The Mystery of the CIRCLING FISHES by Deborah Painter

The giant trevally can catch dolphins and small sharks and perform a mysterious action that no one in the scientific world understands. CREDITS:  Skeeze             

The giant trevally has a large but very fragmented range that includes the Indian Ocean and Red Sea coastline of twelve African nations as well as the Pacific islands of Vanuatu, Wake Island and Hawaii.  It is the largest member of the genus Caranx, and the fifth-largest member of the family Carangidae (exceeded by the yellowtail amberjack, greater amberjack, leerfish and rainbow runner), with a recorded maximum length of 170 cm and a weight of 80 kg. In Hawaii’s coastal waters dwell the largest individuals. The species has excellent panoramic vision.  Its normal habitat is estuaries when a juvenile, and outer reefs and atolls when it is a mature adult.Off Africa’s coast, the diet is similar to the diet of those ranging off Hawaii, consisting mostly of fish, crustaceans and squid. Juvenile turtles and even dolphins were reported being found in stomachs of larger giant trevally.  As a rule, however, the older the individual giant trevally, the more fish it consumes.  It is a very popular fish among recreational fishermen. 

For hundreds of years the only thing the giant trevally, also known as the kingfish or trevally jack, was known for worldwide was its size, strength and voraciousness, and not its mysterious trip up one of South Africa’s rivers.  Caranx ignobilis is an inappropriate name, it would seem, for a fish that does a mysterious and beautiful thing in the waters of the Mtentu River, where it should not occur at all since the river is not the right temperature, depth or pH for this species.

  The giant trevally is listed by the International Union for Conservation of Nature (IUCN)as “Least Concern”, meaning that its global population is of adequate size so that it is not considered near threatened or vulnerable.   It was only in recent decades that the peculiar behavior of schools off the South African coast was properly filmed and chronicled for science.   At the Mtentu River, giant trevally enter the riverine waters led by one individual.  This one could be correctly termed the “King Kingfish”.  They swim for a considerable distance and start to reach shallow waters.  Just when it seems that they have nearly exceeded their safe zone upstream and have almost reached the absolute limits for survival needs for depth, temperature and pH, the fish stop swimming any further upstream and instead begin to circle.  No one knows the purpose of this underwater ballet, as they are not feeding, courting, or spawning. They do not meet any other fishes.

After approximately an hour the school heads back to the Indian Ocean. Scientists and fishermen have no idea why they do this. They are drawn by some unknown motivation.  Since we cannot ask the fish why they make this journey, it may be a long time before we know.

The IUCN Red List: Giant Trevally (Caranx ignobilis).