Glossary of Bycatch Reduction Techniques

Acoustic deterrent devices

Underwater sound-emitting devices (maximum level of intensity equivalent to approximately 175 dB re 1 µPa @ 1m) attached to fishing gear, principally gillnets. ADD’s such as acoustic pingers are now mandated for use in some fisheries in the U.S. Northwest Atlantic, California driftnet, and in Europe. The sound of these devices is believed to alert an animal to the presence of the net and thus decrease the probability of entanglement. Although some studies have shown that pingers can have the unintended consequence of attracting pinnipeds to fishing operations (Bordino et al., 2002) , this may be controllable by raising the emitted frequency of the pingers above seal hearing (Kraus et al., 1997). Other ADD’s emit sounds of such high intensity that they cause pain or alarm in certain underwater species. The minimum sound level is approximately 200 dB re 1 _Pa @ 1m (Olesiuk et al., 2002). Other ADD’s include audio recordings of an animal in distress, or of its predator, played to deter individuals of that species from entering into a fishing area. Jefferson and Curry  concluded that this technique was largely ineffective for reducing marine mammal interactions with fishing activity based on their review of multiple studies. Sounds produced to disrupt the normal echolocation abilities of cetaceans are also considered ADD’s. Preliminary research in Europe has shown some promise that these devices reduce depredation by bottlenose dolphins in gillnets and trammel nets, although habituation may be a challenge (Mooney et al. 2009) . The use of loud explosive devices, including gunshots, to scare non-target species such as sea lions away from a fishing operation can also be used. Deterrence may result from noise or tactile annoyance. Anecdotal evidence from some fishermen suggests this practice is widespread though its efficacy is not backed up by a number of studies, and it obviously threatens animal survival (Matkin 1994).


Alternative bait

Changing the type of bait, such as switching from squid to mackerel, to deter non-target animals (such as sea turtles) that prefer one type of bait versus another (Watson et al. 2005).


Alternative leader design

Traditional leaders used in fishing gears such as longlines, pound nets, gillnets and/or traps are modified with an alternative design. For example, deep water pound net leaders can be modified by replacing the top two-thirds of the traditional mesh panel leader with vertical ropes (DeAlteris et al. 2005).


Alternative offal discharge

Discarding waste away from where bait enters the water to lure non-target species (seabirds) away from baited hooks in longline fisheries (McNamara et al. 1999).


Animal prodding

The physical prodding of non-target species using a pole or other implement to deter them from interacting with a fishing operation (Werner et al. 2006).


Artificial bait

Bait manufactured from non-natural substances as a substitute to natural bait that may render it less appealing to non-target animals (Mejuto et al. 2005).


At-call pop-up buoys

Devices that use an acoustic trigger for releasing a buoy attached to submerged pots that would then float to the surface for gear retrieval. This would eliminate vertical (and potentially entangling) lines in the water column (DeAlteris 1999).



A procedure used by purse seine operators that facilitates the escape of trapped dolphins. The backdown occurs after the majority of the net is on board. At this point net retrieval is stopped, the net is tied to the vessel and the engine is put into reverse. This creates a water current that causes the net remaining in the water to form a long channel. The water current also pulls the end of the channel under water providing an area for dolphins to escape (Bratten and Hall 1996).


Bait-casting mechanisms

Devices that toss the bait beyond the turbulence of longline boat propellers that tend to keep bait buoyant longer where it is more prone to seabird predation (Brothers et al. 1999).


Bird curtain

A barrier, which prevents birds from forming a flight path alongside vessels when baited hooks are being deployed (Gilman et al. 2007).


Bird-scaring devices

A number of devices used to disturb birds from foraging on bait. These include streamers attached to a pole suspended above the area where bait is set or placed in the water, towed buoys, and water jets (Melvin et al. 2010).


Buoy lines with reduced breaking strength

Underwater traps or nets linked to a surface buoy by a weak line. To haul the gear, a messenger device would be sent down the weak line along with a stronger hauling line. The messenger device would attach to the hauling line to the bottom gear for retrieving the gear. The premise is that a large whale would easily break free from a weak line suspended in the water column and the stronger line needed for hauling could be located out of harm’s way (Werner et al. 2006).


