Methylmercury (MeHg) is a neurotoxic pollutant that bioaccumulates and biomagnifies in aquatic food webs, impacting the health of piscivorous wildlife and human consumers of predatory fish. While fish mercury levels have been correlated with various biotic and abiotic factors, many studies only measure adults to characterize the health of locally fished populations, omitting information about how local fish bioaccumulate mercury relative to their growth. In this study, we sought to establish length: total mercury (THg) concentration relationships in juvenile and adult fish of four genera (sunfish, yellow perch, white perch, and killifish) across six freshwater pond systems of Nantucket Island to determine safe consumption sizes across species and environmental conditions. A wide length range (2-21 cm) was utilized to develop linear regression models of ln-THg versus fish length. In most cases, different genera within the same pond indicated similar slopes, supporting that all four genera share comparable features of feeding and growth. Comparing individual species across ponds, differences in ln-THg versus fish length were attributable to known environmental Hg-modulators including surface water MeHg levels, pH, and watershed area. Referencing human health and wildlife criteria, our results confirm that numerous Nantucket freshwater ecosystems contain elevated fish THg levels, which could impact the health of not only piscivorous wildlife in all measured ponds but also recreational fishers in at least two measured systems. Future studies should measure THg levels across juvenile and adult fish to detect potential differences in the slope of THg concentration across fish length relevant for local consumption advice.