Gretchen Jeanette Anderson Hansen

• MS, PhD
• Professor (Assistant) at University of Minnesota

Temperate lakes may contain both coolwater fish species such as walleye (Sander vitreus) and warmwater fish species such as largemouth bass (Micropterus salmoides). Recent declining walleye and increasing largemouth bass populations have raised questions regarding the future trajectories and management actions for these species. We developed a ther...

Freshwaters are being transformed by multiple environmental drivers, creating uncertainty about future conditions. One way of coping with uncertainty is to manage for resilience to unanticipated events while facilitating learning through adaptive management. We outline the application of these strategies to freshwater recreational fisheries managem...

We classified walleye (Sander vitreus) recruitment with 81% accuracy (recruitment success and failure predicted correctly in 84% and 78% of lake-years, respectively) using a random forest model. Models were constructed using 2779 surveys collected from 541 Wisconsin lakes between 1989 and 2013 and predictor variables related to lake morphometry, th...

Rapid transitions in ecosystem structure, or regime shifts, are a hallmark of alternative stable states (ASS). However, regime shifts can occur even when feedbacks are not strong enough to cause ASS. We investigated the potential for ASS to explain transitions between dominance of an invasive species, rusty crayfish (Orconectes rusticus), and nativ...

Controlling invasive species can restore ecosystems while also quantifying species interaction strengths. We experimentally removed invasive rusty crayfish (Orconectes rusticus) from a Wisconsin lake. Rusty crayfish abundance declined by 99% in 8 years and did not significantly increase 4 years postharvest, with no compensatory recruitment response...

Cisco (Otoonapii in Ojibwe; Coregonus artedi Lesueur, 1818), is a widely distributed stenothermic freshwater fish whose embryos typically incubate under ice and in the dark. We used Cisco as a model organism for testing the potential of UV‐induced escape hatching behaviour. Owing to reduced ice cover and increased water transparency in north temper...

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...

Climate change is altering the thermal habitats of freshwater fish species. We analyze modeled daily temperature profiles from 12,688 lakes in the US to track changes in thermal habitat of 60 lake fish species from different thermal guilds during 1980-2021. We quantify changes in each species’ preferred days, defined as the number of days per year...

1. Aquatic invasive species (AIS) are a global threat to freshwater biodiversity and ecosystem services. Documenting AIS prevalence at broad spatial scales is critical to effective management and early detection. However, conventional monitoring for AIS is costly and is rarely applied at the resolution and scale required for effective management. M...

Freshwater ecosystems can serve as model systems that reveal insights into biological invasions. In this article, we summarize nine lessons about aquatic invasive species from the North Temperate Lakes Long-Term Ecological Research program and affiliated projects. The lessons about aquatic invasive species are as follows: Invasive species are more...

Environmental DNA (eDNA) refers to genetic material released by organisms into their surrounding environment. Collecting and identifying eDNA has gained popularity for monitoring and surveillance of aquatic invasive species. Invasive species management is most successful when an invasion is identified early while population size is likely to be low...

Forecasting invasion risk under future climate conditions is critical for the effective management of invasive species, and species distribution models (SDMs) are key tools for doing so. However, SDM‐based forecasts are uncertain, especially when correlative statistical models extrapolate to nonanalog environmental domains, such as future climate c...

Understanding age and growth are important for fisheries science and management; however, age data are not routinely collected for many populations. We propose and test a method of borrowing age–length data across increasingly broader spatiotemporal levels to create a hierarchical age–length key (HALK). We assessed this method by comparing growth a...

Estimating relative abundance is critical for informing conservation and management efforts and for making inferences about the effects of environmental change on populations. Freshwater fisheries span large geographic regions, occupy diverse habitats and consist of varying species assemblages. Monitoring schemes used to sample these diverse popula...

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...

Lake morphometry is a driver of limnological processes, yet digitized bathymetry is lacking for most lakes. Here, we describe a method for efficiently extracting hypsography from bathymetric maps using ImageJ. To validate our method, we compared results generated from two independent users to those obtained from digital elevation models for 100 lak...

