Range expansion and population shifts of estuarine fishes in a changing subtropical estuary

Catastrophic losses (ca. 95%) of seagrass and increased environmental degradation have occurred during the past decade in the Indian River Lagoon (IRL) estuary on the US Atlantic coast of Florida. Changes were observed in the abundance of 2 closely related sparid fishes in these estuarine waters based on fishery-independent monitoring efforts over 22 yr: sea bream Archosargus rhomboidalis significantly increased, while sheepshead A. probatocephalus simultaneously declined. These abundance trends and a northward expansion of sea bream into areas where they had never been documented were associated with an annual minimum water temperature increase of approximately 1.5 degrees C, an annual mean water temperature increase of 0.9 degrees C, and an annual mean salinity increase of 7.6 ppt. Lost seagrass habitat was often replaced by species belonging to the attached macroalgae genus Caulerpa or remained bare. Our findings suggest that sea bream, a tropical herbivore, are expanding poleward into the northern IRL, which may further negatively impact the potential recovery of sheepshead populations via competition. Furthermore, observed declines in seagrasses and populations of dependent species, including sheepshead, will likely continue if anthropogenic perturbations persist or expand. Long-term monitoring is essential for the timely detection of population changes and range shifts to better refine direct fisheries management of existing species and determine the potential need for management of recently expanding fish species should new fisheries emerge. Effective monitoring also allows for more proactive restoration of critical habitats and water conditions to minimize further adverse effects on fishes and other estuarine biota.