Anthony is collecting samples at one of the open water sites

Anthony Pothoff

Anthony has a B.S. in Zoology from North Dakota state University. He graduated with a M.S. degree from North Dakota State University in the Spring of 2003. His research in aquatic ecology in wetlands was supported by  a Water Institute Fellowship. He is seeking a job in the field of research ecology with either a state agency or the federal government.

Anthony is collecting samples at one of the open water sites.

Evaluation of Walleye to Suppress Fathead Minnow Populations in Type IV and V Wetlands

Fellow: Anthony Pothoff, Department of Biological Sciences, NDSU

Advisor:  Malcolm Butler, Professor of Biological Sciences, NDSU

Matching Support: Minnesota Department of Natural Resources, St. Paul, MN

Degree Completed: M.S., September 2003 

M.S. Thesis Abstract:


Potthoff, Anthony Joseph, M.S., Department of Biological Sciences, College of Science and Mathematics, North Dakota State University, August 2003.  Evaluation of Walleye to Suppress Fathead Minnow Populations in Class IV and V Wetlands.  Major Professor: Dr. Malcolm G. Butler.

The greater depth of many prairie wetlands resulting from consolidation of smaller, more ephemeral wetlands has caused a decrease in the frequency and extent of summer and winter anoxia.  As a result, fathead minnows now persist on a more permanent basis, often reaching high population densities, which reduce zooplankton and macroinvertebrate diversity and abundance.  Reductions in zooplankton in turn lead to increased phytoplankton, decreased water clarity, reduced macrophyte abundance, and ultimately decreased waterfowl use.  Effective ways to control fathead minnow populations in Prairie Pothole Region wetlands are needed by wetland managers.  A two-year study to assess walleye stocking as a tool to suppress fathead minnow populations and improve water quality was designed.  Treatments consisted of 6 wetlands stocked with age-0 walleyes, 6 wetlands stocked with adult walleyes, and 6 wetlands that were left unmanipulated (contained only fathead minnows).  The biomanipulation was ineffective in the advanced walleye treatment, as these wetlands did not differ significantly from the control wetlands.  In both 2001 and 2002, the walleye fry treatment had significantly lower densities of fathead minnows and significantly higher densities of cladocerans.  Macroinvertebrate populations in 2001 did not differ significantly but in 2002 some macroinvertebrate groups were significantly higher in the walleye fry treatment sites.  Chlorophyll a and turbidity decreased significantly in 2002 in the walleye fry treatment.  These results indicated the biomanipulation was successful in suppressing fathead minnow populations, creating a trophic cascade which improved water quality in the walleye fry treatment wetlands.

The Problem Addressed by the Research

Fathead minnows (Pimephles promelas) are a native species and the most common fish in wetlands of the Prairie Pothole Region (PPR) of the U.S. and Canada .  However, the distribution of these fish is limited by the harsh conditions of the PPR, particularly anoxia.  Changes to the landscape of the PPR over the last hundred years, primarily due to agriculture, have caused the consolidation of temporary, seasonal, semi-permanent, and permanent wetlands, creating large class IV and V wetlands.  The lower probability of anoxic conditions in these larger and deeper wetlands has permitted substantial expansion of fathead minnow populations on the landscape.  Fathead minnows have a very dynamic life history and under favorable conditions, particularly anoxic free conditions, populations can grow exponentially, often reaching high densities.  Due to this high reproductive potential, fathead minnows in wetlands without periodic anoxia can reach very high densities very quickly.    Fathead minnows’ diet consists predominantly of crustaceans and aquatic insects.  Fathead minnows can use both filter-feeding and particulate feeding methods, making them not only efficient, but also capable of utilizing a broad range of food resources.  Rapid growth and high food consumption of fathead minnows cause them to have critical influences on energy flow in wetlands.

