Aquatic Weeds in Fish Lake

By Louis Berg, DWR Research Biologist

The Utah Division of Wildlife Resources Recently identified the primary species of aquatic plant at Fish Lake in Sevier County as Eurasian water milfoil, an exotic nuisance species never before reported in Utah. That identification was confirmed by expert botanist Dr. Susan Aiken of the Canadian National Museum.

Problems the species has caused in other states have drawn national attention. Eurasian milfoil causes heavy infestations, restricts Water-use activities including boating and fishing, and has the potential to spread rapidly from one water to another by fragments left on boat trailers.

It is impossible to determine how long Eurasian water milfoil has been in Fish Lake. However, increasing abundance of aquatic vegetation there has been a topic of concern for over 20 years. Initially, the problem was considered to be the result of increased nutrient levels. High human use of the area, with its lodges, cabins and campgrounds, was causing enrichment from domestic wastes. In response, a sewer system was constructed to divert nutrients away from the lake. Although that system reduces nutrient input, there is no practical way to reverse the process and actually remove the nutrients from the lake. Thus, weeds remain abundant. Even if costly efforts at weed control were adopted, they would only provide short term relief.

Fish Lake anglers can attest to the fact that fishing has been seriously affected by heavy weed growth. Sixty years ago, Fish Lake was primarily a shore fishery. Today a thick band of vegetation extends around the entire lake, making shore fishing difficult except in a few isolated spots. About 80% of the annual fishing pressure now occurs from boats.

Some changes in fish populations have occurred which could be related, at least in part, to increased aquatic vegetation. Yellow perch, illegally introduced prior to 1970, live and reproduce in the thick weeds and are now very abundant. Utah chubs, which reproduce in shallow, warm water and are preyed upon by yellow perch, have declined in abundance. Lake trout, which in the past utilized Utah chubs as their primary forage, now have a reduced forage base and are eating predominantly rainbow trout. Thus, the rainbow trout fishery has drastically declined. The Utah Division of Wildlife Resources is currently formulating a plan to deal with these fisheries problems.

Experience in other states with Eurasian watermilfoil has shown little success in control. As a result, aquatic weeds will probably always be abundant at Fish Lake. However, we can do something to prevent the spread of Eurasian water milfoil into other waters. Boaters would be wise to remove weeds clinging to trailers before leaving Fish Lake for other destinations.