Relevance of dual selection in grid based selectivity studies Bent Herrmann a*, Manu Sistiaga b*, Eduardo Grimaldo c and Roger B. Larsen b a Denmark Technical University (DTU), North Sea Centre, DK-9850 Hirtshals, Denmark b University of Tromsø. Breivika N-9037, Tromsø, Norway c SINTEF Fiskeri og Havbruk AS, Brattørkaia 17B, 7010 TRONDHEIM, Norway * Equal authorship Traditionally, the selectivity of grid based systems has been measured as a joint grid + codend selection system. However, questions like “what is the relevance of the mesh size in a codend placed subsequent to a grid?” have enhanced the necessity of separating the role of each of the devices in the overall selectivity of the system. Two conditions need to be fulfilled so that a fish can escape through a grid: a) the fish needs to come in contact with the grid and b) the fish should be able to physically pass through the grid. In this study, we introduce the variable “grid contact” (Cgrid), which determines what percentage of fish comes actually in contact with the grid, in a grid based selectivity study. No work has evaluated this so far and previous studies have assumed a grid contact equal to 100%. Figure 1 The data collection for the present investigation was carried with the setup represented in Figure 1, where a cover over the grid and a cover over the codend collected the fish escaping from each of the devices. In this study, a 55 mm Sort-V sorting grid device was alternatively combined with a 135 mm and 140 mm diamond mesh codend. All cod (Gadus morhua L.) and haddock (Melanogrammus aeglefinus L.) above 30 cm were measured in the three compartments of the gear: grid cover (GC), codend (C) and codend cover (CC). The data were analysed using the computer software SELNET. A dual sequence logistic curve, which is the product for the selection curve for the grid and the selection curve for the codend, was fitted to the data. The difference with former studies is that the parameter Cgrid is added into the calculations (see SELNET poster). The contact in the codend is assumed to be 100%. Kvamme and Isaksen (2004) presented study with a similar setup where they estimated the dual selection characteristics of a grid + codend gear. However, they calculated the total selectivity of the gear as the product of the selectivity of both devices (grid and codend); this is similar to assuming that the grid contact is 100%. Figure 2 shows retention and escapement curves for cod and haddock with the grid + 135 mm codend setup. The curve considering the grid contact (grey) shows constantly a better fit (R2) than the curve assuming a contact likelihood of 100% (black). The overall differences in the retention curves estimated with both methods for both species and both codends were found to be between 0.89 / -1.38 cm for L50, -0.12 / -4.82 cm for SR and 0.052 / 0.006 for R2. As expected, Cgrid was constantly higher for haddock than for cod. The mean ± Standard Error was 0.9512 ± 0.0075 for haddock and 0.7802 ± 0.0228 for cod. Haddock is a much more active fish than cod inside the trawl and tends to seek for escapement on top. It is therefore not surprising that Cgrid for haddock is higher than that of cod. Figure 2 With few exceptions, grid selectivity studies have evaluated the overall selectivity of a grid + codend setup incorporating the fish escaping through the grid and the codend as a single compartment. Although data collected in this way can be analysed as dual selection data, the present study reveals differences in the same datasets depending on whether the fish escaping through the grid and the codend are incorporated in the analyses separated (grey) or joined (black) (see Figure 3, which represents the curves for cod with the grid + 135 mm codend setup). The necessity of collecting the data in separate compartments to avoid deviance from the correct estimates is therefore demonstrated. For this reason, dual selection analysis results obtained with joined GC and CC should be interpreted carefully. Taking the grid + 135 mm pooled hauls as example, for cod the difference in the analysis results when the GC and CC data are included separately (taken as reference) or joined is -0.11 cm for L50, 0.36 cm for SR and 17.7% for Cgrid. For haddock on the other hand, the differences were 0.47 cm for L50, 1.40 cm for SR and 63.2% for Cgrid. Figure 3 References Kvamme, C., Isaksen, B., 2004. Total selectivity of a commercial cod trawl with and without a grid mounted: grid and codend selectivity of North-east Arctic cod. Fish. Res. 68, 305–318. ICES WGFTFB Ancona, 2009