Both species have been implicated in the decline of vulnerable native species in the river, particularly juvenile Chinook salmon, says lead author Dylan Stompe, a PhD student and researcher at the University of California at Davis. For eight months in 2017, Stompe and fellow researchers with the California State University at Chico and the California Department of Fish and Wildlife surveyed a 35 kilometer reach of the river outside Chico for the two predatory fish. They baited hooks with with sardines, worms, and chicken liver and fished from an anchored boat. When they brought a pikeminnow or striped bass on board, the researchers performed a gastric lavage, in which pulses of pressurized water are directed into the fish’s esophagus, causing it to evacuate its stomach contents. Thirty pikeminnow, ranging in size from 11 to 22 inches, and 47 striped bass, 9 to 19 inches long, had identifiable stomach contents. Back at the lab, the researchers used diagnostic parts like spinal columns for fish and head parts for crayfish and other macroinvertebrates to count individual prey. With the help of genetic testing they identified the DNA of 17 target species within the heavily digested “soft matrix” expelled from the fishes’ stomachs. After tallying it all up, the researchers discovered that the two predators had very similar diets
composed almost entirely of macroinvertebrates (about 79 percent) and juvenile Chinook salmon (about 16 percent), the latter likely migrating hatchery fish. Sampled pikeminnow and striped bass had also eaten shad, hardhead, lamprey, and sculpin, in much smaller but still relatively similar amounts. Most importantly, Stompe says, the result indicates that, at least in the clear waters of the middle Sacramento River, the non-native bass, valued primarily for sportfishing, may play a very similar ecosystem role to the voracious native pikeminnow—with interesting implications for management. “Their high dietary overlap suggests that control of one species would increase resources for the other,” the paper reads.