Influence of the Atlantic Ocean thermal anomaly on the Longsnout seahorse Hippocampus reidi in a Brazilian estuary.

One of the consequences of climate change is an increase in the temperature of the oceans, which is considered to be one of the greatest impacts on biodiversity. Fish may respond to this impact in several ways, including shifts in their patterns of occurrence. The present study investigated the variation in the structure of a H. reidi population between 2015 and 2017 in the northern Guaíba Island area, highlighting a possible relationship to thermal anomaly associated with the El Niño phenomenon. The seahorse population monitoring was performed monthly, recording sex ratio, abundance, juvenile and adult proportion, depth of occurrence, total length and the holdfast which the seahorse were found attached. The influence of the El Niño event on the study population was evaluated by the correlation of the thermal anomaly data reported for the Tropical South Atlantic Index. Seahorse density on northern Guaíba island was positively and significantly correlated with water temperature, but the sex ratio and number of juveniles were not. The diversity of holdfasts used increased over the study period and was inversely proportional to the thermal anomaly. These results suggest that the thermal anomalies caused by the El Niño in the South Atlantic might trigger migration behaviour in the study species, providing a large aggregation during that period in Guaíba island.