Mitigation Techniques

Using sound to discourage or distract bycatch species from interacting with fishing gear. Auditory deterrents are not generally considered useful in reducing bycatch of seabirds, turtles and sharks, except in limited circumstances. In the main, this is because the feasibility and long-term effectiveness of an acoustic deterrent is affected by habituation. Acoustic deterrents (e.g., pingers) are used with some success for marine mammals, in particular, cetaceans.

Artificial baits are an experimental technology in pelagic longline fisheries. Research in the field has been very limited, with mixed success. Although they have the potential to both reduce bycatch and bring other efficiencies to the fishery (e.g. enhanced selectivity, waste reduction, etc.), much work remains to be done before artificial baits are a viable alternative to natural baits.

Small cetaceans may become trapped when purse seiners fish on free-swimming schools of tunas. The backdown procedure and Medina panel are used in combination with skiffs and rafts to create an escape route and shepherd the cetaceans to safety.

A Bait Casting Machine (BCM) is a hydraulically operated device designed to deploy baited hooks during pelagic longline setting (prior to the development of BCMs, individual hooks were cast by hand). BCMs are commonly used in high seas pelagic fisheries and are an integral part of the line setting process.

Bait condition and bait size play a part in bait sink rate. The faster bait sinks, the less available it is to predatory seabirds during line setting. Bait condition also plays a part in the ease of hooking ('baiting') and whether the bait will stay on the hook (quality).

In pelagic longline fisheries, branchlines can be 40 m long. During hauling, each branchline is hauled individually on, or close to, the surface. At this time, birds will attempt to snatch retained bait. The use of a branchline hauler can speed up the hauling process making it more difficult for birds to catch bait.

The Brickle Curtain is a seabird deterrent device used in demersal longline fisheries. This ‘bird curtain’ forms a protective barrier around the hauling hatch. It is composed of vertically hanging streamers supported by poles fixed to the railing above the hauling hatch. See ‘Haul mitigation’.

The use of chemical attractants or deterrents in longline and purse seine fisheries aims to exploit differences in sensory biology between target and non-target species, improving the specificity of fishing and thus reducing bycatch.

Using wide circle hooks has been shown to significantly reduce sea turtle interactions without compromising catch rates of target species. Switching from J hooks to circle hooks may increase shark catch rates but lower at-vessel mortality rates - this is an area of active research.

Corrodible hooks are fishing hooks composed of material other than stainless steel. They may be made from different alloys, with different coatings, which all affect how long they last. The hook may dissolve quickly, within a couple of days, or more slowly over weeks or months. The premise behind the use of corrodible hooks is that they should improve the mortality rate of bycatch released with a hook attached. However, this needs to be tested through tagging studies.

Shark decoys have been shown to work as sea turtle 'scarecrows', though these decoys also frightened-off target finfish species (tunas, billfish, mahi-mahi). Visual deterrents ('looming-eye buoys', 'scarybird') have had some success in keeping seabirds from entanglement in gillnets but further testing is needed.

Deep setting is a longline fishing technique where hooks are set below a critical depth, out of range of most bycatch species, but within the range that target species are usually captured. Deep setting has been shown to decrease bycatch of sea turtles.

Yamazaki double-weight branchlines are a type of line weighting which has been shown to reduce seabird bycatch in pelagic longline fisheries. They are recommended to be used in combination with streamer (tori) lines and night setting. The double-weight configuration is designed to 1) sink pelagic longline hooks beyond the range of seabird attacks within the aerial extent of a tori line during line setting, and 2) reduce injuries to crew should a hook come free due to bite offs or when the line breaks under tension and recoil back at the vessel.

In theory, dyeing bait blue reduces the contrast between the bait and the surrounding seawater making it more difficult for foraging seabirds to detect, or it may be that seabirds are simply less interested in blue-dyed bait compared with undyed controls. Due to practical issues of dyeing bait at-sea and the inconsistent results of experimental trials, this technique is considered unproven and is not recommended.

Fish aggregating devices, or ‘FADs’, are floating objects, either natural or artificial, that attract and aggregate fish, including tuna schools. Silky and oceanic whitetip sharks are the main bycatch species in purse seine fisheries, incidentally caught when vessels fish on drifting FADs or entangled in the netting beneath FADs. Sea turtles are occasionally caught in seines and may be snared in the netting on, or under, FADs. Bycatch can be significantly reduced through effective FAD design (non-entangling, biodegradable) and management (e.g., limit FAD numbers & fishing seasons, shift a percentage of fishing effort to free schools, fish sharks from the net with handlines and release, target tuna schools greater than 10 tonnes, release sharks from deck using safe handling and release techniques).

Using fish bait (such as mackerel and mullet) instead of squid bait has been shown to reduce sea turtle bycatch. However, depending on the species, it may either increase of decrease shark catch rates. Evidence points to fish bait increasing deep hooking for some shark species.

'Gear configuration - other' is a catch-all for changes in the deployment of fishing gear (aimed at reducing bycatch) that are not covered by other mitigation methods listed in this database. Examples include gear-switching and gear modification.

Experimental weighted hook technology aiming to rapidly sink baited hooks.

'Hook shielding device' is a term used to describe mitigation techniques which - via different methods - protect the point and barb of baited hooks from seabird attack during line setting. There is potential for these devices to reduce sea turtle bycatch but this is yet to be examined. See 'Hook Pod' and 'Smart Tuna Hook'.

