Annual cockle Cerastoderma edule production in the Wadden Sea usually fails to sustain both wintering birds and a commercial fishery

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.

Long-term and large-scale variability in productivity of the tellinid bivalve Macoma balthica on Wadden Sea tidal flats

“…As we explained earlier (Beukema & Dekker 2006), twice-annual sampling at maximum and minimum individual weights would be preferable, but it is theoretically feasible to obtain an unbiased estimate by sampling once annually about half-way between the peak and trough of individual weight. The exact timing should be assessed by following some annual cycles of changes in individual weights and numbers.…”

“…Balgzand estimates of Macoma balthica total somatic production of ~3 or ~4 g AFDM m -2 yr -1 for the entire population and ~3 g AFDM m -2 yr -1 for the post-spat population are less than half the production of cockles Cerastoderma edule in the same area (Beukema & Dekker 2006). The present estimate of M. balthica production would amount to between 5 and 10% of the estimated total benthic secondary production on Balgzand.…”

“…In such species, unbiased estimates of annual net production can be best obtained by sampling populations at annual peaks and troughs of individual weight, as described by Beukema & Dekker (2006). For M. balthica this would involve at least twice-annual sampling: in March and around July 1.…”

“…As explained by Beukema & Dekker (2006), obtaining unbiased estimates of production rates in species with an annual cycle of gain and loss of soft-part weight makes special demands of the data set, including sampling at the right time of the year, i.e. preferably timed to coincide with the 2 annual reversals of weight change.…”

Spatial variation in the abundance and condition of the bivalve Nuttallia olivacea in relation to environmental factors and sublethal predation

“…These results are understandable because bivalves at plots with lower elevation can take in more food because of the longer submersion times. The importance of elevation for growth and production is well known (Peterson & Black 1987, Harvey & Vincent 1991, although the higher production at the lower intertidal is not common: the highest production of some bivalve species is observed at intermediate intertidal levels (Beukema & Dekker 2006, Dekker & Beukema 2007.…”