Geology topics

Daniel A. Isermann

Publications and source records attributed to Daniel A. Isermann.

At least 19 recordsLinked to original sources

USGS2025

Muskellunge spawning habitat characteristics and availability in Green Bay, Lake Michigan

Objective Habitat degradation has been associated with the loss of many self-sustaining Muskellunge Esox masquinongy populations, including those in Green Bay, where stocking has provided an exceptional trophy fishery but restoration goals include establishing self-sustaining populations and there is little evidence of natural recruitment. Our objectives were to determine whether (1) Muskellunge spawning locations and occurrence of successful hatching were related to a suite of habitat characteristics, (2) proportions of Muskellunge spawning in or outside of tributaries to lower Green Bay were different, and (3) Muskellunge showed spawning site fidelity. Methods From 2017 to 2019, adult Muskellunge ( N = 60) were surgically implanted with radio and acoustic transmitters to identify spawning locations, where we measured a suite of habitat variables and attempted to collect eggs and larvae. Side-scan sonar was used to quantify the amount of habitat available to Muskellunge for egg deposition in the Fox and Menominee rivers, which are tributaries to Green Bay. Results Muskellunge eggs were collected at 58 locations, but only two larvae were collected from a single location. Bottom slope, depth, distance to shore, gravel substrate, organic matter, and dissolved oxygen best predicted the presence of Muskellunge eggs. We determined that little habitat associated with Muskellunge egg deposition was available in the Fox and Menominee rivers. However, approximately half of tagged Muskellunge appeared to spawn outside of tributaries. Muskellunge in Green Bay displayed moderate spawning site fidelity. Conclusions Our results suggest that successful hatching occurs at very low levels and the lack of suitable Muskellunge spawning habitat in Green Bay tributaries may be limiting natural reproduction. Changes in spatial allocation of stocked fish and enhancement of known spawning locations may increase egg deposition and subsequent natural reproduction.

Michigan, Wisconsin↗

USGS2025

Lake sturgeon behavioral diversity in the Laurentian Great Lakes: Migratory patterns across populations and habitats

Background Characterizing the diversity of migration behaviors from the individual to the population level is essential for understanding how organisms respond to environmental variation and how these responses affect survival and habitat use. Lake sturgeon ( Acipenser fulvescens ) is a species of special concern in the Laurentian Great Lakes that are long-lived and generally classified as intermittent, adfluvial spawners. Observations of lake sturgeon movements at ecologically relevant spatiotemporal scales have shown that migration behavior often varies among individuals within the same population. However, studies on individual populations, particularly when focused only on a part of the life cycle (e.g., often spawning), provide just a partial understanding of the species’ full migratory scope and processes underlying expression of different migratory behaviors. To better understand lake sturgeon migratory diversity, we characterized and compared migratory behaviors of six lake sturgeon populations occupying environments that varied in habitat availability and connectivity in different Laurentian Great Lakes. Methods Sequence analysis combined with agglomerative hierarchical clustering and visual inspection of daily location data were used to identify distinct lake sturgeon migratory behaviors present in each population. Results Seven distinct migratory behaviors were identified based on differential patterns of lake and river use that encompass spawning and other seasonal periods. Behaviors were categorized as annual spring river, intermittent spring river, intermittent two-step, annual summer river, annual winter river, and annual interlake migrants along with river residents. The presence and frequency of migratory behaviors varied substantially among populations. Conclusions Our study demonstrated that migratory diversity is a general feature of lake sturgeon life history that may be partially shaped by habitat availability and connectivity. Given these results, we propose a conceptual model that links habitat availability and connectivity to migratory diversity and predict a positive association between them. This updated framework provides a cohesive basis for understanding lake sturgeon migratory behavior across variable ecological contexts in the Laurentian Great Lakes and will help promote future research to refute or refine the model.

