Layer by layer, tiny bones tell the life stories of fish

WOODS HOLE -- Using atom-by-atom analysis of small bones found inside fish ears, scientists can now tell where a fish was born, where it grew up, and where it migrated during its lifetime.

The ability to interpret the data -- similar to reading a tree's annual growth rings -- may prove vital for fish conservation efforts, especially in aggressively harvested areas such as offshore New England.

These subtle clues to a fish's travels are found inside its otoliths, three small stone-like objects used to help orient the fish.

The otoliths add a new layer of bone every day, and the chemical composition of each layer reflects the salinity and other telltale features of the water the fish was in. The layers can be analyzed to indicate where the fish originated and where it matured.

``It's sort of like a flight recorder," said Bob Warner, a marine biologist at the University of California, Santa Barbara, who studies otoliths.

Fishermen have been ``reading" these bony organs for a century to tell the age of the fish they catch, but only recently has the technology been developed to look inside such fine detail in fish otoliths. Mass spectrometers can count the relative abundances of chemical elements and their isotopes in each sample, matching the results with the chemical makeup of the local environment.

``The chemistry tells us where the fish was at these different stages in its life," said biochemist Simon Thorrold, a researcher at the Woods Hole Oceanographic Institution. ``It's great to know how old a fish is. But it's really cool to know where it came from, whether it's been in fresh water, salt water, or both."

Researchers like Thorrold and Warner are only beginning to plumb the possibilities of the data, but Thorrold's recent studies of a common East Coast swimmer, the weakfish, successfully identified the coastal estuaries in which different populations hatched. Such data could change how government agencies work to manage the annual catch of weakfish.

Currently, the fish are managed as if they were a single population along the whole East Coast, since they show no genetic variation, Thorrold and a colleague at Woods Hole, Anne Cohen, noted in a Woods Hole publication. These fish hatch in estuaries, spend their adulthood near the bottom in coastal waters, and return to estuaries to spawn.

The two researchers were able to read the chemical signatures of weakfish from each of five large East Coast spawning estuaries, from Georgia to Long Island. They found that most adult fish were returning to their birthplaces to reproduce -- not randomly to any of the five possible estuaries. This means that protecting one or two of the estuaries might not be sufficient to maintain the fish stocks, Thorrold said.

Similar otolith work has also been done by Thorrold and others -- successfully -- on trout and salmon. Thorrold said he was able to track where a salmon originated -- such as the Columbia River system in the Pacific Northwest, and how old the salmon was when it swam from fresh water into salt water.

At present, Warner said, Thorrold is the world's best reader of such hidden data.

Cohen and a research colleague at Woods Hole, Graham Layne, are using a similar approach on the world's most expensive fish -- the Atlantic bluefin tuna.

Highly prized by sushi fans in Japan, a 1,400-pound tuna can sell for $40,000. Such prices have spurred overfishing and near-collapse of the tuna fishery.

The goal with bluefin tuna studies is to see whether there's only one population of the big, powerful fish in the Atlantic or whether two exist -- one in the western Atlantic, the other in the east. Fishermen have argued about whether both sides of the Atlantic need the same rigid regulations now enforced in the west; if there are two populations, then fewer regulations may be needed.

Cohen and Layne are hoping to use the chemical signatures of tuna otoliths to show migration patterns, stock mixing, and spawning habits.

They expect that the entire life history, from birth to death, of a giant 30-year-old bluefin tuna is contained within a single otolith less than one inch long.

Such work is important because, as Thorrold put it, ``marine fish populations are in trouble globally; they're struggling everywhere you look. So we need to be much smarter about how we exploit that resource. We need more science."