Data From: Changes in reef fish assemblages in a cross-shelf euphotic-mesophotic gradient in tropical SW Atlantic

Abstract

Despite a vast number of studies about shallow reef fish assemblages, and an increasing amount of studies about mesophotic ones, there are very few cross-shelf assessments including both, shallow and mesophotic depths. Herein, we analyze changes in reef fish assemblages in a cross-shelf euphotic-mesophotic (19-72.6 m) gradient in tropical SW Atlantic. Data were obtained using a novel low-cost 360° stationary video system. Sixty-four species from thirty families were identified. Depth, distance offshore and proportion of consolidated substrate were the main predictors of fish assemblages’ structure. Four distinct fish assemblages were identified across the depth gradient (20-30, 30-40, 40-50 and 50-75 m), with marked shifts in composition, abundance, and trophic structure. Abundance of invertebrate feeders and large roving herbivores declined across the shelf and with increasing depth, while abundance of planktivores and piscivores increased, particularly below 50 m. These changes were likely caused by changes in food availability and habitat suitability. Our low-cost video system yielded high-resolution images that enabled us to accurately survey reef fish assemblages, being therefore a useful method for studies focusing on both, euphotic and mesophotic reef ecosystems. Our results contribute to a better understanding of how reef fish assemblages change with increasing depth in SW Atlantic reefs and suggest that the breakpoint between euphotic and mesophotic fish assemblages is context-dependent rather than previously defined as 30 m depth.

摘要
尽管学界针对浅海礁鱼群落（reef fish assemblages）已开展大量研究，针对中光层（mesophotic）礁鱼群落的研究也日益增多，但同时涵盖浅海与中光层水深范围的跨陆架评估仍极为匮乏。
本文针对热带西南大西洋海域，分析了跨陆架的透光层（euphotic）-中光层（19~72.6米）水深梯度下礁鱼群落的变化特征。本研究采用一套新型低成本固定式360°视频系统采集数据，共鉴定出30科64种鱼类。
水深、离岸距离以及固结底质占比是调控礁鱼群落结构的核心影响因子。在该水深梯度下共划分出4个特征显著的礁鱼群落（20~30米、30~40米、40~50米及50~75米），其物种组成、丰度及营养结构均存在显著变化。随着跨陆架梯度延伸与水深增加，食无脊椎动物鱼类与大型漫游植食性鱼类的丰度逐渐下降；而浮游生物食性鱼类与食鱼性鱼类的丰度则呈上升趋势，尤其在50米以深水域更为明显。上述群落变化可能与食物可获得性及栖息地适宜性的改变相关。
本研究采用的低成本视频系统可获取高分辨率影像，能够精准开展礁鱼群落调查，因此是适用于透光层与中光层珊瑚礁生态系统研究的有效手段。本研究结果有助于深化对西南大西洋礁鱼群落随水深变化规律的认识，同时表明透光层与中光层礁鱼群落的分界点并非此前认定的30米水深，而是依具体环境条件而定。