| Resumo: | With an increase in anthropogenic activities around the world, negative impacts affecting many species and the environment have also been rising. Greater attention has been given to the global rehabilitation of degraded habitats. Two seahorse species, the short-snouted seahorse (Hippocampus hippocampus) and the long-snouted seahorse (H. guttulatus), inhabit the shallow waters of the ria Formosa lagoon and have shown a substantial decrease in number due to rising anthropogenic impacts and their inherent vulnerability. This study consisted of two main objectives. The first was to design, test and select, under controlled conditions, a new Artificial Holdfast Unit (AHU) to be used in the wild. The second, was to deploy the selected AHU in the lagoon, in an attempt to contribute to the rehabilitation of the seahorses degraded habitats and help restore their numbers to earlier estimates. The three AHUs tested in the first study differed in material and shape. The first AHU used polyethylene strips attached to a mesh base; the second, used sisal rope attached to a mesh base; the third, consisted of an open cube made of welded iron. The sisal rope was later tested against an enclosed iron cube in the presence of two seahorse predators: the green crab and cuttlefish (Carcinus maenas and Sepia officinalis). 100 replicas of the selected AHU were constructed and consequently deployed in a selected Marine Protected Area (MPA) in the ria Formosa. Results showed that the deployed AHUs maintained their structural form and were able to offer a stable habitat for the seahorses in the lagoon. A significant difference was observed between the sisal rope and the other structures being tested, with more than 75% of the seahorses, choosing the natural fibre. After the deployment of the AHUs in the ria Formosa, the abundance of H. guttulatus continued to be significantly higher than H. hippocampus (P=0.04) and significant differences were found between the abundance of seahorses on the three monitorings (P=0.878). This study demonstrated that even in the presence of previously successfully tested polyethylene AHUs, seahorses preferred the newly designed sisal rope structure. The natural fibres of this AHU were also preferred by the seahorses when common predators were present and maintained its structure efficiently when deployed in the ria Formosa.
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