Shark study in Seychelles shows mechanism for peaceful coexistence of two species

The team actively tracked individual sharks' movements and behaviour in the lagoon. (Rainer Von Brandis/Save Our Seas Foundation) 

Juvenile sharks escape from some of the fastest and fiercest fish in the sea by going to shallow waters where the predators cannot go, according to a project done in collaboration with the Save Our Seas Foundation (SOSF) in Seychelles.

In a press release last week, SOSF said that after falling deep into the field of shark research at Bimini Biological Field Station in The Bahamas during her master's research, Dr Ornella Weideli, a former leader of the SOSF Research Centre on D'Arros Island, developed an interest in the dietary behaviour of sharks.

Weideli wanted to not only understand what juvenile sharks eat but also how their behaviour may change if they are sharing their nursery ground with another, ecologically similar juvenile shark species.

The project that she completed with the team at the Research Centre on D'Arros, was conducted in the shallow lagoon of St. Joseph, an atoll of the Amirantes group in Seychelles.   

An aerial view of St Joseph Atoll. (Ryan Daly/Save Our Seas Foundation) Photo License: All Rights Reserved

"The lagoons here are home to a large population of not just one, but two juvenile shark species: the blacktip reef shark and the sicklefin lemon shark and so it really is the perfect location to study how these two species are sharing space and food," explained Weideli.

Theory suggests that the key to a peaceful co-existence seems to be to reduce competition, and therefore conflict, between the species. The slightly smaller blacktips seem to be subordinate to the larger lemon sharks, which Weideli and her team investigated in captive experiments at the Centre of Island Research and Environmental Observatory (CRIOBE), which has research centres in Perpignan, France, and French Polynesia.

To find out how this hierarchy affects juvenile shark behaviour, the team actively tracked individual sharks' movements and behaviour in the lagoon. On foot or by kayak, they tracked the sharks, recording where they spent their time and where they searched for food.  

By flushing the sharks' stomachs, without harming the young animals, Weideli was also able to analyse the stomach contents of 115 blacktips and 188 lemon sharks to find out what the juveniles had been eating.

Dr Ornella Weideli was also able to analyse the stomach contents of 115 blacktips and 188 lemon sharks. Photo License: All Rights Reserved

Her findings were that 84.6 percent of the stomach contents of blacktip reef sharks were fish compared to 94.2 percent in lemon sharks.

The difference came in the fish species they ate – blacktips showed broader diets, eating 32 different species, compared to 17 different fish species found in the stomachs of lemon sharks.

It seems as though this may be because the juvenile blacktips spent more time in deeper, riskier habitats, as a result of being the subordinate species.

"The results support the initial hypothesis – to avoid competition, subordinate sharks eat the food of poorer quality when there's a dominant competitor around! To me it's incredibly fascinating that different species of shark already show diverging diets at this young age," said a delighted Weideli.

She added that "we've assessed for the first time the competitive ability in two co-existing shark species, enabling us to provide evidence that niche partitioning, basically, how sharks share, is at least partially driven by competition between species."