Description: The decrease of mussel stocks in the tidal flats of Lower Saxony in the 1980s and 1990s and in Schleswig-Holstein since the early 1990s was not reversed by a strong spatfall in spring 1996. The decrease was only interrupted and delayed. Mussel fisheries in Schleswig-Holstein and Lower Saxony have been subject to a fishery programme and management plan since 1997 and 1999 respectively. These measures are accompanied by monitoring of intertidal mussel stocks. Monitoring in Lower Saxony showed that blue mussel areas and biomass reached their all-time minimum level in the spring of 1996. After the inventory in spring, a strong spatfall took place which led to the largest growth of mussel stocks in 1999. Following several years with a negative trend, mussel stocks have levelled off at a low level for 7 years. But for the past 5 years, there has been a positive trend in biomass. Mussel stocks in the Wadden Sea area of Schleswig-Holstein remained at an average level until 2001. Values then fell to one fifth of their former maximum values (recorded at the end of the 1980s) and, with 1/10 in 2010, dropped to their lowest level since the beginning of monitoring. Mussel stocks have not recovered so far. The shrinking of mussel bed area and biomass is superimposed by the spread of the Pacific oyster; there has been no general regeneration of blue mussel stocks. Between 1980 and 2000, a small but persistent mussel stock existed in the rocky northern intertidal of the island of Helgoland, which is part of Schleswig Holstein, but data from this period is incomplete. Since 2005/2006 the number of recorded individuals in the monitoring area decreased to only approx. 3% in summer 2011 compared to maximum values in 2005/2006. As blue mussel stocks in the „Hamburgisches Wattenmeer” national park are not monitored continuously (pers. comm. Peter Körber), only information about mussel stocks in Lower Saxony and Schleswig-Holstein is presented.