In the European Union, the collection of discard data is enforced through the Data Collection Regulation or Framework (DCR/DCF) of the European Commission (EC). To comply with this ruling, approximately ten trips of discard-intensive beam-trawlers are being monitored annually since 1999 (Helmond and Overzee, 2010). In 2009, revisions to the DCF (2008/949/EG), required member states to increase sampling intensity to i) improve the precision of their estimates and ii) the number of sampled métiers. To meet this requirement within an affordable budget, the Institute for Marine Resources and Ecosystem Studies (IMARES, part of Wageningen University and Research) set up a collaborative project between the Dutch fishing industry and the research institute to recruit a ‘reference fleet’ of vessel owners willing to participate in a self-sampling programme. This programme complemented the existing observer programme. In the observer programme, vessels were selected quarterly from a pool of available vessels, whereas in the self-sampling programme, trips were pre-determined from a reference fleet of participating vessels. Missing and/or wrong information precluded the inclusion of 17% and 13% of all self-sampled trips in 2009 and 2010. In total, 9 and 10 observer, and 63 and 132 valid self-sampling trips were completed in 2009 and 2010, respectively. For these remaining valid self-sampled trips, procedures were developed to test whether data quality was comparable with i) other self samples from the reference fleet and ii) comparable observercollected data (i.e. temporally and spatially overlapping trips). In addressing i), there were no unusual patterns in the length frequencies of self-sampled discards of European plaice (Pleuronectes platessa), common dab (Limanda limanda), grey gurnard (Eutrigla gurnardus), and whiting (Merlangius merlangus) in 2009 and2010. In addressing ii), no significant differences in the discard rates of plaice between the two programmes were found. There was no evidence that sampling may have been biased at the vessel level, justifying the decision to present all discard estimates independent of the programme type. While in both programmes the majority of observations were done onboard beam-trawl vessels with mesh sizes ranging between 70 and 99 mm, in the self-sampling programme data from four additional beam- and otter-trawl métiers with two 70-99 and 100-119 mesh size ranges and other target species assemblages (i.e mixed crustaceans and/or demersal fish) were collected. This lead, apart from a considerable increase in sampling effort for some métiers, to an increase in the temporal and spatial spread of sampling. Samples from previously unsampled northern and eastern parts of the North Sea were available. The spatial distribution of sampling locations matched that of the total effort of the fleet for intensively-sampled métiers. In all but two métiers, combined fish and benthos discards exceeded the volume of landings. In contrast, large-mesh beam- and otter trawls (100-119 mm) landed on average more than they discarded. The majority of discards was comprised by benthic (invertebrate ) species such as common starfish (Asteria rubens); sand star (Astropecten irregularis); swimming crab (Liocarcinus holsatus); and serpent star (Ophiura ophiura). Most frequently discarded fish species of no commercial value included: dragonet (Callionymus lyra); grey gurnard (Eutrigla gurnardus); scaldfish (Arnoglossus laterna); and solenette (Buglossidium luteum). Among commercially-valuable fish, common dab (Limanda limanda) and European plaice (Pleuronectes platessa) were the most frequently discarded species
