Size selection of European flounder (Platichthys flesus) in the demersal Baltic cod trawl fishery – Theoretical investigations which improve multispecies selection Master thesis by Ulrike Luschtinetz 2012 First supervisor: Prof. Dr. C. Möllmann Institute for Hydrobiology and Fisheries Science, University of Hamburg, Second supervisor: Dr. D. Stepputtis Thünen-Institute of Baltic Sea Fisheries Rostock Table of Content Table of Content Table of Content ........................................................................................................................ II Abstract ....................................................................................................................................... 1 Zusammenfassung ...................................................................................................................... 2 1. Introduction ......................................................................................................................... 3 1.1 Working hypotheses .................................................................................................... 3 1.2 By-catch and discard .................................................................................................... 5 1.3 Species and size selectivity .......................................................................................... 7 1.4 European Flounder (Platichthys flesus) ..................................................................... 10 1.4.1 General information ............................................................................................ 10 1.4.2 Economical importance and fishery ................................................................... 12 1.5 Demersal trawl fishery in the Baltic Sea.................................................................... 15 1.5.1 Bottom trawl ....................................................................................................... 15 1.5.2 Regulations ......................................................................................................... 16 2. Material and Methods........................................................................................................ 20 2.1 Influence of flounder morphology on selectivity – the method in general ................ 20 2.2 Influence of flounder morphology on selectivity – the method in detail ................... 21 2.2.1 Data sampling ..................................................................................................... 21 2.2.2 Measurement of flounder cross-sections ............................................................ 22 2.2.3 Definition of shape models ................................................................................. 23 2.2.4 Fall-through experiments .................................................................................... 25 2.2.5 Simulation of mesh-penetration and selection of a penetration model .............. 27 2.2.6 Prediction of selectivity parameter (Design guide) ............................................ 30 2.3 Comparison of simulated and experimental data ....................................................... 32 2.4 Prediction of selectivity parameters for T90 meshes ................................................. 33 2.5 Influence of twine characteristics on flounder selectivity ......................................... 35 2.5.1 Data sampling ..................................................................................................... 35 2.5.2 Analysis .............................................................................................................. 36 II Table of Content 3. Results ............................................................................................................................... 37 3.1 Influence of flounder morphology on selectivity....................................................... 37 3.1.1 Data sampling ..................................................................................................... 37 3.1.2 Shape model ....................................................................................................... 38 3.1.3 Penetration model ............................................................................................... 42 3.1.4 Prediction of selectivity parameters ................................................................... 44 3.2 Comparison of simulated and experimental selection data for T0 meshes ................ 52 3.3 Analyzing of T90 meshes .......................................................................................... 53 3.4 Influence of twine characteristics on flounder selectivity ......................................... 58 4. Discussion ......................................................................................................................... 60 4.1 Methodology .............................................................................................................. 61 4.1.1 FISHSELECT ..................................................................................................... 61 4.1.2 Cover-codend method......................................................................................... 62 4.2 Morphological description of flounder ...................................................................... 63 4.3 Square and diamond meshes (BACOMA) ................................................................. 65 4.4 Rectangular meshes ................................................................................................... 66 4.5 Hexagonal meshes (T90) ........................................................................................... 66 4.6 Influence of twine characteristics on flounder selectivity ......................................... 67 4.7 Behavioral aspects ..................................................................................................... 68 4.8 Multispecies approach and outlook ........................................................................... 69 5. References ......................................................................................................................... 70 6. Indices of tables and figures .............................................................................................. 77 6.1 Index of tables ............................................................................................................ 77 6.2 Index of figures .......................................................................................................... 79 7. List of Abbrevations .......................................................................................................... 82 8. Acknowledgement ............................................................................................................. 83 9. Declaration of Authorship ................................................................................................. 84 10. Appendix ........................................................................................................................... 85 III Abstract Abstract The European flounder (Platichthys flesus) is the most abundant and the most widely distributed flatfish species the Baltic Sea. Flounder are caught by a direct flounder fishery, as well as by-catch in the demersal cod trawl fishery. Therefore, it is quite important to optimize the codend selection of cod (Gadus morhua), and even to improve the flatfish selectivity. In this study, the theoretical selection potential of Baltic flounder was analyzed for different mesh types, using the FISHSELECT method. For the first time, this method was applied to a Baltic flatfish species. Measured morphological data, systematic fall-trough experiments for defined mesh types, and simulations were used to predict selectivity parameters, L50 and selection range (SR), for defined meshes. Diamond, rectangular, and hexagonal mesh types were analyzed. The specific selectivity parameters were predicted and illustrated for defined mesh parameter, such as mesh opening and opening angle. The diamond meshes of the current legal BACOMA codend, with a mesh opening of 105 mm, have flounder L50 values between 11.10 cm and 23.41 cm. The simulated selectivity parameters correspond with experimental data (125 mm mesh opening), obtained at sea. The square meshes of the BACOMA escapement panel, with a mesh opening of 120 mm, have a predicted flounder L50 value of 21.01 cm. Furthermore, underwater records of hexagonal mesh shapes of a T90 codend were analyzed with the FISHSELECT software. Selectivity parameter, L50 and SR, were predicted based on the observed mesh shapes. The values of predicted L50 values are below minimum landing size of flounder in Germany (25 cm). Additional, the influence of twine characteristics on flounder selection was investigated. A significant influence of twine diameter and number of twine (single or double) was shown for T90 codends. An increase of these parameters results in decreasing L50 values. The determined selectivity data of flounder for defined meshes may be used as a primary basis for decisions regarding mesh and codend developments. Further analyses are essential, especially for additional species, to achieve the long-term aim of an improved multi-species selection. 1 Zusammenfassung Zusammenfassung Die Europäische Flunder (Platichthys flesus) ist die häufigste und die am weitesten verbreitete Plattfischart in der Ostsee. Sie werden durch gezielte Flunderfischerei, als auch