On Tuesday 4 March, the Dyer Island Conservation Trust and Marine Dynamics team were alerted of a stranded eagle ray, Myliobatis aquila, at Franksraal beach. Upon arrival, marine guides Sandra Hörbst, Susy Alexandersen and skipper Hennie Odendaal identified an additional ray stranding of the same species. One individual appeared to be a recent stranding whilst the second was slightly degraded hinting at a longer stranding time. The deceased individuals were collected and transported back to the International Marine Lodge to be frozen for dissection.
Common eagle rays have a widespread distribution in the eastern Atlantic Ocean, found from the British Isles to South Africa at depths from shallow coastal waters to 800m. With a brown colouration and pale belly, the camouflage tactic of countershading allows effective evasion of predators and feeding, with the animals often utillising sandy sediment to conceal themselves. Hexagonal plates arranged in bars through the mouth allow the crushing of hard exteriors of their specific prey items consisting of crustaceans, molluscs and bivalves.
Worldwide distribution of Common eagle rays, indicated by the red line within the Atlantic (Florida Museum).
Friday, 7 March, marine biologist Amy Webber led the dissection of the 2 individuals. Assisted by marine guide Sonja Fleck and Marine Dynamics Academy intern Thomas Merdy, complete measurements were taken for both eagle rays where possible, observed by Dutch Applied Biology students of AERES.
The first individual, of good condition, was identified as a female of 1.51m from snout to the end of the tail and a disc width of 33.8cm. The second ray had a damaged tail so a full measurement was not possible, however, the individual was larger with a disc width of 35cm. Due to the quality of the animal, sex identification was not possible. At maximum length, this specific species can reach up to 1.83m with a disc width of 80cm but most tend to be smaller. At full maturity, these rays display sexual dimorphism, an occurrence in which females grow to be larger than males. Intricate measurements were continued, with 25 specific recordings completed for both individuals.
Before dissection of the first ray, external observations were completed, in an attempt to identify any injuries, either as a result of predation or fishing, and external parasites. No external damage was found, however slight bleeding could be seen from the cloaca of the ray. Due to the densely packed organs at the centre of the ray disc, an incision was made from the mouth to the cloaca to begin an internal investigation. Due to the quality of the deceased ray, the organs were in good condition, providing an invaluable opportunity to investigate the internal biology of the animal. The large double lobed liver, a signature of elasmobranch species, takes up majority of the body cavity and assists the animal with buoyancy. The heart is situated in between the gill slits that can be found on the ventral side just behind the mouth, increasing the effectiveness of oxygen transfer within the bloodstream. The ovaries could be seen at the base of the cavity, confirming the sex. The female can be identified as mature, due to being over 70cm.
Common eagle rays are currently defined as Critically Endangered by the IUCN Redlist, with main threats occurring as a result of the fishing industry. Unselective fishing methods of trawling contribute to cases of bycatch, with targeted fishing of ray species operating to remove the pectoral fins for the purpose of the fin trade. A previous Common eagle ray dissected in 2021 by the Trust was an unfortunate victim of this brutal trade, with the animal missing the pectoral fins at the time of collection. Although the cause of death was unable to be confirmed, it is undeniable that irresponsible and unsustainable fishing practice harms the population stability of these animals, amongst many other marine species, further defining the importance and value of the dissection opportunity.
For any strandings in the wider Overstrand area, please contact the Dyer Island Conservation Trust or Marine Dynamics, public alerts are paramount to the rare opportunities provided that aid research and evidence based conservation.
References:
Common Eagle Ray, Florida Museum: https://www.floridamuseum.ufl.edu/discover-fish/species-profiles/common-eagle-ray/
Common Eagle Ray ID Guide, Shark Trust: https://www.sharktrust.org/faqs/common-eagle-ray-id-guide
