Joseph Mrnak

• BS | MS | PhD
• Research Fish Biologist at Iowa Department of Natural Resources

Ecosystems are abruptly changing due to invasive species and global climate change. In lakes, invasive Rainbow Smelt Osmerus mordax can cause negative ecosystem effects through competitive and predatory interactions with native species leading to food web shifts away from native species dominance, altered zooplankton communities, and the decline or...

Managing fisheries in a changing socio-ecological environment may require holistic approaches for identifying and adapting to novel ecosystem dynamics. Using 32 years of Ceded Territory of Wisconsin (CTWI) walleye (Sander vitreus) data, we estimated production (P), biomass (B), biomass turnover (P/B), yield (Y), and yield over production (Y/P) and...

Invasive species are a global concern. After an invasive species establishes, they often disrupt ecosystems leading to new dynamics and species interactions, making management efforts difficult. Panarchy theory is a conceptual framework to account for the dual and seemingly contradictory characteristics (stability and change) of all complex systems...

Natural reproduction of pallid sturgeon Scaphirhynchus albus has been limited for decades and a recruitment bottleneck is hypothesized to occur during the larval stage of development. In this study, we evaluated the effects of water velocity and temperature on the swimming activity, energy use, settling behaviour and mortality of endogenously feedi...

Culturally, economically, and nutritionally valuable inland fisheries face many new challenges on top of chronic disturbances. In the upper midwestern United States, declines in cool-and coldwater fisheries have been observed, including ogaa/walleye Sander vitreus. In response to population declines, agencies have implemented rehabilitation efforts...

Many walleye populations in the Midwestern United States have declined due to inconsistent or failed natural recruitment, with stocking often failing to rehabilitate natural recruitment or sustain quality fisheries for tribal subsistence and angler harvest opportunity. Numerous factors have been implicated in walleye natural recruitment declines in...

In recent decades, some lakes in the Ceded Territory of Wisconsin have experienced increasing largemouth bass Micropterus salmoides abundance coupled with declining walleye Sander vitreus abundance. These shifts in abundance have become of management concern, as largemouth bass have been hypothesized to negatively influence walleye populations thro...

Coarse woody habitat (CWH) is an essential structural habitat within northern temperate lakes and is known to have distinctive effects across trophic levels, however, its impacts on many resource dynamics are not well understood. Stable isotopes, such as δ13C, δ15N, and δD allow unique insight into food-web positioning, habitat use, and allochthon...

The Ceded Territory of Wisconsin is a lake-rich region that supports important tribal subsistence and recreational fisheries for multiple species. Recently, declines in walleye Sander vitreus production and recruitment, coupled with increases in centrarchid abundance (e.g., largemouth bass Micropterus salmoides and bluegill Lepomis macrochirus) hav...

Managing fisheries in a changing socio-ecological environment may require holistic approaches for identifying and adapting to novel ecosystem dynamics. Using 32 years of Ceded Territory of Wisconsin (CTWI) walleye (Sander vitreus) data, we estimated production (P), biomass (B), biomass turnover (P/B), yield (Y), and yield over production (Y/P) and...

Ecosystems are abruptly changing due to invasive species and global climate change. In lakes, invasive Rainbow Smelt Osmerus mordax can cause negative ecosystem effects through competitive and predatory interactions with native species leading to food webs shifts away from native species dominance, altered zooplankton communities, and the decline o...

Cisco (Coregonus artedi) are a widespread, cold‐water zooplanktivore native to North America. Although Cisco are generally referred to as an “obligate zooplanktivore,” there is some evidence that the species exhibits considerable variability in trophic niche. Here, we assessed how Cisco body size relates to trophic position, that is, trophic ontoge...

The Ceded Territory of Wisconsin is a lake-rich region of northern Wisconsin that has experienced variable changes in its multi-use tribal and recreational fishery in recent decades. Declines in walleye Sander vitreus production and recruitment coupled with increases in warm-water species abundance (e.g., largemouth bass Micropterus salmoides) have...