Laurentian Great Lakes↗

USGS2025

Bright spots for advancing ecological understanding and conservation decision-making

A lot can be learned by studying bright spots—defined as unexpected positive outcomes. In fields like public health, education, and oncology, identifying factors behind bright spots reveals previously unknown drivers of success that can be replicated elsewhere. This concept is being applied in conservation but is hampered by variations in definitions of bright spots and confusion with hotspots—sites with high absolute values of a metric. We developed a framework to clearly define and distinguish between hotspots (e.g., a wetland with high plant diversity) and bright spots (e.g., a biodiverse wetland in a housing development), which outperform conservation expectations. The framework is an iterative cycle, consisting of setting expectations for relative comparisons, classifying systems into bright, dark, hot, and cold categories, and digging deeper to reveal hidden mechanisms and opportunities for intervention. We drew on examples from diverse fields to demonstrate how our framework can generate new knowledge, identify potential interventions, and inform management priorities. Defining conservation and management expectations, often through predictive models, is essential to understanding drivers of success and fosters hypotheses about overlooked factors. Our framework can enhance ecological understanding, guide interventions, and help prioritize actions in conservation and natural resource management.

Conservation Biology↗

USGS2025

Differential responses of coolwater fishes to a whole-lake reduction of a warmwater thermal guild

Climate change is transforming the ecology of lakes at a rapid pace, shifting some lakes toward warmwater-dominant habitats. As a result, warmwater fishes are increasingly becoming more prevalent in lakes where they already existed, altering the patterning and strength of species interactions. Understanding shifting species interactions (e.g., competition, predation), and the role of lake management in shaping these interactions, will be critical for lake stewardship in response to climate change. Here, we present results from an intensive 5-year experimental removal of ~285,000 warmwater fishes from a north-temperate lake. The goal of the experiment was to test whether warmwater fish reduction is effective for rewiring lake food webs to reverse undesirable conditions for coolwater species, leading to increased recruitment and abundance of coolwater fishes. Throughout the experiment, warmwater fishes were resilient to reductions, with biomass declines of 23% averaged across five species. Among coolwater fishes, the top predator walleye showed no biomass response, while yellow perch biomass increased by ~914%. Fish species biomass changes translated to food web shifts, including a yellow perch trophic position decline of 0.4, decreased zooplankton abundances, and increased zoobenthos abundances. Our results highlight differential species responses to a management action aimed at adapting to climate change. Despite similar thermal tolerances, two coolwater species responded differently to removal of warmwater fishes, highlighting the characteristics (e.g., life history strategies, adaptive capacity) that contribute to species resilience. Given the importance of biotic interactions, climate adaptation may need to go beyond a “one-size-fits-all” approach even when species have similar thermal tolerances.

Wisconsin↗

USGS2025

Estimating mortality of Lake Sturgeon in the Lake Winnebago system using traditional age-based approaches and capture–recapture models

Objective The Lake Winnebago system in Wisconsin supports a popular winter spear fishery for Lake Sturgeon Acipenser fulvescens . Setting harvest caps for this fishery relies on estimating instantaneous natural mortality rate ( M ), which can be done using age-based approaches or capture–recapture models that incorporate recoveries of fish with passive integrated transponder (PIT) tags or detections of fish with acoustic transmitters. Our objectives were to determine (1) if recent estimates of exploitation ( u ) have exceeded the 5% harvest cap, (2) if M and total mortality rates are similar among estimation methods that rely on age estimates or capture–recapture methods, and (3) if potential differences in mortality estimates would affect harvest caps. Methods Harvest of PIT-tagged fish was used to evaluate u from 2010 to 2019. Catch curves incorporating corrected fin ray ages were used to estimate total mortality and M for fish collected from 2010 to 2019. Capture–recapture models were used to estimate annual survival and M from detections of fish with acoustic transmitters from 2007 to 2019 and recoveries of PIT-tagged fish from 1999 to 2020. Mortality estimates were used to calculate and compare sex-specific harvest caps among estimation methods. Results Observed u did not exceed 5% for either sex between 2010 and 2019. Estimates of M varied among methods (males: M = 0.001–0.134; females: M = 0.001–0.131), with PIT-based models consistently providing the lowest and telemetry-based models providing the highest estimates. Simulations indicated that female u has limited potential to exceed 5% if M from fin ray ages or telemetry is used to set harvest caps, while PIT-based simulations showed no indication of cap exceedance. Conclusions Harvest management practices in the Lake Winnebago system appear to have kept Lake Sturgeon exploitation below the 5% harvest cap from 2010 to 2019. Capture–recapture models relying on PIT tags appear to provide the most precise approach for setting harvest caps for this fishery.

