Tom Bech Letessier

University of Plymouth | UoP · School of Biological & Marine Sciences

Quantitative field ecologist who contributed to the fundamentals of polar science and pelagic ecology.

Animal body-size variation influences multiple processes in marine ecosystems, but habitat heterogeneity has prevented a comprehensive assessment of size across pelagic (midwater) and benthic (seabed) systems along anthropic gradients. In this work, we derive fish size indicators from 17,411 stereo baited-video deployments to test for differences b...

Global socio-ecological shocks, such as the COVID-19 pandemic, can threaten progress in protecting vulnerable marine environments by altering behaviour of resource users 1 . When government priorities shift from environmental protection towards safeguarding human populations, control of illegal activity in protected areas can alter. Resulting incre...

Bird colonies on islands sustain elevated productivity and biomass on adjacent reefs, through nutrient subsidies. However, the implications of this localized enhancement on higher and often more mobile trophic levels (such as sharks and rays) are unclear, as spatial trends in mobile fauna are often poorly captured by traditional underwater visual s...

Many species are restricted to a marginal or suboptimal fraction of their historical range due to anthropogenic impacts, making it hard to interpret their ecological preferences from modern‐day data alone. However, inferring past ecological states is limited by the availability of robust data and biases in historical archives, posing a challenge fo...

The global network of protected areas has rapidly expanded in the past decade and is expected to cover at least 30% of land and sea by 2030 to halt biodiversity erosion. Yet, the distribution of protected areas is highly heterogeneous on Earth and the social-environmental preconditions enabling or hindering protected area establishment remain poorl...

Decades of unsustainable fishing have caused widespread declines in shark populations leading to calls for increased management and policy. To design successful measures, an understanding of the social context and drivers for trade is needed. This study uses a value chain approach in Sri Lanka, where shark landings have declined rapidly, to map sha...

Aim Coastal fishes have a fundamental role in marine ecosystem functioning and contributions to people, but face increasing threats due to climate change, habitat degradation and overexploitation. The extent to which human pressures are impacting coastal fish biodiversity in comparison with geographic and environmental factors at large spatial scal...

Human exploitation has profoundly depleted animal populations in the ocean, leading to declines in ecosystem productivity, resilience, and contributions to people 1,2 . However, it remains unclear how size structure of fish populations varies across marine habitat and levels of human exploitation while simultaneously underpinning food web architect...

Mobile predators serve important ecological functions, including acting as nutrient vectors between different ecosystems. In coral reefs, pelagic nutrient subsidies are believed to play an increasingly important role under ongoing and projected environmental changes. Here, we combine visual sightings with passive acoustic monitoring to report habit...

The link between biodiversity and ecosystem functioning has been the topic of considerable research, but it remains unclear how biodiversity decline is compromising ecosystem functionality, particularly in the pelagic realm. Here, we explore how pelagic fish species diversity relates to functional diversity by sampling two locations, which, on the...

Seamounts are important marine habitats that are hotspots of species diversity. Relatively shallow peaks, increased productivity and offshore locations make seamounts vulnerable to human impact and difficult to protect. Present estimates of seamount numbers vary from anywhere between 10,000 to more than 60,000. Seamount locations can be estimated b...

The pelagic ecosystem is the ocean's largest by volume and of major importance for food provision and carbon cycling. The high fish species diversity common in the tropics presents a major challenge for biomass estimation using fisheries acoustics, the traditional approach for evaluating mid-water bio-mass. Converting echo intensities to biomass de...

Seamounts are important marine habitats that are hotspots of species diversity. Relatively shallow peaks, increased productivity and offshore locations make seamounts vulnerable to human impact and difficult to protect. Present estimates of seamount numbers vary from anywhere between 10,000 to more than 60,000. Seamount locations can be estimated b...

Quantifying fish species diversity in rich tropical marine environments remains challenging. Environmental DNA (eDNA) metabarcoding is a promising tool to face this challenge through the filtering, amplification, and sequencing of DNA traces from water samples. However, because eDNA concentration is low in marine environments, the reliability of eD...

Assessing the impact of global changes and protection effectiveness is a key step in monitoring marine fishes. Most traditional census methods are demanding or destructive. Nondisturbing and nonlethal approaches based on video and environmental DNA are alternatives to underwater visual census or fishing. However, their ability to detect multiple bi...

Ocean health is fundamental to human prosperity. However, fisheries exploitation, industrialization and climate change imperil our oceans. Marine protected areas (MPAs) have been established in coastal habitats since the 1970s and the ongoing monitoring of these MPAs has shown their general ecological and economic value. Demonstrable benefits can i...

Quantifying the diversity of species in rich tropical marine environments remains challenging. Environmental DNA (eDNA) metabarcoding is a promising tool to face this challenge through the filtering, amplification, and sequencing of DNA traces from water samples. However, the reliability of biodiversity detection from eDNA samples can be low in mar...

The Pitcairn Islands, located in the central South Pacific, contain near-pristine marine ecosystems which support unique fish assemblages, together with both endemic and threatened species. Pitcairn itself is the only inhabited island in the group and, before this study, the environmental impact of local fisheries was unclear, with little data to i...

