Walleye Populations Research Articles

Improving fishery-independent indices of abundance for a migratory walleye population

The primary goal of many biological surveys is to provide an unbiased representation of trends of population abundance. However, there are often factors other than abundance that vary over time and influence catch rates and thus inferences about population trends from surveys. This is particularly true for highly mobile species because of interannual variation in the timing, extent, and duration of movements, and for surveys that are not randomized with respect to space and time. We developed general and generalized linear mixed models to standardize Canadian and United States fishery-independent surveys that provide an index of basin-level walleye (Sander vitreus) population abundance trends in Lake Erie (1983–2008). In Canadian waters, the probability of a non-zero catch was associated with the type of gill net set (canned>bottom sets), the presence of hypoxia (negative trend; −), and secchi depth (−). Positive catch rates were associated with the set type (canned>bottom) and water depth (+). In United States waters, survey catch rates were associated with secchi depth (−) and surface water temperature (+). For each case, the best model included random effects (interactions between year, week, basin, sub-basin) which accounted for a modest amount of the total variation. General abundance trends were similar between the standardized and nominal indices, but substantial annual variation in the direction and magnitude of the difference between indices was observed. We recommend the use of standardized indices for walleye population assessments because these account for factors influencing catch rates other than changes in abundance.

Simulated Equilibrium Walleye Population Density under Static and Dynamic Recreational Angling Effort

AbstractUnderstanding the dynamics of angling effort and how fish populations are affected by angler effort is an important and often unappreciated component of fisheries management. Our objective was to determine the extent to which angling‐induced mortality limits walleye Sander vitreus population density in northern Wisconsin lakes. We developed a simulation model to evaluate the effects on long‐term equilibrium walleye population density caused by effort limitations, density‐dependent angling effort, and angling regulations. Equilibrium density was limited by angling effort when effort was held constant but was generally lower when effort was density dependent. Equilibrium density ranged from 3.46 to 21.79 adults/ha when effort was density dependent, similar to the observed median density in lakes sustained by natural reproduction (8.7 adults/ha). Median equilibrium density was 23.3 adults/ha when effort was zero. Equilibrium density was higher when harvest was regulated by a minimum length limit as opposed to no length limit and with no postrelease mortality than with postrelease mortality. Population collapse was more likely and equilibrium density was lower when the population density and effort/ha relationship was strongest. Because the observed relationship between adult walleye population density and effort/ha was weaker, we conclude that open‐access walleye fisheries in northern Wisconsin are generally self‐regulating. However, populations likely persist at lower population densities than would be expected if angling effort were limited to lower levels than those currently observed. Our results are intuitive; however, they suggest that density increases may be on the order of 100% when effort is severely restricted compared with open‐access fisheries with unlimited effort. Other fish species may experience different expected density increases with restricted angler effort depending on a number of factors, but our results suggest that population density changes can be dramatic and that similar modeling exercises may be useful to managers of other fish species if increasing density is of interest.Received December 19, 2011; accepted June 13, 2012

Bioaccumulation factors and the steady state assumption for cesium isotopes in aquatic foodwebs near nuclear facilities

Steady state approaches, such as transfer coefficients or bioaccumulation factors, are commonly used to model the bioaccumulation of 137Cs in aquatic foodwebs from routine operations and releases from nuclear generating stations and other nuclear facilities. Routine releases from nuclear generating stations and facilities, however, often consist of pulses as liquid waste is stored, analyzed to ensure regulatory compliance and then released. The effect of repeated pulse releases on the steady state assumption inherent in the bioaccumulation factor approach has not been evaluated. In this study, I examine the steady state assumption for aquatic biota by analyzing data for two cesium isotopes in the same biota, one isotope in steady state (stable 133Cs) from geologic sources and the other released in pulses (137Cs) from reactor operations. I also compare 137Cs bioaccumulation factors for similar upstream populations from the same system exposed solely to weapon test 137Cs, and assumed to be in steady state. The steady state assumption appears to be valid for small organisms at lower trophic levels (zooplankton, rainbow smelt and 0+ yellow perch) but not for older and larger fish at higher trophic levels (walleye). Attempts to account for previous exposure and retention through a biokinetics approach had a similar effect on steady state, upstream and non-steady state, downstream populations of walleye, but were ineffective in explaining the more or less constant deviation between fish with steady state exposures and non-steady state exposures of about 2-fold for all age classes of walleye. These results suggest that for large, piscivorous fish, repeated exposure to short duration, pulse releases leads to much higher 137Cs BAFs than expected from 133Cs BAFs for the same fish or 137Cs BAFs for similar populations in the same system not impacted by reactor releases. These results suggest that the steady state approach should be used with caution in any situation where reactor releases are episodic or pulse in nature, even if the magnitude of these releases is small.