Circle hooks

A circular hook design in which the point of the hook is perpendicular to the hook shank. Circle hooks are used widely in many recreational and commercial fisheries and recently have been shown to reduce both the hooking rate and the mortality of turtles that are hooked on pelagic longline gear. As a result of several successful field trials (Bolten and Bjorndal 2005)(Watson et al. 2005), these hooks are becoming increasingly used in longline fisheries.



Devices used for attracting non-target animals away from fishing activity where they might become captured or entangled in gear (ICES 2010).


Decreased soak time

Soak time is the length of time that fishing gear is submerged between hauls; reducing it appears to change bycatch probabilities (Watson et al. 2005).


Deep sets

Baited hooks in longline fisheries set below 100 meters of water to avoid the principle feeding zones of sea turtles and other epipelagic species. Increasing the depth at which pelagic drift nets are set may also reduce bycatch rates of air-breathing vertebrates (Beverly et al. 2004).


Differently colored ropes

Various colored rope or other vertical line material used as 1) buoy lines for crab, whelk or lobster pots, 2) end lines of connected pots or 3) end or anchor lines for gillnet systems (Kot et al. 2012).



One FAD with a solid light at 20 m depth is deployed by a work boat. A second FAD with a flashing light at 100 m depth is deployed by a second work boat. The second work boat moves the FAD with the flashing light away from the first FAD. As it moves away, the main boat encircles the first FAD with the surrounding net (Kawamoto et al 2012). Study on the methods to mitigate the bycatch of juvenile bigeye tuna by introducing Double-FADs with light stimulus for tuna purse seine fishery.


Dummy longline sets

Approaches that include setting longlines in novel patterns (such as in a sinusoidal shape) or using “dummy” sets to mask the presence of a fishing operation (Werner et al. 2006).


Dyed bait

Bait dyed blue to reduce its visibility to non-target species such as seabirds hovering around longlines as baited hooks are deployed (Watson et al. 2002).


Electromagnetic deterrents

Electromagnetic fields created in the vicinity of a fishing activity to deter interaction of non-target species with fishing gear, bait, or target species. For example, rare earth metals, such as cerium or lanthanide, are attached or imbedded into hooks (longline, hook and line etc.) in an effort to deter the incidental bycatch of sharks and rays (Tallack et al. 2009). The main prize of the 2006 Smart Gear competition run by the World Wildlife Fund was for a magnetic shark deterrent to be tested on pelagic longlines. Polet et al (2005) describe evaluations of an "electro-trawl" in which electric charges stimulated shrimp into moving upward from the sea floor into the path of the trawl mouth. In this approach, the space between the groundrope and the benthos might be increased without reducing target catch levels but decreasing the contact the trawl might have with some non-target benthic invertebrates and groundfish.


Excluder devices

These can consist of a grid of metal bars or mesh placed usually within the neck of a trawl that has an opening for escape at either the top or bottom. Examples include “skylights” which consist of a mesh opening located at the top of the codend. ”Skylights” can be combined with guiding funnels that create visual and tactile stimuli to direct finfish towards the “skylight” escape sections. A different but similar type of design is the fisheye (including Gulf and yarrow fisheyes and bigeyes) which are openings typically forming a rigid frame constructed of aluminum or steel rods and placed along the top center portion of the codend (i.e. Catchpole and Revill 2007). Typically, large animals that strike the bar exit through the opening, while the smaller target species pass through the bars and are captured in the net or codend. Examples of excluder devices in trawls are the Nordmore grid, the Turtle Excluder Device (TED), and the Sea Lion Excluder Device (SLED). A sea turtle excluder chain mat is used in the Northwest Atlantic scallop dredge fishery. A new modification to pound nets may reduce sea turtle bycatch by replacing the upper two-thirds of the leader netting with vertical ropes spaced wide enough apart to let sea turtles swim through without becoming entangled (DeAlteris et al., 2005).


Fence or net barriers

Barriers erected in aquaculture and corral-type fishing gear to exclude non-target species. Barrier nets can create a separate bycatch problem based on reports of fatal entanglements that have occurred with California sea lions and humpback whales (Petras, 2003).


