Dead Fish Appearing on Lake Washington Shorelines Due to Bacteria and Warmer Water

by Ronnie Estoque

The Washington Department of Fish & Wildlife (WDFW) has received numerous reports of dead fish washing up on the shores of Lake Washington. According to WDFW King County District fisheries biologist Aaron Bosworth, the dead fish that are appearing are mostly yellow perch, with a few sticklebacks in the mix as well. He conducted a survey earlier this month at the Kirkland shoreline, and counted about 100 yellow perch scattered alongside the area. Yellow perch are an extremely common fish in Lake Washington, and they can also be found in other lakes across the state.

"The birds seem to like to pick them out of the water and leave them on nearby rocks. This is the time of year when many yellow perch have completed their spawning," Bosworth said. "In most years, we see a minor die-off of the yellow perch around now, likely caused by stress and exhaustion from the annual spawning event combined with the warming of the water."

Bosworth adds that yellow perch die-off has been common in Lake Washington for numerous years, but the WDFW does not have the resources to monitor exact numbers vigorously.

"This year seemed a little worse than other years," Bosworth said.

Nearly seven years ago, fish health specialists from WDFW did research on the fish die-off and had determined columnaris, a naturally occurring bacterial infection, was one of the diseases that was affecting the fish population at the time. Bosworth explains that many of the dead perch that are washing up on shore this year show signs of columnaris, which also affects many other fish in Lake Washington.

"The columnaris infection in the weakened post-spawn perch seems to result in a small annual fish kill this time of year," Bosworth said. "The perch, however, are very abundant in the lake, and the dead fish represent a very small fraction of the overall population."

Rachel Schulkin, public affairs manager at Seattle Parks and Recreation, also maintained the WDFW's stance on the occurrence. The WDFW began to receive a large volume of reports in late May, and rising warmer temperatures in Lake Washington have contributed to larger populations of non-native fish, such as yellow perch, compared with native species, such as sockeye and Chinook salmon.

"Water quality conditions for fish in the lake have deteriorated a little bit, and the aspect of water quality that has deteriorated is probably temperature, as we're seeing summers warming up earlier and lasting longer," Bosworth said.

Currently, the Washington State Department of Health DOH recommends that consumption of yellow perch be limited to one meal per month, as PCBs (a group of man-made organic chemicals consisting of carbon, hydrogen, and chlorine atoms) have been identified as a contaminant. According to Bosworth, the public does not need to report instances of yellow perch die-off.

"If there's a large number of fish and you're not sure what species, yeah, then we would appreciate a report, because sometimes fish kills do occur in the lake for other reasons," Bosworth said.

You can find out more about fish consumption advisories across Washington State at the DOH website.

Featured image by Dan Lewis/Shutterstock.com.