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.

Delineating a threshold migration rate for demographic independence is important for understanding connectivity among fragmented populations and defining management units for conservation and harvest regulation. In turn, defining management units is an essential step in sustainable management to avoid unintentional depletion of resources managed for conservation or harvest. The 10% rule of demographic connectivity is a rule of thumb that delineates the threshold of demographic independence when the behavior of two populations shifts from synchronous at >10% to independent at <10%. However, the accuracy of the 10% rule to real world scenarios and application to natural resource management is unknown. We evaluated the 10% rule using simulation for two life history types: Pacific cod, Gadus macrocephalus, a gadid with relatively fast growth, and blackspotted rockfish, Sebastes melanostictus, a long-lived rockfish species. Results were obtained by simulating a real-world tool for evaluating demographic connectivity, positive correlation in estimated population sizes. We assessed the effect of migration on demographic connectivity on otherwise independent populations under one- and two-way migration, and with different population sizes and life history parameters. Sensitivity testing showed that positive correlation in population size does not occur in roughly a quarter of simulations, regardless of the migration rate. When positive correlation in population size does occur, mean migration rates over all simulations were between 5% and 10%: 0.089 (8.9%) for blackspotted rockfish and 0.058 (5.8%) for Pacific cod. However, the range of migration resulting in demographic connectivity was large, 0.02 – 0.44 for blackspotted rockfish and 0.02 – 0.40 for Pacific cod.