainable yield (MSY) is a classic concept from fish population dynamics.  It operates under the assumption that populations have a carrying capacity, or equilibrium size, where the the resources available match the population requirements and if individuals are removed from the population, they will be replaced (i.e., are renewable).  When harvest removes a portion of the population, it  lowers pressures on the population’s associated resources, such as food and habitat, allowing the remainder of the population to benefit from less competition and balance the population back to the carrying capacity.  By this theory, the surplus biomass can be harvested sustainably, without long-term impacts to the population.

MSY is a commonly applied fisheries management tool, and even a guiding principle of the Magnuson-Stevens Fishery Conservation and Management Act, the primary legislation governing marine fisheries management in United States federal waters.  However, it is important to note that there are many situations in which the assumptions of MSY are not met which result in overfishing and unsustainable practices.

Gill rakers

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Gill rakers (above gill arch) from an American Shad Alosa sapidissima are not to be confused with gill filaments (below); they are particularly important for filter-feeding fish (photo credit: NOAA).

While gill rakers have no role in gas exchange, the predominant function of gills,  they do perform an equally important function for filter-feeding fish – food acquisition.  These bony projections serve as a sieve to trap food particles.  They vary greatly in length and number and these characteristic differences have evolved with different feeding strategies for fish.  A plankton feeder, for example, such as American Shad Alosa sapidissima, has very tightly-packed, comb-like, gill rakers to efficiently filter their food from the water column. An omnivore or piscivore, on the other hand, has shorter, more widely-spaced gill rakers, better for larger prey items.  In fact, the morphology of gill rakers is so diverse that they are often used as a taxonomic tool to identify and classify fish species (e.g., gill raker counts can differentiate species on a dichotomous key).