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Fish populations that experience rapid-range shift decline noticeably, up to 50 per cent over a decade. The populations affected most are those living on the northern poleward edges of their species’ range. Researchers combined data from two large databases to examine the population numbers of range-shifting species. Their analysis revealed that extremely fast poleward shifting species, defined as upward of 17 kilometers per year, show marked declines in population, compared to negligible increases in populations that did not shift.