Last weekend, several river users reported seeing numerous dead turtles on the banks of the St. Croix River near Somerset, Wisconsin. A video on social media showed about six dead turtles across from Log House Landing, while a river resident in Marine on St. Croix saw about eight dead turtles across the river near Somerset Landing. Another individual reported more than a dozen near McLeod’s Slough.

The National Park Service and other authorities were contacted, and a ranger collected several of the turtles as part of an investigation. It has now been explained that the kill was caused by Wisconsin Department of Natural Resources staff during a survey of catfish in the river. The staff had deployed numerous “hoop nets” in the river to capture fish, checking them daily, as part of census to monitor the population.

A DNR spokesperson confirmed that about 60 turtles were discovered caught in the nets on August 21, and about a third of them died.

“DNR staff members successfully released the majority of the turtles back into the St. Croix River alive,” explained Garrett Dietz, Wisconsin DNR public information officer. “Fisheries staff attempted to revive and recover all captured turtles, and the turtles that didn’t leave immediately were placed on a sand bank to swim back into the river later. DNR staff checked on the turtles a day later and noticed many swam away. However, approximately 20 turtles did not survive.”

Most of the turtles were spiny softshells, a common species in the St. Croix. Because they have some ability to absorb oxygen from the water, in addition to breathing air, spiny softshells can survive underwater for five or more hours.

“No endangered or threatened species were impacted, and the turtle species impacted are common in the St. Croix River and have a healthy population,” Dietz said. “DNR Fisheries staff members have now removed these nets from this location on the St. Croix River to minimize future incidental turtle catches.”

Collateral damage

The DNR has been conducting similar surveys in the St. Croix for the past four years, but says this was the first time that high numbers of turtles were caught in the nets. The accidental capture of turtles, called “bycatch,” during catfish surveying has been a known issue among fisheries managers decades.

“Because passive fishing nets (i.e., trap nets) are often set for [more than] 24 h[ours], organisms other than fish (e.g., turtles and snakes) may be captured and incidentally killed by drowning,” read a 2004 paper in the Journal of Wildlife Management.

The paper analyzed data including more than 2,300 turtles unintentionally captured in nets in the Upper Mississippi River system and found that about 10 percent died.

Since the problem was discovered, other scientists and fishery managers have explored how to reduce the harm to turtles. One technique is to place nets so they have areas above water where turtles can breathe. Another study found that warmer water resulted in much higher mortality rates — as warm water typically contains less oxygen.

“At a minimum, we recommend taking temperature and dissolved oxygen measurements at sample depth to ensure that enough oxygen is available for turtles to survive a 24-hr set if they are caught as fisheries bycatch,” the 2004 paper reported. “We had no captured turtle mortality in water with temperatures <20 °C [68°F].”

Temperature loggers operated by the U.S. Geological Survey at both St. Croix Falls and Stillwater — the catfish survey was conducted between the two points — showed water temperatures well above 20 degrees Celsius (about 68 degrees Fahrenheit) for the entire span of time the catfish survey was being conducted this month.

On August 21, when the high number of captured turtles were discovered, the temperature was in the 22-23 degree Celsius range at St. Croix Falls, and 23-24 degree range in Stillwater.

Terrapin trouble

Some St. Croix River citizen stewards say they are not satisfied by the DNR’s explanation of the event, and still have questions.

“‘Removing the nets from this location on the St. Croix’ won’t solve the problem — it just moves things away from curious eyeballs!” said Anne Reich, who observed several dead turtles. “How does the DNR plan to change their survey methods to minimize turtle drowning?”

Reich added that she understands fish surveys like what was being conducted on the St. Croix are important for protecting species and managing the fishery, but efforts must be taken to reduce the risk to other animals.

Reich pointed out that further analysis of the incident could also help expand understanding of the river’s complex ecology.

“This unfortunate event provides the WDNR with an opportunity to not only improve their survey methods, but also ask ‘What might this be telling us about the St. Croix River ecosystem?’” She says. “For example: ‘Is the increase in turtle bycatch due to their populations increasing?’ Or, ‘Are turtles hungry and thus more attracted to bait due to some change in their food source?’”

Reich also said the DNR fisheries staff should be consulting with their colleagues in the agency who study and manage non-game species — animals that aren’t harvested but are still an important part of the river.

For its part, the DNR says all wildlife are important to its work, whether or not they are endangered or game species.

“The DNR is committed to its responsibility of protecting and conserving all fish and wildlife species in the state,” the DNR’s Deitz said.