Managing the footprint of highly mobile fishing fleets is increasingly important due to continuing declines in fish populations. However, social‐ecological drivers for fisher behaviour remain poorly understood for many fleets globally. Using the Sri Lankan fleet as a case study, we explored the role of social, environmental and policy drivers of ef...

Area coverage of large-scale marine protected areas (MPAs) (LSMPAs, > 100,000 km²) is rapidly increasing globally. Their effectiveness largely depends on successful detection and management of non-compliance. However, for LSMPAs this can be difficult due to their large size, often remote locations and a lack of understanding of the social drivers o...

The link between biodiversity and ecosystem functioning has been the topic of considerable research, but it remains unclear how biodiversity decline is compromising ecosystem functionality, particularly in the pelagic realm. Here, we explore how pelagic fish species diversity relates to functional diversity by sampling two locations, which, on the...

Seamounts are important marine habitats that are hotpots of species diversity. Relatively shallow peaks, increased productivity and offshore locations make seamounts vulnerable to human impact and difficult to protect. Present estimates of seamount numbers vary from anywhere between 10000 to more than 60000. Seamount locations can be estimated by e...

• Monitoring large marine mammals is challenging due to their low abundances in general, an ability to move over large distances and wide geographical range sizes. • The distribution of the pygmy (Kogia breviceps) and dwarf (Kogia sima) sperm whales is informed by relatively rare sightings, which does not permit accurate estimates of their distribu...

Given the recent trend towards establishing very large marine protected areas (MPAs) and the high potential of these to contribute to global conservation targets, we review outcomes of the last decade of marine conservation research in the British Indian Ocean Territory (BIOT), one of the largest MPAs in the world. The BIOT MPA consists of the atol...

Species-specific bans are increasingly being implemented to stem loss of vulnerable marine species, but there is a paucity of evaluative research into resulting socioeconomic and ecological consequences. In 2012, a blanket ban on landing Alopiidae (thresher) sharks was introduced in Sri Lanka. We used fisher perceptions, shown to influence support...

Spatial and temporal distribution of seabird transiting and foraging at sea is an important consideration for marine conservation planning. Using at‐sea observations of seabirds (n = 317), collected during the breeding season from 2012 to 2016, we built boosted regression tree (BRT) models to identify relationships between numerically dominant seab...

The conservation of marine biodiversity is firmly embedded in national and international policy frameworks. However, the difficulties associated with conducting broad-scale surveys of oceanic environments restrict the evidence base available for applied management in pelagic waters. For example, the Oceanic Shoals Australian Marine Park (AMP) was e...

Seamounts are important marine habitats that are hotpots of species diversity. Relatively shallow peaks, increasedproductivity and offshore locations make seamounts vulnerable to human impact and difficult to protect. Present estimates ofseamount numbers vary from barely 10000 to more than 60000), because locating and identifying them remotely can...

Unmanned aerial vehicles (UAVs) are cost‐effective remote sensing tools useful for generating very high‐resolution (VHR) aerial imagery. Habitat maps generated from UAV imagery are a fundamental component of marine spatial planning, essential for the designation and governance of marine protected areas (MPAs). We investigated whether UAV survey alt...

[This corrects the article DOI: 10.1371/journal.pbio.3000366.].

Since the 1950s, industrial fisheries have expanded globally, as fishing vessels are required to travel further afield for fishing opportunities. Technological advancements and fishery subsidies have granted ever-increasing access to populations of sharks, tunas, billfishes, and other predators. Wilderness refuges, defined here as areas beyond the...

A multi-criteria trial aiming to explore the use of a novel fixed-wing Unmanned Aerial Vehicle (UAV) as an enforcement and monitoring tool in the Turneffe Marine Atoll (Belize), was conducted in partnership between the Turneffe Atoll Sustainability Association (TASA), the Zoological Society of London (ZSL) and the Marine Management Organisation (MM...

Reef sharks are vulnerable predators experiencing severe population declines mainly due to overexploitation. However, beyond direct exploitation, human activities can produce indirect or sublethal effects such as behavioral alterations. Such alterations are well known for terrestrial fauna but poorly documented for marine species. Using an extensiv...

We chose to trial a prototype water-landing fixed-wing UAV in The British Indian Overseas Territories (BIOT) marine reserve (640,000km2). We trialled the UAV in the BIOT reserve for megafauna surveys and illegal unreported and unregulated (IUU) fishery enforcement.

With over 70 contributors from 30 agencies, the NESP Marine Biodiversity Hub has developed a suite of field manuals to describe a nationally consistent and defensible approach to marine data acquisition. This manual relates to pelagic (mid-water) BRUVS, i.e. gear designed to acquire digital video imagery of macro-organisms living in the ocean’s w...

Australia has one of the world’s largest marine estates that includes many vulnerable habitats and a high biodiversity, with many endemic species crossing a wide latitudinal range. The marine estate is used by a variety of industries including fishing, oil & gas, and shipping, in addition to traditional, cultural, scientific and recreational uses....

