At about 50 meters down, in the shadow of floating pens holding penned bluefin tuna off Malta’s north-eastern coast, a remotely operated vehicle flew over a seabed that was moving. In October 2024, researchers steering that ROV logged an estimated 100 to 150 common eagle rays, Myliobatis aquila, gathered on and just above the bottom beneath a tuna ranch — a number the authors describe in Marine Biodiversity as the largest known aggregation of the species recorded to date, and one apparently drawn there by leftover baitfish sinking uneaten out of the cages above.
The paper, titled “Large Myliobatis aquila aggregations associated with tuna ranches in the Maltese Islands,” is built on video rather than nets or tags. That matters for what can and cannot be said about the rays. Nobody caught them, measured them, or examined what was in their stomachs. What exists is footage of a dense assembly of a species usually reported alone or in small numbers, hovering in a place where food arrives on a schedule set by an aquaculture operation.
What the ROV saw under the cages
The survey worked its way through part of a large facility — according to a Times of India account of the study, eight of 24 cages were inspected, along with four reference sites away from the pens for comparison. The ROV filmed at roughly 50 to 55 meters depth. That same account reports fresh food fish lying on the seabed under three of the eight cages surveyed, and the largest ray gathering sitting beneath one of those three. Other rays, including common stingrays, turned up in smaller numbers.
The headline figure is explicitly an estimate. The rays were in motion — swimming across the frame, settling, lifting off, overlapping — and the authors are direct that this made an exact count impossible. “Over 100 individuals” is the abstract’s phrasing; 100 to 150 is the working range. It is a considered read of moving video, not a census, and it should be held that way. A single animal circling back through the field of view can be double-counted; an animal resting outside the light cone is missed entirely. Both errors run in opposite directions and neither can be resolved from the footage alone.
What the video does establish is co-occurrence: many eagle rays, in one place, at the same time, at a site where uneaten baitfish reaches the bottom. The abstract points to opportunistic feeding as the driving factor behind the aggregation. It also declines to close the door on alternatives — mating or other reproductive behavior is not definitively ruled out. That hedge is the authors’ own, and it is the honest position when the evidence is a camera pass rather than a season of observation.
Why this species is so rarely seen in numbers
Common eagle rays are widespread across the eastern Atlantic and the Mediterranean, and they are not cryptic animals: broad pectoral wings, a pointed snout built for rooting out buried bivalves and crustaceans, a whip of a tail. But they are also mid-water travelers over soft bottoms in a depth band that recreational divers rarely work and that survey trawls sample bluntly. Sightings tend to be incidental — a fisher’s bycatch, a diver’s photo, a stray record in a database.
Aggregations of the species have been reported only a handful of times, and the authors note how few such accounts exist. That scarcity is the reason a single ROV transect can plausibly claim a record. It is worth being precise about what “largest known” means here: it is the authors’ framing, drawn from the published literature they surveyed, not an independently verified maximum. Absence of larger reported gatherings is not proof that larger gatherings do not occur — it may equally reflect that almost nobody has been looking at 50 meters, under aquaculture infrastructure, with a camera.
Which is part of what makes the observation useful. The ROV was there to inspect a working site. The rays were an outcome of that inspection, not its target.
What a food subsidy under a tuna pen might mean
Tuna ranching works by capturing wild bluefin and fattening them in sea pens on baitfish — mackerel, sardines, similar oily species — delivered in bulk. Not all of it is eaten. What sinks becomes a predictable, concentrated food source on a seabed that would otherwise offer scattered clams and worms. The pattern in the Maltese footage, with the biggest ray gathering under a cage that had fresh food fish beneath it, is consistent with animals responding to that subsidy.
The open questions follow directly, and the study does not resolve them. Does a reliable pile of baitfish change where eagle rays spend their time, or how far they range? Does a diet shift away from hard-shelled benthic prey matter for animals whose jaws and teeth are built to crush shells? Does concentrating a vulnerable species — the Times of India notes eagle rays are considered vulnerable in the Mediterranean — into a small footprint near fishing and vessel activity create risk alongside the meal? None of that is demonstrated by one survey, and the paper does not claim it is. These remain questions raised by the observation rather than conclusions drawn from it.
For now the fact holds at its own scale: in October 2024, an ROV under a tuna ranch off north-eastern Malta filmed an estimated 100 to 150 common eagle rays at around 50 meters, on a seabed strewn with baitfish the tuna above had not eaten — the largest gathering of Myliobatis aquila anyone has reported, found by a camera sent down to look at something else.