First Insight of Meso- and Bentho-Pelagic Fish Dynamics Around Remote Seamounts in the South Atlantic Ocean

Abstract

Seamounts have long been recognised as hotspots for pelagic productivity and diversity in the world’s open ocean habitats. Recent studies have suggested that productivity may vary greatly between different seamounts, depending on complex interactions between the bathymetric features and local oceanography. These processes may enhance local primary production which support elevated biomass at higher trophic levels. In addition to enhancing local biomass, seamounts may also act as aggregative features, attracting pelagic species from the surrounding waters. Such characteristics make seamounts attractive targets for fisheries. However, as these unique habitats are localised and relatively small, they are vulnerable to overexploitation, which may have detrimental impact on the wider region. Mapping and quantitative assessments of the fish biomass at different seamounts are crucial prerequisites to identifying vulnerable seamounts and will aid toward understanding the dynamics of these important ecosystems and their vulnerability to fishing pressures. We used fisheries acoustics during two expeditions in 2018 and 2019, to investigate the distribution and abundance of fish and micronekton on and around five little studied seamounts of Tristan da Cunha, a remote archipelago in the South Atlantic Ocean. The results confirmed increased productivity at the seamounts, compared to the surrounding open ocean with higher acoustic backscatter values, a proxy for biomass, particularly at the shallower (~200 m depth) seamounts. Fish largely dominated the backscatter on most of the seamounts especially over the plateau areas where large densities of prey fish, primarily the mesopelagic Maurolicus inventionis, were detected. Very large aggregations, thought to consist of bentho-pelagic fish, were also observed over the slope of McNish Seamount that resulted in very high biomass estimates. Aggregations of this size and magnitude, have, to our knowledge, never been mapped or quantified on seamounts, using acoustic methods. Specific physical processes, such as enhanced retention and vertical mixing that were identified by an oceanographic model, may be some of the drivers of the enhanced fish biomass detected at McNish. The characteristics of the seamounts observed in this work suggest that these habitats are highly suitable for the presence of large predatory fish that can utilise these areas as their primary habitat or as important foraging grounds.

Depth and benthic habitat influence shallow and mesophotic predatory fishes on a remote, high-latitude coral reef

“…Previous studies have also linked the distribution and density of top order predatory fishes to prey availability [70,71]. In fact, Boaden and Kingsford [72] suggest that high densities of predators are stronger predictors of prey density than the structure and composition of habitat.…”

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

“…This study will characterize seafloor habitats (seascapes) and the observed fish and cephalopod fauna observed during a series of ROV transects conducted on selected seamounts of the Discovery Rise. These are conspicuous and mobile components of benthic and benthopelagic megafauna that concentrate permanently or temporarily at seamounts often taking advantage of surface-originated food sources and biogenic habitats, formed by cold water corals and sponges (Auster et al, 2005;Porteiro and Sutton, 2007;Morato and Clark, 2007;Campanella et al, 2021). Recent studies have revealed that depth, seafloor habitats, local circulation processes and distribution of suspension-feeding organisms may drive fish and cephalopod species distribution in South Atlantic topographic rises (i.e.…”

“…Recent studies have revealed that depth, seafloor habitats, local circulation processes and distribution of suspension-feeding organisms may drive fish and cephalopod species distribution in South Atlantic topographic rises (i.e. Rio Grande Rise, São Paulo Ridge, Tristan da Cunha seamounts) (Perez et al, 2018(Perez et al, , 2020Campanella et al, 2021). These factors contribute to habitat diversity within a single topographic feature, potentially selecting for distinctive mesoscale megafauna assemblages and affecting overall seamount diversity (e.g.…”