Hydrogeographic Vicariance Determines the Genetic Structure of Northwestern Walleye Populations

AbstractWalleye Sander vitreus is one of the most important freshwater commercial and sport fishes in western Canada. It is an intensively managed species that is under considerable harvest pressure, yet little is known about the patterns of genetic variation and diversity of walleyes between connected water bodies and among river basins in this region. We examined the genetic variation of walleyes from 12 lakes in five different river basins of northern Alberta. Each lake contained a genetically distinct walleye subpopulation nested within a larger population of the river basin in which the lake was situated. Differentiation between subpopulations varied (FST = 0.05–0.29) and exhibited a broad‐scale isolation‐by‐distance pattern. Patterns of genetic divergence aligned closely with the current hydrogeographical landscape, as subpopulations in the same river basin were more similar than those in different river basins. The clear and distinct pattern of genetic structure is likely to have been generated and maintained by historical vicariance and natal philopatry. Because walleye populations are so clearly genetically structured by hydrogeography in western Canada, these data can be used to monitor population status, assess stocking programs, delineate management units, and enable forensic enforcement of harvest restrictions in this region.

Reproductive allocation in exploited lake whitefish (Coregonus clupeaformis) and walleye (Sander vitreus) populations

We examined age-related changes in energetic status and reproductive effort of lake whitefish (Coregonus clupeaformis) and walleye (Sander vitreus) populations in Lakes Winnipeg and Ontario. We predicted, based on life history theory, that both species would exhibit declines in somatic energy stores and increases in the proportion of their energy reserves allocated to reproduction as they aged. These predicted trends were evident for walleye but not for lake whitefish, and the contrast between species was most pronounced for females. Walleye devoted an increasingly larger proportion of their body lipids to gonad development as they got older whereas lake whitefish devoted an increasingly smaller proportion. Between-species differences for some reproductive traits, particularly gonad size (GSI) and relative fecundity, were not consistent between the two lakes. Lake whitefish follow a strategy of lower relative fecundity, larger eggs, and lower egg lipid content compared to walleye. Differences between these species in the gamete quantity and quality of different age classes suggest that age-structured population models parameterized for one species should be applied to other species with caution.

Historical and Anthropogenic Factors Affecting the Population Genetic Structure of Ontario’s Inland Lake Populations of Walleye (Sander vitreus)

Populations existing in formerly glaciated areas often display composite historical and contemporary patterns of genetic structure. For Canadian freshwater fishes, population genetic structure is largely reflective of dispersal from glacial refugia and isolation within drainage basins across a range of scales. Enhancement of sport fisheries via hatchery stocking programs and other means has the potential to alter signatures of natural evolutionary processes. Using 11 microsatellite loci genotyped from 2182 individuals, we analyzed the genetic structure of 46 inland lake walleye (Sander vitreus) populations spanning five major drainage basins within the province of Ontario, Canada. Population genetic analyses coupled with genotype assignment allowed us to: 1) characterize broad- and fine-scale genetic structure among Ontario walleye populations; and 2) determine if the observed population divergence is primarily due to natural or historical processes, or recent anthropogenic events. The partitioning of genetic variation revealed higher genetic divergence among lakes than among drainage basins or proposed ancestries-indicative of relatively high isolation among lakes, study-wide. Walleye genotypes were clustered into three major groups, likely reflective of Missourian, Mississippian, and Atlantic glacial refugial ancestry. Despite detectable genetic signatures indicative of anthropogenic influences, province-wide spatial genetic structure remains consistent with the hypothesis of dispersal from distinct glacial refugia and subsequent isolation of lakes within primary drainage basins. Our results provide a novel example of minimal impacts from fishery enhancement to the broad-scale genetic structure of inland fish populations.