Coarse woody habitat (CWH) modifications to aquatic ecosystems are often a short-term plan for management goals. Positive CWH effects can often be a draw for fish in the area to utilize it for feeding, reproduction, and survival. Though this woody debris can start to degrade and move, further altering the ecosystem, the long-term effects of these m...

Coarse woody habitat (CWH) experiments in lakes have increased in popularity in the midwestern United States to combat negative aquatic ecological effects associated with lakeshore residential development. Previous short-term experiments associate CWH additions with positive effects and CWH removals with negative effects with regards to the fish co...

The Ceded Territory of Wisconsin is a lake-rich region of northern Wisconsin that has experienced variable changes in its joint tribal and recreational fishery in recent decades. Declines in walleye Sander vitreus production and recruitment coupled with increases in warmwater species abundance (e.g., largemouth bass Micropterus salmoides) have led...

Invasive species are a global concern. After an invasive species establishes, they often disrupt ecosystems leading to new dynamics and species interactions, making management efforts difficult. Panarchy theory is a conceptual framework to account for the dual and seemingly contradictory characteristics (stability and change) of all complex systems...

Aquatic ecosystems are transforming due to invasive species and global climate change. Invasive rainbow smelt (Osmerus mordax) can cause negative ecosystem effects through competitive and predatory interactions with native species leading to food webs shifts away from native species dominance, altered zooplankton communities, and the decline and(or...

Information on yellow perch Perca flavescens population dynamics and responses to various abiotic and biotic factors in oligotrophic, north‐temperate inland lakes is limited. Water level fluctuations are known to influence available habitat and biological communities within the littoral zones of lakes, yet research is lacking for yellow perch in Wi...

Walleye population productivity is influenced by water temperature and water clarity, and available evidence suggests that model-predicted thermal-optical habitat area (TOHA) considered optimal for walleye populations has declined in the upper Midwest. Despite apparent relationships between TOHA and walleye populations, estimation of TOHA relies on...

Project updates on thesis research related to thermal-optical habitat area (TOHA) and walleye populations.

Walleye Sander vitreus are culturally and recreationally important in Wisconsin, USA and have experienced population declines in some lakes due to reduced natural recruitment. In 2013, the Wisconsin legislature implemented the Wisconsin Walleye Stocking Initiative, a statewide rehabilitation effort to help declining walleye populations through incr...

Knowledge of species‐specific fish biology, ecology, and habitat use is critical for informing science‐based management. Yellow perch Perca flavescens are an ecologically and recreationally important fish species. Yet, for many north‐temperate systems (e.g. inland lakes of Wisconsin), little is known about yellow perch seasonal habitat use and ecol...

This research attempts to increase our management capabilities by viewing fisheries management and aquatic restoration in an ecosystem and food web context.

Predation by native predators can hinder the success of an invasive species. Bythotrephes, an invasive zooplankton species, established in Trout Lake, Vilas County, Wisconsin, USA, in 2014. However, by 2020, Bythotrephes densities dropped to densities where they were barely detectable. Cisco (Coregonus artedi), a native zooplanktivore, is an abunda...

Available evidence suggests that walleye population status is influenced by water temperature and light levels and that the availability of thermal-optical habitat area (TOHA) considered optimal for walleye growth has changed over time. Despite the apparent connection between TOHA and population status, no previous study has empirically assessed wa...

Inland cisco Coregonus artedi are an energy-dense forage fish native to some Wisconsin northtemperate oligotrophic lakes. Cisco play a critical role within their native food webs as they are a common prey item for important gamefishes (e.g., lake trout Salvelinus namaycush, walleye Sander vitreus, and muskellunge Esox masquinongy) and can couple be...

Understanding how fish populations respond to various regulations is critical for informing management decisions. Indeed, similar regulations (e.g., bag limits, length limits, season closures) may elicit different population-level responses depending on the species and ecosystem. Nebish Lake, a 94-acre oligotrophic lake in northern Wisconsin with a...

