1. Shell production by cockles Cerastoderma edule was studied to examine whether or not the present licensed rate of shell extraction in the Dutch Wadden Sea exceeds the current rate of shell addition to the exploitable stocks. 2. Long‐term data on numbers of cockles and weights of their shells were used to estimate their annual production on Balgzand, a 50‐km2 tidal flat area in the western‐most part of the Wadden Sea. During the 1969–97 period, it amounted to an average of 125 g m–2, including 107 g m–2 of shells large enough to be exploitable for shell‐lime fishery. 3. The very irregular annual recruitment of cockles was the main cause of the wide 95% confidence limits (74 and 140 g m–2 year–1) of this 28‐year estimate. Moreover, high mortality rates in severe winters substantially reduced production per recruit in some year classes. 4. About one‐third of the estimated production does not reach exploitable stocks, because it is fragmented by birds (particularly eider ducks), permanently buried in the sediment, or removed by the fishery for live cockles. 5. During the last few decades, the estimated mean amount added annually to the exploitable stocks was 88 million kg or 132 000 m3 of large cockle shells. This amount compares favourably with the current annual level of removal of 134 000 m3 of shells, three‐quarters of which are cockles. 6. Even at temporarily lower production rates, the exploitation of shell stocks at its present rate is not expected to lead to a rapid exhaustion of the existing stocks in the tidal inlets of the Dutch Wadden Sea, as these stocks will be in the order of a few million m3.
Journal of Applied Ecology – Wiley
Published: Feb 1, 1999
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