Effects of Codend Mesh Size and Twine Number on Nephrops Selectivity. Report on FISA project 03/13 | Scottish Marine and Freshwater Science Volume 6 Number 3

The twin trawl method was used to perform selectivity trails. This is where one net (termed thecontrol) of what is normally a matched pair in a twin-rig setup fishes a small mesh codend whichretains the total population of catch of all species. The other net (the test) differs from thecontrol only in the feature(s) under examination. Here the features under examination are meshsize and number. Catches from the test are compared to catches from the control andexpressed as a proportion retained of the total population. MFV Ocean Trust PD 787 waschartered for the purpose and the trials were undertaken during 11-23 July 2014 on commercialwest coast Nephrops grounds. The target species for all hauls was Nephrops. Certain speciesof commercial whitefish were worked up as available but these were not targeted to theexclusion of Nephrops.

A set of trials were conducted in the West Coast to examine the selectivity of Nephrops (Nephrops norvegicus) with regards to codends with the following mesh size and construction: 80 mm diamond mesh codend of 4 mm single Brezline (PE) twine; 100 mm diamond mesh codend of 5 mm double Brezline (PE) twine; 120 mm diamond mesh codend of 5 mm double Brezline (PE) twine. The results can be summarised as follows: For Nephrops the catch rates of the 3 test gears differed significantly from each other. At a carapace length of 35 mm the 80 mm mesh showed 77% retention, the 100 mm codend showed 61% retention and the 120 mm codend showed 43% retention.For whiting (Merlangius merlangus) the catch rates of the 3 test gears differed significantly from each other with increasing selectivity as mesh size increased. For haddock (Melanogrammus aeglefinus) the selectivity increased as mesh size increased however, that of the 80 mm and 100 mm codends did not differ significantly from each other.