Evaluation Of The Effects Of A Closed Area In The North Sea ('Plaice Box') On The Stock Development Of Plaice (Pleuronectes Platessa)

No abstracts are to be cited without prior reference to the author.The plaice box, a partially closed area in the North Sea, was established in 1989 to reduce discarding of undersized plaice (P1euronectes platessa) in the main nursery areas and thereby enhance the recruitment to the fishery. Despite the plaice box, yield and spawning stock biomass have decreased. This paper evaluates the effect of the plaice box by analysing the relevant factors and processes (natural and anthropogenic) operating in the plaice box that affect the level of recruitment. It is concluded that the overall beam trawl effort in the box decreased to around 6% of the level befoce the establishment of the box reducing the discard mortality rate in the box. However, other processes such as a reduced growth rate and possibly a higher rate of -natural mortality have counter affected the reduced discard rate. The changes in growth and mortality coincide with changes in the North Sea ecosystem that occurred in the early 1990s. The changes may also be due to the changes in level of beam trawl effort which may have led to a reduction in the food supply for plaice. However, as no direct evidence is available, the latter hypothesis needs further study.