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

Accurate and precise monitoring of the absolute density (i.e., number of fish per area or volume unit) of exploited fish stocks would be strongly advisable for deriving stock status and for designing proper management plans. Moreover, monitoring should be achieved at relevant (i.e., sufficiently large) temporal and spatial scales. This objective is particularly challenging for data-poor fisheries, as is often the case for recreational fisheries. Therefore, 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 was estimated along the southern coast of Mallorca Island (nearly 100 km). The median fish density ranged between 111 ind/km2 (Es Molinar) and 14,110 ind/km2 (Cabrera). Absolute density was correlated with fishing exposure, habitat, and depth. Site specific, seemingly long-term, effects of fishing exposure were negatively correlated with fish density, but short-term effects (assessed by the interaction between fishing exposure and before/after the season when recreational fishing occurred in the study area) were not detected. We suggest that the short-term effects of fishing may remain undetected because highly exploited sites could contain fish that are already not vulnerable to recreational fishing gear, irrespective of the short-term fishing pressure exerted. Such a process may explain some hyper-depletion patterns and should preclude the use of fisheries-dependent data for monitoring fish density. The results reported here indicate that monitoring fish abundance with vertical unbaited cameras at large spatial and temporal scales can be a reliable alternative for many species.精准监测开发中鱼类种群的绝对密度（absolute density，即单位面积或体积内的鱼类数量），对于评估种群现状、制定合理的渔业管理计划至关重要。此外，监测需在具有生态学意义的（即足够大的）时空尺度上开展。对于数据匮乏渔业（data-poor fisheries）而言，达成这一监测目标尤为困难，休闲渔业通常便属于此类范畴。为此，本研究以概念验证（proof of concept）为目的，验证了水下视频监测（underwater video monitoring）采用垂直无诱饵相机（vertical unbaited cameras）的方式，用于估算沿岸定栖鱼类（sedentary fish species）绝对密度及其生态驱动因子（ecological drivers）的可行性。本研究在马略卡岛南部沿岸（总长近100公里的海域），对受休闲渔业捕捞的小型鮨科鱼类（serranid，纹斑鲈*Serranus scriba*）的绝对密度进行了估算。鱼类密度中位数介于111尾/平方公里（埃斯莫利纳尔港，Es Molinar）与14110尾/平方公里（卡布雷拉，Cabrera）之间。绝对密度与捕捞暴露度、栖息生境及水深显著相关。位点特异性的捕捞暴露度长期效应与鱼类密度呈负相关，但未检测到短期效应；短期效应通过捕捞暴露度与研究区域休闲渔业季前后的交互作用进行评估。我们推测，未检测到短期捕捞效应的原因可能是：在高强度捕捞位点，无论施加的短期捕捞压力如何，鱼类可能已对休闲渔业渔具产生逃避性，不再易被捕捞。这一过程可解释部分超耗竭（hyper-depletion）模式，同时也说明不应使用依赖渔业的监测数据（fisheries-dependent data）开展鱼类密度监测。本研究结果表明，在大时空尺度下采用垂直无诱饵相机监测鱼类丰度，可作为多数鱼类物种的可靠监测手段。