Dataset: Mesoscale assessment of sedentary coastal fish density using vertical underwater cameras

Accurate and precise monitoring of absolute density (i.e., number of fish per area or volume unit) of exploited fish stocks would be strongly advisable for deriving the stock's status and for designing proper management plans. Moreover, monitoring should be achieved at relevant (i.e., large enough) temporal and spatial scales. This objective is particularly challenging for data-poor fisheries, as it is often the case of recreational fisheries. Therefore, here, the feasibility of underwater video monitoring (vertical unbaited cameras) for estimating, as a proof of concept, the absolute density (and its ecological drivers) of a coastal sedentary fish species is demonstrated. The absolute density of a small serranid (Serranus scriba) targeted by recreational fishing has been estimated alongside the south coast of Mallorca Island (nearly 100 km). Median fish density ranged between 111 ind/km2 (Es Molinar) and 14,110 ind/km2 (Cabrera). Absolute density is correlated with fishing effort, habitat, and depth. Site-specific, seemingly long-term, effects of fishing effort are negatively correlated with fish density, but short-term effects (assessed by the interaction between fishing effort and before/after the season when recreational fishing accumulated at the studied area) were not detected. We suggest that the short-term effects of fishing may remain undetected because highly exploited sites could inhabit fish that are already non-vulnerable to recreational fishing gears, irrespective of the short-term fishing pressure exerted. Such a process may explain some hyperdepletion patterns and should prevent the use of fisheries-dependent data for monitoring fish density. The results reported here support that fish monitoring with vertical unbaited cameras at large spatial and temporal scales can be a reliable alternative for many species.