Using pop-up archival tag data, a team of researchers found that commercial by-catch of juvenile white sharks is the leading cause of mortality, according to findings published in the Journal of Applied Ecology.
Assistant professor at the University of Nebraska and lead author of the study, Dr. John Benson, said that PAT data represented “a widely-available, untapped data source that could dramatically increase our understanding of marine population ecology.”
Benson, who primarily studies terrestrial predators, conducted the white shark study as a postdoctoral researcher at Monterey Bay Aquarium. The aquarium— together with colleagues at California State University, Long Beach; Aquatic Research Consultants in San Pedro; and the Ensenada Center for Scientific Research and Higher Education (CICESE) in Baja California—has been tagging and tracking juvenile white sharks since 2002.
By using data collected, Benson was able to determine the fate of these sharks when the tags stopped collecting information. If a tagged shark was eaten by a predator, or if it died in a fishing net, the tag recorded those data.
“Because the PAT tags record detailed data on temperature and diving, it is possible to reconstruct the fate of the shark in the final minutes of each track,” said Jorgensen. “We’ve looked at these data for years and often noted when a tag ended up on a fishing boat or in the stomach of a larger predator. Surprisingly, nobody had yet connected the dots to use this information to estimate a critical demographic rate: annual survival.”
According to data taken from 37 sharks tagged since 2002:
- The overall estimated annual survival rate for young white sharks was 63 percent. Though this study did not address broad trends in the white shark population in the Northeastern Pacific, the researchers note that protection of white sharks in 1994 has likely resulted in a reduction in fishing-related mortality. The increase in juvenile shark sightings over the last 15 years may be an early indication of a positive sign for population recovery.
- Fisheries bycatch was the main source of mortality for juvenile white sharks in the region, highlighting the need to follow best practices related to incidental catch in coastal commercial and sport fisheries. Only two young white sharks tagged by researchers died of natural (non-fishing) causes.
- Overall mortality risk for young white sharks was lower for larger animals, which could be attributed to smaller sharks being more abundant, or simply more susceptible to capture in gillnets.