Haulback delay

A delay, usually 10-15 seconds in length, between slowing of a trawl vessel and engagement of the winch to haul in the trawl (Broadhurst et al. 1996). This delay may cause changes to the geometry of the codend, creating additional stimuli that directs the bycatch out through fish escape window/separator devices or may simply allow more time for bycatch species to escape through these devices.


High tension rope

Increasing the tension of a fishing rope, such as by simultaneously increasing surface flotation and bottom weight.


Low profile gillnet

A gillnet with reduced vertical curvature (compared to nets typically used in a specific gillnet fishery), an effect produced by not using tie downs. The use of tie downs in general increases the amount of ballooning in the net. Presumably because the net hangs more vertically in the water column, non-target species are more likely to bounce off the net as opposed to becoming entangled within the net “bag” (Price and Van Salisbury 2007).


Metal oxide/barium sulfate nets

Nylon nets infused with barium sulfate or other metal compounds that have acoustical detection features for reducing small cetacean bycatch. These may reduce small cetacean and sea turtle bycatch by increasing the likelihood that these animals would “bounce” off the netting. Experimental results show that they can be effective in reducing the bycatch of harbor porpoise and greater shearwater (Trippel et al., 2003), though it has not been ascertained if this is due to their acoustic reflectivity or increased stiffness over conventional gillnets.


Modified ground gear (mobile)

Modifications made to mobile fishing gear that fishes on the sea floor. An example would be the "Newhaven" spring tooth dredge, used in the Isle of Man queen scallop fishery. This dredge is wider and fishes higher off the benthos than what is typically used, and has a rubber lip instead of metal teeth (Hinz et al. 2009).


Net sleeves

A buoyant net or "umbrella" that is attached to the vertical hook line above the baited hooks. When the mainline is hauled back during gear retrieval, the net sleeve slides down, covering the hooked fish. Net sleeves are used to prevent depredation and hooking non-target animals (Hammer and Childerhouse 2012).


Night sets

The setting of fishing gear at night so that seabirds are less likely to see sets. Lights may also be dimmed to enhance the effect (McNamara et al. 1999).


Noxious bait

Bait that is treated with compounds intended to make it unpalatable to non-target species (Gearin et al. 1988). For example, a type of olfactory deterrent used for seabird avoidance. Biogenic chemical deterrents that can be made from fish liver oil are released behind the vessel on the surface of the water as longlines are being set (Pierre and NOrden 2006).


Passive acoustic deterrents

Objects such as rubber tubes, thick polyester rope, and chains attached to fishing nets to alert a marine cetacean to their presence using echolocation (Hembree and Harwood 1987).


Quick-release gear

A metal wire attached to an outrigger clip on a troll line. The quick-release mechanism of the outrigger clip causes the wire to travel down the bait line when a fish is captured. The metal wire may deter dolphin depredation (Zollett and Read, 2006).


Raised footropes

An alteration to the lower edge of a trawl net in which the “mouth” is raised high enough in the water column to prevent it from dragging across the benthos. Raised footropes are obligatory during certain periods of the year in bottom trawling in Massachusetts to reduce the bycatch of non-target demersal species such as flounder (Chosid et al. 2012).


Reflective/colored buoys

Buoys coated with a material to make them reflect or blend into the natural environment so that they are a less conspicuous signal to sea turtles, which are thought to be attracted to buoys used in fishing operations (Werner et al. 2006).


Scent deterrents

The application of substances that produce odors to deter non-target species from entering into a fishing area (Southwood et al. 2008).


Short sets

Reduce the length of longline set to reduce the risk of depredation and bycatch (Tixier et al. 2010).


Side sets

The placement of fishing gear over the side of a longline vessel rather than the stern. Studies have shown that seabirds avoid going after baited hooks near the vessel hull, and by the time the stern passes them they are deeper in the water than they would be in stern sets (Brothers and Gilman, 2006).


Stiff gillnet

Stiffness of the nylon twine is enhanced by adding a different grade of nylon (Bordino et al. 2013).


