Hundreds of dead Asian carp were observed on an exposed gravel bar near the Maries River confluence with the Osage River. (U.S. Fish and Wildlife Service photo)
Mystery on the Osage River
By Patricia Herman
La Crosse Fish Health Center
Missouri residents utilize their water resources for recreation— whether it is boating, jet skiing or fishing— all year long. With that in mind, it is not uncommon to find remains of fish that were cleaned at the boat ramp and left to compost. It is unusual, however, to find dead fish littering the banks, sandbars and boat ramp parking lots, which prompted a phone call from a University of Missouri graduate student researcher to the U.S. Fish and Wildlife Service.
The seasoned field researcher reported seeing dead and dying Asian carp in and along the banks of the Osage River. She also noticed that the silver and bighead carp were lethargic, even complacent when electrofished, which is extremely unusual behavior for Asian carp. While it was acknowledged that the water temperatures were cold, around 4 degrees Celsius, she was adamant that the Asian carp were displaying notably different behaviors even for those temperatures.
After a quick call to the La Crosse Fish Health Center (FHC) to describe the situation, it was decided that an exploratory trip to the Osage River was warranted. A Columbia Fish and Wildlife Conservation Office crew spent a frigid day electrofishing and taking pictures of dead Asian carp near the Mari-Osa Access, approximately 10 miles upstream from the confluence with the Missouri River.
Upon arrival, the crew discovered piles of bighead and silver carp in the parking lot and along the ramps. These fish had obvious puncture wounds, injuries likely sustained at the hands of anglers gigging for suckers on the Osage River. It seemed curious that flighty fish like silver carp were able to be gigged, especially in the numbers that seen at the access. We did not see many live fish at all in the cold, clear water and after electrofishing for nearly an hour, we stopped at a gravel bar where the Maries River enters the Osage River.
With the low water levels, a large gravel bar was exposed and littered with bighead and silver carp in various states of decay. Given the stages of decomposition, it appeared as though these fish had been experiencing a die-off for several months. Most notably, only Asian carp were observed on the shores or drifting in the current. No other species were found dead. In all, 10 live silver carp were seized while drifting or captured by electrofishing from the Osage River. The fish were packed in ice and shipped overnight to the La Crosse FHC.
Terry Ott from La Crosse FHC was quick to process the fish and report his preliminary findings. Upon visual inspection, the silver carp appeared to have gas bubbles at the base of the fins and, microscopically, aneurysms were observed in the filaments, conditions often associated with gas saturation.
During the necropsy, however, the internal organs indicated a different problem. He noticed that the bile in the gall bladders was black, an indication that these fish had not eaten in over a week. The stomachs and intestines were also empty and nearly all fish appeared emaciated. Bacteria was present in several kidney cultures and cell lines are currently being cultured to determine if viral pathogens are present.
From all outward appearances, Asian carp on the Osage River are starving to death. It is unknown if these fish are starving due to complications from a disease, are experiencing side effects from dissolved gas in the water or if they have simply eaten all available food in the Osage River. Notably, we did not observe other species of fish being impacted in the same way as the Asian carps, though species diversity was low during our electrofishing runs. Additional reports of dead silver carp on the mainstem Missouri River have since been shared with our office. This is truly a mystery, one that we are eagerly hoping to solve.