Longlining
Crown copyright reproduced with the permission of Fisheries Research Services Marine Laboratory, Aberdeen, UK
The hook is tied to a branchline (snood) mounted with numerous other branchlines on a long main line, hence the name.
Longlines can be deployed anywhere in the water column - the surface, the mid-water or the bottom - to catch a wide range of species. However, the longline is mainly used to catch larger, high-value species of finfish.
Although the method is rather primitive, a certain level of mechanisation and automation has occurred in many industrialised fisheries. There are special haulers for the main line, arrangements for taking off, mounting and storing the branch lines, arrangements for baiting the hooks, removing fish off and cleaning the hooks from used bait, and so on.
The bait used is often a different species of fish or squid, and the bait normally has to be very fresh in order to attract the catch. Therefore, bait is often a substantial part of the running expenses of a longlining operation.
There have been several attempts to develop artificial bait for longlining. However, there has so far been little success in producing a cost-effective bait recipe, and a media which can carry the smell for a long time while looking attractive and being fastenable to the hook. In some cases small chemical light flares have provided some success in night fishing.
There are two main varieties of long lining used in New Zealand - surface longlining and bottom longlining.
Surface (and midwater) longlining
A surface longlines consists of a main line that can be many kilometres long, supported in the water by a series of floats; the surface long line is not anchored to the seabed.Attached to this main line are branch lines that are each up to 50-75 metres long. Every branch line carries a baited hook, and there can be up to 3000 hooks on a longline.
The line is set as the boat moves forward, at a speed of five to seven knots, with setting taking from two to six hours. Once the line is fully extended, it is then hauled in. Hauling is done at a slower speed, depending on the amount of catch, and the whole process can take up to 12 hours.
When longlines are used close to the water surface, they are set to target large schooling and migrating species such as tuna and bonito, amongst others. These species are normally found in the upper layers of the water and branch lines of up to 50-75 metres are therefore deployed in the upper 100 metres of water by hanging the main line in big bights between buoys. The distance between the branch lines is 10-50 metres.
Bottom longlining
Crown copyright reproduced with the permission of Fisheries Research Services Marine Laboratory, Aberdeen, UK
At one end of the line is an anchor which is dropped to the sea floor. The other end has a weight attached. Depending on the length of the line a series of hauling lines are attached that come to the surface and are marked with buoys. The line is then set from a moving boat, and left for between six and 12 hours, before being hauled in using the surface lines.
When a longline is deployed on the seabed it is targeting different species - whitefish, flatfish, sharks, and skates. Bottom longlines are also used to catch ling and snapper.
The branch lines are much shorter - 1-3 metres - and are mounted more closely together on the main line: the branch lines are usually 1-3 metres apart.
Various aspects of longlining
Longlining is not a method to be used for catching large quantities of fish which means that it is more often used to catch high-value species and larger fish.To do this, it is crucial for the fishers to find a fishing ground where the big fish occur. In addition, some size selectivity can be achieved using different sizes of hooks and bait. Recent investigations into the shape of the hook have also revealed that efficiency can be improved by selecting the right shape for a certain fishery.
When the fish takes the bait underwater, observations have shown that after a short struggle the fish calm down and remain calm until the line is hauled. The result is that fish caught with a longline is regarded as having the very best quality, with no damage or deterioration caused by the fishing gear.
Longlining has very little impact on the environment, and the vessels do not consume a lot of energy while operating longlines.
Unfortunately, longlines have in the past been seen as susceptible to catching sea birds and mammals as by-catch. The birds are caught while they are chasing the bait when the line is being shot. They either take the bait and are drawn down, or the get entangled under the line where it penetrates the surface.
One mitigating technique is to simply set at night when the birds are less likely to be feeding actively. Various mitigating devices have also been developed over the years. One simple system is to hang coloured bands - called tori lines - over the line while shooting. This technique enables the birds to better see the lines. Another option is to prevent exposure of the line to birds by letting it run through a plastic tube which goes from the line reel to the water surface behind the vessel.
Hook size and shape have in recent studies been found to be crucial in reducing incidental by-catch of sea turtles and sea birds.