Assessing the Effects of Historical Exposure to Endocrine-Active Compounds on Reproductive Health and Genetic Diversity in Walleye, a Native Apex Predator, in a Large Riverine System

In this combined field and laboratory study, we assessed whether populations of native walleye in the Upper Mississippi River experienced altered genetic diversity correlated with exposure to estrogenic endocrine-active compounds (EACs). We collected fin-clips for genetic analysis from almost 600 walleye (13 sites) and subsampled 377 of these fish (6 sites) for blood and reproductive organs. Finally, we caged male fathead minnows at 5 sampling sites to confirm the presence of estrogenic EACs. Our findings indicate that male walleye in four river segments produced measurable concentrations of plasma vitellogenin (an egg-yolk protein and, when expressed in male fish, a biomarker of acute estrogenic exposure), a finding consistent with the presence of estrogenic EACs and consistent with published historical data for at least three of these study sites (Grand Rapids, St. Paul, and Lake City on Lake Pepin). Patterns of vitellogenin induction were consistent for native walleye and caged fathead minnows. No widespread occurrence of histopathological changes, such as intersex was found compared with published reports of intersex at the furthest downstream study site. To assess possible effects of estrogenic exposure on the genetic diversity of walleye populations at the study sites, we DNA-fingerprinted individual fish using 10 microsatellite loci. Genetic differences were observed between populations; however, these differences were consistent with geographic distance between populations, with the largest observed difference in genetic diversity found between fish upstream and downstream of St. Anthony Falls (and/or Lock and Dam 1 of the Mississippi River), traditionally a historical barrier to upstream fish movement. Although the persistent occurrence of endocrine disruption in wild fish populations is troubling, we did not detect degradation of reproductive organs in individual walleye or alteration in genetic diversity of walleye populations.

Analysis of Compensatory Responses in Land‐Locked Alewives to Walleye Predation: A Tale of Two Lakes

AbstractWe compared one established population (Cayuta Lake) and one expanding population (Canadarago Lake) of landlocked alewives Alosa pseudoharengus to evaluate compensatory responses of alewives to predation by walleyes Sander vitreus. Alewives have been present in Cayuta Lake since at least 1977, and during the years 1995 to 2009 the population ranged from 24,470 fish/ha in 2000 to 3,800 fish/ha in 2007. Alewives were first observed in Canadarago Lake during 1999, and the population remained at low densities (<26 fish/ha) until reaching 370 fish/ha in 2006 and 1,050 fish/ha in 2009. Differences in zooplankton reflected higher planktivory rates in Cayuta Lake than in Canadarago Lake. Density of age‐3 and older walleyes was higher in Canadarago Lake (21–24 fish/ha) than in Cayuta Lake (12–14 fish/ha), despite elevated stocking rates of fingerling walleyes in Cayuta Lake during 2002–2006. Alewife density explained 77–84% of the variation in alewife length at age and 48–84% of the variation in condition (ages 0, 1, and 2) among years and lakes. Alewife length at age under low densities in Canadarago Lake was similar to observations in anadromous populations. Alewife recruitment (number of fall age‐0 recruits/spawner) was higher in Canadarago Lake than in Cayuta Lake. The combined data yielded a Beverton–Holt stock–recruitment curve that rose rapidly at low alewife densities, and 50% of the maximum recruitment was predicted to occur at 400 spawners/ha. Compensatory responses of alewives at low densities include decreased cannibalism and increased growth rates of both adults and age‐0 fish. Compensatory responses should be considered when evaluating the piscivore levels needed to control alewife populations or, conversely, when evaluating the risk of alewife population collapse in response to increased predation rates. Although alewife abundance declined in response to the experimental walleye stocking in Cayuta Lake, we were unable to increase the walleye population sufficiently to decrease alewife recruitment.