Sub-surface bait setting

Methods that reduce bycatch by eliminating gear sets at the ocean surface by changing the property of fishing lines so that they are less likely to catch or ensnare animals feeding at the surface or in the mid-water column. These include devices such as setting chutes that place sets below the ocean surface in longline operations where they are less prone to seabird predation, and setting gillnets below the sea surface to reduce entanglement rates of small cetaceans (Hembree and Harwood 1987). They can also be used on longline vessels to increase the speed at which baited lines get below the water’s surface where seabird predation mainly occurs (Robertson et al. 2010). Other examples include low profile line, a kind of rope linking lobster pots that might be suspended deep enough to avoid whale entanglements but with enough floatation to lie above rocky bottoms that tend to abrade them. Weighted mainlines may also increase the sinking ate of pelagic longline gear, making it less likely to capture surface-feeding seabirds (Moreno et al. 2008). Frozen bait can be thawed before it is set in the water to increase the rate at which it sinks in longline fisheries. (The sinking rate can also be increased by puncturing the swim bladder of fish bait) (Gilman et al. 2007).


Sub-surface sets (gillnets)

Methods that reduce bycatch by eliminating gear sets at the ocean surface. These include setting gillnets below the sea surface to reduce entanglement rates of small cetaceans.


Tensioning gillnet

Increasing the overall tension across the surface area of net panels such as by increasing floation on the float line while simultaneously increasing the weight on the lead line (Thorpe et al. 2001).


Thin twine nets

Thin twine monofilament nets are used instead of thick twine nets to reduce the bycatch of species such as marine mammals (Northridge et al. 2003).


Time area closures

Time area closures are used as a tool to reduce the incidental capture of bycatch speices (Kimberly et al. 2000).


Timed release

A digital device that can be programmed to trigger a mechanical arm to move and release gear at a precise pre-set time (hauling ropes and flotation devices) that is being secured at depth until released for hauling.


Trap guards (T-bars, otter guards)

Welded bars, netting or bungee cords placed in pot traps to prevent pinnipeds, small cetaceans, or otters from entering them and preying on the target catch (Noke and Odell 2002). This technique can also be used to reduce the mortality of bycatch species that may become trapped and drown.


Trap-net modification

Traps are modified to reduce the incidental capture of bycatch species. This can be done through measures such as reducing the trap opening size, bent-tunnel openings etc. (Favaro et al. 2012).


Vessel chasing (hazing)

The use of boats to chase non-target marine animals from a fishing area (Werner et al. 2006).


Vessel noise reductions

Structural or operational changes to fishing vessels that would decrease the intensity or signature of their sound output, potentially decreasing the degree to which they attract animals that presumably associate these vessels with a feeding opportunity (Nowacek et al. 2003). At least one study in the Pacific indicated that the noise from longline haulers attracted false killer whales from long distances (J. Watson, pers. comm.).


Visual deterrents

These devices can vary. For example, they could consist of battery-operated lights set at different flicker rates intended to attract fish but not sea turtles (Wang et al. 2007). Other examples include the illumination of gillnets with a light source such as LED lights or chemical lightsticks (Wang et al. 2013). Deploying the shape of a major predator, such as sharks, near fishing gear. Some studies have indicated that shark shapes trigger an escape response in loggerhead sea turtles (Higgins, 2006). Rope consisting of polypropylene blended with a phosphor that glows a bright yellow-green underwater in wavelengths large cetaceans can see is another example. It glows for 48 hours after activation at an intensity a human can see readily at 20 yards (18 m). The design is based on the premise that with increased visibility cetaceans and perhaps turtles would be more likely to avoid rope entanglements at night or at depth. Current research is looking at how to maintain the glowing properties under the rigors of mechanized hauling (CWBR 2006). Opaque mesh netting can be inserted into the upper portion of a gillnet. This material is highly visible to birds and therefore deters them from becoming entangled in the net. This material is also more visible to marine mammals (Melvin and Conquest 1996).


Weak links/ropes

The incorporation of rope attachments manufactured to break at a strength lower than that of the rope used, or otherwise reducing the breaking strength of lines used in fishing.