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Tree Rings for Lobsters
10 December 2012 3:45 pm
Like Hollywood starlets, crabs, shrimp, and lobsters have always been good about hiding their ages. The crustaceans shed their armor, or exoskeleton, every year, making it hard for biologists to determine how old they are. But now researchers have found well-hidden growth bands in the animals that persist through molting and yield reliable ages. The finding should help fishery agencies better manage these commercially important species.
With no previous measures to go on, fisheries biologists have estimated the age of a crustacean based on its body length. Fishermen are allowed to harvest only longer—and thus theoretically older—animals, allowing juveniles to reach sexual maturity and start reproducing before hitting our dinner plates. But such length-based limits are often flawed, because growth rates vary depending on conditions in the ocean. Cool waters, for example, stunt a lobster's growth. In the Gulf of Maine, prime habitat for the American lobster, water temperatures can differ by several degrees in areas just 50 kilometers apart. Even so, biologists still assign the same age-length ratios to lobsters from all locations, says biologist Carl Wilson of the Maine Department of Marine Resources in West Boothbay Harbor.
To establish more reliable ages for crustaceans, marine biologist Raouf Kilada of the University of New Brunswick, Saint John, in Canada and his team looked for annual growth bands in various calcified hard structures in snow crab samples, and finally found promising patterns in the animal's eyestalk. His team sliced up hundreds of eyestalks and divided the slivers of each one onto 40 microscope slides. Colleagues told him, "You are crazy, you are wasting your time," he says. "I was swimming against the currents."
Finally, after 6 months of examining the slices under a microscope, Kilada found definitive age bands. Like tree rings, the markings, which are less than a millimeter thick, each contain a thin, dark rim lined by a thicker, lighter edge. The dark-light pattern could represent changes in seasonal growth rates, but the team still has not confirmed how these differences form. They are confident, however, that each light-dark pair represents 1 year.
Since then, Kilada's team has found similar growth bands in the eyestalks of two shrimp species and within teethlike structures in the stomachs of the American lobster. To confirm their findings in lobsters, the researchers submerged 20 juveniles in a chemical tracer that stained the growth bands as they formed. They set the lobsters aside for 18 months and allowed them to molt three times. Then, the team dissected the lobsters and found the chemical tracer intact within all of their samples, confirming that molting did not erase the bands. Kilada presented the team's results last month at The American Lobster in a Changing Ecosystem conference in Portland, Maine, and in the November issue of the Canadian Journal of Fisheries and Aquatic Sciences.
"The next step is to apply this technique on a wider scale and to implement it in a stock assessment plan," Kilada says. Now, fisheries biologists can collect samples of crustacean populations from different regions with varying growth rates, cut the animals open, look at their growth bands, and recalibrate existing age-length ratios accordingly. Only a small subgroup of each population would need to be sliced and studied to improve age-length models that should enable management agencies to assign better site-specific size limitations.
Wilson, who attended the conference, is impressed with the study. "Is this unexpected? Not particularly," he says. "But someone needed to go through this kind of work and develop these techniques, and they seem to have developed a nice set." Meanwhile, Kilada has received calls from colleagues all over the world -- including Alaska, Australia, and Chile—who are eager to extend his methods to other lobster and crab species.
"It's a big step forward," says Laura Stichert, a fisheries research biologist with the Alaska Department of Fish and Game in Kodiak. She is now working with Kilada to search for growth bands in several economically important Alaskan crab species. Stichert says she's hopeful that age measures will improve crustacean management and stock sustainability worldwide. "I think there will be a lot more coming from this."