The high densities of fathead minnows occurring in the larger class IV and V wetlands extend profound ecological influences on those water bodies including lower abundances, biomass, and species richness of common orders of invertebrates.  Zimmer et al.documented dramatic shifts in invertebrate community composition where minnows were present.  Increases in nutrient levels, phytoplankton biomass, and turbidity were also documented in experimental wetlands stocked with fathead minnows.  Zimmer et al. reported that fathead minnows directly and indirectly affected nutrient partitioning within prairie wetlands, with wetlands supporting low macrophyte abundance and high fathead minnow densities often existing in a turbid state.  The increases in nutrient recycling directly associated with increased in fathead minnow populations provides more available nutrients for phytoplankton, allowing the phytoplankton  populations to increase in density.  In response to an increase in phytoplankton, one might expect, an increase in zooplankton densities, however, the high feeding efficiency of fathead minnows and their high densities prohibits such an increase in zooplankton and results in an overall increase in turbidity.  Also, macrophyte communities deteriorate under these highly turbid conditions.  Plant abundance and species richness decrease due to increased light attenuation.  The high density and high food consumption of fathead minnows contributes to the increase in nutrient recycling rates, furthering phytoplankton growth.  The absence of macrophytes also contributes to increases in nutrient recycling and the stabilization of the turbid water state by favoring sediment resuspension. 

Previous studies have evaluated ways of controlling fathead minnows, but no method has had a high success rate.  In a study conducted by Zimmer, rotenone was applied to ten Minnesota wetlands containing fathead minnows.  In only one treated wetland out of ten was the fathead minnow population successfully eradicated.  In another study, commercial harvesting of fatheads applied to two South Dakota wetlands had little influence on their fathead minnow populations.

The application of “biomanipulation” to control fathead minnow populations is based on the concept of a top-down effect of predation initiating a “trophic cascade”.  The idea is that the introduction of a top predator can alter the entire trophic structure of a system.  In the case of my study wetlands, the top predator is a piscivore (such as walleye), intentionally stocked to decrease and regulate the planktivores (here fathead minnows), allowing the herbivores (zooplankton) to increase.  The flourishing zooplankton, in turn, regulate the phytoplankton by grazing.  This reduction of algae also slows nutrient recycling because nutrients are retained by the zooplankton and piscivores.  The anticipated net result is a shift of the system from a turbid water state to a clear water state.  The clear water state will allow the resurgence of macrophytes, which in turn contribute to suppression of algae via several mechanisms.  The macrophytes will also decrease turbidity reducing sediment suspension from wind-induced wave action.  

The use of piscivorous fish to limit fathead minnow densities and improve water quality has shown some success in previous studies.  A study in a South Dakota pond showed a significant decrease in fathead minnow densities during the summer growing season due to walleye predation.  Northern pike (Esox lucius) and largemouth bass (Micropterus salmoides) also have suppressed fathead minnow populations.  Stocking piscivorous fish in fishless wetlands may have negative effects on the wetland.  Reed and Parsons reported that walleye rearing in a fishless wetland reduced densities of some invertebrate orders, and recommended that fishless ponds not be used for walleye rearing.

Uncertainty surrounding the effects of stocked piscivorous fish on a wetland ecosystem has prevented managers from using this method to control fathead minnow populations. The rising problem of deteriorating wetlands has pushed this issue to the forefront.  There is currently no information indicating negative effects from walleye rearing in wetland ponds already containing fathead minnows.  If the current evaluation of walleye stocking demonstrates improvements in wetland water quality and other ecosystem characteristics, this technique may hold benefits for the management of both wetlands and fisheries. 

Summary and Conclusion of Research

The success of biomanipulation efforts aimed at switching wetlands from a stable, turbid-water state to a stable clear water state depends on two key steps: (1) the trophic cascade, and (2) secondary responses.  The first step is the triggering of a trophic cascade (Carpenter et al 1985 & 1987; Scheffer et al 2001; Hansson et al 1998).  Within the trophic cascade, three key groups must change significantly for this step of a biomanipulation to work: planktivorous fish, large cladocerans, and phytoplankton.  Planktivorous fish biomass must decrease; not only must the adult planktivores be suppressed, but also recruitment of young of the year (Hansson et al 1998).  Large cladocerans, like Daphnia, must increase in response to reduction planktivore biomass.  This should reduce phytoplankton biomass via increased grazing pressure and result in a relatively instantaneous increase in water clarity (McQueen et al 1986; Carpenter et al 1987; Carpenter & Kitchell 1998). 

