Chinook salmon are returning to the Mokelumne River in large numbers this year – a continuing trend that is good news to conservationists and commercial fishery stakeholders alike.
At the Mokelumne River Hatchery just below Camanche Reservoir on Tuesday morning, Manager William Smith was pouring glowing orange eggs into five gallon buckets as his team carried out other legs of the spawning operation.
A group of kids on a field trip peered through a large window to watch hatchery staff sort the males and females before sending them down a metal slide to be harvested for eggs and sperms.
Managed by the California Department of Fish and Wildlife (CDFW) and financed by the East Bay Municipal Utility District (EBMUD), the Mokelumne River Hatchery is one of five major Central Valley hatcheries that raise fall-run Chinook salmon, the most abundant of the Central Valley races. Those locations, according to CDFW, release more than a combined total of 32 million smolts annually that contribute to large commercial and recreational ocean fisheries as well as popular sport fisheries in freshwater streams.
A food source for a diversity of wildlife in both freshwater and saltwater habitats, Chinook salmon are considered a keystone species, meaning they are good indicators of ecological health. The salmon is listed as a Species of Concern under the federal Endangered Species Act due to concerns over population size and hatchery influence on natural populations. The Mokelumne River’s salmon population currently consists of about 22% wild fish, according to local experts, and debate lingers over what impacts those raised in the hatchery may have on wild salmon.
The Mokelumne River Hatchery is expecting about 14,000 salmon in the Mokelumne River this year – “less than our biggest years, but above our average” for the past 10 years, Smith said on Tuesday. This season’s production goal is to take 7.5 million eggs and raise and release 6.8 million smolts into the San Joaquin River and some ocean sites in the spring of 2020.
Over the past five to 10 years, population levels have been on an upswing compared to the hatchery’s historical average of about 4,500 fish, which dates back to the 1940s, according to Michelle Workman, former supervising biologist with EBMUD Fisheries and Wildlife. She’s currently the watershed and recreation manager for the district.
In 2017, the count peaked at 20,000 fish, and remained relatively high at about 17,000 last year.
Smith said the return rate for adult salmon migrating from the ocean to freshwater streams depends on a variety of factors, making the reasons for changing numbers difficult to pinpoint.
The count varies based on conditions at release locations, conditions for rearing habitat, conditions in the ocean, how much the fishery impacted them, flows in the fall and more, he said.
Migrating through the delta can provide its own set of challenges for salmon, let alone the abundance of saltwater predators waiting for them at the end of their swim.
“That’s a daunting place if you’re a salmon, both from temperature to predation issues, there’s a lot of mortality trying to get to the ocean,” said Workman. “Then (there could be a) lack of food in the ocean, ocean currents changing, predator risks in both freshwater and saltwater … Trying to, as scientists, figure out what the most critical piece is, it’s always a moving target. Some years it could be river conditions, some years it could be ocean conditions, the timing of predator populations going out. Every year that’s something fish have to deal with.”
Workman attributes the recent upswing, in part, to various conservation initiatives EBMUD has undertaken to mitigate the impacts of its dam systems blocking Chinook salmon habitat.
In addition to financing hatchery operations, the district has for decades worked with the U.S. Fish and Wildlife Service to lay down over 65,000 cubic yards of spawning gravel in the Mokelumne River to support quality spawning habitat. It has also installed side channel habitat and floodplain habitat for juvenile fish to keep them safe from predators and help them bulk up for the journey downstream.
Reservoir operators also work to conserve the cold water in Camanche Reservoir to provide it to salmon as they need it.
“Salmon are a cold-water fish, and we’re at the southern end of their range, so maintaining cold-water temperatures is one of the challenges we face, but can do that by changing operations and where we release water from,” Workman said.
Additionally, EBMUD manages flows to support salmon runs based on seasonal migrations. Pulse flows in the fall attract adult salmon to the river when conditions are good for spawning, while pulse flows in the spring provide habitat for young fish.
“It really takes a group of stakeholders in a single watershed like the Mokelumne to make these efforts happen,” Workman said. “EBMUD works with the U.S. Fish and Wildlife on habitat restoration programs, with the landowners that give us access to their properties and other concerned citizens that make all of these programs work to help us do our job.”
Workman said these efforts, to date, have primarily supported salmon migrating as far east as Camanche Reservoir, but EBMUD has been in discussion with the Foothill Conservancy, a local river conservation nonprofit, about repopulating the Mokelumne River farther upstream above Pardee Reservoir.
At the moment, the roadblock is that salmon below the dams in the Central Valley carry diseases that could have ecological impacts in the Upper Mokelumne, and state pathologists won’t allow it, Workman said.
“We’re still trying to look at ways we can make that happen,” Workman said.