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).
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.
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.
Recent experimental purse seine bycatch safe handling devices aimed at increasing post-release survival. Encompasses equipment designed to assist with handling larger animals and improve crew safety. Devices include brailer hoppers, sorting grids, bycatch release ramps and conveyor belts, and velcro harnesses.
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.
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.
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.
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.
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.
With sub-surface gillnets the headline of the net is set below a specified depth, usually 2 m in research to date. This method of gear deployment is still in an experimental stage, however, it has been used successfully to reduce sea turtle, shark and cetacean bycatch in Pakistan's tuna gillnet fishery. However, it has not reduced bycatch of deep-dwelling cetaceans in this fishery.
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.
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.
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.
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'.
Deep-set buoy gear is an experimental gear type designed to target swordfish during the day.
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.
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.
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.
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.
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.
'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.
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 . 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.
Research aims to enable tuna fishers to use echosounders and other acoustic equipment to better identify the species, size, and number of tuna and non-tuna at fish aggregating devices (FADs) before they cast their nets, helping to avoid overfishing and reduce bycatch. See FAD design and management
Early experimental work aiming to detect species caught on longlines in real-time and release bycatch species automatically and immediately after capture.
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.