Can codend selectivity of Nephrops be explained by morphology?

1. Improving codend selectivity Based on the findings in this study, selectivity in other codends can be predicted. Selectivity of Nephrops was documented to take place in the entire length of the codend and the relatively closed meshes in the forward part of a diamond mesh codend are therefore assumed to reduce L50 (length at 50% retention) and increase SR (L75 -L25). Simulations were run in which a fraction of the closed meshes were replaced with more open meshes. The range of opening angles (oa’s) in the codend was thus reduced and the output demonstrate that this has a positive effect on selectivity of the codend (Fig. 4). Means to obtain such mesh configurations in a diamond mesh codend could be shortened lastridge ropes which reduce tension in the mesh bars. ICES FTFB 2009, Ancona, Italy • Simulating codend selectivity • Selectivity of each of the three contact modes is assumed to contribute to the resulting selection curve of the codend. To estimate the relative contributions of the different contact modes, a high number of stochastic simulations are run. In each run, the contribution of each of the different modes vary randomly as they are assigned with a combination of auto-generated weighting factors. The output of the simulations is an equally high number of proposed selection curves that are ranked according to their similarity with an experimentally obtained selection curve from a 68 mm square mesh codend (Fig. 2). The combination of weighting factors that are found to be most accurate in reproducing the experimental selection curves, is assumed to reflect the selection process in the specific trawl codend. Fig. 3. Retention data for a 90 mm diamond mesh codend obtained experimentally (blue diamonds) and simulated (red triangles). Meshes with opening angles below 15 degrees were found to explain 83.6 % of the simulated selectivity. Fig. 2. Retention data for a 68 mm square mesh codend obtained experimentally (blue squares) and simulated (red triangles). The optimal contact mode is found to explain 87.5 % of the simulated selectivity. Fig. 4. Prediction of retention data in a 90 mm diamond mesh codend. Best fit of the simulated data as shown in Fig. 3 (red triangles) is used as a base line. The range of opening angles is reduced by removing some of the more closed meshes and attributing all the remaining meshes with equal weighting. The result is an increasing steepness of the curve. • It is however also assumed that the combination of contact modes found to explain selectivity in one codend can be transferred to other types of codend i.e. the contact modes are independent of the mesh shapes in the codend. This assumption allows us to simulate selectivity in a 90 mm diamond mesh codend for which we also have experimental data (Fig. 3). Can codend selectivity of Nephrops be explained by morphology? Rikke Petri Frandsen*, Bent Herrmann*1 and Niels Madsen *Authorship equal. 1 Corresponding author: bhe@aqua.dtu.dk • Selectivity of Nephrops in trawl codends is in general poor with resulting high discard rates and / or loss of legal sized catch. In the present study, the FISHSELECT* methodology has been used to attain a profound understanding of the selection process of the species in order to identify means to improve the selectivity • * FISHSELECT is described in: Herrmann, B., Krag, L.A., Frandsen, R.P., Madsen, N., Lundgren, B., Stæhr, K.-J., 2009. Prediction of selectivity from morphological conditions: Methodology and a case study on cod (Gadus morhua). Fisheries Research 97, 59-71 • Different contact modes • Due to the low swimming speed of Nephrops, their orientation when encountering the netting is assumed to be more or less random. Three modes of contact between the Nephrops and the netting were found to reflect the entire range of selection curves estimated in a covered codend experiment. Measurement of cross-section parameters were performed for 70 individuals and their ability to penetrate 160 different meshes was investigated (Fig. 1). This yielded more than 40,000 results which were the foundation of this study. Depending on the contact mode, 98.2 to 99.7 % of the mesh penetration results could be reproduced from the geometric description of the corresponding cross-section. • Based on these results, the FISHSELECT software was used to simulate mesh penetration from the measured cross-sections and subsequently create a virtual population and set up a model to simulate codend selectivity. Fig. 1. Three different contact modes were found to represent the selective range of Nephrops in trawl codends. The upper image illustrate the contact modes during testing of their ability to penetrate meshes. The corresponding cross-sections are captured by use of a ”MorphoMeter” and by scanning images. Finally, the FISHSELECT software is used to digitize the outline of the cross-sections and fit different geometric shapes to these. The optimal mode for mesh penetration is the one with the smallest cross-section – i.e. the right most panel. DTU Aqua National institute of Aquatic resources: www.aqua.dtu.dk