Spatial overlap between size groups within marine fish populations in response to abundance and life history traits

The degree of spatial overlap between size groups within a population (hereafter, spatial overlap) influences key biological processes, such as connectivity and competition within a population. However, how changes in spatial overlap with abundance vary across populations, particularly in relation to life history traits, is largely unknown. In this study, we analyzed spatial time series data from 1982 to 2019 to investigate how spatial overlap responded to changing abundance in 56 marine fish populations across four regions. Our results show that 33 populations exhibited positive relationships between spatial overlap and abundance in over 50% of size group pairs. In the North Sea and on the Scottish West Coast, certain size group pairs showed a stronger spatial overlap-abundance relationship in populations characterized by slower growth (higher growth coefficient, longer lifespan, later maturity) and larger body sizes (greater maximum and asymptotic size), compared to populations with faster growth and smaller body sizes. This suggests that size groups within slower-growing and larger-bodied populations tend to spatially segregate more rapidly when abundance declines, making them vulnerable to local disturbances such as in hotspots of fishing pressure and habitat destruction. Our analytical approach suggests that the spatial overlap-abundance relationship could serve as a useful vulnerability index for conservation and management efforts.