Wisconsin↗

USGS2025

Integrating acoustic telemetry research into management: successes and challenges in the Laurentian Great Lakes

In the Laurentian Great Lakes, the application of acoustic telemetry to track fish movements has evolved into an important part of multijurisdictional management. Nevertheless, barriers remain in translating telemetry research into management or conservation actions. Here, we synthesize acoustic telemetry literature within the Great Lakes basin to explore factors that have contributed to successes and failures of integrating research with the needs of decision-making processes. Collaboration between researchers and managers, facilitated by consistent opportunities for stakeholder engagement, stood out as one of the most effective means of integration. For example, 79% (95 of 127) of articles published (up to 2023) included co-authorship by both government and academic organizations. Case studies on lake sturgeon ( Acipenser fulvescens ), walleye ( Sander vitreus ), and sea lamprey ( Petromyzon marinus ) further highlight how telemetry has informed management through collaborative engagement among researchers, stakeholders, and managers, as well as ongoing challenges. By exploring facets of acoustic telemetry research and connections to conservation and fisheries concerns, we identify pathways to reduce knowledge–action gaps widely applicable within and outside of the Great Lakes.

Laurentian Great Lakes↗

USGS2025

Responses of larval fish and zooplankton to whole-lake 2,4-D herbicide treatments used to control Eurasian watermilfoil in northern Wisconsin lakes

In North America, Eurasian watermilfoil Myriophyllum spicatum is one of the most widespread non-native aquatic plant species in freshwater ecosystems. Applications of 2,4-dichlorophenoxyacetic acid (2,4-D) herbicides are often utilized to control Eurasian watermilfoil. Herbicide applications may have unintended effects on non-target organisms like zooplankton and fish, but these effects are poorly understood. Our objectives were to determine if whole-lake 2,4-D herbicide treatments (epilimnetic target rate = 0.3 ppm) used to control Eurasian watermilfoil affected (1) density, diversity, and size of zooplankton and (2) density, diversity, and growth of larval fishes in three northern Wisconsin lakes when compared to trends observed in three untreated reference lakes. Average peak concentrations of 2,4-D following applications ranged from 0.152 to 0.257 ppm. We found no statistically significant evidence that the applications influenced the zooplankton and larval fish metrics we examined, but subtle responses to the herbicides may not have been detected given the inherent variation in our response metrics. Additional replication and research are needed to assess responses to higher concentrations and repeated applications over time and to document responses across a broader range of lake types and fish communities.

Wisconsin↗

USGS2025

Reproductive contribution of lake sturgeon transferred upstream of dams on a Great Lakes tributary

Dam construction contributes to declines in the distribution and abundance of many fishes. Increasing connectivity through adult transfer can be demographically and genetically beneficial, but assessing the effects resulting from transfer can be difficult if resident fish exist upstream. Genotypes of adult and larval lake sturgeon ( Acipenser fulvescens ) were used to quantify contributions to larval recruitment from adults transferred upstream of dams on the Menominee River, USA. We evaluated whether transfer timing, sex, and adult size were associated with the odds of reproduction. Elevator transfer operations in Fall 2019, Fall 2020, and Spring 2021 resulted in 152 male and 81 female lake sturgeon transferred upstream. In 2020 and 2021, 580 and 518 larvae were genotyped. We found that 86% (201/233) of adults reproduced and 62.3% (684/1098) of offspring had transferred parents. In total, we estimated that 392 resident adults contributed to offspring production. Mixed matings accounted for 53% of offspring genotyped, increasing levels of offspring genetic diversity relative to offspring produced from resident-only matings. Transferring adults may be a viable restoration alternative for other iteroparous fish in river systems where connectivity to spawning areas has been impeded.

