Effects of a Lifting Bag on Nephrops Selectivity
FISA Project 07/14: Scottish Marine and Freshwater Science Vol 8 No 2
Twin trawl trials were conducted in the West Coast of Scotland to examine the selectivity of Nephrops (Nephrops norvegicus) with regards to codends of the following mesh size and construction: • 80 mm diamond mesh codend of 4 mm single Brezline polyethylene (PE) twine both with and without a lifting bag (here designated 80 mm and 80 mmL respectively). • 100 mm diamond mesh codend of 5 mm double Brezline (PE) twine both with and without a lifting bag (100 mm and 100 mmL). The results can be summarised as follows: • For Nephrops, increasing the mesh size and removing the lifting bag improves the selective performance of the gear and the analysis suggests that the codends can be ranked from least to most selective as follows; 80 mmL, 100 mmL, 80 mm and 100 mm. We are not able to show this in a statistical sense as the side of the twin trawl on which the test codend was fished influenced the number of Nephrops caught and as a result the subsequent analysis was unable to calculate 95% confidence bands. • For whiting (Merlangius merlangus), selectivity improves with increasing mesh size and removal of the lifting bag. There was no side effect. • For haddock (Melanogrammus aeglefinus) only the presence/absence of a lifting bag proved significant. There was no influence of mesh size, which is probably due to the small sizes of haddock caught, and there was no side effect.