Abstract
Distinct areas of seabed, called seascapes, 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. We explored the relationship between seascapes and trends in the biomass density of several North Sea fish species. We divided the North Sea into 10 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 and found independent biomass density trends between adjacent seascapes at a time interval of 0 yr in all species assessed. Long-term causal dependencies in biomass density occurred at time lags of 1-2 yr across different gradients of exchange: (1) both directions from North to South; (2) unidirectional, North-South; (3) unidirectional, South-North; (4) unidirectional, East-West; and (5) no clear direction. Our findings indicate that the separation in (a)biotic conditions between North 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.
Highlights
The dynamics of marine fish populations are typically categorized as regulated primarily by top-down processes or by bottom-up processes (Wollrab et al 2012, Heath et al 2014, Vinueza et al 2014)
Due to the strong link between seabed morphology and community structure of benthic invertebrates (Beaman et al 2005, Kaskela et al 2017), seascapes are often used as a proxy for mapping marine benthic habitats (Kostylev et al 2001, Brown & Collier 2008)
We know little about the extent to which seascapes affect the dynamics of fish populations, despite the fact that the few case studies available show it could help explain the spatial distribution of fish resources (Pittman & Brown 2011, Getsiv-Clemons et al 2012)
Summary
The dynamics of marine fish populations are typically categorized as regulated primarily by top-down processes (i.e. predation) or by bottom-up processes (i.e. resource production) (Wollrab et al 2012, Heath et al 2014, Vinueza et al 2014). Recent research revealed that the central and lower part of the North Sea comprises 8 distinct seascapes excluding the coastal zone (van der Reijden et al 2018), and that these were selectively targeted by commercial fishing vessels This preferential selection provides some indirect link to the availability and distribution of fish resources, a more complete study exploring the relationship between seascapes and the population trends of North Sea fish is missing. If seascapes define distinct abiotic environments that are important for the distribution of fish populations, we expect biomass densities in different seascapes to be unrelated to each other, they can still be related across longer time lags This result could be interpreted as the separation in conditions between seascapes representing relevant barriers to the processes determining the amount of fish biomass observed. Our study tests a new approach to examine intraspecific dependencies in the biomass trends of North Sea fish
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