Michigan, Wisconsin↗

USGS2024

A comparison of survival and behavior of lake whitefish following transmitter implantation using electro- or chemical immobilization

Background The number of telemetry studies focused on lake whitefish ( Coregonus clupeaformis ) in the Laurentian Great Lakes has steadily increased over the last decade, but field tests of immobilization methods used for tag implantation, which have the potential to affect survival and behavior of fish after release, are lacking. We compared post-tagging survival and behavior of lake whitefish that were immobilized for tag implantation using electroimmobilization via a transcutaneous electrical nerve stimulation (TENS) unit or by chemical immobilization via exposure to 10% eugenol. Results Acoustic tags were implanted into 126 adult lake whitefish ( N = 126; N = 67 TENS treatment group, N = 59 eugenol treatment group) collected from the Fox River, Wisconsin, during the spawning period in November 2021. We found no significant differences between treatments in the number of days that lake whitefish spent in the Fox River following tagging (TENS mean = 13.4 days, eugenol mean = 14.7), and also found that the proportions of fish within each treatment group that returned to the Fox River during fall 2022 (51% from TENS treatment group, 49% from eugenol treatment group) did not differ from the proportions for all fish that were confirmed to be alive at that time. The best Cormack–Jolly–Seber model indicated no differences in survival between the two treatment groups (monthly survival = 0.980, 95% CI 0.970–0.987). Fish immobilized using TENS underwent almost immediate induction and recovery from surgeries, while fish immobilized using eugenol had induction times that ranged 167–487 s (mean = 347 s) and recovery times that ranged 51–2358 s (mean = 1242 s). Conclusions Short- and long-term behavior (time to exit of Fox River, return to Fox River in the next spawning season) and monthly survival estimates of lake whitefish did not differ between the immobilization treatments. Either method may be suitable for immobilization during tag implantation, but the additional time needed for induction and recovery of fish when using eugenol may be a limiting factor in some field-based tagging situations.

Wisconsin↗

USGS2024

Use of vehicle counters to index and evaluate potential shifts in angler effort following implementation of more restrictive panfish regulations in Wisconsin lakes

Objective: Understanding angler responses to fisheries management actions such as regulation changes have important implications for the effectiveness and efficacy of such management strategies. We examined the ability of remote vehicle counters to provide a relative index of angler effort and present a case study demonstrating use of vehicle counters to assess potential changes in angler effort associated with implementation of more restrictive panfish regulations in a subset of Wisconsin lakes. Methods: We compared vehicle counts with compulsory creel and game-camera based estimates of angler hours and number of angling parties. During 2016, a series of more restrictive panfish regulations were implemented across 132 Wisconsin lakes. We deployed vehicle counters at a subset of lakes within each regulation type during pre- (2015 and 2016) and post-regulation time periods (2021 and 2022) to assess whether the distribution of angler effort (as indexed using vehicle counters) among regulation types may have shifted in response to regulation implementation. Result: At lakes with paired vehicle counters and compulsory creel data, vehicle counts explained 57-72% of variation in daily angler effort (h) and 65-85% of variation in daily number of angling parties. Across lakes with paired vehicle counters and game cameras, vehicle counters explained 77% of variation in the number of apparent angling parties; however, effectiveness of vehicle counters for explaining variation in number of apparent angling parties varied among lakes. Vehicle counters explained 63-74% of variation in number of apparent angling parties when considering paired vehicle counter-game camera observations pooled within regulation types. We did not observe any systematic shifts in effort indicative of redistribution of angler effort in response to panfish regulations. Conclusion: Results suggest that, given appropriate validation measures, vehicle counters could be used as a cost-effective tool to index angler effort, particularly when interest lies in understanding effort dynamics over large spatial scales. Our findings suggest a localized scale for implementation of specialized regulations may be appropriate given angler behaviors and preferences for Wisconsin panfish.