Walleye (Sander vitreus; WAE) and yellow perch (Perca flavescens; YEP; collectively percids) are freshwater fishes threatened by multiple stressors, including aquatic invasive species. Zebra mussels (Dreissena polymorpha; ZM) and spiny water fleas (Bythotrephes cederströmii; SWF) are aquatic invasive species that reduce pelagic zooplankton biomass,...

Poikilothermic animals comprise most species on Earth and are especially sensitive to changes in environmental temperatures. Species conservation in a changing climate relies upon predictions of species responses to future conditions, yet predicting species responses to climate change when temperatures exceed the bounds of observed data is fraught...

Lake water clarity is an indicator of water quality, trophic status, and habitat condition. Changes in clarity impact lake ecosystems and may reflect land use changes or presence of invasive species. Quantifying temporal changes in water clarity can be challenging because clarity varies seasonally, annually, and spatially within and among lakes. We...

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...

Walleye (Sander vitreus) are an ecologically important species managed for recreational, tribal, and commercial harvest. Walleye prefer cool water and low light conditions, and therefore changing water temperature and clarity potentially impacts walleye habitat and populations across the landscape. Using survey data collected from 1993 to 2018 from...

Managing ecological systems for resilience can increase their capacity to maintain key functions even under global change. Oxygenated coldwater (oxythermal) habitat in lakes is an important ecological resource that is threatened by both climate change and eutrophication. Here, we quantify the resilience of oxythermal habitat in over 10,000 glacial...

Walleye Sander vitreus, Sauger S. canadensis, and Yellow Perch Perca flavescens (referred to as percids herein) are collectively among the most culturally and ecologically important fish species in North America. As ecosystems change in response to environmental drivers, such as climate change, nutrient loading, and invasive species, there is a nee...

The dataset described here includes estimates of historical (1980–2020) daily surface water temperature, lake metadata, and daily weather conditions for lakes bigger than 4 ha in the conterminous United States (n = 185,549), and also in situ temperature observations for a subset of lakes (n = 12,227). Estimates were generated using a long short‐ter...

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...

Walleye Sander vitreus and Yellow Perch Perca flavescens are culturally, economically, and ecologically significant fish species in North America that are affected by drivers of global change. Here, we review and synthesize the published literature documenting the effects of ecosystem changes on Walleye and Yellow Perch. We focus on four drivers: c...

Aim Recurrent species assemblages integrate important biotic interactions and joint responses to environmental and spatial filters that enable local coexistence. Here, we applied a bipartite (site–species) network approach to develop a natural typology of lakes sharing distinct fish faunas and provide a detailed, hierarchical view of their bioregio...

• 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...

Aim Availability of uniformly collected presence, absence, and abundance data remains a key challenge in species distribution modeling (SDM). For invasive species, abundance and impacts are highly variable across landscapes, and quality occurrence and abundance data are critical for predicting locations at high risk for invasion and impacts, respec...

Aquatic invasive species (AIS) are one of the greatest threats to preserving Minnesota’s natural aquatic resources, requiring effective surveillance and early detection to mitigate the threats posed by AIS. In this report, we identify key components of a statewide surveillance and early detection system including, Surveys and Inspections, Partnersh...

Walleye (Sander vitreus) population declines have been linked to climate change, but it is unclear how the growth of this cool-water species may be affected by warming water temperatures. Because warming rates vary among lakes, it is uncertain whether lake characteristics may mediate the temperature effects on walleye growth or may vary as a result...

Understanding invasive species spread and projecting how distributions will respond to climate change is a central task for ecologists. Typically, current and projected air temperatures are used to forecast future distributions of invasive species based on climate matching in an ecological niche modeling approach. While this approach was originally...

Recruitment of age‐0 Walleye Sander vitreus is often indexed using fall electrofishing surveys. However, collecting fish before fall may provide timely information regarding stocking decisions and factors influencing recruitment. We evaluated sampling methods for age‐0 Walleye in northern Wisconsin lakes that could be used to assess recruitment in...

