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Mitigation Techniques

A bycatch mitigation technique (MT) is a method that fishers or fishery managers can useto reduce catch of non-target species. It may involve a change in fishing practice, such as reducing longline soak time and avoiding protected species 'hot spots', or a change in gear, such as using circle hooks instead of J hooks. BMIS descriptions of MT summarize their performance and refinement in experimental situations...more...

A streamer line (also called a tori or bird scaring line) is a line with streamers that is towed from a high point near the stern of a vessel as baited hooks are deployed. They are the most commonly prescribed seabird bycatch mitigation measure for longline fisheries. However, recent evidence shows that they are not fully effective unless combined with other mitigation measures, i.e., branchline weighting and night setting.

Spatial and temporal measures aim to avoid or minimise bycatch by either temporarily or permanently moving fishing out of an area (e.g., time and area closures, marine protected areas), or requiring that particular mitigation techniques be adopted in an area. They are predominantly mandatory (i.e., fisheries regulations) but can be voluntary and the areas that they apply to may be dynamic, e.g., around an ocean front, or static, e.g., around a seamount or below a specified latitude.

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.

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.

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.

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.

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. Leaders made of wire have implications for sharks and seabirds.

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. In early trials at sea, deep setting was shown to decrease bycatch of sea turtles. Recent research challenges this finding.

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.

Management of abandoned, lost, discarded fishing gear (ALDFG) refers to the management of fishing gear - onboard and deployed, e.g., FADs - and retrieval of lost gear as a means of reducing fisheries bycatch. In recent years, tuna RFMOs, to varying degrees, have adopted binding measures and data collection protocols to address the issue.

In theory, dyeing bait blue reduces the contrast between the bait and the surrounding seawater making it more difficult for foraging seabirds to detect. Alternative theories suggest that seabirds are simply less interested in blue-dyed bait compared with undyed controls. Practical issues of dyeing bait at-sea and the inconsistent results of experimental trials suggest that blue-dyed bait is not an appropriate primary mitigation measure.

Underwater setting techniques are means of deploying baited hooks below the surface of the sea, out of the sight and reach of foraging seabirds. They are unproven and not recommended as a mitigation method [1].

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.

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.

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.

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

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.

The Brickle Curtain is a deterrent device that 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.

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.

The hook pod 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.

Double-weight branchlines are designed to reduce seabird bycatch in pelagic longline fisheries when used in combination with tori lines and in some cases 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 while under tension in the landing process and recoil back at the vessel.

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.

'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.

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; strategies are needed to make them less attractive or invisible to turtles. Gillnet illumination has been shown to reduce both seabird and sea turtle bycatch.

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.

Using sound to discourage or distract bycatch species from interacting with fishing gear. Auditory deterrents are not generally considered useful, except in limited circumstances, in reducing bycatch of seabirds, turtles and sharks. In the main, this is because the feasibility and long-term effectiveness of an acoustic deterrent is affected by habituation.

The Seabird Saver is a recently developed technology combining a laser and an optional acoustic deterrent. It has been designed for longline, purse seine and trawl fisheries, among others, with the aim of scaring many different seabird species from interacting with bait, catch or discards. ACAP regards lasers as unproven & not recommended, with concerns about safety for both seabirds & humans.

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.

Depredation - the removal of bait and damage of hooked fish by sharks and cetaceans - is a major issue for pelagic longline fleets, with a negative impact on their economic profitability [1,2]. In the BMIS, ‘Depredation Mitigation Devices (DMDs)’ are principally technologies designed to reduce cetacean bycatch through the use of physical barriers.

'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'.

Shark decoys have been shown to work as sea turtle 'scarecrows', though these decoys also frightened-off target finfish species (tunas, billfish, mahi-mahi). However, there remains potential to develop decoys which maintain target species catch rates while deterring sea turtles from approaching baited longline hooks.

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.

'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. They are generally not well studied, for example, decreasing the number of hooks between floats to decrease shark catch rates.

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.

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.

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).

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.