Drift gillnets (or driftnets), in common with other types of gillnet, are among the simplest and oldest methods of fishing. Such nets operate by gilling or entangling fish in the meshes of a sheet of netting. The netting is held more or less vertically in the water column by means of a buoyant floatline at the top of the net, and a weighted leadline at the bottom of the net. Nets rigged in this way may be used singly or, by joining them together serially, as a fleet of nets. Usually the floatline floats at the surface of the water, and the net hangs below it. Sometimes the floatline may be rigged in such a way that it floats some distance below the surface. The net then fishes a lower part of the water column. In such cases the net is partly suspended by additional buoys at the surface which are attached to the floatline at regular intervals. In some sandy bottomed areas driftnets are even rigged to fish along the bottom of the seabed, for prawns especially.
The net is left to fish passively, fish being caught when they swim into it and the meshes of the net become caught behind their gills. Driftnets may be attached at one end to the boat which is fishing them, or they may be left to drift free of the boat and are recovered later.
Driftnets are only one type of gill net. Set gill nets (‘set nets’, ‘sink nets’, ‘anchored gill nets’ or ‘fixed nets’) are also widely used in coastal waters and differ from drift gill nets in that they are fixed by means of anchors or stakes to prevent them from moving with the water. Driftnets, in contrast, are allowed to drift with the water currents or the wind.
Driftnets are usually fished at night, as the meshes of the net are less visible to the fish. The type of material used in making the nets is also important in controlling the visibility of the net. In recent years monofilament nylon, which is a clear plastic, has been widely used, largely because it is usually more difficult to see, and is therefore more efficient at catching fish.
Multifilament twines are also used in many driftnet fisheries, and have the advantage that they are less rigid than monofilament, which means that once a fish is gilled, it is less likely to be able to escape. They are also less bulky and easier to handle than monofilaments. To overcome their generally greater visibility underwater, multifilament yarns are coloured in accordance with the environment in which they are being used.
On occasion, fish may not be caught simply by being gilled, but may rather become entangled in several meshes of the net. Sometimes nets may be rigged so that the sheet of netting is fixed to the floatline and leadline with plenty of slack to encourage entanglement; this is particularly so when awkwardly shaped fish are targeted. The degree to which a net entangles rather than gills is determined by the ‘hanging ratio’ of the net. This is the ratio of the length of the net when rigged, to the fully stretched length of the net (where the meshes are all closed). Typically for a gill net this ratio might be between 0.5 and 0.7, for example, which results in more or less rectangular mesh openings. The lower the hanging ratio is than this, the more the meshes of the net are distorted from a rectangular shape, and the more likely they are to entangle. If the hanging ratio is low enough the net may more correctly be described as a tangle net.
The vast majority of vessels using driftnets in the world are small boats, often used to supply family or village needs for pelagic fish. Often such gears have been used for generations, and the only modern improvement may be the use of nylon twines rather than cotton or hemp or other vegetable material. Nylon is, weight for weight, stronger than the natural fibres which were used traditionally. The greater strength of nylon compared with cotton or hemp for example, has meant that thinner twines can be used to catch fish of a given size, and this has improved the fishing power of the nets. Nylon is very often cheaper than a traditional alternative (this is not the case everywhere) and this may have lead to an increase in the numbers of small vessels using driftnets. Furthermore, nylon is more durable than traditional materials, which also makes it more economical to use.
The origins of gill net fishing cannot be traced with certainty, but it is clear that gill nets of one form or another have been in continuous use around the world for millennia. The principles and practice of gill netting are both so simple that it seems plausible that gill nets may well have been developed independently in different parts of the world. Certainly, no one nation or region can claim unambiguous credit for having initiated their use.
It is clear, for example, that herring driftnet fishing was established around much of the North Sea by the 11th and 12th centuries, and this was to become one of the most economically important of all gill net fisheries. The fishery was expanded in the 16th and 17th centuries by the Dutch who were the first, in Europe at least, to develop large industrial vessels for driftnetting in the open sea, processing their catch on board.
