Co‐occurrence between size groups within populations decreases with maximum body size across marine fish populations

The degree of co‐occurrence between size groups within a population influences key biological processes, such as connectivity and competition within a population. However, the links between co‐occurrence between size group pairs, abundance, and maximum body size among populations remain largely unclear. In this study, we analyzed spatial time series data from 1982 to 2019 to investigate the co‐occurrence between size groups of 54 marine fish populations across three regions (North Sea, Scottish West Coast and eastern Bering Sea). We show that for all the regions, the probability of co‐occurrence for most size group pairs decreased with increasing maximum body size across populations. This suggests that the size group pairs within larger‐bodied populations tend to segregate more in space, whereas those within smaller‐bodied populations tend to mix and aggregate. The co‐occurrence for most size group pairs within populations did not change with abundance, indicating that the tendency of the size groups to aggregate or segregate was mostly insensitive to abundance. Moreover, across populations and regions, the co‐occurrence‐abundance relationship of most size group pairs did not change with the maximum body size except five size group pairs in the North Sea, suggesting that compared to smaller‐bodied populations, these size group pairs within larger‐bodied populations in this region tend to aggregate more with increasing abundance and segregate more with decreasing abundance. These findings provide new insights into the spatial dynamics of size groups, offering valuable implications for understanding population spatial structure and connectivity in marine ecosystems.