The Hookpod (Hookpod-LED & Hookpod-mini) protects the point and barb of baited hooks from seabird attack during line setting. Branch line weighting at the hook maximises hook sink rate. When a predetermined depth is reached a pressure release system ensures that the pod opens, releasing the hook to begin fishing. ACAP (2023) recommends that approved hook-shielding devices can be used as stand-alone mitigation measure.

Gillnet illumination has been shown to reduce sea turtle bycatch.

High energy lasers have been used to deter seabirds from interacting with bait, catch or discards. However, they have been shown to be ineffective at deterring seabirds from danger areas around fishing vessels and likely damage seabird visual systems, with negative effects on foraging behaviour (ACAP 2023).

Light attractors, including chemical lightsticks and battery-powered light-emitting diodes (LEDs), are attached near baited hooks on branchlines to attract fish. They also appear to attract sea turtles and sharks.  Strategies are needed to make longline lightsticks less attractive or visible to these species groups. Research is limited but has considered factors such as light colour,  hook depth and fishing according to lunar phase.

Another important consideration is the contribution of lost or discarded chemical lightsticks to marine pollution, both the plastic and the toxic contents.

A line shooter is a hydraulically operated device designed to deploy the mainline at a speed faster than the vessel’s forward motion, which removes tension from the longline. This allows the mainline to enter the water immediately astern of the vessel, rather than up to 30 m behind the vessel. It has been demonstrated that variation in tension on the mainline will affect the sink rates of baited hooks and therefore the risks to seabirds. Line shooters are not recommended as a mitigation technique.

Longlines are weighted to get baited hooks rapidly out of the range of feeding seabirds. Line weighting is a primary mitigation measure and a key component in all successful reductions in seabird bycatch in pelagic longline fisheries. Line weighting should be used in conjunction with streamer lines and night-setting.

The use of permanent magnets, electropositive rare earth metals (EPREM) and other electrical measures has been trialled as a means of deterring sharks from approaching baited hooks.

Seabirds are attracted to fishing vessels to feed on processing waste and discarded fish. Discharging offal on the opposite side to the hauling hatch helps to divert their attention away from the area where hooks return to the surface.

Monofilament (nylon) line is used widely in the fishing industry. It is commonly used for both the mainline (the longline) and branchlines (which hang off the main longline and are also known as snoods or gangions/ganglions). Branchlines may incorporate a section of line (of variable length) known as a leader, with a lead weight at one end and the baited hook at the other. Wire leaders have implications for sharks and seabirds.

Night or day setting refers to the times of day when longliners set, soak and haul their lines. These variables are inherently linked to the duration of the soak (the period that the longline is in the water). Timing depends principally on the target species, but also varies among fleets and regions.

Safe handling and release refers to using best practice methods for dealing with bycatch species, to maximise their chances of survival after interacting with fishing gear. It can also include vessel manoeuvring to avoid taking bycatch species, for example, avoiding setting purse seines on whale sharks. Illustrated Guides and more general literature have been collected in the BMIS.

Side-setting requires the longline hook setting operation to move to the side of the vessel. Birds are unable or unwilling to forage for bait close to the side of a vessel. Additionally, side-setting avoids setting baited hooks into the propeller wash, which slows the sink rate of stern set hooks. It has limited proven efficacy in reducing seabird bycatch (in Hawaiian tuna and swordfish fisheries) and must be used in combination with line weighting and streamer lines.

Sliding Leads are an alternative to leaded swivels. They are designed to increase branch line sink rates (to get baited hooks rapidly out of the range of feeding seabirds) and protect crew safety. Sliding Leads slide away from crew during bite offs or when the line breaks under tension, thereby greatly reducing the incidence of dangerous fly-backs towards the vessel, as can occur with leaded swivels.

The Smart Tuna Hook system prevents hooking of seabirds and turtles during line setting by protecting a baited hook with a metal shield, which is held in place with a biodegradable pin. The pin dissolves once the hook is below the feeding depth of seabirds (25 m) and turtles (100 m). Once the pin dissolves, the shield is released and the baited hook is ready for fishing. It is considered a best practice mitigation measure (ACAP 2023).

Note: The similarly named 'Smart Hook' is a hook designed to deter sharks from approaching longline baits; see 'Magnetic, E+ metals and Electrical deterrents'.

Soak duration is the length of time that pelagic longlines remain in the water, between line setting and line hauling. Average soak time varies among fisheries and is dependent on factors such as the target species, number of hooks deployed and the time required to bring them aboard.

Stealth fishing gear refers to fishing gear and bait that have been camouflaged to deceive predatory species. For bycatch species such as marine turtles and seabirds, the aim is to reduce the detection of bait. For target species, such as swordfish and tunas, the aim is to reduce the detection of the fishing gear and thus increase catch rates.

Underwater setting techniques are means of deploying baited hooks below the surface of the sea, out of the sight and reach of foraging seabirds. The 'Underwater Bait Setter (Skadia Technologies)' is a recommended mitigation method, used in combination with best practice line weighting.

Bycatch rates can vary among fishing vessels operating in the same fishery, even after accounting for factors such as vessel size and fishing effort, and these differences can persist from year to year.

'Visual alerts for gillnets - other' captures means of improving the visibility of nets, other than by acoustic or illumination methods. Examples include net colour, reflective net elements, high contrast and patterned nets.

Some vessels have experimented with water cannons or fire hoses to deter birds from approaching the hauling station.

'Weak hooks' refers to weak circle hooks, constructed of thinner gauge stock wire than standard circle hooks of the same size, and designed to straighten at a lower strain (pull) level than standard hooks, thereby allowing large hooked animals to escape while retaining the target catch.