Effects of a partially closed area in the North Sea (“plaice box”) on stock development of plaice

The “plaice box” is a partially closed area in the North Sea, established in 1989 to reduce the discarding of undersized plaice (Pleuronectes platessa) in the main nursery areas, and thereby to enhance recruitment to the fishery. In contrast to the expected positive effects, yield and spawning stock biomass have decreased. The effects of the plaice box are evaluated by analyzing the relevant factors and processes (natural and anthropogenic) that affect recruitment. It is shown that the Dutch beam trawl effort has decreased in two phases. During 1989–1993, when the plaice box was closed only during the second and third quarter, effort was reduced to around 40% of the original level. When the box was also closed in the fourth (1994) and first quarter (1995 onwards), effort decreased to around 6%. The effort reduction would imply a reduction in discard mortality if all other factors had remained constant. However, a reduced growth rate and possibly a higher rate of natural mortality may have counteracted the reduction in fishing effort. The apparent changes in growth and mortality coincided with changes in the North Sea ecosystem that occurred in the early 1990s but may also be related to a response to the change in beam trawl effort.