Development of Catch Control Devices for mid-water trawls

The project "Development of catch control devices for mid-water trawls" was initiated to solve the problem of taken excessively big trawl catches in the Barents Sea demersal fishery which potentially could lead to reduced quality of the catch, and could also be associated with handing problems and other H.S.E related problems. The goal of the project has thus been to develop catch control devices for mid-water trawls that guarantee that excessive big catches are not taken, that are technologically simple and do not compromise the fishing operation, and that are easy to handle on deck. The project consisted of several tasks with different scientific scope and content, including analytical, numerical and experimental activities. The results that have been achieved in the project include scientific publications on framework conditions, description of the working principle and operation of different catch control devices that can be used as for mid-water trawls and bottom trawls depending on the rigging, and documentation on their performance under commercial conditions. In addition, the project was extended to assess the performance of catch control devices working together with mandatory sorting grids, and to start the development of hydro-dynamically improved sorting grid sections for the bottom trawl fishery. Various catch control devices were tested in the last three years. Some of these devices include: 1) semi-detachable codends that can be activated with acoustic releasers, thin twine that breaks due to drag of the catch or expansion of the codend, or a combination of them; 2) side splits in the N-direction in the foremost part of the codend; 3) large meshes that open as the codend fills up. Simultaneously, has the Norwegian Institute of Marine Research tested 4) a motorized gate that opens upon a signal from an operator so that excess fish are guided out of the trawl; and 5) a hatch that is known as the ExFED (Excess Fish Exclusion Device) in the upper panel of the codend that opens when the codend fills up. Although most of the catch control devices have shown potential for controlling catch size, they have often failed when simultaneously tested with sorting grid sections (Sort-V or Flexi grid type). The explanation for this failure appeared to be mainly related to the significant reduction of the water flow inside the grid section caused by the various combinations of grid, lifting and guiding panels. In these circumstances fish stops and accumulates in front and behind the sorting grid, not falling back to the codend until very late in the haul-back operation. Experiments performed in 2013 and 2014 in a flume tank (Hirtshals, Denmark) revealed important geometry problems and also documented significant water flow reduction in the mandatory sorting grid sections (Sort-V). The lifting panel was observed to be over-dimensioned, and consequently, almost all the cross section and the grid surface was blocked. In addition, the sorting grid pressed itself down and blocked the supposedly existing opening for fish to pass towards the codend. Measurements of water flow showed water flow reductions of more than 50-60 % in the mandatory 2-panel grid sections (Sort-V type). Similar trends were observed when the grid section was modified from a 2 to a 4-panel section in an attempt to increase the cross section area. When the lifting panel was removed, significant water flow improvements were obtained inside the grid section and uncovered a blocking effect not earlier considered. Based on the results from the flume tank full scale tests were designed and carried out to assess the selectivity of a 4-panel sorting grid section with and without lifting panel under commercial conditions. The results showed that the presence of the lifting panel increased the contact of fish with the grid (grid contact, Cgrid) by 26%, and thus improve the selective efficiency of the grid section. However, geometry problems were still observed (grid sitting on the lower panel and blocking the passage of fish towards the codend). No fish was observed swimming in front or behind the sorting grid, but, a great amount of fish was observed swimming in the anterior part of the codend, and not falling back to it until very late in the haul-back process. Late entrance of fish into the aft section of the codend provoked that catch sensors did not activate and often led to unexpected big catches. Additionally the testing of a 4-panel grid section in combination with different codend 2 and 4-panel configurations carried out in the flume tank revealed that the deformation of a 2-panel diamond mesh codend (narrowing of the cross section area as the catch increases) provoked a significant reduction in the water flow. This could explain why the fish stops and accumulates at this point.

In the experimental mid-water trawl gadoid fishery, excessive catches constitute an important problem for fishermen as well as for the control and management organizations. Excessive catches imply among others problems such: as poor selectivity of the gea r, lower quality of the processed fish and HSE problems for the crew. Poor selectivity and low fish quality are problems that often lead to increased discards and fish dumping. These are problems of great concern for management institutions because they c ontribute to unaccounted mortality in the fish stocks and ultimately bias in stock assessment models. Herein, we propose a project to further develop catch control concepts that will contribute to minimize the challenges and problems linked to excessive c atches. The catch control concepts included in the project are based on a codend that closes and partially detaches from the rest of the trawl when the codend is full or filled with the desired amount of fish. The fish entering the gear after detachment is then gently released at the same towing depth, increasing their survival. The concepts have been already successfully tested and further development is needed to solve the identified technical problems. The project runs for three years (Jan. 2012 - D es. 2014). The research team will be mainly affiliated with SINTEF Fisheries and Aquaculture (SFH), the Norwegian University of Science and Technology (NTNU), the University of Tromsoe (UiT), the University of Massachussetts, and the Norwegian Directorate of Fisheries (FDir).