Volunteers in marine conservation monitoring: a study on the distribution of seahorses carried out in collaboration with recreational scuba divers.

Seahorses (Hippocampus) live in tropical and temperate waters. Habitat degradation and fishery overexploitation have led to drastic population declines on a global scale. Population monitoring is therefore essential to determine current status and manage conservation. In this first study in Italian waters on the geographical and ecological distribution of the two Mediterranean species, Hippocampus hippocampus and Hippocampus ramulosus, recreational scuba divers were recruited and trained to report sightings. A specially formulated questionnaire was produced and distributed to scuba diving schools and centres. In the 3-year study, 2536 divers spent 6077 diving hours gathering data and completed 8827 questionnaires. Eight percent of the questionnaires showed seahorse sightings, for a total of 3061 sighted specimens, 68% of which referred to Hippocampus ramulosus. The two species had overlapping geographic distributions. Seahorse abundance varied with the northern Adriatic Sea showing greatest abundance followed by the central-southern Tyrrhenian Sea; seahorses were rare in the Ligurian and northern Tyrrhenian Seas. Preferred habitats were shallow areas with either sandy bottoms or Posidonia oceanica (L.) Delile meadows. Seahorse distribution may be correlated with the degree of degradation of P. oceanica meadows. Resource users, like divers, were shown to be willing to take part in biological monitoring and can contribute both in scientific terms by collecting considerable amounts of data over short time periods, and in economic terms by decreasing costs. The greatest limitation with volunteers was the difficulty in obtaining a uniformly distributed sample across time and space. We conclude that recreational divers can play an active part in monitoring the marine environment and that the Mediterranean Hippocampus Mission may be used as a model for biodiversity monitoring.