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 uses to be the case of recreational fishing. Therefore, here, as a proof of concept, the feasibility of underwater video monitoring (vertical unbaited cameras) for estimating the absolute density of coastal sedentary fish species is demonstrated. The absolute density of a small serranid (Serranus scriba) has been estimated with suitable accuracy and precision alongside the south coast of Mallorca Island (nearly 100 km). Fish density ranged between 2 ind/km2 and 59,115 ind/km2. These large differences are explained by exposure to fishing, and by minor site-specificities of habitat and depth, all well within the previously reported environmental range of the studied species. Site-specific, seemingly long-term, effects of fishing are negatively correlated with fish density, but short-term effects (assessed by the interaction between exposure to fishing 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 fishing, 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.