Wisconsin↗

USGS2024

Comparing conventional tagging methods and acoustic telemetry to inform management of Lake Whitefish in Lake Michigan

Objective Studies of fish movement using conventional tags or acoustic telemetry have different benefits and biases that can influence how conclusions are used in a management context. Our objective was to determine whether these two methods provided similar inferences regarding movements and spawning site fidelity of Lake Whitefish Coregonus clupeaformis in Lake Michigan. Additionally, we assessed movement patterns and used telemetry to assess residency time of Lake Whitefish to provide managers with information on which stocks might be exposed to harvest in different regions. Methods Lake Whitefish were tagged during spawning in (1) North and Moonlight bays, (2) Big Bay de Noc, (3) the Menominee River, and (4) the Fox River. Proportions of fish moving between southern Green Bay, northern Green Bay, and Lake Michigan were compared between tag types. Spawning site fidelity was estimated for each tagging site. Seasonal residency indices were calculated using acoustic telemetry detections. Result Estimates differed between the two methods, but overall trends were similar. Fox River fish rarely left southern Green Bay, and fish tagged in North and Moonlight bays rarely entered Green Bay (<10% of individuals). Big Bay de Noc and Menominee River fish moved into other regions more often (>50% of individuals). The residency indices indicated that Big Bay de Noc fish spent most of their time in Lake Michigan while Menominee River fish spent little time in northern Green Bay despite transitioning to the region. Compared to telemetry, conventional tag recoveries underestimated the proportion of individuals moving among regions. Spawning site fidelity estimates (28–100%) varied among tagging groups and between methods. Conclusion Our results suggest that data from conventional tags can inform management at broad geographic scales. However, acoustic telemetry can provide fine-scale information. Information gained from telemetry can be useful in understanding exposure to fishing mortality, which may be valuable for informing management decisions.

Lake Michigan↗

USGS2024

Hatch timing of largemouth bass: Implications for recruitment at the northern edge of their native range

Climate-related shifts in hatch timing could mean that age-0 largemouth bass Micropterus salmoides in north temperate lakes reach larger sizes and experience higher survival, which may contribute to increased prevalence of this species in many lakes. However, information on age-0 largemouth bass life history is lacking for these lakes. We estimated hatch dates, daily growth rates (DGR), and length-based probability of piscivory of age-0 largemouth bass in Wisconsin lakes to: (1) provide baseline data on early life history; (2) compare hatch timing between years with different spring temperature regimes, and (3) project how shifts in hatch timing might influence total length (TL) distributions of age-0 bass if mean DGR remained constant. Most age-0 largemouth bass (>90%) collected from ten Wisconsin lakes ( N = 746) hatched between 23 May and 24 June during 2012 and 2013 and did not switch to piscivory during their first summer. Prevalence of positive correlations (16 of 24 lake-year pairings) between TL in and age indicates hatch timing may influence TLs attained by age-0 largemouth bass in August in some lake-years. Mean daily water temperatures in May 2012 were 3.1 °C warmer than in May 2013 for the five of the six lakes where hatch dates were estimated in both years. On average, earliest and median hatch dates for age-0 largemouth bass in these six lakes were 10–11 d earlier in 2012 than in 2013 and hatch duration was approximately 8 d longer in 2012. Despite differences in hatch timing, mean DGR was relatively consistent (range = 0.61–0.74 mm/d) between lakes and years. Simple simulations suggest that earlier hatch dates could result in more age-0 largemouth bass reaching TLs associated with piscivory and overwinter survival by the end of their first summer if growth rates did not change, which could translate to higher recruitment.

