Greg G Sass

Wisconsin Department of Natural Resources · Office of Applied Science

Fish population surveys in north-temperate lakes are often conducted in the fall or spring when individuals are easy to capture with traditional fisheries techniques. Because some fishes are preparing to spawn or are spawning during these seasons, there is a critical need to better understand the potential influences of these surveys on decisions r...

Declines in Cisco Coregonus artedi populations in some inland lakes have prompted assessments of Cisco occurrence and extirpation risk in relation to various stressors to identify refuge lakes and factors that promote Cisco persistence. However, most previous assessments have focused on presence-absence of Cisco rather than examining how population...

Large-river decision makers are charged with maintaining diverse ecosystem services through unprecedented social-ecological transformations as climate change and other global stressors intensify. The interconnected, dendritic habitats of rivers, which often demarcate jurisdictional boundaries, generate unique complex management challenges. Here, w...

Fish stocks generally exhibit high interannual variability in recruitment, and past research has suggested the truncation of a stock’s size- and age-distribution may exacerbate such variability. Individual female fish often differ in reproductive traits like fecundity, offspring size and annual spawn timing. These traits are thought to be related t...

Recruitment depensation describes elevated juvenile mortality with declining adult population size which can prevent or delay stock recovery. Understanding the factors influencing when a population undergoes depensation provides resource agencies with targets for management action. Using estimates of depensation from 28 walleye (Sander vitreus, Per...

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...

Structural Habitat additions are a common fisheries management tool used to mitigate the loss of natural physical structure in lakes and reservoirs. Although habitat additions can concentrate fish, it is poorly understood whether such additions increase prey and fish production. Coarse woody habitat (CWH) additions to littoral zones can increase th...

The Tennessee River basin and its cascade of reservoirs are home to some of the most diverse freshwater fish assemblages in the world. This unique system is threatened by the ongoing invasion of Silver Carp Hypophthalmichthys molitrix and Bighead Carp H. nobilis, hereafter bigheaded carps. Bigheaded carps may directly compete for food resources wit...

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 eco...

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

Understanding seasonal differences in species-specific vulnerabilities to recreational angling can be important for informing sustainable fisheries management practices, like fishing seasons or season-specific regulations. However, comparisons of angler catch and harvest rates among seasons with disparate modes of fishing, like open water and ice a...

Recruitment depensation describes elevated juvenile mortality with declining adult population size which can prevent or delay stock recovery. Understanding the factors influencing when a population undergoes depensation provides resource agencies with targets for management action. Using estimates of depensation from 28 walleye (Sander vitreus, Per...

Determining whether shifts in fish population demographics influence reproductive success is important for understanding population resiliency to environmental change and harvest. Previous studies have suggested that larger female walleye may experience greater reproductive success because of higher fecundity and larger eggs. Whether these larger f...

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...

Voluntary catch-and-release (CR) by anglers and stocking have emerged as a recreational fisheries management panacea in many regions. Anglers and managers often assume that CR practices and stocking will allow more individuals to survive to older ages and larger lengths. Taken together, the increasing prevalence of CR fishing and long history of st...

Walleye (Sander vitreus) survival to the fall recruitment stage has notably declined over time within the Ceded Territories of northern Wisconsin with many hypothesized and potentially interactive causes. Failure to recruit can have cascading effects within lake ecosystems, which can be exacerbated by other sources of walleye decline such as produc...

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...

Declining body size is believed to be a universal response to climate warming and has been documented in numerous studies of marine and anadromous fishes. The Salmonidae are a family of coldwater fishes considered to be among the most sensitive species to climate warming; however, whether the shrinking body size response holds true for freshwater s...

In commercial and recreational fisheries, catch rate is often assumed to be proportional to stock size and is used by managers and fishers as an indicator of fishery sustainability. If catch rate is proportional to stock size, it can signal a decline of stocks and managers can impose restrictive harvest policies or recreational anglers can move to...

The first year of the COVID-19 pandemic in 2020 was associated with an “anthropause” in many industries, initially reducing emissions and other negative anthropogenic influences. Outdoor recreation, however, has exploded in popularity in response to closures of indoor recreation options, increased free time, and/or increased levels of stress. We te...

Simple theoretical models predicting recreational fisheries’ ability to self-regulate often fail in practice. These models may be misinformed because they do not account for anglers’ limited knowledge of fish abundance across a suite of potential fishing opportunities. We present results from a novel creel survey specifically designed to elicit ang...

Anticipating and managing climate-induced ecosystem transformations in large rivers is particularly challenging given their inherently complex socio-ecological dynamics that often cross jurisdictional boundaries. We examine how the Resist-Accept-Direct (RAD) framework can facilitate informed decision making and a more cohesive and proactive approac...

In commercial and recreational fisheries, catch rate is often assumed to be proportional to stock size and is used by managers and fishers as an indicator of fishery sustainability. If catch rate is proportional to stock size, it can signal a decline of stocks and managers can impose restrictive harvest policies or recreational anglers can move to...

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...

