Spatial Variation in Pelagic Wildlife Assemblages in the Ascension Island Marine Protected Area: Implications for Monitoring and Management

Abstract

Bathymetric features such as islands and seamounts, as well as dynamic ocean features such as fronts often harbour rich marine communities. We deployed mid-water baited remote underwater video systems on three expeditions in Ascension Island’s Exclusive Economic Zone (EEZ), surveying the waters associated with six different bathymetric and dynamic ocean features: Ascension Island, two shallow seamounts (summits ≤ 101 m), one deeper seamount (summit > 250 m), apparent fronts, and haphazardly sampled open ocean areas. At Ascension Island, the pelagic assemblage consisted of a moderate proportion of predators and a diverse range of other taxa, including turtles, dolphins, and large non-piscivores. At the two shallow seamounts, sharks, tunas, billfish, and other large pelagic predators formed the vast majority of the assemblage, contributing > 99.9% of biomass and > 86% of abundance. At the deeper seamount, the pelagic community was comparatively depauperate, however the functional composition of its assemblage indicated some similarities to the shallow seamounts. Apparent fronts did not significantly differ from random offshore sites for metrics such as total abundance and taxonomic richness. However, they harboured assemblages with more abundant sharks, tunas, and large piscivores than random ocean open locations and these differences may be driven by certain front-associated species. Our results illustrate that pelagic assemblages vary markedly among different physical and oceanographic features and that seamounts appear particularly important for pelagic predators. The diversity and abundance of the assemblage, as well as the threatened status of many of the species observed, serve to highlight the conservation value of the Ascension Island EEZ. Our results also provide important baseline information of pelagic wildlife assemblages against which the performance of the recently implemented Ascension Island Marine Protected Area can be evaluated.

Impacts of Climate Change on the Ascension Island Marine Protected Area and Its Ecosystem Services

“…The AIMPA's pelagic zone is home to an abundance of marine megafauna including sharks, tunas, billfish, rays and cetaceans (Thompson et al., 2021). Many of these are threatened species (International Union for Conservation of Nature, 2023), and the creation of the AIMPA may play a substantial role in the protection of these species.…”

Remote seamounts are key conservation priorities for pelagic wildlife

“…This is especially the case in remote regions where scientific information is even more scarce. We do know, however, that pelagic species associate strongly with bathymetric features such as seamounts, undersea canyons, and oceanic islands (Bouchet, 2015; Morato et al., 2010; Thompson et al., 2021). Both physical and biological drivers make these features important locations for foraging, reproduction, navigation, and other essential functions of pelagic wildlife (Garrigue et al., 2015; Maguire et al., 2023; Rogers, 2018).…”

“…The protection of seamounts thus represents a significant conservation opportunity. Importantly, different bathymetric structures support distinct assemblages (Thompson et al., 2021) and thus a range of remote structures should be included to ensure representative protection. The protection of subsurface features will benefit pelagic biodiversity, as well as providing significant benefits in the conservation of important demersal fish assemblages (Galbraith et al., 2021) and many vulnerable benthic marine ecosystems such as deep reef communities given their higher concentration at these features (Rogers, 2018).…”

An Assessment of Economic Viability of the Ascension Island Tuna Longline Fishery Management: Implications for Marine Protected Area Planning and Future Fisheries Management

“…Away from the island, features including seamounts and canyons which are often hotspots of pelagic biodiversity and attract higher abundances of commercially targeted species (Ascension Island Government, 2021;Thompson et al, 2021) are found within the Ascension Island Exclusive Economic Zone (EEZ). In addition to the seamount complex's, four hydrothermal vents exist, distributed along the mid-Atlantic ridge at depths ranging from 1,700 to 3,600 m, they support highly specialized ecosystems that have adapted to survive in the extreme conditions and live off the chemicals emitted from the vents (Vrijenhoek, 2010).…”