Selectivity Evaluation for Two Experimental Gill‐Net Configurations Used to Sample Lake Erie Walleyes

AbstractWe used length frequencies of captured walleyes Sander vitreus to indirectly estimate and compare selectivity between two experimental gill‐net configurations used to sample fish in Lake Erie: (1) a multifilament configuration currently used by the Ohio Department of Natural Resources (ODNR) with stretched‐measure mesh sizes ranging from 51 to 127 mm and a constant filament diameter (0.37 mm); and (2) a monofilament configuration with mesh sizes ranging from 38 to 178 mm and varying filament diameter (range = 0.20–0.33 mm). Paired sampling with the two configurations revealed that the catch of walleyes smaller than 250 mm and larger than 600 mm was greater in the monofilament configuration than in the multifilament configuration, but the catch of 250–600‐mm fish was greater in the multifilament configuration. Binormal selectivity functions yielded the best fit to observed walleye catches for both gill‐net configurations based on model deviances. Incorporation of deviation terms in the binormal selectivity functions (i.e., to relax the assumption of geometric similarity) further improved the fit to observed catches. The final fitted selectivity functions produced results similar to those from the length‐based catch comparisons: the monofilament configuration had greater selectivity for small and large walleyes and the multifilament configuration had greater selectivity for mid‐sized walleyes. Computer simulations that incorporated the fitted binormal selectivity functions indicated that both nets were likely to result in some bias in age composition estimates and that the degree of bias would ultimately be determined by the underlying condition, mortality rate, and growth rate of the Lake Erie walleye population. Before the ODNR switches its survey gear, additional comparisons of the different gill‐net configurations, such as fishing the net pairs across a greater range of depths and at more locations in the lake, should be conducted to maintain congruence in the fishery‐independent survey time series.Received January 5, 2011; accepted May 13, 2011

Empirical assessment of potential interactions between walleye and smallmouth bass

Summary The expansion of smallmouth bass Micropterus dolomieu throughout North America has created substantial changes in some food webs and created novel interactions with native predators in many systems through competition and predation, causing declines in native predator populations. In this study, we examined potential interactions between smallmouth bass (introduced) and walleye Sander vitreus (a native predator) in two Missouri River reservoirs and four natural lakes in South Dakota, USA, using historical standardized sampling data, which indexed four fitness-related factors (relative abundance, size structure, condition, and growth) of both species. Of the 24 total relationships tested among all water bodies, three (13%) were determined to be significant (size structure in Pickerel Lake: P = 0.06; condition in Lake Kampeska: P = 0.06, and growth at Brant Lake: P = 0.02). Of these three, two relationships were negative as would be expected if negative interactions exist between these two species (size structure in Pickerel Lake: r = )0.61; condition in Lake Kampeska: r = )0.61); the remaining relationship was positive (growth at Brant Lake: r = 0.75). Further, no water body consistently demonstrated significant negative interactions in all relationships tested. The results of our study did not support the concept that introduced smallmouth bass interfere with walleye populations in South Dakota waters. Rather, it is more likely that other environmental factors influence the observed variability in walleye demographics and dynamics observed in this study. At this time, we believe that both fisheries will continue to co-exist. However, continuous monitoring of both species is warranted as changing environmental conditions in the future could allow these interspecific relationships to shift toward the dominance of smallmouth bass over walleye.

