50 dead carp found in Petaluma’s Capri Creek

Petaluma’s Bruce Cohn coaxed his kayak slowly forward through the still waters of Capri Creek, edging carefully through the lush foliage lining the narrow channel in order to train his camera on a grim sight.

Floating pale and motionless on the surface of the dark waters were the bodies of a number of dead fish, his video showed, haphazardly strewn about the shaded waterway.

“There have been dead fish for the last quarter of a mile,” said Cohn, narrating the Sept. 3 excursion.

According to a duo of city studies, the morbid sight is just nature taking its course.

Examinations of a die-off of common carp centered on the confluence of Capri Creek and the Petaluma River have found a culprit in a combination of natural phenomena occurring in the area, said Robert Wilson, environmental services supervisor.

The factors appear to include occasional low dissolved oxygen levels measured in waters around the confluence, as well as the recession of tidally-influenced waters that occasionally strand fish along the shore, he said.

As a non-native species, the common carp is likely less to be suited to the specific conditions of the Petaluma River watershed, he said. The United States Geological Survey describes the fish as a pest species, one that can destroy vegetation and upset habitat and water quality.

“It’s good if people see stuff to let us know, but in this case, it’s the wrong fish in the wrong place,” Wilson said.

City inspectors visited the site, which is near the Petaluma outlet mall, on Sept. 6, and observed approximately 50 large carp carcasses floating along the creek. They confirmed a nearby sewer pump station was intact, tested for a number of possible pollutants and determined that conditions in the channel were generally normal, he said.

Noting that live carp were also in the area, inspectors observed five dead or dying animals stranded outside of the water in low tide. The Petaluma River is in fact a tidal slough leading to the San Pablo Bay, rising and falling throughout the course of the day.

“This small section of Capri Creek is still tidal. The carp are getting stranded,” Wilson said.

The dead fish are later swept up by the rising tide and pushed elsewhere in the channel, he said.

In August, staff also measured low levels of dissolved oxygen in one pocket around the confluence, likely the result of the demand of a number of aquatic animals. While levels were far higher during the September study, the occasional occurrence of those conditions could also be too much for weaker carp.

“It’s a combination most likely of both - sometimes there are pockets of low dissolved oxygen, and sometimes it’s carp being left out on the surface,” he said.

As a non-native species, the animals may be a victim of their own evolution, unadapted to the exact conditions they face in Petaluma’s waterways, Wilson said. The fish are likely attracted to the channel as part of their natural spawning behavior, drawn to conditions like water temperature.

The watershed is meanwhile the natural home to a number of other fish species, including salmonids like steelhead.

“You’re talking about an educational moment here. It’s good to investigate these things, because we don’t want fish kills. But the fish are non-native - they never evolved to the river’s natural ebbs and flows,” he said.

Wilson said state wildlife officials were not concerned with the occurrence involving the carp, and agreed with Petaluma’s findings. Predators in the area seemed unphased by the plight of unlucky fish.

“The tide goes out, and the animals feed on the carp laying on their sides - easy picking,” he said.

(Contact Eric Gneckow at eric.gneckow@arguscourier.com. On Twitter @Eric_Reports.)