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Fish Follow Edges of Erratic Dead Zones in Great Lakes

10 April 2015

US - Dead zones caused by oxygen depletion can be extremely variable and fast-moving in Lake Erie, but fish tend to cluster around the zone edges, according to new research from the United States Geological Survey.

New research found that dead zones caused by hypoxia sometimes disappear and re-emerge elsewhere in the matter of hours, greatly affecting fish distributions, catch rates and the effectiveness of fishing gear.

Scientists also found that fish like yellow perch cluster at the edges of these areas. The discovery of erratic dead zones can help commercial fishers and scientists determine where and how to effectively catch and study fish.

Richard Kraus, a USGS scientist and the lead author, said: "We were amazed by how quickly hypoxic areas moved during our study.

"These findings can help managers sustain valuable fish populations in Lake Erie, which is one of the world's largest commercial fisheries."

Hypoxia at the bottom of Lake Erie occurs during the summer as a result of biological activity in the colder bottom layer that consumes dissolved oxygen in the water.

The warmer surface layer floats on top, preventing oxygen from mixing down to the bottom.

The researchers used sensors to measure oxygen levels and lakebed temperatures, and found that dead zones are frequently moving as a result of internal waves in the lake.

Although bottom waters might be a refuge for cool-water fish species, hypoxia can force fish to seek less suitable habitats.

Acoustic surveys during the study revealed that when fish shift because of seasonal hypoxia, they cluster at the edges of dead zones rather than avoiding hypoxic areas entirely.

Using fishing gear like trawls and nets, the scientists caught fish at the highest rates along dead zone boundaries.

The Illinois-Indiana Sea Grant also funded the study, and their Great Lakes Ecosystem Specialist Paris Collingsworth said: "Our study shows that Lake Erie contains a patchwork of low and high-density fish populations.

"This understanding of fish distributions can inform policy decisions, such as how many walleye, yellow perch and others can be fished from the lake."

The study, conducted near Fairport Harbor, Ohio, during August and September of 2011-2013, was recently published in the Canadian Journal of Fisheries and Aquatic Sciences.

Further Reading

You can view the full report by clicking here.

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