Pikeminnow Survey Shows ‘Big Drop’ in Population in 2019

This is a press release from the Eel River Recovery Project:

The Eel River Recovery Project (ERRP) completed its fourth annual assessment of the non-native Sacramento pikeminnow in a key index reach of the South Fork Eel River on July 18-19 and found that the population had dropped by more than half in 2019, when compared to the three previous years.  The findings suggest that high flow events may be a factor that limits the pikeminnow population, and may have shaped population dynamics over the 40 years since their introduction.

Sacramento pikeminnow close up. 7/18/19.

The Sacramento pikeminnow is the largest member of the minnow family in North America and was formerly known as the squawfish.  Introduced into Lake Pillsbury in 1979, this non-native predator spread throughout the 3600 square mile Eel River basin within a decade and attained a population that likely numbered well over 1 million.  They were perfectly suited to the warm stream temperatures characteristic of main Eel River channels that had been flattened and were filled in by the 1964 flood.

Studies by U.C. Davis and the U.S. Forest Service Redwood Sciences Lab in Arcata established that pikeminnow were a major predatory force threatening Eel River salmon and steelhead. Native Sacramento sucker and sculpin species were also recognized as severely depressed as a result of pikeminnow predation.  The 12-mile reach of the South Fork Eel River that extends from the mouth of Rattlesnake Creek to Standish Hickey Park in Mendocino County was chosen because it produces a large number of juvenile steelhead, coho salmon, and Chinook salmon. Studies also verified significant predation by pikeminnow on salmonids in this reach.

Phill Georgakakos of UC Berkeley and Americorp Stewards Kaitlin Southall and Jessica Comer from the Arcata BLM office take a break from dive survey. 7/18/19.

ERRP has a dedicated core of volunteers, including fisheries professionals that participate annually, so counts are fairly precise.  No fish under 4” are counted because they could easily be mistaken for California roach, which can be abundant.  Size classes of pikeminnow that divers count are 4-8”, 8-12”, 12-16” and greater than 16” long.  Pikeminnow over 16” are of greatest concern, because they develop huge mouths and are the most voracious predators.  Larger fish are also almost exclusively females and, as with other fish species, the number of eggs produced increases with size.

The 2019 dive count found a total of only 614 pikeminnow over 4” long in the index reach, less than half the average of the previous three years when the population ranged from 1173-1414.  The most dramatic drop was in the size classes less than 12”, which fell by 2/3.  This suggests that sustained high flows in late February 2019 may have swept the smaller fish downstream and caused widespread mortality.  While larger pikeminnow greater than 12” declined from 370 to 311, they still constitute a significant threat to native fishes in the index reach.

ERRP has applied for a Scientific Collectors Permit from the California Department of Fish and Wildlife to harvest large adult pikeminnow in the index reach in late summer 2020.  Expert divers will be recruited to use Hawaiian sling spear guns and deploy in the deepest pools of the index reach where dive counts have shown fish greater than 16” to be congregated.  Spear fishing would take place annually for ten years, which is the life span of pikeminnow, and would result in their long-term suppression and the rebound of native fishes. Pikeminnow have site fidelity and typicaly do not move into other reaches. If the removal of pikeminnow in the index reach is successful in controlling the species, similar tactics could be employed throughout the Eel River basin.

A major decline in Eel River pikeminnow had occurred between 1994 and 2012. Otter predation appeared to be a major factor as their population expanded to capitalize on the substantial food base that the non-native fish supplied.  The 2019 findings, however, suggest that high flow years between 1995 and 2012 may also have contributed to the decrease in the pikeminnow population during that period.

Day-two dive team (l to r) Willie Grover, Will Lennox, Pat Higgins, Doug Parkinson, Alex Carey, Phill Georgakakos and guide Eric Stockwell. 7/19/19.

The 2019 pikeminnow monitoring and reporting was sponsored by the Northern California River Guides Association from its residual fund granted through the Humboldt Area Foundation.  ERRP would like to thank volunteer divers Phill Georgakakos and Alex Carey from the University of California, Berkeley (UCB) and Americorp Stewards Katelyn Southall and Jessica Coming with the Arcata Bureau of Land Management (BLM) office.  Seasoned ERRP volunteer Willie Grover was joined by new recruit Will Lennox, and professional fisheries biologists Doug Parkinson and Pat Higgins rounded out the team.

To read the full 2019 pikeminnow report, go to www.eelriverrecovery.org.