The abundance of ichthyoplankton in samples from dedicated plankton surveys by Cefas with positional and sample data. Surveys took place off the Western Coast of the UK and Ireland between 1986 and 2004.

Series of cruises undertaken to contribute to the estimation of the spawning stock biomass of the western mackerel and horse mackerel stocks by plankton survey. The triennial mackerel egg surveys were begun in 1977 to estimate the SSB of the western mackerel stock. Since 1986 the surveys have also been used to estimate the SSB of horse mackerel. Plankton sampling is undertaken to estimate the egg production and trawling is carried out to estimate the mean fecundity of the mature female fish.

Various designs of Gulf VII type samplers have been used with various apertures of nosecones and 270 micron nets. Samplers are now standardised to the 53cm version, fitted with 20cm aperture nosecones.

Analysis at Cefas involved separating all fish eggs and larvae from samples. Where possible all eggs were identified. Eggs lacking identifiable features were measured. Where >100 eggs were found, sub-sampling was undertaken. Eggs that were unmeasured were apportioned across the size distribution of measured eggs. All mackerel and horse mackerel eggs were staged.