Ageing studied are underway to determine age and growth of shortfin mako shark sampled by Japanese longline from widely distributed localities in the Pacific, and the common thresher shark from the California-Oregon drift net fishery. Biologists age sharks by counting the concentric calcified bands in x-rays of cross sections of the shark's vertebra, much like foresters count the rings in cross sections of a tree trunk. The purpose of the mako ageing project is to help clarify movements of and effects of high seas exploitation on different mako age groups. They also hope to resolve existing problems in interpreting the timing of the formation of these calcified rings. This is a joint study between scientists from the Japanese National Research Institute of Far Seas Fisheries at Shimizu, Japan, and scientists within the Fisheries Resources Division at La Jolla, CA to independently age a sample of about 250 subadults and adults mako sharks from the western and central north Pacific.
Thresher shark vertebrae are also being aged at the La Jolla Laboratory using x-radiography techniques. The purpose is to expand and refine previous thresher ageing work using a larger sample size from the drift net fishery with accompanying information on sex and maturity stage.
Tetracycline marking and release of tagged juvenile makos and juvenile and subadult threshers off California has begun to verify the timing of 'annual' ring formation. Tetracycline deposits were new calcification is occurring at the time of injection, and makes a fluorescent time mark in the vertebrae that can be examined in relation to new vertebral growth when a fish is recaptured a year or more after injection and release.