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Causal links between North Sea fish biomass trends and seabed structure

Mark Rademaker*, Isabel M. Smallegange, Anieke van Leeuwen

*Corresponding author:

ABSTRACT: Distinct areas of seabed are known to shape benthic habitats and communities, yet little is known about the extent to which they affect the dynamics of marine fish populations. In this study we explore the relationship between distinct areas of seabed, called seascapes, and trends in the biomass density of several North Sea fish species. We divided the North Sea up into ten seascapes using standardized methods. Time series of fish biomass density were derived from the North Sea International Bottom-Trawl Survey (NS-IBTS), and aggregated to the seascape level. We analysed the interdependencies between these time series using a causal association network. We found independent biomass density trends between adjacent seascapes at the time interval of zero in all species assessed. Long term causal dependencies in biomass density occurred at time lags of 1-2 years across different gradients of exchange: (1) both directions from North-South, (2) unidirectional from North-South, (3) unidirectional from South-North, (4) unidirectional from East-West, and (5) no clear direction. Our findings indicate that the separation in (a)biotic conditions betweenNorth Sea seascapes can represent relevant barriers to the processes determining the observed fish biomass density. We found that non-fusiform morphology and demersal habitat preferences best explained short term causal dependencies. This combination is particular to the flatfish and ray species included in the present study. Contrarily, the movement of large, long-lived, benthopelagic species best explained long term causal dependencies. Our work highlights how causal association networks can be used to study the temporal dependencies between spatial time series in ecology.