It has long been known that natural floating objects, such as logs, palm leaves or dead animals, attract fish. Fish Aggregating Devices (FADs), made of either natural or man-made materials, are deployed to exploit this phenomenon and help catch pelagic fish. In fisheries management literature, the term 'FAD' often encompasses both man-made, deployed FADs and naturally occurring flotsam .
FADS may be anchored or drifting. Anchored FADs are attached to the sea bottom using a weight such as a concrete block. A rope made of floating synthetics such as polypropylene attaches to the mooring and in turn attaches to a buoy. The buoy can float at the surface or lie below the surface to avoid detection and hazards such as weather and ship traffic. Fishers commonly equip drifting FADs with satellite (GPS) buoys so that they can be tracked remotely . Some FADs include sonar capabilities as well, allowing the operator to remotely estimate the aggregation underneath. This improves fishing efficiency, helping fishers to decide where and when to fish to maximise catch during a trip .
It is important to distinguish between two categories of FADs, i.e., 'oceanic' and 'coastal' FADs or, alternatively, 'industrial' and 'artisanal' FADs .
Oceanic (Industrial) FADs
Oceanic FADs are mostly drifting (d-FADs), set far from shore and predominantly used by large-scale, industrial vessels fishing for tuna, usually with purse-seine nets . Pole-and-line and troll vessels can also benefit from oceanic FADs (in the WCPO, oceanic FADs are regularly fished by pole-and-line vessels operating in the region). Purse seiners also fish on free schools of fish but fishing on FADs is very efficient; generally more tuna are caught more quickly on a given trip and sets on FADs are almost always successful while sets on free schools have around a 50 percent chance of success .
Coastal (Artisanal) FADs
Coastal FADs are usually anchored or tethered (t-FADs) within range of small motor boats and canoes . They are used to improve the food security of people who catch fish to feed their families or sell in small amounts at local markets, as well as for domestic fisheries development. They may also be used for recreational and game fishing . The fishing methods used around coastal FADs, such as handline and trolling, select only species that the fishermen want and only a small proportion of the fish around the FAD are caught.
This mitigation method description is concerned with oceanic (industrial) FADs.
Sea turtles are occasionally caught in purse seines in tuna fisheries (from a few tens up to a couple of hundred individuals per year) [8,20,24]. Most interactions (over 70 percent) occur when turtles associate with floating objects, which offer the turtle diverse prey items and some protection, and are captured when the object is encircled . Most (more than 90 percent) are released alive . However, sea turtles become entangled in the webbing that fishers attach to the top and underside of FAD rafts to increase their attractiveness and/or visibility [8,10]. While the number of marine turtles entangled by FADs is likely to be small, it may still be a significant source of mortality for these animals, which are characterised by small populations with low biological productivity .
In the case of sharks, recent research suggests that mortality due to entanglement in FAD webbing is significant. For example, Filmalter et al. (2012) estimated entanglement mortality of silky sharks, Carcharhinus falciformis, in the Indian Ocean at 480 000-960 000 individuals, 5-10 times that of the known bycatch of this species from the region’s purse seine fleet. Using non-entangling FADs is the straightforward solution to this problem (see below).
As intimated above, purse seining on FADs is responsible for major catches of sharks [4,6]. Catch rates of sharks are higher on FADs than on unassociated sets. In an analysis of purse seine data in the WCPO it was found that catches of sharks, in particular silky and oceanic whitetip sharks, were much higher from sets around floating objects (drifting and anchored FADs, logs and animals) than on unassociated sets . Amande et al. (2008) found that FAD-associated fishing sets caught substantially more silky sharks compared with free schools. However, catches were highly variable due to local oceanographic conditions, prey availability, differences in the tuna school composition or size, or other factors. Kondel and Rusin (2007) point out that as the stock status of silky and oceanic whitetip sharks is largely unknown, the import of the observed levels of bycatch is difficult to assess. For sharks such as these however, where reported interactions are relatively high and biological productivity is low, current levels of interaction with the fishery may be resulting in detrimental impacts upon their populations .
In recent years a number of different FAD design and management strategies aimed at reducing bycatch of turtles, sharks and rays have been proposed and evaluated, with some promising outcomes . These strategies are listed below and discussed.