Two primary goals in fisheries research are to (i) understand how habitat and environmental conditions influence the distribution of fishes across the landscape and (ii) make predictions about how fish communities will respond to environmental and anthropogenic change. In inland, freshwater ecosystems, quantitative approaches traditionally used to...

Invasive species represent a threat to aquatic ecosystems globally; however, impacts can be heterogenous across systems. Documented impacts of invasive zebra mussels (Dreissena polymorpha) and spiny water fleas (Bythotrephes cederströmii; hereafter Bythotrephes) on native fishes are variable and context dependent across locations and time periods....

The rapid growth of data in water resources has created new opportunities to accelerate knowledge discovery with the use of advanced deep learning tools. Hybrid models that integrate theory with state‐of‐the art empirical techniques have the potential to improve predictions while remaining true to physical laws. This paper evaluates the Process‐Gui...

Climate change has unprecedented effects on inland glacial lake fisheries in the upper Midwest of the United States, presenting a complex challenge for managers. Here, we provide a regional perspective on managing for climate change to guide management of heterogeneous and interdependent fishery resources. We identify approaches that may promote th...

Tingley RW III, Paukert CP, Sass GG, Jacobson PC, Hansen GJA, Lynch AJ, Shannon PD. 2019. Adapting to climate change: Guidance for the management of inland glacial lake fisheries. Lake Reserv Manage. XX:XXX–XXX. Climate change is altering glacial lake fisheries in the United States, presenting a complex challenge for fisheries managers. Here we pro...

Magee, MR, Hein CL, Walsh JR, Shannon PD, Vander Zanden MJ, Campbell TB, Hansen GJA, Hauxwell J, LaLiberte, GD, Parks TP, Sass GG, Swanston CW, Janowiak MK. 2019. Scientific advances and adaptation strategies for Wisconsin lakes facing climate change. Lake Reserv Manage. XX:XXX–XXX. Climate change threatens inland lakes, which are highly valued for...

Successful management of natural resources requires local action that adapts to larger‐scale environmental changes in order to maintain populations within the safe operating space (SOS) of acceptable conditions. Here, we identify the boundaries of the SOS for a managed freshwater fishery in the first empirical test of the SOS concept applied to man...

Successful climate adaptation for inland fisheries requires approaches that promote ecological resilience, support resilient management strategies, and establish or support long-term monitoring efforts. These broad concepts provide guidance to management agencies but are daunting tasks that are often difficult to implement given multiple barriers t...

The effects of climate change on inland recreational fisheries are often measured in changes in species distributions or biomass. However, the monetary costs associated with these ecological changes to recreational fisheries can be a valuable tool in conveying the ramifications of climate change to the public and assessing the overall value of mana...

Eutrophication and climate warming are profoundly affecting fish in many freshwater lakes. Understanding the specific effects of these stressors is critical for development of effective adaptation and remediation strategies for conserving fish populations in a changing environment. Ecological niche models that incorporated the individual effects of...

Model variable values. Lake specific data used for ecological niche models developed from 1,577 Minnesota lakes. Metadata describing each data field are described in the first portion of the Table. (XLSX)

Taxa-specific model responses. Generalized additive model responses of mean annual temperature (MAT °C) and mean summer epilimnetic total phosphorus concentrations (TP μg/l), depth (m), area (ha), and alkalinity on the relative abundance of 25 fish species sampled in 1,577 Minnesota lakes. Species codes are defined in S1 Table and effective degrees...

Taxa-specific model significance. P-values and percent deviance explained for generalized additive models of the relative abundance of 25 fish species predicted by mean annual temperature (MAT) and mean summer epilimnetic total phosphorus concentrations (TP), depth, area, and alkalinity sampled in 1,577 Minnesota lakes. (XLSX)

Walleye (Sander vitreus) populations are declining in Wisconsin and neighboring regions, motivating broader interest in walleye biology amidst ecological change. In fishes, growth integrates variation in ecological drivers and provides a signal of changing ecological conditions. We used a 23-year data set of length-at-age from 353 walleye populatio...