Trophic cascades were achieved in the summer of 2001 and maintained in the summer of 2002 in the walleye fry treatment wetlands, but were never developed in the advanced walleye treatment wetlands.  The walleye fry treatment wetlands were successful because the walleye fry were able to suppress the fathead minnow populations by depleting both juvenile and adult fathead minnows and limiting young-of-year fathead minnows through predation and indirectly by lowering recruitment. This allowed for dramatic increases in large cladoceran populations, followed by a decrease in phytoplankton and an increase in water clarity by the second year of the study.  Suppression of larval fathead minnows is key, because most adult fathead minnows perish after breeding limiting their direct effect on the wetland ecosystem to only a portion of the summer.   Thus, fathead minnow populations as whole become dependent on the success of the young-of the-year. In addition, the influences of fathead minnow populations on wetland ecosystems quickly switch from the activities of fish which are a year or older, to those of young-of-the-year fish.  The advanced walleye treatment was unsuccessful in initiating a trophic cascade because of the large walleye’s inability to sufficiently suppress adult fathead minnows and lower the reproductive potential of the fathead minnow population overall.  Further more, adult walleye do not prey heavily on larval fathead minnows.  As a result fathead minnow populations continue to grow, making it even more unlikely for adult walleyes to control their populations.  Thus, even when adult fathead minnow populations were low fathead minnows were still influencing wetlands through young-of-the-year fish, preempting any response by large cladocerans, phytoplankton, and water clarity.

The second step needed for a biomanipulation to be successful in creating or maintaining a stable clear water state is the establishment of secondary responses to the trophic cascade.  Secondary responses include the reestablishment or increase of aquatic macrophytes, the reduction of internal loading, and the reduction of resuspended sediments.  The key secondary response are the reestablishment or increase in aquatic macrophytes.  Once plants become dominant in a wetland, they directly and indirectly influence the other secondary responses. 

The walleye fry and advanced walleye treatments did not initiate any significant change in the aquatic macrophytes, but wetlands stocked walleye fry are posed for changes in 2003 due sustained trophic cascades and improvements in water clarity throughout 2002.  Water clarity started to improve in some of the walleye fry treatment wetlands at the end of summer in 2001 and significant improvements were observed in 2002.  Unfortunately the macrophytes were not able to respond that quickly, but a response may develop in 2003.  If a macrophyte response develops in the walleye fry treatment wetlands then other secondary responses should follow that will promote further increases in water clarity and greater stability of the clear water state.  The advanced walleye treatment wetlands never succeeded in producing a trophic cascade, thus no secondary effects are expected.

At the end of the two-year study the walleye fry treatment succeeded in switching wetlands from a turbid water state to a clear water state.  The stability of the clear water state is still questionable, and will remain so until there is positive improvement in macrophyte abundance.  Another important positive result from the walleye fry treatment is the overall higher abundance of most macroinvertebrates resulting from the trophic cascade.  One negative result from the walleye fry treatment appeared during the second year of the study.  There was a trend toward decreasing abundance of benthic chironomids and Chaoborous in the second (even lower than the control treatment wetlands).  The decrease is likely due to an increase in walleye fry predation upon chironomids and Chaborous during the second year because of the decline in fathead minnows, the prefferred food source for the walleye fry.  A possible solution to dealing with this problem might be to stock walleye fry every other year.  Overall in this study, using wetlands to rear walleye fry does not appear to have any significant negative effects on the wetland ecosystems, and in fact can be beneficial to turbid wetlands by switching them to a clear water state with higher abundance of zooplankton, macroinvertebrates, and potentially macrophytes.  These documented benefits from the walleye fry treatment indicate that using rearing walleye fry can be used as a tool by wetland mangers to switch turbid wetlands to a clear water state. 

The advanced walleye treatment was not successful in initiating a trophic cascade. During most of the study, the advanced walleye treatment wetlands did not deviate significantly from the control treatment wetlands in most of the parameters measured.  During the first year of the study, the advanced walleye treatment had lower abundance of many of the invertebrate groups relative to control treatment wetlands, indicating a slight negative impact on these wetlands.  The results from the advanced walleye treatment indicate that stocking year-plus walleyes in wetlands will not positively affect the wetlands and may even have a negative impact.


Minnesota Chapter of American Fisheries Society Annual Meeting 2002 - Poster Presentation

North Dakota Wildlife Society Annual Meeting 2002 - Poster Presentation

National Wildlife Society Annual Meeting 2002  - Contributing Paper (Speaker)

Dr. Malcolm G. Butler

Advisor: Dr. Malcolm G. Butler

Professor of Zoology

North Dakota State University