2,4-Dichlorophenoxyacetic acid (2,4-D) is commonly used to control invasive aquatic macrophytes, including Eurasian watermilfoil (Myriophyllum spicatum) (EWM). Potential influences of 2,4-D on non-target organisms are poorly understood; however, research has suggested the possibility of lethal effects on certain fish species. Lake Ellwood, Wisconsi...

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...

In response to declines in coarse woody habitat (CWH) and fish productivity in natural lakes and reservoirs, agencies and stakeholders have used woody and artificial habitat enhancements to slow or reverse the effects of habitat loss from aging or shoreline development. Given that natural lakes and reservoirs differ in physical and biological condi...

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...

Lake ecosystems are shifting due to many drivers including climate change and landscape-scale habitat disturbance, diminishing their potential to support some fisheries. Walleye Sander vitreus (Mitchill) populations, which support recreational and tribal fisheries across North America, have declined in some lakes. Climate change, harvest, invasive...

Large-scale modeling and prediction provide insight into general influences of climate change on inland recreational fisheries; however, small-scale dynamics and local expertise will be key in developing explicit goals for managing recreational fisheries as the climate changes. The resist-accept-direct (RAD) framework encompasses the entire decisio...

Climate change is leading to shifts in not only the average timing of phenological events, but also their variance and predictability. Increasing phenological variability creates a stochastic environment that is critically understudied, particularly in aquatic ecosystems. We provide a perspective on the possible implications for increasingly unpred...

Decision-makers in inland fisheries management must balance ecologically and socially palatable objectives for ecosystem services within financial or physical constraints. Climate change has transformed the potential range of ecosystem services available. The Resist-Accept-Direct (RAD) framework offers a foundation for responding to climate-induced...

Understanding influences of angler proximity on fisheries resources, the distances anglers are willing to travel, their success rates after they arrive, and seasonal ecological patterns could influence fisheries resiliency, license sales, and management. The Northern Highlands Fishery Research Area near Boulder Junction, Vilas County, WI maintains...

Understanding influences of angler proximity on fisheries resources, in the distances anglers are willing to travel for angling opportunities and their success rates after they arrive, could influence fisheries resiliency and management applications. The Wisconsin Department of Natural Resources, Northern Highland Fishery Research Area (NHFRA), Vil...

Understanding influences of human proximity to natural resources, the distances recreationalists are willing to travel, and their success after arrival could influence open-access resource resiliency and management. The Northern Highlands Fishery Research Area near Boulder Junction, Vilas County, WI maintains a compulsory creel census for five rese...

Understanding influences of angler proximity on fisheries resources, the distances anglers are willing to travel, and their success rates after they arrive could influence fisheries resiliency and management. The Northern Highlands Fishery Research Area near Boulder Junction, Vilas County, WI maintains a compulsory creel census for five research la...

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...

Recruitment depensation or elevated juvenile mortality with declining adult stock size, can prevent or delay or stock recovery from overharvest and/or other factors. In most sportfish, juvenile survival is assumed to increase as adult stock size declines (i.e., compensatory recruitment). However, recruitment depensation, can occur if adult stock...

Bullheads Ameiurus spp. are found throughout much of the United States yet remain a relatively understudied species. However, Black A. melas and Yellow Bullheads A. natalis, are capable of becoming a nuisance and dominating the biomass in north temperate systems, which can influence native community structure. Further, empirical evidence suggests t...

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...

The Bois Brule River is a renowned, spring-fed, western Lake Superior tributary that supports five naturally reproducing populations of salmonid species: native brook trout Salvelinus fontinalis, brown trout Salmo trutta, rainbow trout Oncorhynchus mykiss, coho salmon Oncorhynchus kisutch, and chinook salmon Oncorhynchus tshawytscha. This sympatric...

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...

Climate change is influencing the timing of ecological events (in other words, their phenology) in natural systems globally. In lakes, the timing of important events like ice off, fish spawning, and plankton blooms are not only shifting earlier; they are also becoming much more unpredictable in their phenology from year to year. Such variability ha...

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...

Threats to inland recreational fisheries include warmer water, habitat loss, and expanding populations of predators. These threats are not directly confronted by the usual tools of fishery management. In view of this mismatch, the safe operating space or SOS approach aims to “manage what you can to protect what you want”. In some cases, reduced har...

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...

Sex differentiation in teleost fishes is complex and often dictated by genetics, environmental conditions, and population density during critical embryonic/larval development periods. Walleye Sander vitreus have a long propagation and stocking history, and sex ratios of hatchery-reared Walleye have been variable, typically skewed towards males, and...

Invasive Bighead Carp Hypophthalmichthys nobilis and Silver Carp H. molitrix have infested and caused largescale ecological and economic damage to the Illinois, Mississippi, and Ohio rivers. We compiled demographic data from 42,995 fish from 23 pools in the Illinois, Mississippi, and Ohio rivers, which universities and management agencies previousl...

Small, isolated populations present a challenge for conservation. The dueling effects of selection and drift in a limited pool of genetic diversity make the responses of small populations to environmental perturbations erratic and difficult to predict. This is particularly true at the edge of a species range, where populations often persist at the...