Tuna, billfish, and oceanic sharks [hereafter referred to as 'mobile oceanic fishes and sharks' (MOFS)] are characterised by conservative life-history strategies and highly migratory behaviour across large, transnational ranges. Intense exploitation over the past 65 years by a rapidly expanding high-seas fishing fleet has left many populations depl...

[This corrects the article DOI: 10.1371/journal.pone.0177374.].

The distribution of large pelagic predators can be highly heterogeneous. Investigating pelagic predator-prey interactions around localised geo-physical features (reefs, seamounts) helps elucidate and predict potential foraging mechanisms that support relatively persistent levels of elevated biomass. The Chagos Archipelago (British Indian Ocean Terr...

Reef sharks are declining world‐wide under ever‐increasing fishing pressure, with potential consequences on ecosystem functioning. Marine protected areas ( MPA s) are currently one of the management tools used to counteract the pervasive impacts of fishing. However, MPA s in which reef sharks are abundant tend to be located in remote and underexplo...

Aim Protected areas have become pivotal to the modern conservation planning toolbox, but a limited understanding of marine macroecology is hampering their efficient design and implementation in pelagic environments. We explored the respective contributions of environmental factors and human impacts in capturing the distribution of an assemblage of...

A baited videographic survey at Pitcairn Island has significantly extended the known range for two reef-fish species: blue-lined triggerfish Xanthichthys caeruleolineatus and greater amberjack Seriola dumerili, with an additional minor extension noted for giant trevally Caranx ignobilis. This highlights the importance of further research in this re...

We investigated drivers of reef shark demography across a large and isolated marine protected area, the British Indian Ocean Territory Marine Reserve, using stereo baited remote underwater video systems. We modelled shark abundance against biotic and abiotic variables at 35 sites across the reserve and found that the biomass of low trophic order fi...

The seamounts of the southern Indian Ocean remain some of the most poorly studied globally and yet have been subject to deep-sea fishing for decades and may face new exploitation through mining of seabed massive sulphides in the future. As an attempt to redress the knowledge deficit on deep-sea benthic and pelagic communities associated mainly with...

Understanding the processes driving the distribution of mid-water prey such as euphausiids and lanternfish is important for effective management and conservation. In the vicinity of abrupt topographic features such as banks, seamounts and shelf-breaks, mid-water faunal biomass is often elevated, making these sites candidates for special protection....

A total of 68 cephalopod species belonging to 26 families (10–11% of the total known cephalopod diversity) were collected onboard R/V Fridtjof Nansen during a research survey on Southwest Indian Ocean Ridge in November–December 2009. This relatively small area extends from the Tropical front to the Subantarctic front with four distinctive cephalopo...

Krill (Euphausia superba) are fundamentally important in the Southern Ocean ecosystem, forming a critical food web link between primary producers and top predators. Krill abundance fluctuates with oceanographic conditions, most notably variation in winter sea ice, and is susceptible to environmental change. Although links between local krill availa...

Despite being identified as a driver of mobile predator aggregations (hotspots) in both marine and terrestrial environments, topographic complexity has long remained a challenging concept for scientists to visualise and a difficult parameter to estimate. It is only with the advent of high-speed computers and the recent popularisation of geographica...

In the open ocean, the movements and habitat use of large mobile predators are driven by dynamic interactions between biological and physical variables and complex predator–prey relationships. Understanding the spatial and temporal distributions of pelagic fishes and sharks is a critical component of conservation and fisheries management. Here, we...

In situ characterization of krill morphometry, behaviour and orientation is not yet routinely feasible, yet is critical to understanding swarm characteristics. A first step is to measure individual and aggregation behaviour. We report on successful use of a robust, low-cost underwater stereo video camera system to observe live Antarctic krill (Euph...

In contrast to generally sparse biological communities in open-ocean settings, seamounts and ridges are perceived as areas of elevated productivity and biodiversity capable of supporting commercial fisheries. We investigated the origin of this apparent biological enhancement over a segment of the North Mid-Atlantic Ridge (MAR) using sonar, corers,...

In contrast to generally sparse biological communities in open-ocean settings, seamounts and ridges are perceived as areas of elevated productivity and biodiversity capable of supporting commercial fisheries. We investigated the origin of this apparent biological enhancement over a segment of the North Mid-Atlantic Ridge (MAR) using sonar, corers,...

Trophic relationships and vertical distribution patterns of dominant mesozooplankton (2-20 mm) and macrozooplankton (>20 mm) invertebrates (Euphausiacea, Copepoda, Decapoda, Amphipoda, Thecosomata and Lophogastrida) were investigated within the epi- and meso-pelagic zone (0-200 and 200-800 m depth), north (54 degrees N) and south (49 degrees N) of...

New baseline information is presented on biogeography, abundance and distribution of euphausiids from discrete depth samples collected throughout the water column to 3000 m at 42 locations along the Mid-Atlantic Ridge (MAR) between Iceland and the Azores. Eighteen species were recorded, with Euphausia krohni and Thysanoessa longicaudata being most...