Abstract
We investigated density-dependent mortality within the early months of life of the bivalves Macoma balthica (Baltic tellin) and Cerastoderma edule (common cockle) in the Wadden Sea. Mortality is thought to be density-dependent in juvenile bivalves, because there is no proportional relationship between the size of the reproductive adult stocks and the numbers of recruits for both species. It is not known however, when exactly density dependence in the pre-recruitment phase occurs and how prevalent it is. The magnitude of recruitment determines year class strength in bivalves. Thus, understanding pre-recruit mortality will improve the understanding of population dynamics. We analyzed count data from three years of temporal sampling during the first months after bivalve settlement at ten transects in the Sylt-Rømø-Bay in the northern German Wadden Sea. Analyses of density dependence are sensitive to bias through measurement error. Measurement error was estimated by bootstrapping, and residual deviances were adjusted by adding process error. With simulations the effect of these two types of error on the estimate of the density-dependent mortality coefficient was investigated. In three out of eight time intervals density dependence was detected for M. balthica, and in zero out of six time intervals for C. edule. Biological or environmental stochastic processes dominated over density dependence at the investigated scale.
Highlights
Survival during the early life phase plays a central role in population dynamics of marine invertebrates [1]
After extra error was added to logDt+1 to reach the original residual deviance, the average modelled slope changed only slightly compared to the slope with measurement error only, which is expected when error is added only to the y-variable and not to the x-variable
Density-dependent decrease was detected for pre-recruit Macoma balthica in three of eight investigated time periods, but there was no evidence for density dependence in Cerastoderma edule
Summary
Survival during the early life phase plays a central role in population dynamics of marine invertebrates [1]. Juvenile mortality typically leads to asymptotic stock-recruitment curves. This shape of the relationship points towards density-dependent mortality in the pre-recruit phase. Recruitment is the subjectively defined stage at which juvenile survivors are regarded as added to a population. In our study system in the Wadden Sea, juvenile bivalves are usually termed recruits at sampling in August Pre-recruit and early post-recruit survival of Wadden Sea bivalves is higher in years with low densities [5,6,7]. The recruitment period is the key to marine population dynamics, as it largely sets year class strength. We do not know much about the causes of density-dependent mortality in this early life phase, how prevalent it is in the first place, and when exactly mortality is density-dependent during this critical period of the life-history
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