A stratified sampling programme by gear and port was carried out by observers on board vessels in 2000 to study the discard behaviour of trawler fleets operating off Spanish North Atlantic coasts. No relationship between discards in weight or numbers and haul duration was found, so the results in weight are presented in kilograms per haul instead of kilograms per hour, as is usual for catches. The data were analysed in order to find groups of ports with a similar behaviour and to reduce the number of strata (30 as a result of the crossing of 10 main ports and 3 types of towed gears). The reduction in the number of strata (from the 30 original strata to the 9 new groups obtained) would mean a high improvement in efficiency for raising purposes, for stock assessment needs and for new Spanish sampling programmes. Significant differences in discards between groups of ports were found. Species composition of discards by stratum is given as a percentage of the total weight, based on data collected by the observers on board vessels. There are also differences in the discarded species composition for the different groups of ports obtained.