More than 100 common eagle rays were recorded gathering beneath a tuna farm off Malta in what scientists believe could be the largest aggregation of the species ever documented.
The unusual gathering was observed during underwater monitoring in October 2024, when researchers used a remotely operated vehicle to survey the seabed beneath tuna cages at depths of about 50–55 metres. An estimated 100–150 common eagle rays (Myliobatis aquila) were seen around one of the cages.
Researchers surveyed eight of the farm’s 24 tuna cages along with four nearby reference sites. The largest concentration occurred beneath a cage where leftover fish used to feed the tuna had accumulated on the seabed.
The video footage showed the rays actively feeding on the dead fish. By contrast, only isolated rays were recorded beneath cages where no food remains were present, suggesting that the abundant food source was the main reason for the unusual gathering.
The group included rays of different sizes, with some measuring more than 80 centimetres across while others were around 20 centimetres. The researchers also observed mainly females among the individuals whose sex could be determined, although males may have been present elsewhere in the group.
Common eagle rays typically feed largely on molluscs, although fish can also form part of their diet. The easy availability of food beneath the tuna cages may therefore be temporarily changing their feeding behaviour and encouraging them to remain near the farm.
The discovery is significant because the common eagle ray is considered vulnerable in the Mediterranean. Large, predictable gatherings could provide feeding opportunities but could also make the animals more vulnerable to fishing.
Researchers said continued monitoring is needed to determine whether the Malta aggregation was a one-off event, a seasonal gathering or a recurring response to food availability around tuna farms. They also suggested that additional fishing restrictions around such sites could help protect the rays.
The findings highlight how aquaculture operations can influence the behaviour and movement of wild marine species, even when those species are not directly targeted by the farms.