Although scientists have long considered sharks to be loners, new research shows that sand tiger sharks exhibit behaviours typically seen in mammals and only rarely observed in fish.
Sand tiger sharks, top predators that live in coastal waters off the Eastern United States, have experienced drastic population declines over the past several decades. Understanding how these sharks move and interact could help biologists better conserve this species and determine how vulnerable they are to human activities. During summer, the sharks congregate in the shallow waters of Delaware Bay but are highly migratory, travelling as far south as the Carolinas and Florida during the winter and early spring.
Using acoustic tracking devices to trace the movements of over 200 individual animals in the open ocean for over a year, researchers found that sand tiger sharks form complex social networks. Initial data from two individual sharks showed they encountered nearly 200 other sand tigers throughout the year, as well as several individuals from other shark species.
The researchers even identified a number of “best friends”. This status was afforded to those sharks that met the animals in question more than 20 times over a 12-month period. While this finding is quite surprising in itself, the researchers were even more intrigued by the seasonal variation of the sharks’ social behaviour, which seemed to peak in the summer before dropping off in late winter and early spring.
These sharks exhibit what is known as fission-fusion social behaviour, meaning that the number of sharks in a group and the individuals that are part of the group change by location and time of year. Groups would stay together for certain times of the year and fall apart during other times.
The researchers also found that sand tiger sharks re-encounter the same sharks throughout the year. Another surprise was a sudden lack of encounters with other sand tiger sharks in the late winter and early spring.