A novel approach to estimate the distribution, density and at-sea risks of a centrally-placed mobile marine vertebrate

Abstract

Formulating management strategies for mobile marine species is challenging, as knowledge is required of distribution, density, and overlap with putative threats. As a step towards assimilating knowledge, ecological niche models may identify likely suitable habitats for species, but lack the ability to enumerate species densities. Traditionally, this has been catered for by sightings-based distance sampling methods that may have practical and logistical limitations. Here we describe a novel method to estimate at-sea distribution and densities of a marine vertebrate, using historic aerial surveys of Gabonese leatherback turtle (Dermochelys coriacea) nesting beaches and satellite telemetry data of females at sea. We contextualise modelled patterns of distribution with putative threat layers of boat traffic, including fishing vessels and large ship movements, using Vessel Monitoring System (VMS) and Automatic Identification System (AIS) data. We identify key at-sea areas in which protection for internesting leatherback turtles could be considered within the coastal zone of Gabonese Exclusive Economic Zone (EEZ). Our approach offers a holistic technique that merges multiple datasets and methodologies to build a deeper and insightful knowledge base with which to manage known activities at sea. As such, the methodologies presented in this study could be applied to other species of sea turtles for cumulative assessments; and with adaptation, may have utility in defining critical habitats for other central-place foragers such as pinnipeds, or sea bird species.Although our analysis focuses on a single species, we suggest that putative threats identified within this study (fisheries, seismic activity, general shipping) likely apply to other mobile marine vertebrates of conservation concern within Gabonese and central African coastal waters, such as olive ridley sea turtles (Lepidochelys olivacea), humpback dolphins (Sousa teuszii) and humpback whales (Megaptera novaeangliae).

Using Cumulative Impact Mapping to Prioritize Marine Conservation Efforts in Equatorial Guinea

“…These results are not particularly surprising as Equatorial Guinea hosts an important fisheries sector (Belhabib et al, 2016), as well as offshore petrochemical infrastructure and shipping, that are largely concentrated on the continental shelf. Similar trends are also evident in neighboring countries like Gabon and the Republic of Congo, where continental shelf waters are subject to more persistent pressure from human activities such as shipping and fisheries than offshore waters (Dawson et al, 2017;Metcalfe et al, 2018;Pikesley et al, 2018).…”

Research Priorities to Support Effective Manta and Devil Ray Conservation

“…Recent improvements to geolocation and subsequent state space models are quantifying and reducing the levels of uncertainty associated with lightbased geolocation (Patterson et al, 2008;Jonsen et al, 2013;Gredzens et al, 2014;Braun et al, 2018). It is likely that with the increasing capacity to make fine-scale observations, movement patterns will indicate significant overlap with anthropogenic threats or sensitive areas (Braun et al, 2015;Kessel et al, 2017;Pikesley et al, 2018). Researchers could therefore incorporate metrics such as the Human Impact Index (Halpern et al, 2008) in future mapping of core mobulid utilization areas to identify time periods and locations where mobulid populations are likely to be impacted by human activities.…”

Critical information gaps remain in understanding impacts of industrial seismic surveys on marine vertebrates

“…Not only will such studies help reveal impacts of seismic surveys on marine turtle distribution and abundance, but they may reveal insight into displacement vs. habituation/tolerance in marine turtles. Recent studies demonstrate progress on methodological approaches to the study of sea turtle distribution and abundance in the context of anthropogenic activity (Pikesley et al 2018).…”