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January 28, 2013
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CGA professor redefines sea scallop growth
Study questions the traditional method of determining age
A sea scallop shell with samples removed for stable isotope analysis. Credit: Tony Chute, National Marine Fisheries Service
In an article recently published in the Journal of Shellfish Research, Dr. Sam Wainright, a U.S. Coast Guard Academy science department professor, and colleagues from the National Marine Fisheries Service reported new insights into the seasonal growth patterns of sea scallops off the mid-Atlantic Coast, challenging conventional wisdom.
Sea scallops, like trees, develop thin, visible growth rings on their shells, which can be used to estimate their age. Fast growth results in rings spaced widely apart, whereas slow growth leads to rings spaced closely together. Prolonged periods of slow growth result in so many rings spaced closely together that they appear as a dark band on the shell. These bands are called annual growth rings because they are thought to develop once a year when cold winter temperatures slow scallop growth.
Ages derived from annual growth rings are used in computer models that estimate the size of scallop populations in the future and, ultimately, the number of scallops that can be harvested by commercial fisheries in a sustainable manner.
In research conducted over the last eight years, Wainright and his colleagues found compelling evidence for a pattern of annual growth ring development different from the one already described. Instead of counting annual growth rings to estimate scallop age, they determined age from the ratio of stable oxygen isotopes (18O:16O) found in the scallop’s calcium carbonate (CaCO3) shell. Because the amount of 18O incorporated into the shell when it is first produced is temperature dependent, samples carefully taken from numerous points along the shell can be used to develop a picture of water temperature over the lifetime of the scallop. By noting seasonal fluctuations in temperature, age can be calculated.
The most surprising result of the study was that annual growth rings were produced not during the coldest time of year as previously thought but during the warmest, in late summer through early autumn. This is the breeding season for scallops off the mid-Atlantic Coast, suggesting that stress related to spawning (not low water temperature) causes growth rates to decline and annual growth rings to develop.
This could have serious implications for managing the commercial sea scallop fishery. For example, population models used to estimate sustainable catch limits may be overestimating growth rates and underestimating mortality associated with spawning activity. Moreover, it calls into question the traditional method of determining scallop age. Some sea scallops are known to spawn multiple times per year and might produce more than one annual growth ring, confusing models based on population age structure.
Given these implications, Wainright was excited to share the results with the scientific community, but he acknowledged the potential for resistance. “It was exciting to write the paper and submit it for publication," he said. "But I knew that it might get hammered by reviewers because it was counter to widely held beliefs.”
Another unexpected finding of the study contributes to our basic understanding of sea scallop biology. Before now, very little was known about the growth rate of young scallops because they do not develop annual growth rings until they are -- in Wainright’s words—“the size of a Chips Ahoy cookie.” By studying oxygen isotope ratios in shell material developed early in life, he and his colleagues found that young scallops grow astonishingly fast relative to adults. They hypothesized that fast growth is favored by evolution because it minimizes the amount of time that scallops spend in size classes most vulnerable to predation.
While the recent publication marks the end of the sea scallop project, Wainright is looking forward to future collaborations with colleagues at NMFS and other institutions. Stable isotope analysis has been the cornerstone of his research for more than 20 years, and he has applied the technique to a wide variety of problems involving photosynthesis, food web dynamics and marine pollution. Asked about the most rewarding part of research, Wainright replied that it was, “taking something that many people may find to be esoteric and applying it to a very practical problem, such as managing our marine resources.”
More information on Wainright’s research can be found in the article, Chute, A.S., S.C. Wainright, and D.R. Hart. 2013. Timing of shell ring formation and patterns of shell growth in the sea scallop Placepecten magellanicus based on stable oxygen isotopes. Journal of Shellfish Research 31(3): 349-662.
Written by Associate Professor Lucy Vlietstra, U.S. Coast Guard professor, science department, for the Center for Advanced Studies
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