Reducing discards in Shrimp fisheries with the Letterbox

Two gear adjustments for reducing discards in brown shrimp fisheries were compared; the sievenet and the letterbox. Sievenets are already used in the Netherlands since 2002 and are cone-shaped nets inserted into standard trawls, which direct unwanted by-catch to an escape hole in the body of the trawl. The letterbox is a new gear adjustment that consists of a release hole transversely over the net. The idea is that the shrimps go over the hole in the net, while flatfish can escape through the release hole. In this research paired comparisons were made between the letterbox and the sievenet to see if the letterbox is at least equally effective as a sievenet in reducing discards juvenile flatfish in shrimp fisheries. As flatfish are not the only bycatch in shrimp fisheries also information on other bycatch was collected to see how the letterbox performs compared to the sievenet with regard to by-catch of (round)fish and benthos. A total of 6 research trips were conducted in 2 different periods in 2010 (May/June and September/October). The letterbox seemed to perform well in the first period of the research, especially with regard to plaice the bycatches were significantly less in the letterbox as compared with the sievenet. However there was also a relative large loss of marketable shrimp, which needed to be solved. A small gear adjustment seemed successful: in the second period comparable amounts of marketable shrimps were caught with both nets. In the end we could conclude that the letterbox is at least equally effective as a sievenet in reducing discards of juvenile plaice. Also we have reasons to believe that the letterbox performs better than the sievenet in reducing discards of juvenile flatfish. However the effects of net modification and period are confounded, which made comparison of results between period 1 and 2 rather difficult. Therefore it is recommended to conduct another experiment in spring 2011 when high abundances of juvenile plaice are present, using the newest net design. For the other by-catch species the results were variable; the letterbox was for some species at least equally effective as a sievenet in reducing several other discards, but for other species the letterbox was not as effective in reducing all discards. The letterbox can be a good alternative for the sievenet, especially in spring, when there is high abundance of juvenile plaice in the Wadden Sea. It should be taken into account, however, that the adjustment is not as effective as the sievenet for all species.