MULTIANNUAL TACS AND MINIMUM BIOLOGICAL LEVELS

ICES has focused attention on minimum biologically acceptable levels ( MBALs) as a basis for ensuring the safety of stocks and providing advi ce for management. Simultaneously the European Commission has proposed the use of multi-annual management strategies. This study models the effect of TACs, set constant for different periods of years, on the le vels and variability of spawning stock biomass (SSB) and yield. The Ce ltic Sea cod is used as an example. It is shown that, even with annual updating of TACs, target SSBs have to be substantially greater, possi bly more than seven times the MBAL, to avoid the risk of stocks fallin g below the critical level. Even if the stock is allowed to fall below the MBAL for 5% of the time, the target stock sizes need to be 3-10 t imes the MBAL if TACs are updated at 2-10 year intervals. As the perio d of stable catches is increased the problems of safeguarding the stoc k from falling below the MBAL also increase, and stability has to be p aid for by lower target fishing mortalities. Specifically for the Celt ic Sea cod, a balance of high yield and safety could be achieved with average catches of 7900 t and a target spawning stock biomass of 30 00 0 t. (C) British Crown copyright 1994