Recreational fisheries managers must cope with trends that are beyond their control such as changes in climate, loss of habitat, or social factors that affect angler behavior. The safe operating space (SOS) of a recreational fishery is the multidimensional region defined by levels of harvest, angler effort, habitat, predation, and other factors in...

Responses in lake temperatures to climate warming have primarily been characterized using seasonal metrics of surface-water temperatures such as summertime or stratified period average temperatures. However, climate warming may not affect water temperatures equally across seasons or depths. We analyzed a long-term dataset (1981–2015) of biweekly wa...

The Safe Operating Space (SOS) of a recreational fishery is the multidimensional region defined by levels of harvest, angler effort, habitat, predation and other factors in which the fishery is sustainable into the future. SOS boundaries exhibit trade-offs such that decreases in harvest can compensate to some degree for losses of habitat, increases...

Climate change has already influenced lake temperatures globally, but understanding future change is challenging. The response of lakes to changing climate drivers is complex due to the nature of lake-atmosphere coupling, ice cover, and stratification. To better understand the diversity of lake responses to climate change and give managers insight...

Lakes respond heterogeneously to climate, with implications for fisheries management. We analyzed walleye (Sander vitreus) recruitment to age-0 in 359 lakes in Wisconsin, USA, to (i) quantify the relationship between annual water temperature degree days (DD) and walleye recruitment success and (ii) identify the influence of lake characteristics — a...

Non-native species are a major component of global environmental change, and aquatic systems are especially vulnerable to non-native species impacts. Much of the research on aquatic non-native species impact has occurred at the local or site level. In reality, non-native species impacts play out across multiple spatial scales on heterogeneous lands...

Patterning of the presence/absence of food web linkages (hereafter topology) is a fundamental characteristic of ecosystems that can influence species responses to perturbations. However, the insight from food web topology into dynamic effects of perturbations on species in individual systems is potentially hindered because most described topologies...

Climate change is rapidly warming aquatic ecosystems including lakes and reservoirs. However, variability in lake characteristics can modulate how lakes respond to climate. Water clarity is especially important both because it influences the depth range over which heat is absorbed, and because it is changing in many lakes. Here, we show that simula...

Natural resource decision makers are challenged to adapt management to a changing climate while balancing short-term management goals with long-term changes in aquatic systems. Adaptation will require developing resilient ecosystems and resilient management systems. Decision makers already have tools to develop or ensure resilient aquatic systems a...

Invasive species have substantial impacts across the globe. While management efforts should aim to minimize undesirable impacts, we have a poor understanding of how impacts of a given invasive species vary spatially. Here, we develop a framework for considering heterogeneity of invasive species impacts that allows us to explore the range of possibl...

In Wisconsin, the management of Walleyes Sander vitreus relies on a set of log‐linear regressions to predict Walleye abundance and to set safe harvest. The regression models predict mean Walleye abundance from lake area, but they ignore variability among years; they also predict equal Walleye populations in lakes with the same size and recruitment...

Temporal patterns of species abundance, although less well-studied than spatial patterns, provide valuable insight to the processes governing community assembly. We compared temporal abundance distributions of two communities, phytoplankton and fish, in a north temperate lake. We used both 17 years of observed relative abundance data as well as res...

Largemouth Bass (LMB) Micropterus salmoides is one of the most popular sport fish in the United States and is intensively managed across much of its range. Beginning in 1989, Wisconsin implemented more restrictive harvest regulations for LMB, including greater minimum length limits, reduced bag limits, and a catch-and-release-only season during the...

Water temperature observations were collected from 142 lakes across Wisconsin, U.S.A. to examine variation in temperature of lakes exposed to similar regional climate. Whole lake water temperatures increased across the state from 1990 to 2012, with an average trend of 0.042 °C yr−1 ± 0.01 °C yr−1. In large (> 0.5 km2) lakes, the positive temperatur...

Linkages between benthic and pelagic habitats occur in both freshwater and marine systems across multiple spatial and temporal scales, and are influenced by a number of chemical, biological, and physical forces. We identified three major mechanisms of benthic-pelagic coupling: (1) organism movement, (2) trophic interactions, and (3) biogeochemical...

