Fish and Game completes fish surveys following quagga mussel treatment

Quagga mussels, an invasive species capable of causing harm to waterways, fish populations and fisheries, were found in the Snake River near Twin Falls by the Idaho Department of Agriculture in September 2023. Immediately afterwards, Idaho Fish and Game assisted the Department of Agriculture in their evaluation of potential treatments to eliminate the mussels, and Idaho Fish and Game quantified the fish populations found in the six-mile stretch of river where the mussels were found, and where the treatment would occur. 

Chelated copper was identified as the best product that when applied to river systems will kill quagga mussels but also would potentially kill fish, aquatic insects, amphibians, and aquatic plants as an unwanted side effect. 

With no time wasted, the rapid response from quagga mussel detection to implemented eradication treatment was unprecedented and demonstrated the value of partnerships between state and federal agencies, local governments, and private industries when attempting to eradicate this unwanted pest.

Pre-treatment fish surveys

Once chelated copper was chosen as the preferred treatment option, Fish and Game surveyed multiple sections of the treatment area, which totaled 11-miles of shoreline habitat of the river to better understand impacts to fish populations. These surveys were conducted using electrofishing boats which allowed for relatively quick assessments of the river.

To assess potential fish mortalities, over 4,000 fish were shocked, netted, identified to species, marked, and released alive. Comparison of the number of marked and unmarked fish in follow-up surveys is one way that fisheries biologists learn about fish numbers. 

The pre-treatment fish survey was completed over a two-day period, just days before the start of the treatment on October 3, 2023.

The majority of the fish sampled in pretreatment surveys were largemouth bass, yellow perch, various sunfish species, and smallmouth bass. Other sampled fish included common carp, largescale suckers and northern pikeminnow. 

No white sturgeon were sampled because electrofishing is generally ineffective for this species. However, the Idaho Power Company completed a sturgeon survey in this same reach of river in 2022, and the results of that survey were used by Fish and Game biologists for a pretreatment population estimate. It was estimated that the white sturgeon population in this reach included 49 fish greater than two feet long.

Treatment impacts to fish

As was anticipated, large numbers of fish mortalities were observed on the river within two days of the start of the mussel-killing treatment. 

Most of the fish mortalities were largescale suckers, northern pikeminnow, common carp and yellow perch. Fish and Game surveyed the river for fish mortalities throughout the treatment, and within a few days examined approximately 3,500 dead fish. Very few had been marked during pretreatment surveys, suggesting large populations of these non-game species.

Fish and Game staff handled approximately six to seven tons of dead fish over the two-week treatment. Five tons of the total was largescale suckers, followed by one ton of combined biomass of common carp and northern pikeminnow. All other fish species were less than 1,000 pounds. It is unlikely that all of the fish mortality was observed from the surface and many more dead fish likely were unobserved.

Sturgeon mortality

Based on observed mortalities and pretreatment surveys, fisheries biologists estimated 100 percent sturgeon mortality within the six-mile treatment area. In total, 48 white sturgeon were detected during the treatment. Biological data was collected from each fish. All sturgeon mortalities were hatchery-origin, based on fish marking, fin conditions, PIT tag information, or previous sampling history. The treatment reach is not a sturgeon population in which natural production currently occurs, so the observation of hatchery sturgeon was anticipated.

Approximately 50 percent of the sturgeon were recently stocked and relatively small, 8 years or younger, and the other 50 percent was between 28 and 35 years old. Overall, the average length of the sturgeon mortalities was approximately 4.5 feet long. The biggest sturgeon detected was eight feet and the smallest was about 2.5 feet long. Sturgeon 8 years or younger varied in length from 2.7 ft to 4.4 ft. Sturgeon between 28 and 35 years old varied in length from 4.7 ft to 8.0 ft. 

No sturgeon mortalities were detected downstream of Auger Falls.

Post-treatment fish survey

Fisheries biologists began to conduct additional follow-up surveys in the river two weeks after the treatment ended. Using electrofishing techniques identical to pretreatment surveys, biologists quickly learned that the treatment caused high mortality in certain species, while others survived quite well. 

Surveys showed nearly 100 percent mortality on largescale suckers, northern pikeminnow and yellow perch, along with the white sturgeon. Common carp and smallmouth bass may have been moderately impacted by the treatment and additional follow up surveys are planned in the spring to determine the extent of mortality associated with these populations. 

On the other hand, largemouth bass, bluegill and green sunfish were found in similar numbers in electrofishing surveys before and after the treatment. Also, very few of these species were found dead during mortality surveys which seems to support this observation of low treatment mortality for these species. 

Survey results tended to show that fish mortalities were similar throughout the entire treatment area. Downstream surveys, well below the main treatment area, near Niagara Springs, showed a higher abundance of fish than in the pretreatment surveys. This suggests that some fish may have moved downstream to avoid the treatment and that mortality did not occur downstream from the treatment area.

Now what?

Certain game fish appeared to have tolerated the chelated copper treatment, with numbers of largemouth bass and panfish looking very similar between the two electrofishing surveys. Translocations of fish such smallmouth bass may be needed within certain segments of the treatment area to jumpstart the recovery.

Natural recolonization of largescale suckers, northern pikeminnow, and yellow perch may occur in the reach downstream of Pillar Falls from fish moving upriver from areas downstream of Auger Falls. Also, we expect fish from upstream reaches to get washed down during high spring flow periods. Fisheries biologists will continue to monitor the fish communities over the next few years to better understand the natural recolonization of species back into the reach. 

White sturgeon populations will take time to rebuild due to their slow growth rates. The department is planning to stock hatchery sturgeon back into the treatment area to rebuild the population over the coming years. In addition, Fish and Game will likely translocate sturgeon from other reaches of the Snake River.

At this point, it’s too early to know when or even if additional fish translocations or stocking efforts are needed. These decisions will be informed by follow up sampling for quagga mussels and additional fish surveys during the next couple years.