In 1990, Utah used the world’s largest rotenone treatment on Strawberry Reservoir; over 99% of fish biomass vanished and the rebuilt fishery later produced 715,000 catches a year

In August 1990, Utah wildlife officials carried out one of the largest chemical fish-removal projects in fisheries history, wiping out almost every fish in Utah’s Strawberry Reservoir to save its collapsing ecosystem. The operation, led by the Utah Division of Wildlife Resources (UDWR) and described in publications from the American Fisheries Society and UDWR research reports, removed more than 99 per cent of the lake’s total fish biomass. By clearing out the non-native fish, biologists were able to rebuild the food web and create a healthier sport fishery that later produced more than 715,000 angler catches a year. Located 23 miles east of Heber City, the high-elevation reservoir had long been one of the Intermountain West’s best cold-water fishing destinations. But by the late 1980s, large numbers of introduced Utah chub (Gila atraria) and redside shiners had overwhelmed native game fish. They consumed much of the available plankton and contributed to a major decline in trout numbers.

Overwhelmed reservoir needed a drastic reset

Wildlife managers had already tried a smaller rotenone treatment at the original Strawberry Reservoir in 1961. But when Soldier Creek Dam expanded the reservoir in 1973, increasing its surface area to more than 17,000 acres, illegally introduced Utah chub quickly spread through the newly flooded areas. By 1989, UDWR gill-net surveys showed how serious the problem had become. Non-native forage fish made up more than 90 per cent of the total fish weight in the reservoir. Attempts to stock trout were failing. Millions of hatchery-raised trout fry were either eaten by adult chubs or struggled to find enough food because of competition for tiny aquatic organisms. With the reservoir becoming less productive and fewer people visiting to fish, state biologists concluded that partial netting, commercial fishing and biological controls would not solve the problem. A complete chemical treatment was considered the only practical way to clear the lake.

Military-scale operation across huge watershed

The project required a major logistical effort involving hundreds of people from state and federal agencies. The treatment covered the 17,000-acre reservoir, its many bays and more than 170 miles of streams flowing into the Strawberry River drainage. To reduce the amount of chemical needed and lower costs, water managers first lowered the reservoir to its lowest operating level. Crews then applied rotenone, a plant-based chemical that stops cellular respiration in fish and other gill-breathing organisms and breaks down quickly in sunlight. According to historical American Fisheries Society records, teams used heavy machinery, specialised barges, deep-water pumps, all-terrain vehicles and aircraft to spread thousands of gallons of liquid rotenone and dry powder. Dripper stations were placed along mountain streams to make sure the chemical reached side channels, marshes, springs and beaver ponds connected to the main reservoir. Within days, the treatment moved through the entire system. Millions of Utah chub, redside shiners and other non-native fish died and either washed onto the shore or sank to the lake bottom. Follow-up biological surveys showed that more than 99 per cent of the reservoir’s fish biomass had been removed, leaving the lake with a largely empty food web that could be rebuilt.

Predatory trout restore the balance

After the chemical naturally broke down in late autumn 1990, UDWR began a large restocking programme aimed at preventing non-native fish from taking over again. Biologists introduced millions of native Bear Lake cutthroat trout (Oncorhynchus clarki utah) along with sterile rainbow trout. Bear Lake cutthroat were chosen because they become strong fish-eaters as adults. Unlike earlier trout strains used in the reservoir, the cutthroat naturally prey on smaller fish in open water. This gave the lake a permanent predator that could help control any Utah chub that survived the treatment. The plan worked. With less competition for food, the stocked cutthroat grew quickly. By the mid-1990s, fishing records and UDWR surveys showed a major recovery. State fishery data recorded annual catches of up to 715,000 trout during peak years. Strawberry Reservoir regained its reputation as one of Utah’s top trout fisheries, drawing more than one million angler hours each year and generating tens of millions of dollars for the local economy.

Barriers protect the recovery

To protect the recovery, UDWR built permanent fish traps and barriers along important spawning streams, including the Strawberry River and Indian Creek. The structures allow biologists to catch, count and sort spawning fish while preventing non-native species from moving upstream to reproduce. Today, strict fishing rules and slot limits help protect large predatory cutthroat trout, ensuring enough mature fish remain to keep chub numbers under control. The 1990 Strawberry Reservoir treatment remains an important example of modern freshwater management. It showed that large-scale chemical fish removal, combined with carefully planned predator stocking, can restore a heavily damaged aquatic ecosystem.