Wisconsin↗

USGS2024

Assessing factors related to Walleye stocking success in the Midwestern United States

Objective The objective of this study was to evaluate stocking success of Walleye Sander vitreus in lakes and reservoirs across the Midwestern United States to inform stocking practices for state agencies. Demand for Walleye stocking may increase if climate change limits the potential for natural recruitment in lakes. Consequently, the strategic distribution of Walleye stocking may maximize fishing opportunities. Methods We synthesized data from 2226 Walleye fry and fingerling stocking events on 653 lakes in the Midwestern United States and used random forest algorithms and mixed-effects linear models to identify abiotic and biotic factors related to Walleye stocking success. Result Latitude and year explained relatively little variation in stocking success compared to within-lake variation. Relative abundance of Largemouth Bass Micropterus nigricans was an important indicator of Walleye stocking success for fry and fingerlings, with stocking success generally decreasing with increased bass abundance. There was an interaction between lake surface area and growing degree-days, as large lakes (>2500 ha) seemed to be more conducive to Walleye stocking success regardless of growing degree-days. The models that we developed did not accurately predict exact levels of Walleye stocking success but were 92–94% accurate in predicting whether the stocking success of both fry and fingerlings would be at or above the 50th percentile. Conclusion These findings may help to inform the management and stocking allocation of Walleye and suggest that future increases in Largemouth Bass abundance and growing degree-days could limit the effectiveness of stocking in some lakes.

Illinois, Indiana, Iowa, Michigan, Minnesota, Nebr↗

USGS2024

Understanding the role of recreational angling technology in angler expectations of catch, trip catch, and angler satisfaction

Rapid technological advancement often receives a mix of criticism and welcome implementation. Fishing technologies, such as sonar, are believed to enable anglers to be more efficient and effective in their angling. There are concerns from anglers and managers of increased catch by technology users. We assessed the relationships between technology use—defined as the use of imaging technology such as sonar and underwater cameras—and catch, angler expectations of catch, and trip satisfaction using a dual intercept creel survey. Angling technologies were used by 80% and 79% of intercepted boat and ice anglers, respectively, but only 3.9% of shore anglers. Fishing technologies increased expected catch for game fish anglers, but not panfish anglers, and had no effect on actual catch for either group. Most anglers caught fewer fish than expected, and technology did not improve their ability to meet expectations. Technology use was associated with decreased overall satisfaction among panfish and game fish anglers. These results suggest that concerns about fishing technology increasing catch may not be warranted. Rather, technology use may affect angler expectations and negatively impact angler satisfaction, potentially influencing angler behavior.

Wisconsin↗

USGS2024

Quantifying the relative importance of biotic and abiotic factors in landscape-based models of stream fish distributions

Lotic fish species distributions are frequently predicted using remotely sensed habitat variables that characterize the adjacent landscape and serve as proxies for instream habitat. Recent advancements in statistical methodology, however, allow for leveraging fish assemblage data when predicting distributions. This is important because assemblage composition likely provides better information about instream habitat compared to landscape-derived metrics and therefore may improve predictions. To better understand the value of using multi-species fish data in species distribution modeling, we fit two conditional random fields (CRF) models to quantify the relative importance of fish assemblage co-occurrence, landscape-derived habitat variables, and interactions between these two predictor groups (i.e., effects of co-occurrence could be context-dependent) at over 1200 stream catchments in Pennsylvania, USA. We first compared predictive performance of CRF models against traditionally used single-species logistic regressions (generalized linear models; GLMs) and found that inclusion of fish assemblage data often improved predictive performance. The multi-species CRF models performed significantly better at predicting occurrence for 63% of species with an average percent increase in AUC of 25% compared to GLMs. Furthermore, the CRF identified species co-occurrences as more informative, and thus relatively more important, at predicting occurrence than the other effect types. The CRF also suggested that allowing these biotic effects to be context-dependent was important for predicting occurrence of many species. These findings illustrate the value of fish assemblage data for landscape-scale species distribution modeling and leveraging this information can improve predictions and inferences to help inform the management and conservation of freshwater fishes.

