Some comments to the problem of adjusting TACs to mesh size changes.
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Original versionThis report is not to be quoted without prior consultation with the General Secretary.
Mesh assessments are usually carried out by calculating "short term losses" and "long term gains" expected from a change in the minimum legal mesh size, assuming no change in total fishing effort or its distribution by area and season (Gulland, 1961) • At the time these methods were developed, minimum legal meshand/ or landing sizes were the only regulatory measures applied on most demersal trawl fisheries. After the introduction of TACs one is faced with the problem of what effect a change in mesh size should have on the recommended TAC. Often this has been dealt with by simply calculating the catches corresponding to a range of fishing mortalities for the alternative mesh sizes and recommend TAC's corresponding to a certain value of the fishing mortality on the fully exploited age groups, identical for all mesh sizes. By this approach the "short term loss" is automatically transferred into the TAC, making the TAC for an increased mesh size smaller than the TAC for the present mesh size by an amount approximately equal to "short term losses". After a brief discussion of the assessment of short and long term effects of mesh size changes, this paper will mainly deal with the appropriateness or in-appropriateness of the usual methods applied for selecting TAC's and discuss alternatives.