The interaction between grey seals (Halichoerus grypus) and the Baltic gillnet fishery for herring (Clupea harengus) during the period 2000-2005 was investigated, by comparing and contrasting 3 sources of information: data from the European Union (EU) official logbook system, data from a voluntary logbook system and data from field studies. While the EU official logbooks suggest that seal disturbance occurred in 30% of all herring gillnet fishing events, data from the voluntary logbook gavea figure of 60%. There was a pronounced seasonal variation in the frequency of seal-disturbed fishing efforts, with least interference in the early summer, and greatest at the end of the year. This variation is likely dependent on the life cycle of the seals and their main prey, the herring. Analysis of the EU logbook information also showed that catches were significantly higher on fishing days when there was no seal interference recorded, compared to days when there was such interference. Field experiments demonstrated that herring catches were reduced by 240 kg per fleet of net and fishing occasion when seals were present, which is much more than the observed seals in the area could possibly have consumed, and a very small number of fish remains were found in the nets after seals had been present. These observations suggest that the mere presence of seals does affect catch levels negatively. Nets baited with marked fish were used to estimate hidden losses, i.e. fish removed from the nets, leaving no trace. Seals were assumed to have visited the experimental nets in 14 of the 19 trials. In 11 of these, more than 95% of the marked fish went missing. The 3 different data sources altogether show that the herring gillnet fishery in the northern Baltic is severely disturbed by interaction with grey seals.