One thing we've all learned since white sharks started frequenting the waters around Cape Cod is that we don't know nearly as much about these fascinating creatures as one might think.
Add this to the list: We don't know how quickly they grow, or how long they live. We've had working estimates. But new research by scientists at Woods Hole Oceanographic Institution, Northeast Fisheries Science Center, and Massachusetts Department of Marine Fisheries suggests that our understanding is in need of serious revision.
Biologists have traditionally estimated sharks' ages by counting growth rings inside their vertebrae, much the way tree rings are used. But it's never been entirely clear that sharks' growth rings were annual, that one and only one new ring was produced each year.
The new study tested that idea and found that, indeed, the rings weren't always annual. Just as humans grow rapidly early in life, then slow down and eventually stop, white shark growth rings appeared to be annual for up to forty years. After that point, rings appeared to have been laid down less frequently. The upshot is that we've been significantly underestimating how long white sharks live, and overestimating how rapidly they grow (at least later in life).
That has ramifications for shark ecology and fishery management, although the nature of those ramifications is still a bit murky. If female white sharks reach sexual maturity at, say, age ten and bear young every other year for the rest of their lives, that could be good news. But if, instead, it turns out that white sharks mature much later in life, it could mean current management strategies are based on a falsely optimistic idea of how quickly they can replenish their populations.
How Dr. Li Ling Hamady and her colleagues figured this out is possibly even more fascinating than the result, itself. Hydrogen bomb testing in the 1950's and early 60's released radioactive carbon into the atmosphere. It then made its way into the ocean, and into the plants and animals living there. Hamady used vertebrae from a handful of sharks with known times of death and estimated ages that would indicate they'd lived through the period of testing. She then looked to see if radiocarbon showed up in the bands that would correspond to those years, assuming each ring corresponded to a year.
Call it the scientific version of turning lemons into lemonade.