In 1560 there were 1000 Dutch vessels driftnetting in the North Sea, and 2000 by 1620. Many of these were in excess of 15 metres in length, with a crew of 10–15 and storage capacity of as much as a million fish (Butcher 1979). The North Sea herring driftnet fishery expanded further during the 18th and 19th centuries, so that by 1908 it was estimated that more than half a million tonnes of herring were being taken annually by the driftnet fleets (Samuel 1918). In the autumn of 1913 there were over 1700 drifters operating from just two English ports, each deploying around 3km of netting, 11 metres in depth, in the southern North Sea every night.
At this time driftnets were widely recognised as being an efficient and selective type of gear. They were targeted on schooling pelagic fish which were found in very dense concentrations at certain times of year, and it was recognised that the meshes of the driftnet allowed smaller fish to swim through and escape. Towards the end of the 19th century, however, steam trawlers became popular, and this was, at the time, a considerable cause for concern. At the Great International Fisheries Exhibition in London in 1884, several speakers denounced the wasteful and unselective nature of the recently introduced trawlers.
Subsequently, of course, trawlers and later purse seiners, out-competed the driftnet fisheries, as densely schooling fish can be caught more quickly and efficiently with a trawl or a purse seine. For several decades use of driftnets declined, not just in the North Sea but around the world, as they were superseded by more efficient means of harvesting fish. During the past two decades, however, driftnets and gill nets more generally, have undergone a revival in their use, especially in traditional and modern small scale fisheries.
The present advantages of gill nets are easy to see. They are relatively cheap and easy to use, principally because relatively low powered vessels can be used to deploy them, which makes them fuel efficient. It is ironic, however, that the very gear which was, a hundred years ago, seen as environmentally benign, has in the later part of the 20th century, aroused so much controversy. In the 1980's and 90's it has been the driftnets which have been criticised as both wasteful and unselective, as trawlers were before.
Much of the present controversy surrounding driftnet fisheries derives from the expansion of Japanese and Taiwanese driftnet fisheries for tunas into the South Pacific. The increase in driftnet fishing in this area in the late 1980's was seen by the coastal states of the region as a threat to the long term sustainability of some of the important local fish resources (particularly albacore), as well as representing a serious threat to certain non-target species which were being caught. This situation led to the nations of the South Pacific Forum adopting first a declaration (Tarawa, Kirbati, July 1989) and then a Convention (Wellington, New Zealand, November 1990) banning the use of long driftnets. The issue has also been discussed in numerous fishery and other fora, most notably at the United Nations General Assembly, where a resolution (44/225) on the matter was adopted in December 1989.
The major criticisms of driftnet fisheries can be divided broadly into two main categories. Firstly, there have been several competitive conflicts between driftnet and other fisheries. The efficiency of driftnets in catching fish, particularly when targeted on species which are found at low densities, has meant that they have out-competed vessels using some other types of gear, by catching more fish at lower cost. The fact that driftnets can economically exploit fish at relatively low densities also leads to the fear that an uncontrolled increase in fishing effort by this method may lead to sudden depletion of target stocks (hence the alarm caused by the sudden expansion of effort by this fishing method in the South Pacific).
The fact that a proportion of fish caught in gill nets will fall from the net before being hauled on board, and a further proportion will escape from the net injured, has also lead to the charge from competing fisheries that driftnet fisheries are wasteful of fishery resources.
In addition to these conflicts, driftnets often catch species which are valued by other fisheries, and which are not the main target of the driftnet fishery. Salmon are particularly susceptible to capture in this way. The capture of such high value species in driftnets not primarily targeting them has provoked some strong criticisms. In many areas regulations have been imposed to limit the extent of conflicts such as these. In several parts of the North Atlantic, for example, driftnets have been banned altogether, often to prevent the capture of salmonids.
Other inter-fishery conflicts are reported less frequently. Taiwanese squid jiggers, for example, have reported driftnets being set to encircle their vessels at night, when their powerful lights are attracting squid, so that part of their catch may be intercepted. Driftnets may also physically restrict the use of other surface fishing gears in some areas, and are reported to impede the passage of other vessels, including merchant shipping and pleasure craft.
