The IBTS (International Bottom Trawl Survey) campaigns are part of an international program. They are coordinated with the majority of countries bordering the North Sea in the Council Regulation (EC No. 1543/2000 of 29 June 2000 and N ° 1639/2001) which stipulates that member states must make scientific surveys at sea to evaluate the abundance and distribution of stocks, independently of data from commercial fisheries.

The IBTS campaigns can therefore obtain a diagnosis in real time of the target population, counting of young is used for a calculation of recruitment indices. To achieve this, a rigorous methodology has been defined at the international level: for example, the use of a standard method and area coverage by two different research vessels. To assess indices of larval abundance (group 0 for herring and sprat), sampling at night MIK net (Methot Isaac Kidd) covering the same area as trawls, is also performed according to the same protocol standard.

For nearly 20 years, the sampling area allocated to France for the southern half of the North Sea, however, since 2007, the Eastern Channel has been integrated into the sampling area.

All abundance observations of herring larvae for the area made by the MIK net that are available between 2001 and 2011, have been used. They were first assembled in km² and then transformed (log10 (x +1)) to create an average distribution map interpolation (kriging after variographic study).