The great white shark (Carcharodon carcharias) is considered by many people to rank amongst the world’s most fearsome predators. Accurate insight into great white shark numbers has been hard to obtain, nevertheless, scientists say that its population has fallen significantly over recent years.
The great white shark population decreases have now been reinforced by the results of the first census of the great white shark, carried out as part of a research program led by the University of California Davis and Stanford University. The census has revealed that there are far fewer great white sharks off central California than scientists had previously reported.
What makes this great white shark census study particularly significant is that it is the first rigorous scientific estimate of great white shark numbers in the northeast Pacific Ocean. It is also the best estimate of great white shark population numbers among the world’s three known great white shark populations, with the other two populations near Australia and South Africa.
Researchers went to places in the Pacific Ocean where great white sharks are known to congregate. They lured the great white sharks using a seal-shaped decoy on a fishing line, so the sharks could be photographed close up. From 321 photographs of the uniquely jagged edges of great white sharks’ dorsal fins, the team was able to identify 131 individual great white sharks.
From this data, the scientists estimate that there are 219 adult and sub-adult great white sharks in the region. Great white sharks are classed as sub-adults when they reach about 8 feet to 9 feet in length and their dietary focus shifts from eating fish to mostly marine mammals. The great white sharks are then considered adults when they reach sexual maturity – for males, that is at about 13 feet long; for females, it is when they reach approximately 15 feet long.
“This low number was a real surprise,” said UC Davis doctoral student Taylor Chapple, the study’s lead author. “It’s lower than we expected, and also substantially smaller than populations of other large marine predators, such as killer whales and polar bears. However, this estimate only represents a single point in time; further research will tell us if this number represents a healthy, viable population, or one critically in danger of collapse, or something in between.”
“We’ve found that these white sharks return to the same regions of the coast year after year,” added study co-author Barbara Block, a Stanford University marine biologist and a leading expert on sharks, tunas and billfishes. “It is this fact that makes it possible to estimate their numbers. Our goal is to keep track of our ocean predators.”
Satellite tagging studies have demonstrated that white sharks in the northeast Pacific make annual migrations from coastal areas in Central California and Guadalupe Island, Mexico, out to the Hawaiian Islands or to the “White Shark Café,” a region of the open ocean between the Baja Peninsula and Hawaii where great white sharks have been found to congregate — and then they return to the coastal areas.
The full paper about this research, published in the journal Biology Letters, is available here.