Mitigation Techniques

    ALDFG - management of abandoned, lost, discarded fishing gear

    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 and environmental damage. Tuna RFMOs have adopted binding measures and data collection protocols, as well as encouraging voluntary measures, to address the issue.

    Species Groups
    Marine Mammals, Sea Turtles, Seabirds, Sharks and Rays
    Fishing Gear
    Longline, Purse Seine, Gillnet

    Artificial bait

    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.

    Species Groups
    Marine Mammals, Sea Turtles, Sharks and Rays
    Fishing Gear
    Longline

    Auditory deterrents and attractors

    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.

    Species Groups
    Marine Mammals, Sea Turtles, Seabirds, Sharks and Rays
    Fishing Gear
    Longline, Purse Seine, Gillnet

    Bait caster

    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.

    Species Groups
    Seabirds
    Fishing Gear
    Longline

    Brickle curtain

    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’.

    Species Groups
    Seabirds
    Fishing Gear
    Longline

    Circle hooks

    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.

    Species Groups
    Marine Mammals, Sea Turtles , Sharks and Rays
    Fishing Gear
    Longline

    Corrodible 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.

    Species Groups
    Marine Mammals, Sea Turtles, Seabirds, Sharks and Rays
    Fishing Gear
    Longline

    Decoys

    Shark decoys have been shown to work as sea turtle 'scarecrows', though these decoys also frightened-off target finfish species (tunas, billfish, mahi-mahi). Trials with 'Looming Eye Buoys', to deter seabirds from gillnets, are underway.

    Species Groups
    Sea Turtles, Seabirds
    Fishing Gear
    Longline, Gillnet

    Deep / shallow setting

    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. Deep-set buoy gear has been trialed recently in a US swordfish fishery, with promising results.

    Species Groups
    Sea Turtles, Sharks and Rays
    Fishing Gear
    Longline

    Depredation Mitigation Device

    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] and a risk of mortality to the animals from either retaliation by fishermen or hooking or entanglement in fishing gear [3]. In the BMIS, ‘Depredation Mitigation Devices (DMDs)’ are principally technologies designed to reduce cetacean bycatch through the use of physical barriers to protect target catch.

    Species Groups
    Marine Mammals, Sharks and Rays
    Fishing Gear
    Longline

    Double-weight branchlines

    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.

    Species Groups
    Seabirds
    Fishing Gear
    Longline

    Dyed bait

    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.

    Species Groups
    Sea Turtles, Seabirds
    Fishing Gear
    Longline

    FAD design & management

    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).

    Species Groups
    Sea Turtles , Sharks and Rays
    Fishing Gear
    Purse Seine

    Gear switching

    Gear switching in a fishery refers to changing from using a relatively high-threat to a relatively low-threat gear type. For example, switching from traditional gillnets to either sub-surface gillnets or longlines to reduce cetacean bycatch in Pakistan's tuna fisheries.

    Species Groups
    Marine Mammals, Sea Turtles, Seabirds, Sharks and Rays
    Fishing Gear
    Longline, Purse Seine, Gillnet

    Haul mitigation

    Seabirds are attracted to longliners during hauling to feed on discards, offal and spent bait. Birds can easily become hooked in the bill, foot or wing, as the line returns to the surface or swallow hooks left in discards or bait. Haul mitigation refers to strategies and devices that can reduce such interactions.

    Species Groups
    Seabirds
    Fishing Gear
    Longline

    Hookpod

    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 (October 2021) recommends that approved hook-shielding devices can be used as stand-alone measures or with other measures, such as bird-scaring lines and night setting.

    Species Groups
    Sea Turtles, Seabirds
    Fishing Gear
    Longline

    Lightsticks / light attractors

    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.

    Species Groups
    Sea Turtles, Seabirds, Sharks and Rays
    Fishing Gear
    Longline

    Line shooter

    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.

    Species Groups
    Seabirds
    Fishing Gear
    Longline

    Monofilament / Wire Leaders

    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.

    Species Groups
    Sea Turtles, Seabirds, Sharks and Rays
    Fishing Gear
    Longline

    Night / day 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.

    Species Groups
    Marine Mammals, Sea Turtles, Seabirds , Sharks and Rays
    Fishing Gear
    Longline, Gillnet

    Safe handling & release

    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.

    Species Groups
    Marine Mammals , Sea Turtles , Seabirds , Sharks and Rays
    Fishing Gear
    Longline, Purse Seine, Gillnet

    Seabird Saver

    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.

    Species Groups
    Seabirds
    Fishing Gear
    Longline, Purse Seine

    Side-setting

    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.

    Species Groups
    Seabirds
    Fishing Gear
    Longline

    Sliding Leads (Lumo Leads)

    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.

    Species Groups
    Seabirds
    Fishing Gear
    Longline

    Smart Tuna Hook

    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'.

    Species Groups
    Sea Turtles, Seabirds
    Fishing Gear
    Longline

    Soak duration

    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.

    Species Groups
    Marine Mammals, Sea Turtles, Sharks and Rays
    Fishing Gear
    Longline

    Spatial & temporal measures

    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, 'move-on' guidelines), or requiring that particular mitigation techniques be adopted in an area. They include fleet or vessel communication schemes, such as the Hawaii-based 'Turtlewatch', a dynamic means of avoiding bycatch 'hotspots'.

    Species Groups
    Marine Mammals , Sea Turtles , Seabirds , Sharks and Rays
    Fishing Gear
    Longline, Purse Seine, Gillnet

    Stealth fishing gear

    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.

    Species Groups
    Sea Turtles, Seabirds
    Fishing Gear
    Longline

    Weak Hooks

    '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.

    Species Groups
    Marine Mammals
    Fishing Gear
    Longline