Invasive species are a global concern. After an invasive species establishes, they often disrupt ecosystems leading to new dynamics and species interactions, making management efforts difficult. Panarchy theory is a conceptual framework to account for the dual and seemingly contradictory characteristics (stability and change) of all complex systems...

Walleye, Sander vitreus (Mitchill), natural recruitment has declined in northern Wisconsin lakes over time. Age-0 and age-1 walleye relative abundance (catch per unit effort; CPE) data from northern Wisconsin (1986-2019) was used to test for abiotic (i.e., lake characteristics, temperature variables) and biotic (age-0 and age-1 CPE) factors influen...

Walleye Sander vitreus natural recruitment has declined in northern Wisconsin lakes over time. Several factors have been implicated to explain Walleye natural recruitment declines in Wisconsin including climate change, Largemouth Bass Micropterus salmoides interactions, less desirable fish communities, production overharvest, and depensatory effect...

Age-0 and age-1 Walleye (Sander vitreus) natural recruitment has declined in many populations within the Ceded Territory of Wisconsin (CTWI) over the past 20 years. Using CTWI age-0 and age-1 Walleye relative abundance (CPE) data during 1990-2019, we tested for density-dependent compensatory mortality between age-0 and age-1 Walleye. Age-1 Walleye...

Black Bullheads Ameiurus melas are an environmentally tolerant omnivorous fish species that are found throughout much of North America and parts of Europe. Despite their prevalence, Black Bullheads are an infrequently studied species making their biology, ecology, and life history poorly understood. Although limited information has been published o...

Yellow Perch (Perca flavescens) are an ecologically and recreationally important fish species across their native range. Yet, for many north-temperate systems (e.g., inland Wisconsin lakes), little is known about the species seasonal habitat use and ecology. To further our understanding of this important cool-water sportfish, we characterized Yello...

Walleye Sander vitreus is a prized sportfish in Wisconsin, that has been experiencing recruitment and hence population declines. In 2013, the Wisconsin Walleye Initiative began in attempt to offset declining Walleye populations by increasing stocking efforts. Hence, determining which factors may be affecting the survival of these stocked Walleyes i...

Fish natural recruitment is influenced by abiotic and biotic factors. Natural recruitment has declined in northern Wisconsin Walleye Sander vitreus populations over time. Previous research has suggested that White Crappie Pomoxis annularis may negatively influence Walleye recruitment. We used Black Crappie Pomoxis nigromaculatus and age-0 Walleye r...

Invasive species are a global concern for aquatic ecosystems. Though prevention is the best management practice for invasive species control, detection often occurs after the species has established within a system. Post-establishment, new ecosystem dynamics and species interactions become rampant. These new dynamics and interactions may lead to an...

Information on Yellow Perch (Perca flaevscens) population dynamics and response to various biotic and abiotic factors in inland north-temperate lakes is sparse. Water level fluctuations are known to influence habitat and biological communities within the littoral zones of lakes, yet this information is generally lacking for Wisconsin Yellow Perch o...

Black bullheads Ameiurus melas are an environmentally tolerant omnivorous fish species that are found throughout much of North America and parts of Europe. Despite their prevalence, black bullheads are an infrequently studied species making their biology, ecology, and life history poorly understood. Although limited information has been published o...

• Understanding where, when, and how native species persist in the face of invasive species-driven ecosystem change is critical for invasive species management and native species conservation. In some cases, ecological interactions among native and invasive species are spatially structured, and spatial segregation can be a key coexistence mechanism...

When estimating fish population abundance, it is important to recognize that differing habitat use may cause one gear type to be more effective and less biased than another. We generated and compared population abundance estimates (PE) for adult Yellow Perch in Crystal Lake, Wisconsin using a spring mini fyke net mark-recapture survey and summer hy...

Information on Black Bullhead (BB; Ameiurus melas) fecundity and reproductive potential is lacking in current literature. However, BB can dominate the biomass in north temperate lakes, alter water quality, and are known to negatively affect desirable fish species (e.g., Walleye Sander vitreus, Yellow Perch Perca flavescens). Thus, gaining a better...

