The alarming decline of spawning aggregations across the world highlights the need to improve their long-term monitoring and protection. Here, we examined the ability of diver operated stereo-video to characterize reef fish spawning aggregations, using an opportunistic encounter with the first spawning aggregation of the Mexican hogfish, Bodianus diplotaenia, recorded in the Galapagos Marine Reserve. We aimed to provide estimates of total density, length and reproductive behaviour, using quantitative surveys and 3D representation of the spawning event. 3D scenes of aggregating fish in relation to reef substrate were produced to compute distances between individuals (of the same and different sex), and between individuals and the substrate. Males swam significantly higher in the water column and further away from each other when compared to females, which displayed sociable behaviour closer to the bottom. The mean fork length of males (39.79 ± 6.98 cm) was nearly twice the size of females (24.14 ± 5.39 cm). We show that diver operated stereo-video system can provide accurate baseline measures of fish length, density and reproductive behaviours. Our study offers a new method to characterize fish mating systems, and contributes to expanding the range of easily repeatable low-impact tools for the long-term monitoring of spawning events.

全世界产卵聚集的惊人减少表明，有必要改善其长期监测和保护。在这里，我们通过与墨西哥猪鱼Bodianus diplotaenia的第一个产卵集合的机会相遇，研究了潜水员操作的立体视频表征礁鱼产卵集合的能力，记录在加拉巴哥海洋保护区。我们旨在通过使用定量调查和产卵事件的3D表示来提供总密度，长度和生殖行为的估计。生成了与珊瑚礁基质有关的鱼类聚集的3D场景，以计算个体（同性别和不同性别）之间以及个体与基质之间的距离。与雌性相比，雄性在水柱中游动明显更高，并且彼此之间的距离更远，后者表现出的社交行为更接近底部。雄性的平均叉长（39.79±6.98 cm）几乎是雌性的平均长度（24.14±5.39 cm）的两倍。我们显示，潜水员操作的立体声视频系统可以提供鱼长，密度和生殖行为的准确基线测量值。