Inferring fish escape behaviour in trawls based on catch comparison data: Model development and evaluation based on data from Skagerrak, Denmark

During the fishing process, fish react to a trawl with a series of behaviours that often are species and size specific. Thus, a thorough understanding of fish behaviour in relation to fishing gear and a scientific understanding of the ability of different gear designs to utilize or stimulate various behavioural patterns during the catching process are essential for developing more efficient, selective, and environmentally friendly trawls. Although many behavioural studies using optical and acoustic observation systems have been conducted, harsh observation conditions on the fishing grounds often hamper the ability to directly observe fish behaviour in relation to fishing gear. As an alternative to optical and acoustic methods, we developed and applied a new mathematical model to catch data to extract detailed and quantitative information about species- and size-dependent escape behaviour in towed fishing gear such as trawls. We used catch comparison data collected with a twin trawl setup; the only difference between the two trawls was that a 12 m long upper section was replaced with 800 mm diamond meshes in one of them. We investigated the length-based escape behaviour of cod (Gadus morhua), haddock (Melanogrammus aeglefinus), saithe (Pollachius virens), witch flounder (Glyptocephalus cynoglossus), and lemon sole (Microstomus kitt) and quantified the extent to which behavioural responses set limits for the large mesh panel's selective efficiency. Around 85% of saithe, 80% of haddock, 44% of witch flounder, 55% of lemon sole, and 55% of cod (below 68 cm) contacted the large mesh panel and escaped. We also demonstrated the need to account for potential selectivity in the trawl body, as it can bias the assessment of length-based escape behaviour. Our indirect assessment of fish behaviour was in agreement with the direct observations made for the same species in a similar section of the trawl body reported in the literature

Cod-end selectivities of a modified bottom trawl for three fish species in the Aegean Sea

WOS: 000077631600003Recently, the catch per unit effort of conventional bottom otter trawl nets used in the Aegean Sea has decreased and the mean body length of the major species has become smaller. The reason may be partly the poor selectivity of conventional trawl nets. It may be possible to modify trawl net design to improve selection and protect demersal fish stocks. This study presents selectivity results for the modified design. God-end selectivity of red mullet (Mullus barbatus L.), annular sea bream (Diplodus annularis L.) and axillary sea bream (Pagellus acarne Risso) were measured using the hooped covered cod-end method. God-end mesh selectivity experiments were carried out using 36, 40, 44 and 48 mm diamond and square mesh cod-ends both in Turkish and international waters of the Aegean Sea in 1995 on board RN "Egesuf". The cover mesh size was 24 mm. A total of 85 successful tows were made, the raw data were assessed haul by haul and the valid hauls for each cod-end were combined. The selection of cod-ends was found from the linear logistic function and the selection curves and parameters were derived from these combined data by the maximum likelihood method. The square mesh cod-ends (36, 40 and 44 mm) improved selectivity for red mullet. Fifty percent retention length and selection factors were higher for square mesh cod-ends than diamond mesh cod-ends. However, this did not apply to annular sea bream or axillary sea bream. For these species, it was found that there was no significant difference between diamond and square mesh cod-ends and the diamond mesh cod-ends even gave a slightly higher 50% retention length and selection factor than square mesh cod-ends for some mesh sizes. This result may be explained by the different body shapes and behaviour of the individual species. (C) 1998 Elsevier Science B.V. All rights reserved