Pennsylvania↗

USGS2024

Demographic patterns of walleye (Sander vitreus) reproductive success in a Wisconsin population

Harvest in walleye Sander vitreus fisheries is size-selective and could influence phenotypic traits of spawners; however, contributions of individual spawners to recruitment are unknown. We used parentage analyses using single nucleotide polymorphisms to test whether parental traits were related to the probability of offspring survival in Escanaba Lake, Wisconsin. From 2017 to 2020, 1339 adults and 1138 juveniles were genotyped and 66% of the offspring were assigned to at least one parent. Logistic regression indicated the probability of reproductive success (survival of age-0 to first fall) was positively (but weakly) related to total length and growth rate in females, but not age. No traits analyzed were related to reproductive success for males. Our analysis identified the model with the predictors' growth rate and year for females and the models with year and age and year for males as the most likely models to explain variation in reproductive success. Our findings indicate that interannual variation (i.e., environmental conditions) likely plays a key role in determining the probability of reproductive success in this population and provide limited support that female age, length, and growth rate influence recruitment.

Wisconsin↗

USGS2024

Estimating internal transmitter and external tag retention by Walleye in the Laurentian Great Lakes over multiple years

Objective Both electronic tags (e.g., acoustic and radio transmitters) and conventional external tags are used to evaluate movement and population dynamics of fish. External tags are also sometimes used to facilitate the recovery of internal electronic tags or other instrumentation because healing can make it difficult to identify fish with internal tags based on appearance alone. With both tag types, tag shedding and failure of electronic tags can affect accuracy and precision of study results. Methods We used a decade (2011–2021) of recapture data for Walleye Sander vitreus tagged in the Laurentian Great Lakes, where fish were double- or triple-tagged with external tags (T-bar, loop, or internal anchor tags) and internal acoustic transmitters, to quantify external tag and internal transmitter shedding and transmitter failure rates. Result In total, 1125 (33%) Walleye were recovered that had retained at least one external tag or internal transmitter. No confirmed cases of transmitter shedding were observed; 15 of 899 transmitters (2%) that were checked for functionality failed prior to the expected battery expiration. The retention of external T-bar tags 1 year after release differed depending on whether the tag was placed anterior or posterior to the secondary dorsal fin (anterior, fish length = 420 mm: 73% retention; anterior, fish length = 700 mm: 73%, posterior: 63%) but was <26% after 4 years for both tag positions and fish sizes. Internal anchor tags had an 88% 1-year retention probability and 81% 4-year retention probability. Loop tags had the highest 1-year retention (89%) but after 4 years retention (28–34% depending on agency) was comparable to that of T-bar tags. Conclusion Better understanding of tag retention characteristics through long-term tagging studies such as this can inform study design, be considered in model design, and ultimately improve inferences from mark–recapture studies.

North American Journal of Fisheries Management↗

USGS2023

The incredible HALK: borrowing data for age assignment

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 and mortality metrics to those estimated from lake–year age–length keys ages using seven common freshwater fish species across the upper Midwestern United States. Levels used for data borrowing began most specifically by borrowing within lake across time and increased in breadth to include data within the Hydrologic Unit Code (HUC) 10 watershed, HUC8 watershed, Level III Ecoregion, and finally a species-wide data ALK using all available data with our study for a species. Median deviation in mean length of age-3 fish was within 1 cm for the most specific HALK levels, and median deviation in total annual mortality was close to 0 for most species when borrowing occurred within HUC10 and HUC8 watersheds. Percent error in growth curves increased with data borrowing, but plateaued—or even decreased—for some species when data borrowing expanded across spatial levels. We present the HALK as a method for gaining age information about a fishery when age data are unavailable.

Illinois, Indiana, Iowa, Michigan, Minnesota, Sout↗