What’s eating the salmon?

A salmon smolt near Woodland on Friday, February 19, 2016.
A salmon smolt near Woodland on Friday, February 19, 2016. rbenton@sacbee.com

Water managers have been saying for years that California’s salmon population is under attack by non-native predatory fish. Now there is science to prove it.

In a report on April 19 to the State Water Resources Control Board, the National Oceanic and Atmospheric Administration’s Sean Hayes, whose doctorate is in ecology and evolutionary biology, talked about the multiple stressors that affect the life cycle of salmon. Hayes and his team from the NOAA’s Southwest Fisheries Science Center conducted acoustic tagging of baby salmon over the course of five years.

Their results confirmed similar studies over the past 25 years, showing that only 3 percent of the salmon smolts migrating to the ocean survive their journey in the two to three weeks following their release from upstream hatcheries.

Striped bass and largemouth bass, as well as channel catfish and white catfish, were identified as the predators doing the most damage to young salmon. Hayes estimated the current bass population needs about 27,500 tons of food in a year to sustain itself. Of that, about 264 tons is salmon.

That means this predator has the capability of decimating the entire salmon population in any given year.

In the water board meeting, Hayes described the changing nature of the Delta as a difficult challenge. Bass were introduced to the Delta from the East Coast 130 years ago to populate a new western commercial fishery. These non-native fish, as well as critical habitat loss, Asian clams that compete for food, and water hyacinth all contribute to poor conditions for native fish like salmon.

In addition to predators, the study identified three other factors that affect the salmon life cycle: water diversions, water temperature and contaminants.

Water diversions have been regulated for 20 years and the result has been little, if any, identifiable improvement in salmon populations.

Water temperature is getting more attention lately, at least in the upper reaches of the Sacramento River for the benefit of naturally reproducing salmon. Cold-water releases from Shasta Lake are designed to keep temperatures in the river low enough to benefit newly hatched salmon smolts.

Eliminating predatory bass and catfish altogether likely won’t solve the problem, according to the report. But it does identify “hot spots” in numerous sections of the Delta where salmon numbers suffer their greatest losses and where Hayes believes more attention to predator control needs to be focused. Allowing unrestricted fishing for bass or other predators in those areas would reduce the risk to salmon.

Another area where improved management can help is restoration of tidal habitat. Young salmon need a place to grow before migrating to the ocean. The Delta that existed before development began 150 years ago had vast swaths of tidal marshland that served as incubators for young fish. Today it is estimated that just 15 percent, and some say as little as 4 percent, of natural areas remain unchanged.

Recent experiments in the Yolo Bypass west of Sacramento show that restored habitat can help salmon grow stronger, which Hayes said would improve their survival chances as they migrate to the ocean.

No single tactic will fix all the problems in the Delta. The failed strategy of restricting water deliveries for the past 20 years proves that singling out one activity to help endangered salmon won’t work. It will take the collective effort of federal fishery agencies, the state water board, public water agencies such as irrigation districts and local landowners to address all the things Hayes discussed in the report.

Only then can we expect positive results in the effort to rebuild salmon populations to a sustainable level and deliver the water supplies we need to grow our food.

Mike Wade, a Modesto resident, is executive director of the Sacramento-based California Farm Water Coalition.