The present section provides a very basic description of gillnets and longlines. Section 7 provides more detail on important parameters and ways of rigging, which are reviewed with respect to their influence on selection and catch rates. A comprehensive description of commercial gear types, the ways of rigging, vessel design and operations procedures can be found in Sainsbury (1996).
A gillnet consists of netting attached between a head-rope and a foot-rope. The net is kept open vertically by the differences in buoyancy between the two ropes. The head-rope is given positive buoyancy by using various floating devices. In shallow waters, floating is typically applied by attachable cork or styrene floats or by using head-ropes where styrene is embedded in the rope. For deep-water fisheries, hollow metal or hard plastic rings are used to provide buoyancy. Weight is most simply applied to the foot-rope by embedding lead into the rope but may also be applied by using various sinkers, e.g. metal rings. By regulating the net's overall buoyancy gillnets may be designed to float (used for pelagic fishes e.g. tuna, salmon or small pelagic fish) or to stand at the bottom (used for demersal species e.g. cod, flatfish). In ordinary gillnets the netting consists of a single net sheet. In trammel nets the net is constructed by joining three parallel sheets of netting where the two outer sheets are made of netting with large mesh and the inner sheet with small mesh.
Gillnets are very size selective: a specific mesh size tends to catch fish of a limited size range. The mesh size may therefore be considered the most important characteristic of a gillnet. Mesh sizes are either given in bar length (i.e. measured from knot-to-knot) or as stretched mesh (i.e. the sum of two bars). The bar length measure is often used by commercial fishermen and by net manufacturers. The stretched mesh measure is normally used in scientific literature and is used in the present work. Mesh sizes are commercially available from about 30 mm (for targeting small pelagic fish) to more than 300 mm (e.g. for targeting tuna, halibut and lumpfish).
Modern gillnets are made of monofilament, multimonofilament or multifilament nylon. Monofilament nets are made by simply using the monofile nylon thread. Combining a number of such monofilaments in parallel makes a multimonofilament. Multifilament consists of thin nylon fibres twisted together.
A net may be rigged with varying degrees of slack, which is primarily regulated by the hanging ratio. The hanging ratio measures how tightly the net is stretched along the head and foot rope. The hanging ratio may theoretically vary between the value 0 (all meshes mounted at the same point on the ropes, so the net has no length dimension) and a value of 1.0 (the netting is fully stretched out, so the net has no height dimension, see Fig. 2.1). In commercial fisheries hanging ratios are normally found between 0.25 and 0.65. Slackness may also be introduced by vertical snoods between the head and foot rope (see Machiels et al. 1994).
Figure 2.1 Various hanging ratios, from left to right: 0, 0.4, 0.67 and 1.0
Gillnets are mainly used for catching bony fishes, but they are also used to catch squid and crustaceans (e.g. Fujimori et al. 1996, Collins and Dunning 1981). In arctic waters gillnets are used for catching seals and cetaceans (Kapel 1975, Heide-Jørgensen 1994).
A longline consists of a groundline (also called mainline) supplied with gangions (also called snoods) carrying baited hooks. The groundline is typically made of various synthetic materials typically of a diameter of 0.5–1 cm, while the gangions are thinner. The length of gangions and their spacing differ considerably depending on the fisheries. The buoyancy of the materials used determines whether the longline is to float at the surface (e.g. when targeting tuna and salmon) or to dwell near the bottom (e.g. when targeting cod or flatfish).
Hooks are manufactured in an extensive numbers of models and sizes. Quinn et al. (1985) remark that the number of models exceeds the number of fish species. Overall one may distinguish between traditional J-shaped hooks and more modern circle shaped hooks introduced in the late 1970s (Fig. 2.2).
Figure 2.2 Shape of main hook types. A traditional J-shaped hook (left) and a circle-shaped hook (right).
The size of hook may be described by linear measures, the shank length and gape width (the distance between the point and the shank, e.g. see Takeuchi and Koike 1969, Cortez-Zaragoza et al. 1989) or by squared distance measures in the form of the product of width and length (e.g. see Ralston 1982, Otway and Craig 1993). The gape width measure, which may conveniently be related to the size of the fish mouth, is used in this work. Commercially used hooks vary considerably in size, ranging in gape-width from less than 0.5 cm (e.g. for targeting freshwater eel) to more than 10 cm in some shark fisheries.
The bait is believed to be the single most important factor determining the catch efficiency of longlines. The bait is a priori selected for its attracting properties (e.g. freshness, fatness) and for its mechanical properties (i.e. ease of baiting and durability in water). Wide variations in types and sizes of bait are used in the various commercial line fisheries.