Study Suggests Improved Method for Measuring
A recent study questions the accuracy of the long-used technique of scale morphology when estimating fish age and timing for juvenile chinook salmon migrating into the Columbia River estuary.
By comparing scale morphometrics -- simply thought of as fish scale analysis -- with the more accurate scale and otolith chemistry, the study sought to validate scale morphometrics, but found that the simple scale analysis underestimated the proportion of estuary residents by 37 percent. That includes some fish that entered the estuary as fry and were thought to have been lost to the system.
The researchers further determined that scale chemistry, although somewhat less sensitive than otolith chemistry, is a good alternative to scale morphometrics for some life history studies, particularly where lethal sampling of at-risk fish, which is required by otolith chemistry, would be required.
Still, scale morphometrics is used widely in the Columbia Basin to age returning salmon in rivers and on spawning grounds, according to Lance A. Campbell, fish aging and otolith lab director at the Washington Department of Fish and Wildlife.
He added that this information is accurate and used to assess population structure of the various salmon species as well as to assist in forecasting future returns, but the accuracy declines when measuring the residence of juvenile chinook salmon in the estuary.
"In our study we were testing the ability of scale morphometrics to describe other life history shifts in migrating juvenile salmon," Campbell said, particularly entrance into the saline portion of the Columbia River estuary. "In that more narrow use, it does offer evidence that interpretations of scale patterns to describe estuary entrance needs validation."
"In this case we offered evidence that the correspondence between entrance into the saline portion of the estuary and the formation of an estuary scale check was not good," he said. "All scientific results are endangered when they rest on untested assumptions."
"Correspondence between Scale Morphometrics and Scale and Otolith Chemistry for Interpreting Juvenile Salmon Life Histories" was published online Dec 10 in the Transactions of the American Fisheries Society.
In addition to Campbell, co-authors are Daniel L. Bottom, research fishery biologist with NOAA Fisheries' Northwest Fisheries Science Center; Eric C. Volk, chief research scientist, Commercial Fisheries Division, Alaska Department of Fish and Game; and Ian A. Fleming, professor, Memorial University of Newfoundland. Fleming was with the Coastal Marine Experiment Station and Department of Fisheries and Wildlife, Oregon State University, Hatfield Marine Science Center at the time the study was conducted.
Scale morphometrics is fish scale analysis that measures, counts and classifies scale circuli and annuli. Circuli are the calcified rings and ridges that help hold scales in place on the fish. They form and grow in relation to the fish's growth making them useful for studies of life history growth and age, according to Campbell.
"The annuli are a collection of circuli that form during slower growing periods (winter) and are interpreted to occur over an annular cycle (thus an age estimate can be made)," he said.
The study examines the use of an even finer level of scale analysis -- scale chemistry -- and otolith chemistry to interpret juvenile chinook salmons' entrance into the saline portion of the Columbia River estuary. When using scale chemistry, researchers measure the amount of strontium and calcium deposited in the scales. Strontium presence becomes stronger with salt water.
It concluded that the number of juveniles tested that would not have been identified by scale analysis is because scale morphometrics misses fish that enter the estuary at a small size when their scales are just forming or that the fish had just recently entered the estuary before the new scale material could be deposited and calcified, according to the report. In other words, there is often a lag time between entry to the estuary and the changes that can be visually observed in scales.
Campbell said they were surprised to see the numbers of very small juvenile salmon in the 35 millimeter to 45 millimeter range in their study.
"Very small fry that enter the Columbia River estuary appear to be surviving and recruiting to later life stages . . .," he said. "This is relevant because the prevailing thought has been these small fish, once departed from their natal stream do not survive."
Although the fish were much larger when captured, they were able to determine the size of the fish on entry to the estuary through otolith chemistry. Otoliths are the "earstones" of fish, according to Campbell, located in the bony section of their heads.
"This back-calculation of size is possible because of the relationship between the fork length of the fish and the size of the otolith and because seawater has elevated levels of the element strontium that is incorporated in the calcium carbonate matrix that otoliths are primarily comprised of," he said. "These facts allow us to use a laser and a mass spectrometer to test specific regions of the otolith or scale to determine if a fish has moved into and for how long it has resided in saline waters."
The results do not rule out the possibility that scale morphometrics could be used in other ecosystems, particularly in coastal rivers with small estuaries and limited tidal influences upriver.
"Our findings suggest that scale morphometrics must be applied cautiously because the factors that influence fish growth and scale formation are likely to vary in each ecosystem," the report says.
Campbell added that otolith and scale chemistry techniques offer a "powerful tool for studies of life history and growth."
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