EU carries out a stratified random summer bottom trawl survey in Flemish Cap, Division 3M of the NAFO Regulatory Area, since 1988. Since 1991, the survey was made with the R/V Cornide de Saavedra. In 2003, this vessel was replaced for the R/V Vizconde de Eza, so, in order to maintain the series obtained from the old vessel, a two year comparative fishing trial betweel the two vessels was made in the years 2003 and 2004. This work presents the methods used for the transformation of the series and their results for biomass and length distribution for the principal species in the area: Atlantic cod, American plaice, Redfish (Sebastes fasciatus, Sebastes marinus, Sebastes mentella and Sebastes juveniles), Greenland halibut, Roughhead grenadier and Northern shrimp. For all the species except for Roughhead grenadier the R/V Vizconde de Eza is more efficient that the R/V Cornide de Saavedra for the biomass, with values of FPC varying between 1.0159 (Greenland halibut) and 1.4455 (Juvenile redfish). In lengths, in all species except for Roughhead grenadier too, the new vessel is more efficient catching the smallest individuals and less or equal efficient for the biggest lengths.