Solea, 696. Reise
Dauer: 12. bis 24. November 2014
Fahrtgebiet: Ostsee
Zweck der Reise: Comparative study of T90 codends with standard and alternative closure system / Testing the FLEX (FLatfish EXcluder), a new and simple concept for flatfish bycatch reduction in demersal trawls
1 Comparative study of T90 codends with standard and alternative closure system (RT1)
The codend selectivity is determined by the mesh size, but also by the shape of open meshes. This shape is influenced by several parameters, such as net-type, hanging ratio, catch-size, or drag of the trawl. In normal designs, the codend is tied using the codline and the meshes are squeezed and no longer open, restricting any selectivity process in the surroundings of the closure area. Under the framework of the collaborative project (Contract MARE/2010/11, LOT 1 (SI2.601456)), a Polish netmaker proposed a new closure system based on an alternative net cutting in the rear part of the codend (see Figure 1b), with the aim of reducing the compression forces acting in the closure net area, to keep more open meshes during towing. The codend using the new codline system is called “Envelope codend”.
An “Envelope” T90 codend was already tested during sea trials performed between the 27 and the 29 of November 2012, and 9 of January 2013, onboard R/V Clupea. Information collected was exclusively qualitative using UW video cameras (GoPro, 1080p/30fps), mounted in the roof of the trawl about 2m in front of the cod-line. The UW video recordings showed most of the meshes around the cod-line opened. Unfortunately, the rate of open meshes was difficult to estimate once fish started to accumulate in the codend. How this new closure technique might affect the selectivity of the codend is therefore unknown. The effect of the envelope-type closure on size selection for the available species will be assessed in the present cruise by using quantitative catch data. The main aim is to estimate the selectivity properties of “Envelope” T90 codend, and to compare the functional form of the resulted size selection curve to the curve estimated for the “Standard” T90 codend.
2 Testing the FLEX (FLatfish Excluder), a new and simple concept for flatfish bycatch reduction in demersal trawls (RT2)
In 2013 the Institute of Baltic Sea Fisheries developed a new species selection device which takes advantage of differences in morphology of roundfish and flatfish to reduce flatfish by-catches. The so-called FRESWIND uses body shape as differential criteria between both groups of species, assuming that all fish entering in the area where the device is mounted can meet the escaping window. Sea trials demonstrated the significant reduction of of flatfish catches when using this device.
The FLEX is a new technological solution conceived in the institute to address the problem of flatfish bycatch in roundfish fisheries using a different strategy as in case of FRESWIND. The FLEX concept relies on taking advantage on the differences in swimming behavior assuming that in the area of the trawl the device is mounted, the flatfish will swim in close connection to the lower panel of the net, while roundfish will use the water column in their way to the codend. In addtion to flatfish, FLEX concept could also reduce the unwanted benthic invertebrates commonly caught in demersal mixed fisheries around europe.
The FLEX implies simple alterations of the gear avoiding steel-made components or fundamental redesigns in the extension piece (the tunnel ahead of the codend). The design relies on creating a “funnel effect” in the lower net panel of the extension to drive flatfish downwards to the exclusion hole. Instead of using a single net panel to build the lower side of the extension piece, we will use two segments of different length placed longitudinally one after another. The first segment is pushed downwards-backwards using weights to create the “funnel effect” while the posterior segment (longer than the previous one) is pulled upwards-frontwards using floats to mask the exclusion hole created by the lower panel.
Reisebericht (PDF, deutsch)
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