A second major criticism of driftnet fisheries is that they may have a detrimental impact on non-commercial species and the marine environment. This criticism has arisen from the observation that in many driftnet fisheries a wide variety of non-target species are caught in addition to those targeted.
Driftnet fisheries are by no means alone in catching non-target species. The widely practised trawl fisheries for tropical shrimps, for example, cause considerable mortality to juvenile forms of important food fishes and have a fish by-catch, most of which is discarded, of five to ten times the shrimp catch. In the Mediterranean, longline fisheries take very large numbers of turtles, and this may also be having a detrimental impact on local populations. In the extreme, destructive fishing methods such as dynamiting and poisoning, which are practised despite prohibition, may kill everything in the area in which they are used.
The impact of driftnet fisheries on non-target species should therefore be seen in the context of a wider impact which fisheries have on the marine environment. Nevertheless, the fact that driftnets operate generally near to the air-surface interface makes them especially likely to catch air breathing animals, and some of these species, such as the mammals, are long-lived, slow growing and vulnerable to population depletion through relatively low levels of removal. It has been suggested that high seas driftnet fisheries may also pose a threat to the oceanic pelagic ecosystem, which is relatively species-poor.
The resistance of nylon to decay, and the large amounts of netting which are deployed in some driftnet fisheries, has lead to a further fear that segments of netting which are inevitably lost during fishing, or which are deliberately discarded, may continue to fish for an indefinite period of time, or at least to be capable of entangling birds and marine mammals near the surface.
Although most of these criticisms might be levelled at any or all driftnet fisheries, it is the so-called ‘large scale’ or industrial high seas fisheries which have in fact been most heavily criticised.
Large scale driftnet fisheries have been defined by a number of governments and other organisations to include, for example, only those using nets of more than a certain length, such as 2.5 km (Wellington Convention) or only those fisheries conducted outside EEZ's. Defining fisheries in these terms can, however, be misleading; there are several ‘small scale’ fisheries which use nets of more than 2.5 km for example. Similarly there are several large scale operations which are or were conducted inside EEZ's and some ‘small scale’ fisheries fishing outside EEZ's. The difference between large scale and small scale driftnet fisheries is simply one of degree.
From the management perspective, however, there is clearly a distinction to be made between those fisheries operating within national jurisdiction and those operating in international waters. The expansion of distant water fisheries in international waters which has occurred in the past decade or two has largely been caused by increased fishing effort in coastal waters, which has lead both to decreased catch rates in some areas, and to the adoption of 200 mile EEZ's by most coastal states. Competition for resources within EEZ's must be seen as one of the major reasons for increased fishing pressure on high seas resources, largely due to the fleets of industrial nations.
At present some 90% of the world's exploited marine fishery resources come under the jurisdiction of coastal states. The remaining 10%, however, are still available as an open access resource, which presents problems of regulation, control and enforcement. According to the provisions of the 1982 United Nations Convention on the Law of the Sea, high seas fishing states are under an obligation to conserve the living resources of the high seas, to co-operate with other states on taking conservation measures and to generate and contribute scientific information. With increasing competition for fish resources, there is a need to conclude agreements and conventions regarding the use of high seas stocks. Only a few such conventions exist so far.
The large scale driftnet fisheries which have generated much of the recent controversy over this fishing method should therefore also be seen in the wider context of a world fisheries, in which high seas resources are the subject of increasing competition, generated by, among other things, an overall increasing demand for fish worldwide, and the exclusion of distant water fleets from the EEZ's of coastal states.
There are therefore, a wide range of concerns which surround the issue of driftnet fishing. This report reviews the state of driftnet fisheries around the world and concentrates in particular on the larger scale driftnet fisheries. In it, an attempt has been made to assemble what is known about the scale of driftnet operations, including their catches of target species, and to describe the extent to which they catch or ensnare species of animal which they are not targeted on. Each of the world's main oceans areas is examined in turn, and in the light of what is found, some of the criticisms listed above are re-examined, and in particular the likely and possible impacts of this fishing method on non-target species are reviewed.