Fishing nets - Passive fishing gear

Gillnets are used to catch fishes in both freshwater and in marine environments. Gillnets belong to the family of passive fishing gear and their catching success is fully dependent on fish activity. After installation, gillnets create a wall of netting, in which the fish is enmeshed. One of the biggest advantages of gillnets is their wide range of catchable species as well as sizes. Furthermore, it is possible to set gillnets to various habitats. These features of gillnet sampling has made this to be one of the most often used fishing tools for sampling of fish assemblages.

The importance of gillnets was reflected by the European Standard Document (EN 14 757 Water quality - Sampling of fishes with multimesh gillnets). This Document was accepted by EU in 2005 and in 2006 it was also accepted as the Czech National Document (ČSN EN 14 757 Jakost vod - Odběr vzorků ryb tenatními sítěmi). The Document describes the methodology of fish stock assessment in terms of species occurrence, size composition, quantitative relative abundance and biomass expressed to unit of effort (Catch Per Unit of Effort, CPUE) of fish assemblages in lentic waters of the temperate region. This Document further contains a design of gillnets together with the summary of mesh sizes and their thread diameters.

Based on the experience of scientists from the Institute of Hydrobiology, Academy of Sciences of the Czech Republic, the series of mesh sizes given by the Document has been extended for four larger mesh sizes in order to catch bigger individuals of carp, pikeperch, pike and wells. The larger mesh sizes are as follows: 70 mm/0.25 mm; 90 mm/0.25 mm; 110 mm/4 x 0.15 mm a 135 mm/6 x 0.15 mm. The size of the meshes are given from knot to knot; the number behind the slash denotes the thread diameter; the last two mesh sizes are multifilament; the series represent a continuation of the geometric series with the factor app. 1.25, which is commonly used in multimesh series.

Scientists have developed a new design for the installation of gillnets which makes the sampling easier, especially with respect to a large number of samples. This method connects four gillnets together by lines of 30 m in length.

Gillnets can be divided into two basic types - benthic and pelagic gillnets. Benthic gillnets are installed on the bottom and their height is 1.5 m. The lines between four gillnets is made from the non-looping material. Pelagic gillnets are of three kinds - epipelagic gillnets are fully floating and are installed just from the surface. The second kind is semi-floating mesopelagic gillnets that can be installed anywhere in the upper part of the water column thanks to adjustable length of the lines between the float line of the gillnet and the floats. The last kind of pelagic gillnets is represented by bathypelagic gillnets which are installed in the lower part of the water column and the installation depth is determined by the length of lines between the load line and the weights. The line connecting four pelagic gillnets is made from floating non-looping material in the case of the epipelagic gillnets and from semi-floating non-looping material in the case of the mesopelagic and bathypelagic gillnets.

Inspection of the bottom by echosounder should precede installation of gillnets, especially benthic gillnets. The bottom should be flat without roughness and obstacles in order to preclude damage to the net. Where the bottom is a slope, gillnets are set along the contour line (along the shore). Gillnets are not installed along the fall line (along the slope).

Design of gillnet installation in a sampled habitat