Killing Native Fishes for Fun and Predator Control

Teejay A. O’Rear, John R. Durand, and Peter B. Moyle recently published a blog post on the California WaterBlog. See the full article below or at

by Teejay A. O’Rear, John R. Durand, and Peter B. Moyle

A recent posting of a short film on a 2017 fishing derby (FISHBIO 2018a) is disturbing to those of us interested in conserving our native fishes.  The film glorifies killing Sacramento pikeminnow and hardhead for reducing predation on juvenile Chinook salmon and for attracting more people to sport-fishing.  The idea is for anglers, from senior citizens to kids, to catch and kill as many pikeminnow and hardhead as possible for prizes.  On derby day in 2017, 638 fish were killed, some appearing to weigh more than four pounds.  The big fish were likely 15-20 years old.

Supposedly, removing these fish as predators will increase the number of adult salmon returning to spawn a few years later. In fact, little scientific evidence exists to support the notion that hardhead and pikeminnow affect numbers of returning adult salmon.

The sponsor referenced the bounty program instituted on the Columbia River for northern pikeminnow (a related but different species than Sacramento pikeminnow), implying that a similar program may have similar benefits in the Sacramento-San Joaquin Watershed.  The Columbia River consists of a series of hydropower dams along the mainstem river and major tributaries, creating a series of slow-flowing reservoirs through which salmon have to migrate and within which most predation of northern pikeminnow on salmon is concentrated (Vigg et al. 1991).  These Columbia River reservoirs differ considerably from tailwater rivers where Sacramento pikeminnow and hardhead in the video were targeted.

The Columbia River bounty program is notable because it was instituted after considerable research (e.g., Poe et al. 1991Vigg et al. 1991) that examined metabolic rates of predators, predator diets, predator abundances, habitat types, and daily/seasonal predation rates.  The research suggested that reducing northern pikeminnow numbers could improve salmon survival.  No such research has been performed for Sacramento pikeminnow or hardhead in the Sacramento-San Joaquin Watershed.

Hardhead have jaws deep in the throat with big, flat teeth (like human molars) used for grinding up hard-shelled prey such as crayfish.   While pikeminnow may feed on naïve hatchery-released salmon until full, their slow digestive rate (Vondracek 1987) indicates the population effect is likely small, even when hundreds of pikeminnow are present.   The account of the derby states Sacramento pikeminnow are known for “disturbing salmon redds to scavenge for eggs.”  In fact, the pikeminnow mainly consume eggs that failed to get buried, eggs doomed in any case. Of course, steelhead/rainbow trout do the same thing (e.g.Johnston 2018) but they are not castigated for such “bad behavior,” being game fishes.

Various studies demonstrate the futility of Sacramento pikeminnow and hardhead control.  Females of both species, even relatively small ones, produce thousands of eggs, so a few individuals can restore a population quickly.  In the 1980s, a UC Davis team evaluated an effort to eradicate pikeminnows, hardhead, suckers, and other native fishes from the North Fork Feather River, which were thought to compete with trout.  The California Department of Fish and Game (as it was then) killed 99.9% of the fish with a poison, an effort repeated previously at about 10-year intervals. Examination of the ages of the dead fish showed that most large individuals were spawned a year or two after the previous poisoning operation (Moyle et al. 1983).

Clearly, the few fish that survived the operation had banner reproduction in the following years.  A saying goes that “Nature abhors a vacuum,” which this illustrates well.  Likewise, in a reach of the Mokelumne River, all predatory fish were removed by electroshocking, but predator numbers tripled after two weeks due to new predators moving into the vacant space (Cavallo et al. 2013Grossman 2016).  In the Columbia River, the northern pikeminnow removal co-occurred with increased numbers of Caspian Terns, Double-crested Cormorants, and marine mammals, all of which consume high numbers of salmon (Carey et al. 2012).

