Fish mortalities up in New Jersey waters due to low oxygen levels

Rutgers underwater robots, or "gliders," being deployed in 2023 to observe water quality along the New Jersey coastal shelf.

Dead fish, lobster, and crab were found in the ocean off the U.S. state of New Jersey in the summer of 2023, and the suspected cause of death was low oxygen and pH levels, according to a report by Rutgers University researchers.

Lower dissolved oxygen levels alone are not uncommon in summer months, as they are a natural part of the seasonal stratification of warmer and cooler waters off the U.S. Mid-Atlantic, but 2023 was notable for both lower than usual oxygen and a drop in pH – the measure of relative acidity in the water – the study found.

A team headed by Rutgers Associate Professor Grace Saba and Rutgers Professor Josh Kohut at the Rutgers Center for Ocean Observing Leadership (RUCOOL) used two of the university’s Slocum electric gliders – autonomous undersea robots – to track water quality in August and September 2023.

Much of the bottom water sampled from Sandy Hook near lower New York Harbor south to Tuckerton, New Jersey, at depths of 50 feet to 200 feet, showed dissolved oxygen concentrations less than 5 parts per million (ppm) and pH values less than 7.75.

“Coast-wide, hypoxic levels of dissolved oxygen (less than 3 ppm) were observed at shallower, more inshore locations,” Saba and Kohut reported. Normally, optimal seawater levels include dissolved oxygen over 7 ppm and a pH of 8.1, they wrote.

Dissolved oxygen levels below 5 ppm are a problem for marine life. Concentrations between 3 ppm and 5 ppm may not directly kill fish, but “research focused on New Jersey species has identified other negative impacts such as reduced metabolism, feeding, growth, and reproduction at these levels,” the Rutgers team noted. Extreme low oxygen below 3 ppm has been seen with fish kills, particularly during a 1976 event off New Jersey that covered 4,600 square miles.

During August and September 2023, “when low dissolved oxygen and pH were observed, numerous mortalities of fish, lobsters, and crabs within the sampling area were reported, [affecting] lobsters, Jonah crab, Atlantic rock crab, spider crabs, black sea bass, and tautog …not only in pots where trapped organisms would not have been able to escape poor conditions but also on the open bottom,” the Rutgers team reported. That suggests the conditions were widespread enough that fish over a large area could not swim away in time, they wrote.

The connection between lower oxygen and pH and fish mortality showed that poor conditions in 2023 “could have been more pronounced than in previous years,” the report said.

“Faculty, students, and staff at Rutgers are continuing to analyze the data collected to identify the specific causes of this summer’s low dissolved oxygen and pH event,” it said. “Historical analysis of summer-time dynamics related to low bottom water-dissolved oxygen are also underway. Events such as these that may prevent the ability to sustain normal populations of marine organisms are concerning, not only for the ocean ecosystem but also for the local economy and commercial and recreational fishing industries.”

Photo courtesy of Rutgers University


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