Hakai Magazine

ocean quahog shells on the beach
Ocean quahogs can live for hundreds of years. The marine mollusks build their shells layer by layer, with each reflecting the environmental conditions at the time it was formed. Photo by Buiten-Beeld/Alamy Stock Photo

A 450-Year Record of North Sea Herring, Pried from Clams

Chemical analysis of the shells of long-lived North Sea ocean quahogs unveils hundreds of years of environmental change.

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by Rachel Fritts

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Scientists have reconstructed a detailed account of North Sea herring stocks that stretches back more than 450 years. This is the first time researchers have modeled recruitment—a measure of the number of eggs that survive to become young fish—for herring living before the 20th century. This lengthy record of herring health stems from measurements taken from a wholly unlikely source: the ocean quahog.

Ocean quahogs, palm-sized clams that live in the North Atlantic, might seem like unorthodox record keepers. But the shellfish have two very useful traits: they live for an exceedingly long time—the oldest on record was 507 when it died in 2006—and their shells have visible growth increments, much like tree rings, with a new band forming each year.

Juan Estrella-Martínez, a paleoceanography doctoral student at Bangor University in Wales, led the research. He and his colleagues used quahog shells collected from Scotland’s Fladen Ground in the North Sea to produce a data series showing how the ratios of oxygen and carbon isotopes in the shells and in the water changed from 1551 to 2005.

The isotope ratios reflect shifts in environmental conditions, such as water temperature. Providing a year-by-year account stretching centuries, the quahog shell data offers a way to better understand long-term climate patterns, such as the North Atlantic Oscillation and the Atlantic Multidecadal Oscillation, which cause changes including large-scale variations in rainfall, hurricane activity, and fish populations.

Estrella-Martínez wanted a useful application for his new centuries-long record, and decided to see if his carbon isotope data could be used to help understand long-term variability in important fisheries.

“I started looking at the herring fishery because it had the most historic data available,” he says. “Without historical records, the most we could have done was speculate.”

At first, Estrella-Martínez struggled to tie the two species together. But a breakthrough came when he found a previous study linking the ratio of carbon isotopes in quahog shells to the level of dissolved inorganic carbon in the water. Another came when a colleague pointed out that dissolved inorganic carbon levels change with the amount of photosynthesis by phytoplankton and other photosynthetic organisms. This was the missing link. When more photosynthetic organisms are present, more herring hatchlings survive to enter the fishery, since hatchlings primarily eat small copepods that rely on these organisms for food.

Estrella-Martínez worked with his colleagues to develop a model of North Sea herring that survived the larval stage each year since the mid-1500s, and compared his findings to historical catch data. The model was a perfect fit. In fact, it worked so well that the scientists were able to use the quahog-derived data to “predict” a stock crash in the North Sea herring fishery that had occurred in the 1970s. “We were hoping that it would fit, but this was all new, so when it did fit very well we were very surprised and very happy,” Estrella-Martínez says.

Reg Watson, a fisheries ecologist at the University of Tasmania in Australia who was not involved in the study, says that research like this is very important as we face an uncertain future due to human-induced climate change.

“It is vital that we know how important fisheries stocks and the marine ecosystems that support them have responded during periods of change in the past,” he says.

Estrella-Martínez hopes to inform fishery management by continuing his work using long-term data to create increasingly accurate models of how environmental change affects fisheries.