 As discussed in a previous blog about striped bass predation (Moyle et al. 2016), top predators such as Sacramento pikeminnows can potentially help salmon populations by consuming other predators and competitors, including their own young (Brown and Moyle 1981).  Ironically, the derby sponsor’s website contains an earlier report (FISHBIO 2018b)on attempts to control numbers of the invasive green crab, which resulted in an increase in the population.  Control was focused on removing large adult crabs, to reduce reproduction. However, the major factor limiting survival of juvenile crabs was abundance of adults, which were highly cannibalistic. Simplistic solutions to complex problems rarely work!

The website claims the derby educates people about the harm done by predatory fish to salmon.  However, the main area in the Sacramento Valley where pikeminnow predation has been seriously considered a problem was at the Red Bluff Diversion Dam, which created near-optimal conditions fish and bird predation on salmon (Vondracek et al.1991). The problem was largely resolved once the diversion dam’s gates were left open during salmon emigration (Moyle 2002). Many other studies, such as Sabal et al. (2016), have also noted that artificial structures enhance predation.

Predation is a two-partner relationship.  Salmon and steelhead in the Central Valley are largely hatchery-supported, with little genetic difference between wild and hatchery-produced fish (Pearse and Garza 2015Satterthwaite and Carlson 2015).  Effects of hatcheries on genes can be profound, and one of the most common effects of hatcheries is reduction of predator-avoidance behaviors (Berejikian 1995).   Hatcheries also create naïve fish by spraying food pellets onto the water surface of concrete-lined raceways crowded with fish. When released into rivers, these domesticated juveniles become easy prey as they encounter myriad water-diversion structures that concentrate and disorient them on their way to the ocean. Minimizing hatchery influences and redesigning water-control structures would better improve juvenile survival. Using predation as an excuse to indiscriminately kill native fishes and not take other major actions is shameful.

The goal of killing large numbers of Sacramento pikeminnow and hardhead implies that these fishes are of little value.  But Sacramento pikeminnow and hardhead are native fishes restricted to northern and central California, part of the native biotic community that California Department of Fish and Wildlife’s (CDFW) Ecosystem Restoration Program (CDFW 2018) seeks to restore.  Further, hardhead are a California Species of Special Concern because of their long-term decline in numbers (Moyle et al.2015).  Hardhead and large pikeminnow are good game fish, and anglers who catch them when fishing for other species are often surprised at their sporting qualities.  Properly prepared, both species can be good eating as well; scaling, filleting, scoring the fillets to cut through the intermuscular bones, then breading and frying gives tasty fish nuggets (Buffler and Dickson 1990).

The website states that one of the derby’s goals was to get more people fishing – sport-fishing – because fewer people are sport-fishing in California these days.  Implicit in the definition of sport-fishing is fair chase and restraint from killing unnecessarily.  With no evidence of Sacramento pikeminnow or hardhead harming salmon or steelhead populations in the Central Valley, coupled with hardhead’s conservation status, the killing of hardhead and pikeminnow in their native watershed appears to be highly antithetical to the spirit of sport-fishing.

We – anglers and conservationists – find the justifications for the derby lacking in evidence, overly selective and simplistic, and degrading to the spirit of sport-fishing.  Rather than killing any species found with a few salmon in their guts, a more productive approach is to create a better ecosystem for salmon.  Such a holistic approach – for example, using rice fields and duck ponds for growing fish food, increasing winter and spring river flows, restoring tides to Delta islands – promises benefits to not only salmon but also other native species, including hardhead and Sacramento pikeminnow. As remnants of wild, native California, a much better place for hardhead and Sacramento pikeminnow is swimming freely in their home rivers, rather than in a dog-food bowl or flowerbed.

Teejay O’Rear is a fish ecologist at the Center for Watershed Sciences and lab supervisor for Dr. Peter Moyle. His research interests include the application of the reconciliation-ecology concept in the Sacramento-San Joaquin Watershed, with a particular focus on non-mainstem and/or generally ignored habitats (e.g., managed wetlands, water-supply reservoirs, agricultural ditches, and dead-end sloughs) that may benefit both native and desirable non-native species – including people. John Durand is a researcher specializing in estuarine ecology and restoration at the Center for Watershed Sciences. Peter B. Moyle is a UC Davis Professor Emeritus of fish biology and an associate director of the Center for Watershed Sciences.


