Effects of mat morphology on large Sargassum-associated fishes: observations from a remotely operated vehicle (ROV) and free-floating video camcorders

Abstract

Vagile large juvenile and adult fishes are often under-represented in traditional sampling of Sargassum–associated fishes in the open ocean. We used underwater video recordings from free-floating camcorders and a remotely operated vehicle (ROV) to assess the relative abundance of large mobile fishes under large Sargassum mats (> 10 m diameter), under dispersed clumps of Sargassum (< 1 m diameter), and in open water without Sargassum as a reference. In addition, we conducted dipnet sampling in each Sargassum treatment for a comparison to traditional methods. All samples were obtained in September 1992 along the western wall of the Gulf Stream off Cape Hatteras, North Carolina. A total of 31 fish taxa were identified from both video and dipnet collections. Only 8 taxa were identified in both video and dipnet collections, while 11 taxa were seen only in video and 10 were only found in dipnet collections. Dipnet collections were dominated by juvenile balistids and other small, cryptic fishes, while the video observations were mainly of larger, rapidly-moving carangids. Fish diversity increased with the amount of continuous Sargassum habitat: four taxa were observed when no Sargassum was present, 12 under clumps, and 19 under mats. Our results indicated that mat morphology significantly affects the Sargassum–associated fishes, and that both video and traditional capture methods are complementary and should be used together to accurately census this community.