Microsatellite and mitochondrial DNA markers show no evidence of population structure in walleye (Sander vitreus) in Lake Winnipeg

Walleye (Sander vitreus) are the main contributor to Manitoba's multi-million dollar commercial fishery, with Lake Winnipeg providing the largest and most profitable catch. In some systems, walleye show evidence of returning to the same spawning site each spring (i.e., philopatry), which can lead to the existence of multiple genetically-discrete spawning groups (i.e., stocks) within a lake. This study used nine microsatellite loci (N=932) and one mitochondrial restriction fragment length polymorphism assay (N=123) to investigate possible genetic divergence among walleye from 13 spawning sites in Lake Winnipeg, and compared these with those collected from two hatchery locations and other locations outside of Lake Winnipeg. Microsatellite loci resolved little population structure within Lake Winnipeg, with the exception of two sites (Grand Rapids and the Icelandic River), which showed low but significant levels of genetic divergence with most other sites (Fst=0.0066–0.0457). Slight to moderate genetic divergence was found between Lake Winnipeg walleye and sampling locations outside of Lake Winnipeg (Fst=0.0037–0.1054, P=0.0002). Mitochondrial DNA analysis indicated colonization of Lake Winnipeg by walleye from two glacial refugia (Missourian and Mississippian), but no population structure was detected among sites. The lack of stock structure found in Lake Winnipeg possibly indicates a low degree of natal philopatry, an amount of straying sufficient to obscure genetic structure, or obscured structure as a result of historical and current stocking. Overall genetic variation, using both mitochondrial and microsatellite markers, appeared comparable to that found in other walleye populations.

Genetic differentiation between the blue and the yellow phenotypes of walleye (Sander vitreus): An example of parallel evolution

Phenotypic change in closely related lineages frequently follows a common pathway in response to similar environmental conditions. This process, termed parallel evolution, results in the evolution of similar but independently derived ecotypes. Individuals with a blue phenotype are observed in some populations of walleye (Sander vitreus) from Laurentian Shield lakes. The blue phenotype may represent a case of parallel evolution: previous morphological studies revealed significant differences compared with the yellow phenotype as well as numerous lake-specific characteristics among blue individuals. The genetic variability of blue and yellow walleye phenotypes from 6 Laurentian Shield lakes was estimated using AFLP. Results clearly indicate that, whatever their phenotype, individuals from a given lake were more similar genetically than those from other lakes. However, blue and yellow phenotypes represent different populations within each lake. These results suggest the colonization of each lake by a single group of walleye followed by the parallel origin of the blue phenotype. The parallel evolution of the blue walleye phenotype could represent an excellent model for studying mechanisms underlying divergent selection and reproductive isolation.

Resolving Some of the Complexity of a Mixed‐Origin Walleye Population in the East Basin of Lake Erie Using a Mark–Recapture Study

AbstractAt least two genetically distinct populations of walleyeSander vitreusreproduce in Lake Erie: one west‐basin‐origin population and one east‐basin‐origin population. Each year, some west‐basin‐origin walleyes migrate to the east basin and create a mixed‐origin walleye population. Uncertainties associated with this migratory behavior make it difficult to describe the dynamics of the east‐basin‐origin population. We used mark–recapture analysis to estimate the dynamics of the east‐basin‐origin walleye population and to measure the contribution of west‐basin‐origin walleyes to the total walleye harvest in the east basin. Compared with the west‐basin‐origin walleyes, the east‐basin‐origin walleyes experienced lower fishing pressure, lower natural mortality, and a higher survival rate. On average, the west‐basin‐origin walleye migrants comprised about 90% of the annual harvest in the east basin. The number of the west‐basin‐origin walleyes migrating to the east basin was linearly related to their abundance. Walleyes showed a strong fidelity to their spawning sites. This study provided an approach to the assessment of population dynamics and the management of walleye fisheries in the east basin of Lake Erie.Received June 7, 2010; accepted December 30, 2010

Exploitation of walleye in a Great Plains reservoir: harvest patterns and management scenarios

Abstract This study assessed exploitation and evaluated management options for walleye, Sander vitreus (Mitchill), in Glen Elder Reservoir, Kansas. A total of 2429 walleye varying from 240 to 687 mm was tagged with Carlin dangler tags during 2000–2003. After correcting for tag loss and non‐reporting, exploitation of walleye was estimated as 68.3%. More than 85% of the tagged walleye were harvested during April to June and 75% were harvested within 6 months after tagging. A Beverton‐Holt yield‐per‐recruit model was used to evaluate six minimum length limits varying from 250 to 610 mm. Given current exploitation rates, population demographics and harvest regulations (381‐mm minimum length limit), the walleye population is probably experiencing recruitment overfishing. Model results suggested that a 610‐mm minimum length limit would be required to prevent growth overfishing and a 533‐mm or longer minimum length limit would prevent recruitment overfishing.

