MEPS 309:189-204 (2006)  -  doi:10.3354/meps309189

ABSTRACT: Low cockle Cerastoderma edule stocks in the Dutch Wadden Sea over the last ~15 yr have caused ecological problems, including declines in numbers of some specialized bird species and a sharpening conflict between nature conservationists and fishermen. To study why cockle production and resulting biomass of adult cockles failed, we analyzed long-term (1973 to 2003) data on annual and cohort production of cockles in the Balgzand tidal flat area in the westernmost part of the Wadden Sea. Somatic production was estimated from summed growth increments per half-year period and expressed in ash-free dry weight (AFDW). In adults, positive values in April to August regularly alternated with negative values in September to March, when up to 60% of individual weight gains in the preceding season were lost. Life-time production averaged 0.06 g AFDW for ~3 mo old recruits and 0.21 g for ~10 mo old recruits. The numerical recruit density of a cohort as observed after their first winter predicted subsequent life-time cohort production within narrow limits. The 31 yr mean of net somatic production amounted to 7.3 g AFDW m^–2 yr^–1. The period of largest production during the lifetime of a cohort occurred during their second growing season (when cockles grow from spat to adult size). Years with high production values occurred 1 yr after a highly successful reproductive season, and high biomasses were achieved 1 to 3 yr after a highly successful season. In contrast, low values were observed when a successful recruitment had been >3 yr in the past, and also after severe winters (causing high cockle mortality). Local variation in mean annual production was primarily related to local variation in recruitment success; however, production per recruit was significantly higher at low than at high intertidal levels. The main cause of low biomass values observed in recent years was failing recruitment in off-shore areas. As a consequence, in nearly all of the last 15 years, cockles have been too scarce in the western Wadden Sea to sustain both wintering birds and a commercial fishery.

KEY WORDS: Long-term data · Seasonal growth · Annual recruitment · Secondary production · Tidal flats