Low oxygen levels cause a big problem for aquatic ecosystems. When oxygen falls below two milligrams per liter, the area is classed as “hypoxic,” a condition that can be driven by pollution such as agricultural runoff. Hypoxia has an effect on marine life that’s pretty relevant to fisheries: low oxygen in the environment slows down the growth of individual animals, meaning that populations are made up of smaller creatures.

Research published in PNAS this week, led by Martin D. Smith at Duke University, uses a new method that takes a big step toward being able to quantify the economic impacts of the pollution that causes hypoxia. The approach could give policymakers a better tool to understand the costs and benefits of various pollution controls. Their technique could also help researchers to observe the effects of marine disturbances in other areas.

Smith and his colleagues looked at data from the Gulf of Mexico, which has the world’s largest area of seasonal hypoxia, peaking in the summer. The area is home to an important brown shrimp fishery, which should show the effects of hypoxia—but it’s not quite so simple to detect this.

“Although studies demonstrate ecological effects of hypoxia, economic consequences have not been determined in this fishery,” the authors write. Determining these economic consequences is a vital step in informing policy decision. It’s expensive to control pollution upstream of hypoxic areas, so knowing the economic effects of hypoxia help to determine whether the costs are worth it on a purely economic basis.

To get a clear picture of the causal effects of hypoxia, what you really need is a natural experiment: one area that becomes hypoxic, where you can trace the impact on fisheries over time; and another area that isn’t hypoxic to act as a control or baseline. The problem is that ships aren’t static—if one area is producing better harvests, people will just move there. That makes it impossible to clearly compare the hauls from hypoxic areas to non-hypoxic areas.

Where you can expect to see a difference is in the prices of shrimp. If the populations of shrimp in hypoxic areas have smaller individuals and fewer large ones, that should make larger shrimp more scarce, which in turn makes them more expensive. Smaller shrimp will be more abundant, driving their price down. Conveniently, shrimp are generally sold by size, making this relatively easy to track, but not entirely easy: local shortages can be quickly overcome by imports, which means that any price fluctuations will be short-lived.

To see whether our expectations actually appeared in the data, Smith and his colleagues looked at seasonal hypoxia, tracking shrimp prices on a month-by-month basis from 1990 to 2010. They found a strong relationship between prices and hypoxia: when hypoxia peaked, so did the prices of large shrimp, while the prices of smaller categories of shrimp dipped.

This finding is wider-reaching than just the prices of shrimp from the Gulf of Mexico. The method that Smith and his colleagues have developed could be used in other fisheries to determine the impacts not just of hypoxia, but also other environmental disturbances—assuming the right kinds of data can be found. Because environmental impacts on fisheries run a lot deeper than just shrimp and just hypoxia, the cumulative economic impacts could slowly be revealed by multiple analyses.

If the goal is to provide evidence that there’s an economic incentive to do something about hypoxia, there’s a lot more information to gather. “An ideal claim for policy analysis would be something like “reducing nutrient runoff X percent leads to economic benefits for shrimp (and other fisheries) of $Y,” the authors write. For the moment, there isn’t enough data to make a claim like this.

PNAS, 2016. DOI: 10.1073/pnas.1617948114 (About DOIs).