FISH, cephalopods and associated habitats of the Discovery rise seamounts, Southeast Atlantic

Abstract

In February 2019, seamounts of the Discovery Rise, SE Atlantic (41–45°S, 3°W - 3°E), were explored in support of the application of the Ecosystem Approach to Fisheries Management in the Southeast Atlantic Fisheries Organization (SEAFO) Convention Area. Video records of the seafloor were produced by the Video-Assisted Multisampler System (VAMS) along 15 valid transects conducted in plateau and flank areas of Shannon, Tablemount, Discovery and Heardman seamounts at depths ranging from 394 to 1839 m. Nine benthic seascapes were classified and described based on substrate hardness, texture, slope, physical and biological modifiers as observed in the video images. Predominant water masses were estimated from temperature, salinity and dissolved oxygen vertical profiles obtained by CTD casts in the vicinity of each transect. A total of 366 fishes were seen (total observation time = 25.0 h) and classified in 32 morphotypes, included in 9 orders and 12 families. Most fish morphotypes (14) were included in the Macrouridae family. Family Moridae, on the other hand, included 65.3% of all fish records (239), with two particularly abundant morphotypes: Laemonema sp. (116) and Guttigadus sp. (92). Thirteen cephalopods were observed and classified in five morphotypes; the oegopsid squid Moroteuthopsis ingens was the most abundant of them (6). Despite the taxonomic uncertainties associated with video identifications, the explored region was found to contain a mixture of tropical – subtropical and subantarctic faunas of the Atlantic. Similarities of fauna composition and non-directional beta diversity estimates revealed some degree of seamount identity, but 57.5–61.9% of morphotypes were shared among seamounts (Jaccard = 0.425, Sørensen = 0.381). Fishes and cephalopods were more frequently observed on the shallower plateau areas under the influence of warmer and more oxygenated Antarctic Intermediate Waters. Depth and related factors did not influence richness, but dissimilarities in fauna composition between video transects increased with increasing depth intervals. Spatial habitat heterogeneity may have accounted for the increased beta diversity within seamounts. The most widely explored Tablemount seamount contained the most diverse observed sites, but also a considerable spatial variability likely associated with seascape heterogeneity. In two sites (transects 11 and 14), there were exceptionally high aggregations of the morid cods Laemonema sp., at the shallowest ‘gravely’ site explored (397 m), and Guttigadus sp., at a site of intermediate depth (1020 m) covered mostly by soft sediments. Neither of these species (or congenerics) have been previously classified as ‘seamount-aggregating” species. On the other hand, the Patagonian toothfish (Dissostichus eleginoides) and other commercial species known to aggregate in these seamounts were not found in the explored areas.