Seamount effect on circulation and distribution of ocean taxa in the vicinity of La Pérouse, a shallow seamount in the southwestern Indian Ocean

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Abstract

The La Pérouse seamount (60 m depth) has so far been poorly studied despite it being a short distance (160 km) from Réunion Island. As part of the MADRidge project, a multidisciplinary cruise was conducted to evaluate the effect of this shallow seamount on the local hydrology and ecology. Current measurements, temperature and chlorophyll-a profiles, and mesozooplankton and micronekton samples were collected between the summit and 35 km away. Micronekton data were supplemented with stomach content of pelagic top predators as well as fisheries statistics from the domestic longline fleet operating from Réunion. Vertical current profiles revealed distinct patterns between the offshore and seamount-flanked stations, giving evidence of topographical induced flow instabilities, notably on its leeward side (west) relative to the east flank. Distinct patterns in temperature and chlorophyll-a vertical profiles suggest the formation of convergent and divergent circulation cells as a result of the irregular and crescent-like summit topography. Spatial differences in zooplankton abundance were detected with higher biovolumes on the leeward flank. The overall acoustic backscatter for micronekton over the summit was weaker than offshore, but highly concentrated in the upper layer. Albacore tuna and swordfish dominate the longline catch west of Réunion, seemingly in association with a deep (900 m) topographic feature. Yet the largest catch is not directly associated with La Pérouse which would be too shallow for top predators to aggregate around in the long term. Enhanced levels of phytoplankton or zooplankton enrichment at La Pérouse were not demonstrated in this study, nor was there notable diversity of micronekton species. This might explain the relatively limited importance of this seamount to the tuna fisheries in this region.

Introduction

The subsea topography of the South West Indian Ocean (SWIO) is rugged, formed of many banks, ridges (Mascarene Plateau, Mozambique Plateau, Madagascar Ridge, South West Indian Ridge, among others) and isolated seamounts rising from plateaus or from the deep abyssal plains (Tomczak and Godfrey, 1994; Demopoulos et al., 2003; Ingole and Koslow, 2005). The summits of these many seamounts peak at various depths, ranging from >2000 m to just a few metres below the sea surface. Seamounts are known as biodiversity hotspots and are generally rich in demersal and pelagic seamount-associated fish of high commercial value (Fock et al., 2002; Clark et al., 2007). Historically, fish inventories on seamounts worldwide have been developed based on the catches of commercial trawl surveys, such as the Soviet expeditions carried out in the Indian Ocean in the 1970s (Romanov, 2003). Understanding seamount ecosystems was not a priority during this fishing expansion phase. However, following rapid depletion of stocks within a few years of exploitation, in particular stocks of long-lived, slow-maturing species such as the orange roughy (Hoplostethus atlanticus) and the alfonsinos (Beryx spp.) (Morato and Clark, 2007), the need to generate integrated ecosystem knowledge was recognized in order to develop management and conservation strategies around these seamounts (Rogers et al., 2008; Clark et al., 2012).

The MADRidge project conducted between 2016 and 2017 (see Roberts et al., 2020) was designed to investigate the productivity associated with shallow seamounts, in particular the possible contribution of flow-topographic interactions to enhanced productivity. The project focused on three seamounts located between 19°S and 34°S ― (1) La Pérouse (60 m depth), (2) an unnamed pinnacle on the Madagascar Ridge (hereafter named MAD-Ridge, 240 m) and (3) the Walters Shoal (18 m). Multidisciplinary ship surveys were conducted at each seamount. The cruise to La Pérouse was made in September 2016 (DOI: 10.17600/16004500) and aimed to study physical and biological processes that potentially drive important ecological interactions around this seamount, which otherwise is located in an oligotrophic biome ― the Indian South Subtropical Gyre (ISSG).

La Pérouse is an extinct volcano rising from the abyssal plain (5000 m depth) up to the surface euphotic layer. Its formation is thought to have occurred 8–10 million years ago, in the same epoch as Réunion (G. Barruol, pers. comm.), as a result of an intraplaque hotspot (Barruol and Sigloch, 2013). La Pérouse, as an active volcano, was probably an island in the past. However, due to erosion and seafloor subsidence, it has transitioned to an atoll before becoming a shallow seamount (Deplus et al., 2016). La Pérouse is well-known by recreational fishers who venture to the seamount from Réunion, and by professional fishers operating the Réunion-based longline fleet. Apart from fish resources, significant knowledge gaps exist on the ecosystem and physical environment at and near La Pérouse. The research cruise in 2016 represents the first integrated approach to study the La Pérouse ecosystem.

In this paper, we present and analyse the cruise data that reveal different components of the near and far environment of La Pérouse ― including the circulation, hydrology, plankton, micronekton, large pelagic fish and megafauna taxa ― to assess whether a “seamount effect” underpins an enhanced ecosystem at this isolated topographic feature on the Mascarene Plain.

Section snippets

Material and methods

The R/V Antea, owned by the French Institute of Research for Sustainable Development (IRD), was used for the survey. It is a 35-m catamaran equipped for physical and biological investigations in the coastal and deep ocean. La Pérouse is located 160 km northwest of Réunion, at latitude 19°43′S and longitude 54°10′E (Fig. 1a). The R/V Antea left Réunion on September 16, 2016 and took 15 h to reach the study site, which was then investigated for 12 consecutive days. The cruise ended in Réunion on

Altimetry and regional circulation

The altimetry field covering the region between Réunion and La Pérouse, for the duration of the cruise, was characterized by moderate geostrophic gradients not exceeding 0.36 m (Fig. 2; sea level anomalies ranged from −0.26 m to +0.10 m). During the first part of the cruise (16–25 September), La Pérouse was located on the edge of a cyclonic eddy which moved towards the northwest at an average speed of 12 km d−1 (Fig. 2a). From 16 to September 18, 2016, the S-ADCP measurements between Réunion

Discussion

The research cruise carried out in September 2016 around La Pérouse covered many aspects of the seamount ecosystem (topography, hydrology, plankton and micronekton distribution) and generated new datasets on a poorly known topographic rise, only discovered in 1962. The high-resolution map of La Pérouse produced during this cruise gave evidence of a crescent-shaped summit, possibly as a result of a huge landslide that occurred on its eastern flank in the distant past. The newly acquired

Concluding remarks

This paper presents new physical and biological data collected in the vicinity of the La Pérouse seamount in an attempt to assess whether there is a ‘seamount effect’ on the various trophic levels of the surrounding ecosystem. The data collected clearly showed that this steep, crescent-shaped topographic feature, located in the oligotrophic province of the Indian South Subtropical Gyre, strongly influences the circulation around the summit flanks in the upper 400 m. Consequently, it is likely

CRediT authorship contribution statement

Francis Marsac: Supervision, Conceptualization, Formal analysis, Writing - original draft, Writing - review & editing, Supervision. Pavanee Annasawmy: Conceptualization, Formal analysis, Writing - original draft, Writing - review & editing. Margaux Noyon: Conceptualization, Formal analysis, Writing - original draft, Writing - review & editing. Hervé Demarcq: Writing - original draft, Writing - review & editing. Marc Soria: Funding acquisition, Investigation, Writing - original draft, Writing -

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank the officers and crew of the R/V Antea for their assistance during the La Pérouse cruise, which was achieved thanks to the financial support from the Conseil Régional de La Réunion, the Flotte Océanographique Française (French Oceanographic Fleet) and the Institut de Recherche pour le Développement (IRD). We are also grateful to the UMS Imago for the logistical support provided in cruise preparation, to Pierre Rousselot for his assistance in the processing of ADCP data, and to François

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