NEWPORT, Ore. - Scientists at Oregon State University's Hatfield Marine Science Center have found a link between the growth rings of trees and the growth rings from the ear bones of long-living rockfish species, and their findings could provide new information for resource management.
The study also may provide clues about the impact of climate on-shore and at sea. Results will be published in the forthcoming issue of the Canadian Journal of Fisheries and Aquatic Sciences.
In trees, growth rings have long been used to determine the effects of climate on tree growth, and to establish the precise age of each tree. Climate induces synchronous growth patterns in all trees at a site, the researchers say, and drought years are characterized by a narrow ring, while mild or wet growing conditions lead to the formation of a wider ring.
"Fluctuations in climate over time form a distinct growth pattern, much like a bar code, that can be cross-matched among the trees," said Bryan Black, an assistant professor at OSU who works at the center's Cooperative Institute for Marine Resources Studies.
Cross-matching is important, Black said, because some rings may be subtle. The technique allows dendrochronologists to confirm that all growth increments are accounted for.
"This pattern matching, or 'cross-dating' is an extremely powerful tool for precise dating and is the fundamental procedure of any tree-ring study," Black said. "We've found that the same concept applies to otolith (ear bone) growth increments and, for the first time, we're able to apply that same level of precision to fish."
Just as tree rings reflect precipitation and temperature, otoliths faithfully record variations in ocean temperature and upwelling, among other factors. The year 1983, for example, showed up as a narrow band in nearly all of the rockfish otoliths. Not coincidentally, that was the year of one of the strongest El Nino occurrences of the century.
El Nino years typically result in poor upwelling and prey production, limiting rockfish growth. By contrast, strong upwelling leads to rapid growth. By statistically isolating the climate-induced "bar codes," Black has been able to precisely date all growth increments in these fish.
And the fish are old, according to George Boehlert, director of OSU's Hatfield Marine Science Center and co-principal investigator for the study.
"About 20 years ago, radiometric dating first revealed that rockfish can live much longer than previously thought," Boehlert said. "The splitnose rockfish, for example, grows only to 18 inches and can live as long as 90 years, though they reach their maximum size in 20-25 years."
Boehlert, a fish ecologist, said that using cross-dating techniques allow researchers to identify the exact age of each fish. "Accurate age data are crucial for fisheries management," he said.
The OSU researchers have been able to link these growth increment chronologies to various aspects of ocean variability as a way to investigate ocean ecology. The growth of fish, Boehlert says, is generally an indicator of environmental conditions.
"With fish, there are a lot of factors, such as temperature, upwelling and prey availability that affect fish growth," Boehlert said. "But if you go back and look at the historical data, you can tease apart the different factors and get an idea of how the fish responded to environmental factors."
Analysis of the climate-growth relationship clearly shows that rockfish grow best in cool ocean conditions with plenty of upwelling, especially in the winter and spring months. The tie between ocean variability and fish growth is as strong as the relationship between temperature or precipitation and tree growth in many tree ring studies, Black pointed out.
"The strength of the correlation is surprising - and encouraging," he said.
The visual environmental impact of the ring studies gets even more interesting for scientists when they plug in "big-scale" events like the Pacific Decadal Oscillation - multi-year general warming and cooling trends in the northern Pacific. In the growth rings, this can show up in jumps from a series of narrow bands to a number of wide bands, or vice versa.
"These longer-term oscillations are especially useful for management," Black said. "The ocean tends to stay in a warm or cool phase for many years, so we know that if we're in a warm phase one year, we'll probably be in a warm phase the next year and rockfish growth may be lower than average."
Though Boehlert and Black worked primarily with splitnose and canary rockfish for their study, they say other long-lived fish with otoliths may also be valuable as environmental indicators. Likewise, some clams like geoducks have rings on their shells that correspond to age and perhaps environmental conditions. Some long-lived freshwater mussels and deep-sea corals have similar patterns.
The researchers say the otolith biochronologies from fish are directly comparable to tree-ring chronologies on land. "Our next goal is to better establish the linkage between marine and terrestrial ecosystems using these chronologies," Black said.
Already some patterns are apparent, he added. When ocean conditions are warm, the winter is less severe and the growing season for trees in the Cascades starts earlier and lasts longer. Tree rings become wider, not narrower - just the opposite signature from the rockfish.
"The Pacific Ocean has an enormous influence on environmental conditions," Black said. "The inverse connection between tree growth at 5,000 feet in the high Cascades, and rockfish living hundreds of feet below the ocean's surface, is fascinating."
The research is part of an effort by OSU's Hatfield Marine Science Center to broaden its marine science research to examine linkages with terrestrial ecosystems, Boehlert said.
"Cross-dating these rings - and comparing them with established data sets - offers tremendous potential for learning more about the impacts of climate on living things, terrestrially and in the ocean," Boehlert said. "They're not laying down those rings for us, but we're sure going to use the information they provide."