I had an opportunity to participate in the renovation of a small lake in north-central Nebraska during my graduate work. The 158-acre, shallow, prairie lake had an abundant common carp population. This lake certainly had been affected by the carp, as it contained almost no submergent aquatic plants and algal blooms were abundant. The water often looked like chocolate milk. As part of an ongoing research project, we decided to remove the entire fish community. Three airboats were used to apply the liquid rotenone (a plant-derived fish poison) and one prop boat was used to distribute some powdered rotenone as well. In addition, an amphibious vehicle equipped with a plant sprayer was used to get into the hard to reach backwater areas and cattails beds of the lake to ensure the chemical was distributed throughout. We didn’t want to leave any hiding spots for the fish we were trying to eliminate. Two small carp of opposite genders left alive could ruin an expensive operation!
The first small fish started surfacing within an hour of first adding the chemical. Toward the end of the day numerous adult carp were observed struggling at the surface. The biggest part of our job came after the rotenone application. We needed to estimate the number and total weight of all fish species in the lake. We walked certain areas of the lake shoreline and counted every dead fish. These counts were then used to estimate how many fish were in the entire lake. We know that lakes have many fish living in them but it sure is impressive to see all of those fish at one time! By the third day we wondered if we should be on the “Dirty Jobs” television program as the fish definitely were getting ripe! All in all, we believe that the lake contained about 5,412 adult common carp and 2,658,249 fathead minnows. Plus, these are probably underestimates because many fish sink to the bottom and may not float in time to get counted!
It will be interesting to watch potential changes in the lake after the treatment. Will the water clear up and new aquatic vegetation sprout up next year? Only time will tell. However, we certainly hope so! Both the sport fishes and the waterfowl should benefit from increased amounts of submergent aquatic vegetation. Without the use of rotenone to remove the undesirable fish, we could never have attempted this project. Breaking news on this ongoing project – after several years (post- carp removal) without noticing any increase in vegetation – all sampling sites now have an abundance of aquatic vegetation. It looks like the lake might be “recovering”!
Malheur Lake and Malheur National Wildlife Refuge has a carp problem. A big one. Malheur Lake alone (not counting its tributaries) roughly averages 37,500 acres. If the carp density was the same as the lake in Nebraska (34 carp/acre), extrapolating for lake size, Malheur Lake could contain almost 1.3 million carp! In terms of biomass, at a rate of 234 pounds/acre, this translates into almost 8.8 million pounds of carp swimming in the lake. Just think if you could convert this biomass of carp into plants and invertebrates! The lake is denuded of vegetation and only produces a quarter of the ducks that it did in the ‘40s. Although there are likely several potential reasons for this, the lack of vegetative habitat and associated invertebrate prey items is surely related. Earlier this year a coworker and I attended a workshop at Malheur National Wildlife Refuge where refuge staff and some of the best “carp minds” in the country gathered to discuss Malheur’s problems and brainstorm an attack plan. A thorough assessment of the problem is ongoing. A recent article in The Oregonian also highlighted the problem: http://www.oregonlive.com/news/index.ssf/2010/05/fish_invaders_are_eating_birds.html. Finally you can visit Malheur NWR’s Aquatic Health page to learn more about this issue: http://www.fws.gov/malheur/wildlife/aquatichealth.htm.
Submitted by Jeff Jolley.
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