Reef Check Med - key Mediterranean marine species 2001-2020

The dataset consists of abundance records of 43 marine taxa in the Mediterranean Sea from 2001 to 2020. Selected taxa include some protected species, non-indigenous species, ecosystem engineers, species sensitive to climate change, and/or threatened by human activities. Taken together, these taxa represent key ecological aspects of Mediterranean subtidal habitats (Cerrano et al., 2017) and can be used to calculate the MedSens, an index of the environmental status of subtidal rocky coastal habitats based on species sensitivity (Turicchia et al., 2021). Observations were carried out by the EcoDivers (trained scuba divers, freedivers and snorkelers) according to a rigorous protocol (RCMed U-CEM) developed by the association Reef Check Italia onlus. Records include geographic coordinates, site name, date, survey depth range, prevailing habitat, searching time, taxon name, abundance category (including absence), occurrence depth range and the name of the EcoDiver who made the identification.

The Reef Check Med Underwater Coastal Environment Monitoring (RCMed U-CEM) protocol is a Citizen Science initiative, developed by the Italian non-profit association Reef Check Italia onlus, aimed to assess the ecological status of the Mediterranean marine coastal habitats (Cerrano et al., 2017; Turicchia et al., 2021). Taxa were selected based on a combination of criteria including identification easiness and being key indicators of the shifts Mediterranean subtidal habitats may undergo under local pressures and climate change. When it is not easy to discriminate between species, genus or higher level was chosen, as in the case of the two protected Mediterranean seahorses. Before diving, each EcoDiver (i.e. the trained volunteer scuba diver, freediver or snorkeler) chooses which and how many of the 43 taxa, included in the protocol, will be searched for, according to the expected habitat typology and personal motivations. Abundance (using numerical or descriptive categories according to the countability of organisms) and occurrence depth range (min and max, in metres) of searched taxa are recorded. Taxa actively searched for, but not encountered, are recorded as absent. The adopted abundance category are (in brackets the corresponding descriptive categories): Category 0: 0 specimens (absent) Category 1: 1 specimen (isolated specimen) Category 2: 2 specimens (some scattered specimens) Category 3: 3-5 specimens (several scattered specimens) Category 4: 6-10 specimens (a crowded area) Category 5: 11-50 specimens (some crowded areas) Category 6: > 50 specimens (several crowded areas) Diving sites are localised using global positioning system (GPS) receivers, nautical charts or known points (e.g., mooring buoys at MPAs). Geographical coordinates (WGS84) are recorded with ± 6 arc-second (i.e. about 200 m) accuracy, which is in the distance range usually explored by divers in a single dive. The prevailing habitat is identified according to the following categories: - Coastal rocks - Offshore rocks - Rocky cliff - Posidonia - Posidonia and sand - Posidonia and rocks - Cave - Metal wreck - Sand - Mud - Breakwaters and ports - River mouth - Coastal lagoon Records include geographic coordinates (decimal degrees, WGS84), site name, date, survey depth range, prevailing habitat (from the list), the sampling size in term of searching time (minutes), taxon name, abundance category (from 0 to 6, see categories description), occurrence depth range (min and max, in metres) and the name of the EcoDiver who made the identification. Data passed through quality control, based both on automatic filters (e.g. consistency among dive and observation depth ranges) and on manual procedures (e.g. matching between the site name and geographic coordinates), are made freely available (CC BY 4.0).