OAKDALE — The once plentiful population of rainbow trout in the Stanislaus River has declined by 75 percent from that of six years ago according to a study by fisheries consulting firm FISHBO communications.
The study, released Thursday, estimated about 5,000 trout remained in the river in 2015. The key reason for the decline is believed to be warmer river temperatures that resulted from the California drought.
As the group prepares to do this year’s survey, it expects to find even worse numbers. That is because trout numbers tend to decline a year after a hot summer. River temperatures were warmer in 2015 than in 2014.
The average daily water temperature in the Stanislaus River reached 69 degrees in August, 2015. That is higher than any summer temperature since 1998. The temperatures were due in large part to low water levels in the New Melones Reservoir upstream. The reservoir was at 12 percent of capacity in 2015 which was too low to remain cool.
While the reservoir has recovered to 24 percent, water temperatures have remained warm. For young trout to survive, the temperature of the water should be 60 degrees or colder.
Oakdale and South San Joaquin irrigation districts and the Tri-Dam Project have funded surveys by scientists at FISHBIO since 2009. The scientists snorkel through the river to get an accurate count which allows the team to track the fish’s abundance over time and across a range of conditions.
“Last summer, there was not enough cold water from deep in New Melones Reservoir to send down the river and lower the temperature because it had been released earlier,” said Steve Knell, general manager of the Oakdale Irrigation District in a press release.
Since 2009, the survey has found an average of about 20,220 fish in the river. It has been one of the largest populations of trout in the Central Valley. Prior to 2014, the fish count had never dropped below 14,000.
The scientists have found that water temperature, not river flow, seems to have the biggest impact on the fish. With the exception of extremely high flows that were released for flood control in 2011, higher water releases have yielded little in the way of more fish.
“This drought is a good reminder that sometimes saving water for later release is better for fish than releasing too much at once,” Doug Demko, president of FISHBIO stated about the study. “We now have evidence that releasing stored water in a way that depletes cold water reserves can significantly reduce rainbow trout abundance.”
In 2009, the National Marine Fisheries Service (NMFS) mandated large releases of water pursuant to the Endangered Species Act (ESA) be sent down the river annually to protect rainbow trout and steelhead. FISHBO’s research indicates that may have been a flawed strategy.
The irrigation districts continue to challenge the current release strategy.
“Federal officials naively believed that ‘more water equals more fish.’ The districts warned, and NMFS agreed, that the severity and occurrence of dry-year conditions only increase under the ESA releases, and that more dry years would hurt the O. mykiss fishery,” Peter Rietkerk, general manager of the South San Joaquin Irrigation District said in a release.