ABSTRACT: Predation by birds on mussel beds was evaluated in the tidal flats of the East Frisian island Spiekeroog (Lower Saxony, Germany) based on measurements in 1991 and 1994. In May 1991, 32 locations with blue mussels Mytilus edulis covered an area of 5.2 km<+k-15>2<+k0>, 311 t AFDW blue mussels being available in the whole study area. Areal extent decreased thereafter. In May 1994, there were 20 locations covering an area of only 2.3 km², a total biomass of 48 t AFDW being available. After the strong spatfalls in the summers of both years, 1290 t biomass were available in the study area in 1991, and 550 t in 1994. Herring gulls Larus argentatus, oystercatchers Haematopus ostralegus and eiders Somateria mollissima are the main predators on blue mussels. During one year (1992), oystercatchers were present on 3.7 million bird days, eiders on 1.1 million bird days and herring gulls on 0.7 million bird days. The annual consumption of blue mussels by birds in the intertidal flats was estimated at 165 t AFDW. The highest proportion of total consumption was by oystercatchers (54%), while eiders consumed 39% and herring gulls 7%. Consumption by all 3 species amounted to 32 g AFDW per m² of mussel bed in 1991 and to 71 g AFDW per m² in 1994. Predation could be compensated by 12% of production in 1991 and by 29% in 1994. In order to evaluate the relevance of predation for the mussel stocks, the share of the total biomass eliminated annually by predation was assessed on the basis of standing stock and annual production. Predation was responsible for 7% of total elimination in 1991 and for 15% in 1994. Thus, factors other than predation were responsible for 85 to 93% of elimination of mussel biomass.

KEY WORDS: Predation · Tidal flats · Lower Saxony · Blue mussels Mytilus edulis · Oystercatchers · Haematopus ostralegus · Eiders Somateria mollissima · Herring gulls Larus argentatus

Published in MEPS Vol. 146 (1997) on January 30
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1997