Can the Standard Ices Co-Ordinated Ibts-Mik Survey Provide Reliable Data on Herring Recruitment and Spawning Locations in the North Sea?

In February and March an ICES co-ordinated ichthyoplankton (IBTS-MIK) survey is conducted in the North Sea to estimate the number of herring larvae. The standard sampling utilises an MIK (2m mid-water ring trawl: 1.6 mm mesh size), designed to catch larger autumn spawned herring larvae. The MIK North Sea herring recruitment index ignores larvae below 11 mm. However, in recent years the numbers of small herring larvae has increased due to the increased winter spawning component and a prolonged spawning season. The international herring larvae surveys (IHLS), in September to December, use a Gulf VII high-speed plankton sampler (280 µm) for catching recently hatched larvae. This survey finishes too early for newly hatched larvae in February.

Recently an MIKey-M net (20cm ring-net: 335 µm) attached to the MIK ring, was developed to sample ichthyoplankton. In addition to catching small herring larvae, the MIKey-M gives the possibility of undertaking winter spawning sampling on the IBTS.

In this study the catchability of fish eggs and herring larvae are compared between gears. This examines the utility of the IBTS-MIK survey to provide a reliable index of recruitment for the North Sea herring stock and the possibility of providing data for determining spawning locations.