Biologist Ricardo Gomes, from the Mar Urbano Institute, argued the absence of other dead species on the beach indicates the incident was likely not caused by pollutants or a lack of oxygen in the water.
"This makes the hypothesis of trawling more evident in the cases of these stingrays," he said.
Trawling is a fishing technique that involves pulling a net through the water behind one or more boats.
"This is not (the stingray's natural) place, so this is strange even for the people who are here and who live off fishing," said environmentalist Isabeli Delois.
"Something very bad happened," she added.
