Duration: September 19 – 28, 2020
Area: North Sea
Purpose: Investigations on Innovative Fishing Techniques (DRopS)
The research cruise will be used to investigate innovative fishing techniques in the beam-trawl fishery targeting Brown shrimp (Crangon crangon) in the North Sea. From the socio-economical point of view, this is one of the most important fisheries in the North Sea, supporting an international fleet of approximately 560 vessels, employing more than 1,000 fishermen, and producing yearly revenues of up to 100 million €.
One key aspect for the sustainability of the fishery is the development of more efficient, selective, and environmentally friendly beam trawls. The present program includes two different research topics (RT). Each of the RTs is described in the following lines.
RT 1: Modified ascending trawl
One approach within the framework of the project DRopS (Dolly Rope Suspension) is a modification of the trawl design. Conventional beam trawl nets are cut in a shape that ensures the codend in standard configuration to hover just above the seabed or even grind on it. To counteract this issue, a so-called "ascending trawl" was constructed based on the design of the conventional net. This modification was already tested during the cruises 755 (Oct/Nov 2018) and 758 (Jan 2019), where the ascending trawl was compared directly to a conventional beam trawl. In contrast to the conventional net, which dragged on the bottom during fishing, the lower edge of the ascending net hovered on average 0.3 m above the seabed. Initial catch evaluations showed that the percentage of shrimp in the total catch was very similar in both nets.
In case heavy components (mussel shells, sand etc.) enter the codend, the ascending shape of the net will probably not be able to keep the codend away from the seabed neither. It is therefore important to avoid catching such components and to ensure that they can exit the net before reaching the codend. In addition to changing the overall shape of the net, it is also possible to increase the size of the meshes in the lower panel. This allows such heavy components to leave the net before entering the codend. However, as a loss of shrimp is also possible, it is necessary to investigate the effect of large meshes in the lower panel on catches of both heavy components and shrimp. Such investigation is the aim of RT1.
A similar trawl was already tested during cruise 767 in September 2019. However, the tested trawl was inefficient and further development of the design and constructive details was required. Therefore, a further modified trawl shall be tested on this cruise. Both, an ascending cut trawl with standard mesh sizes and one with a section of big meshes in the under panel will be tested each against a standard beam trawl as control.
RT 2: Investigating the influence of dolly rope abrasion protection on codend size selection and catch efficiency
Dolly ropes are polyethylene fibres, often used in the fishery to protect the lower panel of the codend from ground abrasion. During the fishing process, these ropes fray and parts of them tear off. These torn pieces contribute to marine waste, therefore negatively affecting marine species and ecosystems. However, the effect of dolly ropes on the selectivity and catch efficiency of brown shrimp beam trawls is unknown. With the age and use, dolly Ropes lose flexibility and the individual elements tend to get entangled, becoming matted. This results in accumulation of sand and gravel, potentially affecting the selectivity of the codend. Therefore, the current research cruise will investigate the effect of dolly ropes in codend size selection and catch efficiency of beam trawls. This investigation will be conducted using the paired-gear method.
Scientific cruise leader: