What is longlining?
Thousands of hooks waiting to be baited and set astern
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Longlining gets its name from the length of the lines that are used. In broad terms a longline consists of a main line to which many branch lines are attached. Each branch line has a baited hook at its end.
Longlines are especially appropriate for catching large, high quality, high value fish such as bluefin and yellowfin tunas, swordfish and Patagonian toothfish.
In the 1980s, longlining became an increasingly popular method of fishing, partly in response to the increasing demand for high-quality, high-value fish for upmarket restaurants, but also following the global ban on drift nets.
Some longliners set lines of over 100km. Attached to it are literally thousands of baited hooks. An estimated 1 billion hooks are set annually by the world's longline fleets.
Depending on the type of longline, buoys, floats and/or weights are used to position the line on the seabed or at the required depth in the water.
There are four basic ways in which longlines are set:
1. Demersal
This is the most commonly used method for bottom fishing. Two systems have been developed, namely single (Fig. 1) and double lines. The single system is simple to operate and consists of one line with weights and buoys at either end, and weights at intervals along the main line which keep it on the seabed. The double or Spanish system is the same but it has a second, safety line. It is used in areas where strong currents and rough floors otherwise cause high gear loss. The safety line is un-weighted and floats above the main line, attached at several points. This allows the main line to be retrieved even if it breaks.
Figure 1. Demersal (bottom set) longline with anchor (a), buoy line (bl), buoys (b) and marker buoys (mb) with weight (w) and flag. Redrawn from Bjordal and Lokkeborg 1996.
2. Semi-pelagic
Three methods are employed. The most widely used method involves lines with buoys and buoy-lines of variable lengths (Fig. 2). Weights are used to keep the line from drifting from its mid-water position The second method is the only "throwaway" gear, with a thin monofilament mainline attached to many floats. All floats are hauled before the line is hauled, resulting in irreparably tangled lines. Only the pollock fishery in Korea uses this. The third method is known as overhauling. Lines are set permanently and each day the line is hauled, fish removed, rebaited and redeployed simultaneously. Small-scale cod fishers off Newfoundland use this.
Figure 2. Semi-pelagic longline with anchor (a), buoy line (bl), buoy (b) and marker buoy (mb) with weight (w) and flag, longline (ll), float (f), dropper (d) and topline (tl). Redrawn from Bjordal and Lokkeborg (1996).
3. Pelagic
Primarily used by tuna and billfish fisheries, the main line is attached at either end to buoys, and floats are placed at regular intervals (Fig. 3). Sometimes the branch lines are weighted, but this method usually relies on the mainline sinking under its own weight to get to the required depth.
Figure 3. Pelagic longlines showing the shape of a regular tuna longline (above) with 6 branch lines (bl) and hooks (h) attached to the main line (ml) between each float (f) and float line (fl), and a deep tuna longline (below) with 13 branch lines between the floats. Redrawn from Suzuki et al (1977).
4. Vertical
Uncommon, it is used mainly off Madeira for black scabbard. It consists of a single line with a float at one end and a weight at the other. Effective on steep shelves.
Both the type of bait and the size (whole or sections) affect the catch. These are changed to target different species as well as to exclude smaller individuals within a target species.
Bait
Many fish species, e.g. mackerel and sardine, as well as squid, shrimp, crab, lobster, octopus, mussels, whelk sand worms are used. Some fisheries use a combination of baits to be more attractive to the target fish.
Usually whole bait are used, but occasionally sections for larger fish, squid and octopus. Baiting machines allow smaller pieces of bait to be used without adding much to the time spent setting the line. This cuts the cost of bait significantly.
Hooks
There exists a huge array of shapes and sizes of hooks. As with bait, different sizes and shapes are used to catch different fish. There is some evidence to suggest that different hooks can affect seabird mortality differently. Smaller hooks can be swallowed whole (and are thus more likely to catch) by big and small birds alike, whereas big hooks can only be swallowed by large birds, excluding smaller species from the bycatch.
Acknowledgements
Much of the information on this page was taken from an excellent publication on the technical aspects of longlining: Bjordal and Lokkeborg 1996. Longlining. Fishing New Books, University Press, Cambridge.