Researchers studying spotted wobbegongs in Sydney have found that these sharks gather in social groups rather than associating randomly or according to the amount of food available.
The study, led by researchers at Macquarie University, reveals the potentially damaging consequences of fishing on shark populations – as, instead of randomly catching individuals, important members of social networks could be removed.
“If these aggregations are not random but in fact reflect more complex social relationships, then the impacts from historical fishing may not be random, but may have continuing consequences,” said Nicolette Armansin, an animal behaviour expert who led the research.
“Wobbegongs are a large bottom-dwelling ambush predator often found in large groups close to shore where they have been heavily exploited – the last place most people would have encountered one was at their local fish and chip shop,” she said.
Data was collected from the IMOS Animal tracking network of acoustic receivers, to track the movements and associations of individual sharks in Cabbage Tree Bay Aquatic Reserve in Manly on the northern beaches of Sydney, New South Wales.
They found that groupings were not sex- or age-related, meaning younger animals may not be sticking together for safety.
With the stability of these aggregations not yet known, removing individuals from a group could have a significant impact.
The study, which has been ongoing for over a decade, found that sharks grouped together with preferred partners, even in different areas across the reserve, while others never associated.
“What we found was a complex network of associations,” said Robert Harcourt, leader of the IMOS Animal tracking facility and co-author of the paper.
The findings were published this month in the journal Animal Behaviour, and suggest that even lone predators such as as the wobbegong could demonstrate complex social relationships.
“Social behaviour might be expected in sharks that feed together on large schools of fish, but the existence of these associations in even bottom-dwelling predators that individually ambush their prey points to a much higher level of social complexity than previously imagined, and to possible common evolutionary mechanisms across multiple animal groups,” said Harcourt, who added that sharks are a far more complex species than we understand.
“Like mammals, birds and reptiles, it appears that sharks inhabit a far richer social world than we would have imagined, or deemed necessary,” he said.
These same wobbegongs, when not at Cabbage Tree Bay Aquatic Reserve in Manly, have been recorded moving up and down the coast on IMOS Animal Tracking acoustic lines. They have been recorded out to the edge of the continental slope in waters 200m deep at the very end of the IMOS Animal Tracking Acoustic Bondi line.
This news item has been adapted from an article by George Meredith in 'Australian Geographic', 4 May 2016.