Water level fluctuations have great influence on ecosystem processes. With climate change predicted to increase the frequency and intensity of precipitation events, a better understanding on the influence of water level on desirable game fish populations is warranted. To address this, we used a 39-yr dataset (NTL-LTER) to examine Yellow Perch (Perc...

Invasive species are a global concern, particularly for aquatic ecosystems. Though prevention is the single best management practice for invasive species control, detection often occurs after the species has established within a system. Post-establishment, new ecosystem dynamics and species interactions become rampant, making management efforts dif...

The long-term persistence of an invasive species depends on the biotic and abiotic conditions of the recipient ecosystem. Predation by native species represents a major barrier invasive species must withstand in order maintain an established population. In our study, we focus on the apparent disappearance of an invasive zooplankton, spiny water fle...

Invasive species are a global concern, particularly for aquatic ecosystems. Invasive rainbow smelt have established in many freshwater systems across North America and cause ecosystem level effects through competitive and predatory interactions. Negative effects include shifted food webs away from native species dominance, altered zooplankton commu...

Invasive rainbow smelt Osmerus mordax negatively affect ecosystems that they colonize, primarily through competitive and predatory interactions with native species. Negative effects include shifting food webs, altering zooplankton communities, and the decline or extirpation of native cool- and cold-water fishes (e.g., yellow perch Perca flavescens,...

Invasive rainbow smelt Osmerus mordax have negatively affected ecosystems that they have colonized, primarily through competitive and predatory interactions with native species. Negative effects observed have included shifting food webs, altering zooplankton communities, and the decline or extirpation of native cool- and cold-water fishes (e.g., ye...

Pallid Sturgeon Scaphirhynchus albus are a federally endangered species endemic to the Missouri River basin. Natural reproduction has been limited for decades and a recruitment bottleneck is hypothesized to occur during the larval stage of development. Understanding factors that affect survival of Pallid Sturgeon larvae is key given their critical...

Pallid Sturgeon Scaphirhynchus albus are a federally endangered species endemic to the Missouri River basin and the lower Mississippi River. Successful natural reproduction has been limited for decades and in addition, a recruitment bottleneck is hypothesized to occur during the endogenous-drift phase of development. Understanding factors that affe...

Assessment of the angling and tribal spearing Walleye (Sander vitreus) fisheries in the Ceded Territory of Wisconsin (CTWI) is critical for the sustainability of this resource. Key to these assessments is an understanding of harvest demographics, exploitation, catch and harvest efficiency, and relationships between catch/harvest and adult density....

Pallid Sturgeon Scaphirhynchus albus are a federally endangered species endemic to the Missouri River basin and the lower Mississippi River. Successful natural reproduction has been limited for decades and in addition, a recruitment bottleneck is hypothesized to occur during the endogenous-drift phase of development. Understanding factors that affe...

Pallid Sturgeon Scaphirhynchus albus are a federally endangered species endemic to the Missouri River basin and the lower Mississippi River. Successful natural reproduction has been limited for decades and in addition, a recruitment bottleneck is hypothesized to occur during the drift phase of endogenous larvae. Understanding factors that affect su...

Continual assessment of the joint tribal spearing and angling walleye fisheries in the Ceded Territory of Wisconsin is critical for the sustainability of this economically and culturally important resource. Key to these assessments is an understanding of harvest demographics, catch and harvest efficiency, and relationships between catch, harvest, c...

An index of biotic integrity (IBI) provides fishery professionals a single value to rapidly assess aquatic communities and evaluate restoration efforts within those communities. An IBI accounts for the structure, composition, and functional organization of the biological community. To address the difficulty of characterizing fish assemblages in lar...

Implemented and released in the fishWiDNR package in December, 2016.

In the Laurentian Great Lakes, nonnative Rainbow Smelt (Osmerus mordax) provide forage for Lake Trout (Salvelinus namaycush) and other predatory fish and are potential competitors with and predators of young Coregonus species. In recent years, their populations have declined across the Laurentian Great Lakes. This study is centered on Whitefish Bay...