Changes in water temperatures resulting from climate warming can alter the structure and function of aquatic ecosystems. Lake-specific physical characteristics may play a role in mediating individual lake responses to climate. Past mechanistic studies of lake–climate interactions have simulated generic lake classes at large spatial scales or perfor...

Predicting recruitment is a long-standing goal in fisheries management. To guide management of Wisconsin’s Walleye (Sander vitreus), we predicted the probability of successful recruitment, defined as fall young-of-year Walleye catch rates greater than 10/mile, based on 2,210 surveys in 303 lakes conducted from 1989-2012 in Wisconsin’s Ceded Territo...

We determined if variation in biological performance indicators (i.e., ω, mean total length (TL) at age 3, mean age at 50 cm TL, age classes present, age class diversity [H], catch-per-effort [CPE] of age-0 walleyes, coefficient of variation [CV] in age-0 CPE, targeting angler catch rate, and the mean TL of the 10 smallest mature females) observed...

Long-term overharvest and environmental change can depress fish productivity and lead to a collapse of stocks. In particular, selective harvest of large individuals in recreational fisheries may induce changes in individual-level traits (e.g., size-at-maturity) and in turn reduce population sustainability. We assessed region-wide trends in body siz...

Adaptive management (AM) promotes learning about managed systems by implementing regulations experimentally, monitoring the response, evaluating outcomes, and adapting. Despite its appeal, AM has infrequently been implemented successfully. Fisheries management provides opportunities for AM, as fisheries management actions are often replicated acros...

Invasive species are leading drivers of environmental change. Their impacts are often linked to their population size, but surprisingly little is known about how frequently they achieve high abundances. A nearly universal pattern in ecology is that species are rare in most locations and abundant in a few, generating right-skewed abundance distribut...

Largemouth bass (LMB) are one of the most popular and intensively managed fish species in the United States. Beginning in 1989, Wisconsin implemented more restrictive harvest regulations for LMB including greater minimum length limits, reduced bag limits, and a catch-and-release-only season during the spawning period across much of northern Wiscons...

Background/Question/Methods In this presentation we draw on research from the North Temperate Lakes Long Term Ecological Research program in Wisconsin to show how long-term studies can provide a framework for predicting future ecological change. In particular, we focus on how climatic forcing has led to quasi-cyclical patterns of lake water levels...

long-term research on freshwater ecosystems provides insights that can be difficult to obtain from other approaches. Widespread monitoring of ecologically relevant water-quality parameters spanning decades can facilitate important tests of ecological principles. Unique long-term data sets and analytical tools are increasingly available, allowing fo...

Invasive species abundance is variable, and understanding the mechanisms driving this variation is a major research and management goal. The rusty crayfish (Orconectes rusticus) is an invasive species that impacts all trophic levels of aquatic ecosystems, although rusty crayfish do not always achieve high densities and negatively affect native biot...

The US National Science Foundation–funded Long Term Ecological Research (LTER) Network supports a large (around 240) and diverse portfolio of long-term ecological experiments. Collectively, these long-term experiments have (a) provided unique insights into ecological patterns and processes, although such insight often became apparent only after man...

The rusty crayfish (Orconectes rusticus) is an invasive species that impacts all trophic levels of aquatic ecosystems. However, in some systems they do not achieve high densities and do not negatively affect native biota. We conducted a whole-lake experiment to remove rusty crayfish from a northern Wisconsin lake to induce a shift from a high-densi...

Systematic approaches to site selection for marine protected areas (MPAs) are often favored over opportunistic approaches as a means to meet conservation objectives efficiently. In this study, we compared analytically the conservation value of these two approaches. We locate this study in Danajon Bank, central Philippines, where many MPAs were esta...

Communicated by J. Ellen Marsden Index words Laurentian Great Lakes Eradication Monitoring Early detection Ballast water Risk assessment Ballast water regulations implemented in the early 1990s appear not to have slowed the rate of new aquatic invasive species (AIS) establishment in the Great Lakes. With more invasive species on the horizon, we exa...