Berejikian, B. A.  1995.  The effects of hatchery and wild ancestry and experience on the relative ability of steelhead trout fry (Oncorhynchus mykiss) to avoid a predator.  Canadian Journal of Fisheries and Aquatic Sciences 52(11):2476-2482.

Brown, L.R. and P. B. Moyle. 1981. The impact of squawfish on salmonid populations: a review. North American Journal of Fisheries Management 1:104-111

Buffler, R., and T. Dickson.  1990.  Fishing for Buffalo.  Minneapolis: University of Minnesota Press.

Carey, M.P., B. L. Sanderson., K. A. Barnas, and J. D. Olden. 2012. Native invaders–challenges for science, management, policy, and society. Frontiers in Ecology and the Environment 10: 373-381.

Cavallo, B., J. Merz, and J. Setka, 2013. Effects of predator and flow manipulation on Chinook salmon (Oncorhynchus tshawytscha) survival in an imperiled estuary. Environmental Biology of Fishes 96: 393-403.

CDFW.  2018.  Ecosystem Restoration Program.  CDFW.  July 25, 2018.

FISHBIO.  2018a.  Fishing for pikeminnow: a native predator removal derby.  The Fish Report. July 10, 2018.

FISHBIO.  2018b.  Crab wars: the invasive European green crab.  The Fish Report. July 22, 2018.

Grossman, G.D. 2016. Predation on fishes in the Sacramento–San Joaquin Delta: current knowledge and future directions. San Francisco Estuary and Watershed Science 14(2).

Johnston, R.  2018.  Rivers: the lower Sacramento River and the Feather River.  RJ’s Fly Trips.  July 31, 2018.

Moyle, P.B. 2002. Inland Fishes of California. Berkeley: UC Press.

Moyle, P.B., R. M. Quiñones, J.V.E. Katz, and J. Weaver. 2015.  Fish Species of Special Concern in California.  3rd edition.  Sacramento: California Department of Fish and Wildlife.

Moyle, P., A. Sih, A. Steel, C. Jeffres, and W. Bennett.  2016.  Understanding predation impacts on Delta native fishes.  California WaterBlog.  July 25, 2018.

Moyle, P. B., B. Vondracek, and G. D. Grossman.  1983.  Responses of fish populations in the North Fork of the Feather River, California, to treatments with fish toxicants.  North American Journal of Fisheries Management 3:48-60.

Pearse, D. E., and J. C. Garza.  2015.  You can’t unscramble an egg: population genetic structure of Oncorhynchus mykiss in the California Central Valley inferred from combined microsatellite and single nucleotide polymorphism data.  San Francisco Estuary and Watershed Science: 13 (4).

Poe, T.P., H. C. Hansel, S. Vigg, D. E. Palmer, and L.A. Prendergast. 1991. Feeding of predaceous fishes on out‐migrating juvenile salmonids in John Day Reservoir, Columbia River. Transactions of the American Fisheries Society 120: 405-420

Sabal, M., S. Hayes, J. Merz, and J. Setka. 2016. Habitat alterations and a nonnative predator, the Striped Bass, increase native Chinook Salmon mortality in the Central Valley, California. North American Journal of Fisheries Management 36: 309-320.

Satterthwaite, W. H., and S. M. Carlson.  2015.  Weakening portfolio effect strength in a hatchery-supplemented Chinook salmon population complex.  Canadian Journal of Fisheries and Aquatic Sciences 72: 1860-1875.

Vigg, S., T. P. Poe, L. A. Prendergast, and H. C. Hansel. 1991. Rates of consumption of juvenile salmonids and alternative prey fish by northern squawfish, walleyes, smallmouth bass, and channel catfish in John Day Reservoir, Columbia River. Transactions of the American Fisheries Society 120:421-438.

Vondracek, B. 1987. Digestion rates and gastric evacuation times in relation to temperature of the Sacramento squawfish, Ptychocheilus grandis. Fishery Bulletin 85(1):159-63.

Vondracek B, S. R. Hanson, and P. B. Moyle. 1991. Sacramento squawfish predation on Chinook salmon below a diversion dam on the Sacramento River. Unpublished manuscript files.