Lakes and reservoirs are aging with associated declines in structural habitat and fish production. In response, management agencies and stakeholders have used a suite of woody and artificial habitat enhancements to slow or reverse aging effects. However, variability in the types of structures used and differences in the physical and ecological cond...

Behavioral responses of fishes to littoral zone habitat enhancements are relatively understudied in diverse fish communities but are critical for understanding overall fish community responses. To advance knowledge on effects of coarse woody habitat (CWH) littoral zone enhancements, we initiated a long-term study on Sanford Lake, Vilas County, Wisc...

During the COVID-19 pandemic, recreational angling is an obvious choice of activity for anyone who wishes to spend quality time outdoors and away from crowds. The pandemic may therefore be influencing rates of participation, the demographics of recreational anglers, and the intensity and distribution of recreational fishing effort. Recruitment, ret...

Changing climatic conditions effect recreational fisheries in variable and uneven ways. These climate impacts will manifest themselves at different times in different waterbodies depending on the waterbody’s specific characteristics. Some systems may be strongly affected by climate in the near future while others may be resilient for decades before...

Knowledge of density-dependent responses of fish populations to exploitation is important for the sustainable management of fisheries and in structuring fish populations to meet angler desires. To better understand the density-dependent responses of Walleye Sander vitreus to exploitation, we conducted a ten-year, 50% annual exploitation experiment...

On June 20, 2021, the Northern Highland Fishery Research Area (NHFRA) celebrated its 75th anniversary of continuous fisheries population monitoring and compulsory angler creel census on five lakes in northern Wisconsin. In 1946, five lakes were designated as experimental fisheries research lakes and all anglers have been required to adhere to the c...

Angler trip success and catch rates are dependent upon a fishes’ vulnerability to angling. Angling vulnerability can be influenced by angler-specific attributes (i.e., bait choice, lure size, use of a guide), and individual fish traits (i.e., boldness, aggression, stress responsiveness, and memory retention). The mechanisms that function in a fishe...

Invasive bighead carp Hypophthalmichthys nobilis and silver carp H. molitrix have infested and caused largescale ecological and economic damage to the Illinois, Mississippi, and Ohio rivers. We compiled demographic data from 42,995 fish from 23 pools in the Illinois, Mississippi, and Ohio rivers, which universities and management agencies previousl...

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...

Fall electrofishing surveys for age-0 and age-1 Walleye (Sander vitreus) have shown a decline in natural recruitment for many populations in the northern third (Ceded Territory) of Wisconsin over the past 20 years. Our data set spanned from 1985 to 2017 with estimates of age-0/age-1 catch per effort (CPE) and survival/mortality of age-0 and age-1 W...

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...

Angler experience has suggested that environmental factors (season, lunar phase, wind direction) effect trip success and catch rates. Most studies have focused on whether lunar cycles effect catch rates, with limited information on the effects of other environmental factors. Our objective was to test for the influence of multiple environmental fact...

Stocking of fish is an important tool for maintaining fisheries but can also significantly alter population genetic structure and erode the portfolio of within-species diversity that is important for promoting resilience and adaptability. Walleye (Sander vitreus) are a highly valued sportfish in the Midwestern United States, a region characterized...

Bullheads Ameiurus spp. are found throughout much of the United States and are an infrequently studied species. Although limited information has been published on Black Bullheads Ameiurus melas and Yellow Bullheads A. natalis, it has been shown that bullheads can dominate the fish biomass in some north temperate USA lakes resulting in a fish commun...

Exploitation can be a stressor on fish populations directly or indirectly through other exacerbating variables such as climate change, habitat loss, or species-interactions. Estimates of sustainable exploitation rates for walleye (Sander vitreus) have been highly variable (range 15-84%). The current limit reference point exploitation rate for Ceded...

An understanding of angler catch rate characteristics related to proximity of residence to lakes (local, non-local, non-resident) is important for informing fisheries management. The Northern Highland Fishery Research Area compulsory creel census records zip code data from all anglers fishing Escanaba, Nebish, and Pallette lakes near Boulder Juncti...

2,4-D is a chemical commonly used to control invasive aquatic macrophytes, including Eurasian Watermilfoil (EWM). Effects of 2,4-D on non-target organisms are poorly understood; however, recent research has suggested the potential for lethal effects on certain fish species. Lake Ellwood, Florence County, Wisconsin was chemically treated with 2,4-D...

The COVID-19 pandemic has resulted in social and economic hardship nationwide. Outdoor recreational activities, however, have remained popular among Americans. Recreational fishing is one obvious possible choice of activity for anyone who wishes to spend quality time outdoors and away from crowds. The pandemic may therefore be influencing rates of...

Information about species-specific angler catch rates during the open water versus the ice season are limited. We used all available Wisconsin Department of Natural Resources creel survey data during 1990-2020 to test for differences in Black Crappie, Bluegill, Northern Pike, Walleye, and Yellow Perch angler catch rates between open water (May-Octo...

Natural recruitment has declined in northern Wisconsin Walleye populations over time. Several factors have been implicated to explain Walleye natural recruitment declines including climate change, centrarchids, imbalances in fish communities, overexploitation, and cultivation/depensation effects. Anecdotal evidence has suggested that Walleye and Bl...