The western stock of Atlantic mackerel (Scomber scombrus) is an important fishery resource in European waters with a catch potential of about 500,000 tonnes. Since 1977 the spawning stock size has been estimated every 3 years using the annual egg production method (=total fecundity
method). In 1989 the daily egg production method (=batch fecundity method) was applied for the first time. Daily egg production in the middle of the spawning season was estimated from a subset of the data collected for the regular annual production survey. Fish were captured by trawl for measurement
of batch fecundity and daily spawning fraction. The daily egg production method gave a biomass estimate 15% higher than the annual egg production method. This could be explained by a loss of eggs from the potential fecundity during the course of the season. The daily egg production method
overcomes the problem of indeterminate fecundity in this species and requires only one, instead of five plankton surveys. Suggested improvements to the annual production method to measure changes in annual fecundity incur penalties in ship-time requirement. It is concluded that in the future,
on the grounds of improved precision and lower cost, the daily egg production method will be preferred for estimation of biomass in this species.
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