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.

Ample evidence has indicated shifts in distribution of fish populations in response to environmental stress. However, most studies focused at the whole population scale. This neglects the spatial dynamics between groups of different body size (body size groups), that fundamentally shapes the spatial structure of a population. Here, we explored the mechanisms that modulate spatial dynamics of body size groups, and applied our analyses to three North Sea fish populations which experienced severe declines in biomass from 1977 to 2019: Atlantic cod (Gadus morhua), haddock (Melanogrammus aeglefinus), and whiting (Merlangius merlangius). All three populations exhibited strong declines in the overlapped area between body size groups in winter over 43 years, yet their mechanisms differed. These declines were either due to (1) different magnitudes of contraction of the distribution area of body size groups; and/or (2) different speeds and directions of spatial shift among various body size groups, both increasing spatial segregation within populations. These patterns were either associated with ocean warming, and/or declining population biomass, and such associations often varied according to distinct body size groups. Our analytical approach provides a powerful tool for identifying vulnerable populations under environmental stress and can be generalized to study a variety of size/age structured populations at various ecosystem types.