Growth as a measure for environmental conditions and relatedness of North Sea fish stocks

Abstract

Information about the ecological connections between species is needed to make the shift from fisheries management strategies centred around single species to ecosystem-based fisheries management. Growth rates of fish strongly depend on the environment. Individual growth curves could therefore contain valuable information about the environmental conditions experienced by an individual throughout its life. In this study, we explored whether estimated growth curves contain information about the competition within and between species. We estimated growth curves for 10 species in 10 North Sea roundfish areas based on length-at-age data from the North Sea International Bottom Trawl Survey (NS-IBTS) fish survey by fitting a von Bertalanffy growth model with varying asymptotic length. From these model fits, we extracted a relative measure for the temporal variation in growth in body size of a stock, which can be used as an indicator of the experienced environmental conditions. We found signs of density-dependent growth in at least 6 out of the 10 species for some specific areas as well as all North Sea areas combined. In addition, we used cluster analysis to explore the relatedness in ecological growth conditions between species and areas. Ecologically related species tended to show more similar temporal variation in ecological growth conditions than ecologically more distant species. Likewise, areas that were geographically closer showed more similarity in ecological growth conditions compared to geographically more distant areas. This suggests that information on ecological conditions derived from growth curves provide a useful way to distinguish ecological groups or regions without the need for additional stomach sampling or ecological studies.