Idaho Fish and Game completes fish surveys after processing quagga mussels | Idaho

Idaho Fish and Game completes fish surveys after processing quagga mussels |  Idaho

BOISE — The quagga mussel, an invasive species capable of causing harm to waterways, fish populations and fisheries, was found in the Snake River near Twin Falls by the Idaho Department of Agriculture in September. Immediately thereafter, Idaho Fish and Game helped the Department of Agriculture evaluate potential treatments to eradicate the mussels, and Idaho Fish and Game measured fish populations in the six-mile stretch of river where the mussels were found, and where the the sea. The cure will happen.

Chelated copper has been identified as the best product when applied to river systems it will kill quagga mussels but may also kill fish, aquatic insects, amphibians and aquatic plants as an undesirable side effect.

Pre-treated fish surveys

Once chelated copper was selected as the preferred treatment option, Fish and Game surveyed multiple sections of the treatment area, totaling 11 miles of river coastal habitat to better understand the impacts on fish populations. These surveys were conducted using electric fishing boats which allowed relatively quick assessments of the river.

To assess potential fish mortality, more than 4,000 fish were stunned, netted, species identified, tagged, and released alive. Comparing the numbers of marked and unmarked fish in follow-up surveys is one way fisheries biologists learn about fish populations.

The pre-treatment fish survey was completed over two days, just days before processing began on October 3.

The majority of fish sampled in the pre-treatment surveys were largemouth bass, yellow perch, various species of sunfish, and smallmouth bass. Other fish sampled included common carp, large-sized suckers, and northern pikeminnow.

White sturgeon were not sampled because electrofishing is generally ineffective for this species. However, Idaho Power Company completed a sturgeon survey in the same river area in 2022, and Fish and Game biologists used the results of that survey to estimate the pre-treatment population. It is estimated that the white sturgeon population in this range includes 49 fish longer than 2 feet.

Effects of treatment on fish

As expected, large numbers of fish were killed in the river within two days of the start of the mussel killing operation.

Most of the fish kills were largemouth bass, northern pikeminnow, common carp and yellow perch. Fish and Game surveyed the river for fish kills throughout the treatment period, and within a few days had examined nearly 3,500 dead fish. Very few were identified during pre-processing surveys, indicating the presence of large numbers of these non-game species.

Fish and Game staff handled approximately six to seven tons of dead fish during the two-week treatment period. Five tons of the total were large suckers, followed by one ton of biomass collected from common carp and northern pikeminnow. All other fish were less than 1,000 pounds. It is unlikely that all fish kills will be observed at the surface, and there are likely to be many dead fish that go unnoticed.

Sturgeon mortality

Based on observed mortality rates and pre-treatment surveys, fisheries biologists estimated 100 percent sturgeon mortality in the six-mile treatment area. In total, 48 white sturgeon were detected during treatment. Biological data were collected from each fish. All sturgeon mortalities were of hatchery origin, based on fish tags, fin conditions, PIT tag information, or previous sampling history. The scope of treatment does not include a population of sturgeon in which natural production is currently occurring, so monitoring of sturgeon in hatcheries was expected.

Approximately 50% of the sturgeon stocked recently are relatively young, 8 years old or younger, and the other 50% are between 28 and 35 years old. Overall, the average length of sturgeon kills was about 4.5 feet. The largest sturgeon discovered was 8 feet long and the smallest was about 2.5 feet long. Sturgeon that are 8 years old or younger range in length from 2.7 feet to 4.4 feet. Sturgeon that are between 28 and 35 years old range from 4.7 feet to 8 feet in length.

No sturgeon mortalities were detected downstream at Auger Falls.

Wipe the fish after treatment

Fisheries biologists began conducting additional follow-up surveys in the river two weeks after the treatment ended. Using electrofishing techniques matched to pretreatment surveys, biologists quickly discovered that treatment caused high mortality in some species, while others survived well.

Surveys have shown nearly 100% mortality in largemouth suckers, northern pikeminnow, and yellow perch, as well as white sturgeon. Common carp and sea bass may have been moderately affected by the treatment, and additional follow-up surveys are scheduled for the spring to determine the extent of mortality associated with these populations.

On the other hand, greengill bass and green sunfish were found in similar numbers in pre- and post-treatment electrofishing surveys. Also, very few of these species were found dead during mortality surveys, which seems to support this observation of low processing mortality for these species.

Survey results tended to show that fish mortality was similar throughout the entire treatment area. Surveys downstream, which is below the main treatment area, near Niagara Springs, showed greater fish abundance than in pretreatment surveys. This suggests that some fish may have moved downstream to avoid treatment and that mortality did not occur downstream from the treatment area.

What now?

Some game fish appear to have tolerated chelated copper tackle, as numbers of largemouth bass and panfish appear very similar between the two electrofishing surveys. Translocations of fish such as smallmouth bass may be needed within certain parts of the treatment area to begin the recovery process.

Natural recolonization of large suckers, northern pikeminnow, and yellow perch may occur in the Pillar Falls downstream area from fish moving upstream from areas below Oger Falls. We also expect fish coming upstream to be washed up during periods of high spring flow. Fisheries biologists will continue to monitor fish communities over the next few years to better understand the natural recolonization of species that have become within reach.

White sturgeon will take time to rebuild due to their slow growth rates. The department plans to restock sturgeon in the processing area to rebuild their numbers over the coming years. In addition, Fish and Game will likely move sturgeon from other tributaries of the Snake River.

At this point, it is too early to know when or even if additional fish translocations or stocking efforts will be needed. These decisions will be based on sampling quagga mussels and conducting additional fish surveys over the next two years.

(Tags for translation)Ichthyology

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