Scientific Papers

The occurrence of mobulid rays at St Helena Island, Cardno seamount and Bonaparte seamount in the South Atlantic and its significance for conservation | Marine Biodiversity Records

369 sightings of mobulid rays were documented between April 1999 and April 2019. 363 sightings were of M. tarapacana consisting of solitary individuals (67 %, n = 244) and schools of up to 14 (33 %, n = 119). Four sightings were of unidentified mobulid species and two sightings were of oceanic mantas M. birostris.

All 70 sightings that occurred at Bonaparte and Cardno seamounts were of M. tarapacana. These encounters were recorded from a boat, either as part of commercial fishing activities or during marine research conducted by St Helena Government biologists. Twenty nine of these sightings were recorded between 2015 and 2019, a period during which 10 % (14 of 142) of all offshore voyages from St Helena were monitored by onboard fisheries observers. These encounters were limited due to the time able to be spent offshore by the vessels. However, data from both inshore and offshore of M. tarapacana shows they are present at St Helena throughout the year (Fig. 2), but with a strong seasonal signal where in animals were more commonly seen in the warmer months. Behaviours reported by observers included cruising, leaping, and feeding both singularly and in groups.

Fig. 2
figure 2

Frequency of M. tarapacana sightings inshore around St Helena’s waters and offshore at Cardno and Bonaparte seamount throughout the year recorded between April 1999 and April 2019

Of the 299 sightings that occurred inshore, 59 % (176) of encounters occurred in water by either swimmers, snorkelers, free divers and scuba divers. Another 38 % (113) were encounters from a boat either as part of commercial, sports or recreational fishing activities as well as wildlife tour operators and recreational boat users. The remaining 3 % (Essumang 2009) of encounters were sightings made from land, shore side or overlooking the sea from cliffs (Fig. 3). Of the in-water encounters, 93 % (168) were whilst actively scuba diving. The frequency of encounters in the sea over the past five years has dramatically increased.

Fig. 3
figure 3

Yearly mobulid encounters inshore from boats, land and in water

Two different solitary oceanic manta (M. birostris) sightings occurred on two separate days during the study period. The first sighting occurred on the 28th of July, 2018. A solitary oceanic manta ray was seen at the surface, swimming in a north easterly direction 150 m from the coastline below Ladder Hill Signal House near James Bay. A different solitary oceanic manta ray was seen two days later on the 30th July 2018 100 m east of the buoys off Rupert’s Bay, from a fishing boat on the north side of the island at the surface. Photographic evidence (Fig. 4) confirmed species identification according to the characteristics in Marshall et al. (2009), while differing patterns of white colouration on the pectoral fins indicated that two separate individuals were involved. Cephalic lobes were seen to be unfurled, which might indicate that the animals were feeding.

Fig. 4
figure 4

Photographs of two individual sightings of solitary oceanic mantas M. birostris off St Helena’s coastline. Both photos show key characteristics of M. birostris as described in Marshall et al. (2009) notably T shaped black on white coloured shoulder patches on the dorsal supra-branchial region, without “hooks” connecting to the spiracles as in Mobula alfredi. a Encounter recorded 28th July 2018 showing dark dorsal colouration and dark pectoral fins. b Encounter recorded 30th July 2018 showing chevron shaped marking anterior to dorsal fin; dark colouration around mouth and white colouration on the pectoral fin tips

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