Mesh Size Measurement Revisited
In 1291 Philip IV the Fair, King of France, forbade “de pescher avec engins de file de quoy la maille (n ait) la moule d’un gros tournois d’argent” or, to fish with nets with meshes smaller than the size of a silver coin of that time (Hovart, 1985). This silver coin can be seen as a predecessor of the present-day wedge gauge used to check whether the meshes of fishing nets comply with modern technical regulations.
A mesh gauge developed by C. J. W. Westhoff under the auspices of the ICES Comparative Fishing Committee became the standard gauge for research activities in ICES countries in 1962 (ICES, 1962a) and became known as the ICES gauge (Figure 1). To make a measurement the ICES gauge exerts a fixed longitudinal measuring force on the mesh. The recommended measuring force is 4 kilogramforce (kgf). When the ICES gauge is correctly used, the measurements are free of human influence. Since its introduction the ICES gauge has been generally used in selectivity experiments, to provide scientific advice on minimum regulated mesh sizes. However, since 1962 a wide range of new twines and netting types have been adopted in the fishing industry. These modern twines vary significantly in thickness and stiffness, characteristics which affect both mesh size and selectivity.
For fisheries inspection the legal mesh gauge is the much simpler wedge gauge (Figure 2). The wedge gauge is normally operated by hand force and this makes the measurements liable to human influences. Therefore, a weight or dynamometer is used to control the measuring force in case the measurements are contested. Because this procedure generally yields lower mesh openings than the hand force, it is hardly ever requested by the fishermen.
It is generally acknowledged that the measurements made with the ICES gauge yield lower mesh openings than the wedge gauge used by fisheries inspectors (Ferro and Xu, 1996; Fonteyne et al., 1998). This difference implies that a codend with the legal minimum mesh size, measured with the wedge gauge, will have a lower selectivity than anticipated, since the proposed minimum mesh size was based on experiments carried out with the ICES gauge.
The question as to whether a 4 kgf load is still appropriate to exert sufficient force to stretch the mesh fully lengthwise in modern netting types was first raised during an EU-sponsored Concerted Action Project to evaluate mesh measurement methodologies (Fonteyne et al., 1998). The participants in this project (scientists, fisheries inspectors, fishermen representatives, netting manufacturers) recognized the need to consider the adoption of a standard mesh measurement method for use by enforcement agencies, scientists and the fishing industry. In 1998/1999 the ICES Working Group on Fishing Technology and Fish Behaviour (WGFTFB) established the need to refine mesh measurement methodologies to take account of the wider range of twines and netting types used in the fishing industry since 1962. To deal with this request, ICES adopted Council Resolution 1999/2B02 and established a Study Group on Mesh Measurement Methodology (SGMESH) under the Fisheries Technology Committee.
This ICES Cooperative Research Report reviews relevant definitions and current mesh measurement practices and reviews the activities of SGMESH that ultimately led to the proposals for a new mesh measurement methodology.
Published under the auspices of the following ICES Steering Group or Committee