bycatch

Longline

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(Diagram credit: Australian Fisheries Management Authority)

A longline a type of fishing gear that is generally used to target pelagic species, such as large tuna and billfish.  Baited hooks are attached to a long fishing line via shorter branch lines called snoods or gangions.  Longlines can be anchored or freely adrift (with tracking devices so they can be retrieved).  In certain open ocean fisheries, these lines can be miles long and hold thousands of hooks.  They can be located at the surface or deeper in the water column.

Due to the nature of the gear, longlines are often subject to bycatch of unintended species.  Management strategies to reduce incidental take of sharks, birds, turtles and other species include setting the lines quickly, at deeper depths, and at night.  Sound deterrents can also help ward off seabirds.

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.

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).

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.

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.

TED (Turtle Excluder Device)

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TEDs significantly reduce turtle bycatch

TEDs significantly reduce turtle bycatch

Turtle Excluder Devices (TEDs) are a specific type of bycatch reduction device (BRD) that are a modification to a trawl net to keep turtles, which are not targets of the fishery, from being caught in the net.  TEDs are generally composed of a set of bars over the trawl net.  While small target animals, such as shrimp, pass easily between the bars into the trawl net, larger animals, such as turtles or even sharks, can escape through a side opening in the net after they have hit the grid of bars.  TEDs were developed to eliminate turtle mortality from trawl fisheries.  Because turtles and other bycatch, or non-target catch, species often come from threatened or endangered populations, the fisheries had strict regulations on how much bycatch was allowed.   TEDs, by reducing bycatch, allows the shrimp trawls to operate more with more ecological and economic efficiency.

For more information, please visit: http://www.fisheries.noaa.gov/pr/species/turtles/teds.htm

Bycatch

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1280px-Shrimp_bycatch

Shrimp fisheries have historically had very high bycatch

Bycatch is non-target catch within any fishery.  Bycatch can be either different species from the target of the fishery or individuals from the target species, generally juveniles, which are not of a marketable size.  Most bycatch is tossed back because it is illegal to possess or it has no commercial value.  In certain fisheries, however, a certain amount of marketable bycatch can be kept to sell.  Including bycatch reduction devices (BRDs) that have been specifically developed for some fisheries can have significant impacts on the amount of bycatch and/or bycatch survival.  In cases where fisheries have bycatch quotas, the use of BRDs, such as Turtle Excluder Devices (TEDs), can often allow fisheries to operate longer because the quotas are not exceeded quickly.

Because bycatch is difficult to quantify, it makes it very difficult to assess the population status of bycaught species.  Overfishing for these species is sometimes not recognized until the population is severely overfished.  Including bycatch in stock assessment techniques is helping to improve sustainability of these species.