Techniques to identify and mitigate bycatch before arriving at a FAD
- Instrumented buoys
- Non-Entangling FADs
Techniques to avoid bycatch before setting a net
- Pre-estimation of catch (e.g., composition by species, size, quantity)
- Animal behaviour (e.g., temporal/spatial segregation, excursions)
- Double FADs
- Attraction of sharks, non-target fish away from FADs or net
Instrumented buoys and pre-estimation of catch
Technology is still advancing but the hope is that sonar can be improved such that it enables fishers to determine the age class and quantities of tuna, sharks or other species under a FAD before reaching the fishing zone. Recent research has sought to examine the many issues related to catch and bycatch estimation which span the period from before the vessel reaches the FAD to when the catch is unloaded in port . Work is underway to determine whether catch estimation skills can be documented in a form that is transferrable between vessels (i.e., development of best practices) and whether particular instruments or equipment can improve pre-set estimates, in order to avoid undesirable catch .
Non-entangling or ecological (Eco) FADs
As described above, sea turtles and sharks become entangled in the webbing - typically surplus purse seine fishing net - used in FAD construction. A number of research projects have investigated alternative FAD designs that reduce incidental entanglement and catch of sharks and sea turtles, while maintaining the FADs' ability to aggregate fish [7,15]. These have demonstrated that to reduce entanglement of turtles on the FAD itself, the surface structure should not be covered, or only covered with non-meshed material . Where the surface structure is covered then a log-shaped (i.e. cylindrical) or spherical floating object will deter turtles from climbing onto the FAD .
If a sub-surface component is used, it should not be made from netting but from non-meshed materials such as ropes or canvas sheets . Entanglement can also be substantially reduced by other methods such as tightly wrapping the hanging nets with ropes, resulting in a tight cylinder, however, these eventually unfurl and become an entanglement risk. More detail on the construction of non-entangling and eco FADs can be found in the following (the ISSF guidebooks are also available in other languages at their website):
The recommendations include advice on the use of biodegradable materials (such as jute, palm leaves, and bamboo), which help to avoid ghost fishing [4,13,19] and to reduce the amount of synthetic marine debris.
The behaviour of target and non-target species on drifting FADs and when encircled by purse seine gear has been explored to determine whether it is possible to isolate and release, in good condition, sharks and other non-target species [21,24]. Research in the Indian Ocean has shown that silky sharks and tunas exhibit very similar temporal patterns: all make excursions away from FADs at similar times (usually during the night) . This suggests that adjusting fishing time to reduce shark catch while maintaining tuna catch is not an effective solution .
The term ‘double FADs’ refers to a FAD with two vertically or horizontally separated sections to split the aggregation underneath. Preliminary experiments suggest that most species tend to select one FAD, and that it is not always the same FAD that gathers all species. Further experiments were recommended .
Attraction of sharks and non-target fish away from FADs or net
The potential for using audio, visual and chemical stimuli to lead or repulse sharks away from from FADs prior to or after setting nets has been of recent interest to researchers [11,25]. In one pilot study, it was shown that sharks can be attracted hundreds of meters away from FADs by towing a bag of chum (bait) away from the FAD . However, reactions of sharks varied greatly, with some showing no reaction and others following the chum for up to 500m . Many factors were thought to be at play, including whether the FAD had been fished recently, the size of the multi-species aggregation and feeding motivation . It was apparent that the FAD always remains a very strong attraction stimulus. Further experiments looked at attracting sharks already encircled by the purse seine net by placing chum on the FAD and towing it out . Passive drifts were found to better at attracting sharks than when the FAD was towed, however slowly .
These studies have led to investigations of other methods to avoid encircling sharks or to release sharks from the net . The use of escape panels (a large window in the net that can be opened or closed) to release sharks from the net is being tested . In addition, the "backdown manoeuvre" used to release dolphins in tuna-dolphin aggregations in the eastern Pacific is being tested for efficacy in releasing sharks and other bycatch from the net .
Other management options
As yet, there is no comprehensive system or database in place that tracks the number of FADs, FAD types or their location. However, tuna Regional Fisheries Management Organisations (tRFMOs) are beginning to recognize the value of this information for managing fish stocks; several have implemented rules for the collection of data on FAD usage . Furthermore, some RFMOs have established working groups to assess the use of FADs in tropical tuna fisheries, developments in FAD technology, the collection of additional FAD data, FAD marking and identification, and FAD monitoring, tracking, and control . The working groups will identify management options.
FAD management for bycatch reduction can incorporate spatial and temporal measures, for example, avoidance of agreed zones for agreed periods as improved data on fisheries-bycatch interactions becomes available. Measures of this type are already in place for tuna [e.g. 26,27].
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