Underwater photo-identification of marine megafauna: an identity card catalogue of sperm whales (Physeter macrocephalus) off Mauritius Island

Abstract

The long-term monitoring of long-lived animal populations often requires individual identification. For cetacean populations, this identification is mostly based on morphological characters observable from a boat such as shape, spots and cuts of the back, caudal and dorsal fins. This is well suited for species easily displaying their caudal fins, such as the humpback whales Megaptera novaeangliae, or those whose skin pigmentation patterns enable individual identification. However, for elusive or shier species such as the sperm whales Physeter macrocephalus, this approach may be more challenging as individuals display a rather uniform skin pigmentation. They also do not show very often their caudal fin that must be photographed perpendicularly to the water surface, vertically and fully emerged, uneasing the individual identification from a boat. Immature sperm whales that usually have a caudal fin without any distinctive marks may sometimes be excluded from photo-identification catalogues. Within the framework of the Maubydick project, focusing on the long-term monitoring of sperm whales in Mauritius, passive underwater observation and video recording were used to identify long-lasting body markers (e.g., sex, ventral white markings, cut outs of fins) to improve individual identification. A catalogue of individual identity cards was developed and 38 individuals were recorded (six adult males, 18 adult females and 14 immatures). This catalogue was used in the field and enabled observers to record some nearly-daily and yearly recaptures. Advantages and disadvantages of this method are presented here. Such catalogues represent a robust baseline for conducting behavioural, genetic and acoustic studies in marine megafauna social species. Benefits of such newly acquired knowledge are of first importance to implement relevant conservation plans in the marine realm.

Long distance runners in the marine realm: New insights into genetic diversity, kin relationships and social fidelity of Indian Ocean male sperm whales

“…A total of 26 adult males were identified based on their body length by underwater observations between 2011 and 2020 off Mauritius (Table S3). Males were observed in 2011, 2013 and yearly since 2015 when the observation effort significantly increased [40]. Since then, adult male sperm whales were sighted each year with a maximum of 10 different individuals observed in 2019.…”

“…But social and breeding behaviours of male sperm whales remain largely unclear, especially in terms of geographical and social fidelity. Here, we studied sperm whales off Mauritius under the auspices of the Maubydick project [21, 40] and off Crozet/Kerguelen [19, 43, 44]. This allowed us to document the presence of different males visiting the focal mostly matrilineal sperm whale social unit, the Irène group, to identify several recaptures of males with the Irène group over years, to decipher some paternal kinships as well as to capture a diagram of kin relationships at a larger geographic scale.…”

“…Field work took place off the western coast of Mauritius (Mascarenes Islands, Indian Ocean) between latitudes 20.465°S 57.334°E and 19.986°S 57.605°E, up to 15 km off the coast [21]. Sea surface and underwater observations have been carried out since 2011, under the auspices of a project called Maubydick led by the MMCO (Marine Megafauna Conservation Organization, [21, 40]. Since 2015, fieldwork has been conducted almost daily between February and May, and some sporadic observations made during the rest of the year, except in January.…”

“…In the Indian Ocean breeding grounds, sperm whales have been less studied than in the Pacific and the Atlantic. Several social groups have been observed [11, 39, 40], and photoidentification campaigns and satellite tracks confirmed that sperm whales are common near the Mauritius and La Reunion Islands [40–42]. The predominant matrilineality of a particular social group, the “Irène’s group” has been recently demonstrated near Mauritius [21].…”

“…The predominant matrilineality of a particular social group, the “Irène’s group” has been recently demonstrated near Mauritius [21]. But except for some photo-identified individuals [40], male sperm whales encountered within the breeding grounds of the Indian Ocean are very poorly known. More knowledge comes from the feeding grounds of the Indian Ocean, and in particular from Crozet/Kerguelen [19, 43, 44], however the movement patterns between feeding and breeding grounds are not known.…”