Report of the Working Group on Mackerel and Horse Mackerel Egg Surveys (WGMEGS)

The ICES Working Group on Mackerel and Horse Mackerel Egg Surveys (WGMEGS) met in Reykjavik from 7–11 April 2014, chaired by Cindy van Damme (IMARES, the Netherlands) and Finlay Burns (MSS, Aberdeen, Scotland), to evaluate the results of the Mackerel and Horse Mackerel Egg Survey in 2013 and to plan the North Sea Mackerel Egg Survey in 2014. Its main objective is to relate the number of freshly spawned eggs found in the water column to the number of females having spawned these eggs. With the estimated fecundity of the females, this provides an estimate of the spawning-stock biomass. In 2013 the Faroe Islands, Iceland Portugal, Spain, Scotland, Ireland, the Netherlands, Norway and Germany participated in the survey. Despite technical and weather problems, temporal and spatial coverage was sufficient in order to deliver a reliable estimate of mackerel and horse mackerel annual egg production. The application of an alternating transect survey design enabled survey effort to be deployed over the wide spatial area, necessary due to the continued expansion of the spawning area and season. Despite fewer fecundity and atresia samples being taken than planned, good spatial and temporal distribution of the sampling was achieved, and was sufficient in providing an estimation of realized fecundity.
The estimate of total mackerel egg production was 3.12 *1015 which is an increase of 47% with respect to 2010 (2.12*1015). The analyses of potential fecundity gave a value of 1257 eggs per gram female for mackerel for the western and southern components combined. The overall prevalence of atresia as a percentage of the population was 22% and the potential fecundity lost in the spawning season was 48 eggs /g. This reduced the potential fecundity by 4%. Spawning-stock biomass (SSB) for the NEA mackerel stock was estimated using the realized fecundity estimate of 1209 eggs/g female, a sex ratio of 1:1 and a raising factor of 1.08 (ICES, 1987) to convert spawning fish to total fish. This gave an estimate of spawning-stock biomass (SSB) in 2013 of 4.29 million tonnes for western component, 1.28 million tonnes for southern component and a combined estimate of 5.57 million tonnes. This represents an overall increase of 1.28 million tonnes (~30%) compared to 2010.
The western horse mackerel stock was found to have spawned less eggs in 2013; 3.95 x 1014. This is a decrease of almost 64% compared to 2010 (1.09 × 1015), and the lowest production over the time-series.
Peak spawning for mackerel in the western area in 2013 was observed during period 2 with the evidence strongly suggesting that spawning had already commenced prior to sampling in the western area. It is therefore accepted that in 2013 the start of mackerel spawning in the western area was in January or early February and consequently was missed by the MEGS survey. In order to address this issue MEGS survey participants are requested to pursue additional opportunities for plankton and fecundity sampling in January, February and March in the years up to and including 2016 in the Bay of Biscay. For the southern mackerel the spawning period is still adequately covered by the egg survey.
Norway has decided to withdraw from the 2014 North Sea egg survey and possibly also from the Atlantic egg survey. Without Norwegian participation, or other nations to replace them, the surveys ability to adequately survey all periods will be significantly reduced. The Netherlands will be the sole participant in the 2014 North Sea egg survey and the objective as always will be to provide as comprehensive coverage as possible during the mackerel spawning season given the limited survey effort available. Preliminary results of the North Sea egg survey will be reported to WGWIDE in August 2014.