Mediterranean monk seal body condition and dolphin demographics using drone-photogrammetry

Written by Fabien Vivier.

Did you know that three species of monk seals have been identified in the world? The three species arthe Mediterranean monk seal (Monachus monachus); the Hawaiian monk seal (Neomonachus schauinslandi); and the Caribbean monk seal (Neomonachus tropicalis), unfortunately extinct since the middle of the 20th century.

With only ~700 Mediterranean monk seals and ~1400 Hawaiian monk seals left in the world, both species, listed as Endangered in the IUCN Red List, are two of the world’s most threatened pinnipeds. This means they are facing a very high risk of extinction in the wild. For this reason, further research on the behavior, ecology, communication, and interactions with human-induced activities of these animals is required to better protect them. If you would like to learn more about Hawaiian monk seal vocalizations, please read our colleague Kirby Parnell’s research blogs (Blog 1, Blog 2, Blog 3). Click here to read more about the Mediterranean monk seal.

Left: Aerial and right: boat-based photograph of a mediterranean monk seal. Credits: F. Vivier and J. Gonzalvo.

In 2021, the Marine Mammal Research Program, Hawai’i (MMRP) started a collaboration with the Ionian Dolphin Project (IDP) by the Tethys Research Institute in the context of a project aimed at Allowing Coexistence of Monk Seals with Tourism, through Science-Based Management, in the Inner Ionian Sea Archipelago; an area designated as a Special Area of Conservation (SAC) within the EU Natura 2000 Network in March 2011. This project is supported by Foundation Segré, member of The Monk Seal Alliance.

Left: Boat-based and right: aerial photograph of a mediterranean monk seal. Credits:J. Gonzalv and F. Vivier.

In August 2021, MMRP PhD candidate Fabien Vivier visited the IDP for 14 days. The specific goals of this collaboration were to test the feasibility of using Unoccupied Aerial System (UAS)-or drone-photogrammetry to assess the behavior and the health status of individual Mediterranean monk seals inhabiting the SAC. The SAC constitutes a habitat of global importance to the Mediterranean monk seal, where it can be predictably and regularly encountered, and ~30 seals have been photo-identified by the IDP since 2012. During the four days when the weather allowed the team to survey for monk seals, the drone was operated for 3h49min to collect aerial imagery of six monk seals, of which at least one was a pregnant female who has given birth since. IDP’s monk seal work is supported by Fondation Segré.

Example of morphometric measurements collected through drone-photogrammetry on a Mediterranean monk seal

The collaboration with the IDP expands also to the resident population of common bottlenose dolphins (Tursiops truncatus) of the Gulf of Ambracia, also known by its Greek name as Amvrakikòs Kolpos. Over the past 30 years, the IDP has collected abundant information on this population (i.e., photo-identification, habitat use, fishery interactions) and estimated that a total of ~150 individuals inhabit the Gulf. As part of a pilot study, the IDP and the MMRP collaboration aimed to test the feasibility of using drone-photogrammetry to assess the Ambracian dolphin population’s age structuration (i.e., proportion of calves, juveniles, and adults) and the health of its individuals.

Thanks to IDP’s research efforts and invaluable dataset, this population represents the ideal scenario to test the new methodology currently being developed by Fabien as part of his PhD thesis research. As a short summary, drone-photogrammetry allows the estimation of an object’s length based on the drone altitude, and photo dimensions (e.g., pixel size). When it comes to dolphins, it is possible to estimate their total length (from the tip of their rostrum, or beak, to the notch in their flukes) if a full elongated body is displayed at the surface of the water. When this is not the case, the distance between the blowhole (or nose) and the insertion of the dorsal fin of each individual acts as a proxy allowing for the estimation of its total length. Once the body length has been estimated, an age-class (newborn, calf, juvenile or adult) can be inferred for each animal, hence looking at the age-class structuration of the population sampled.

A group of bottlenose dolphins in the Gulf of Ambracia, Greece. Credit: F. Vivier.

Over the course of four days in the field, 11h41min were spent in the presence of dolphins, of which 10h20min were spent operating a drone over the animals during 36 drone flights. A total of 117 individuals (including nine calves and 10 juveniles) were both identified from the research vessel using photo-identification and sampled by the drone. Linking fast-moving and diving animals between boat-based photo-identification and drone images is rather challenging, but the information gathered does not only allow for sizing / aging specific individuals, but also to access to their life history, which in turn will help understand better the growth trend in this population. IDP’s work in the Gulf of Ambracia is supported by OceanCare.

Last but not least, if you come across a Mediterranean monk seal, please be SMART, respect some very basic guidelines, contribute to science by reporting your invaluable encounter to the Ionian Dolphin Project, and enjoy. Mediterranean monk seals can be identified by taking photographs of their head and scarring on their back (see the image below).

Facial and back scars constitute good identification features for Mediterranean monk seals. Credit: J. Gonzalvo.

Tethys Research Institute activities in Greece are conducted under research permit (ΑΔΑ: ΨΕΤΥ4653Π8-694) issued by The Greek Ministry of Environment and Energy. UAVs flight permits granted by the Hellenic Civil Aviation Authority under the Hellenic Republic Ministry for Infrastructure and Transport.