Effects of artificial holdfast units on seahorse density in the Ria Formosa lagoon, Portugal

A significant decline in the seahorse populations in the Ria Formosa has been recently reported and holdfast availability suggested as a particularly important variable that influences the abundance of the long-snouted seahorse (Hippocampus guttulatus) and the short-snouted seahorse (H. hippocampus). In order to test the usefulness of artificial holdfast units (AHU) to enhance habitat recovery, several of these structures were deployed at four different locations with distinct environmental characteristics and surveyed for seahorse abundance during a 6 month period. All AHU were colonized by seahorses within a month after deployment, reaching a maximum density of 13.1 seahorse m− 2 at one of the sites. Results suggest that these AHU have the potential to aggregate seahorses in damaged habitats but have limited effect when placed close to natural high complexity habitats. The results from this experiment provide useful guidance in the use of artificial structures to improve degraded seahorse habitats in other similar situations, as part of management plans for seahorse population recovery.