A novel underwater visual census: Seahorse population survey as a case study

Two different Underwater Visual Census (UVC1 and UVC2) were performed at three sites in the Ria Formosa, using 30 meter transect belts, and compared for differences in seahorse density and holdfast availability. Each UVC differed in transect placement and total survey area coverage. The observed Hippocampus guttulatus density was significantly higher (P<0.05) using UVC2 for Site 1 and Site 2, but not in Site 3. No statistically significant differences were found in holdfast availability (P>0.05) between the two UVC methods. We conclude that the UVC2 method is more effective in quantifying abundance of seahorses compared with UVC1, particularly in low population density scenarios. This method can also be used in assessing other seahorse species' populations, in similar habitats.