Population genetics of walleye and yellow perch in the St. Marys River

We assessed the genetic diversity of walleye ( Sander vitreus) and yellow perch ( Perca flavescens) in the St. Marys River system. Walleye samples were collected from four current spawning populations, one historic population, and the two sources used for stocking the St. Marys River. Non-spawning perch were collected from ten sites along the St. Marys River. Five microsatellite loci were used for screening each species to create a genetic baseline to which future populations can be compared and to assess the impacts of past stocking on the walleye populations. Resulting allele and genotype frequencies were characterized to quantify genetic variation and differentiation of the respective populations. Both the walleye and yellow perch populations in the SMR exhibited some degree of population structure. Although stocking has modified the genetic diversity of walleye, there is still at least one unique population (Bar River, Ontario) remaining in the St. Marys River system. Both species had unique populations in Lake George, suggesting that this area may have been isolated in the recent past and should be protected from future stocking events to preserve this uniqueness. Further work is encouraged to understand the nature of this possible genetic isolation.

Abundance, growth, and life history characteristics of sympatric walleye (Sander vitreus) and sauger (Sander canadensis) in Lake Winnipeg, Manitoba

Walleye (Sander vitreus) and sauger (Sander canadensis) are the two most economically valuable and sought-after species in the Lake Winnipeg fishery, the second largest freshwater commercial fishery in Canada. We compared recent temporal trends (1979–2003) in population and life history characteristics of these species across the three main management zones of the lake (north basin, channel region, and south basin). Walleye abundance (gill net catch-per-unit-effort) increased relative to sauger abundance, particularly in the north and south basins, and the ages and sizes of the sampled fish exhibited greater spatial and temporal variabilities in sauger than in walleye. Walleye of the south basin and channel region exhibit a bimodal growth pattern, previously unreported for walleye populations. Growth rates of both walleye and sauger increased, and ages and sizes of maturity generally decreased from 1979 to 2003. However, walleye showed much greater flexibility in these traits, both spatially and temporally, than sauger. Sauger utilize a reproductive strategy of younger age and smaller size at maturity and higher fecundity with smaller, more lipid-rich eggs relative to walleye. Recent environmental and/or harvesting conditions on the lake appear to favour walleye over sauger, and differences in their life histories could make the sauger population more vulnerable to the impacts of commercial harvest than the walleye population.

Consistent patterns of maturity and density-dependent growth among populations of walleye (Sander vitreus): application of the growing degree-day metric

Growing degree-days (GDD, °C·days) are an index of ambient thermal energy that relates directly to an ectotherm’s cumulative metabolism but is rarely used to describe growth and development in fish. We applied GDD to length and maturity data from 416 populations of walleye ( Sander vitreus ) from Ontario and Quebec, Canada (mean annual GDD = 1200 to 2300 °C·days). On average, males matured after they had experienced 6900 °C·days and reached 350 mm total length (L) (n = 77 populations), and females matured after 10 000 °C·days and at 450 mm L (n = 70). Across 143 populations, GDD accounted for up to 96% of the variation in the length of immature walleye but also revealed a twofold difference in growth rate that was indicative of variation in food availability. When applied to data from eight populations in which walleye abundances have changed dramatically over time, GDD revealed a 1.3-fold increase in immature growth rate when abundance was low compared with when it was high. Our results both demonstrate the explanatory power of GDD with respect to fish growth and maturity and inform the development of regional management strategies for walleye.

Maternal influences on population dynamics: evidence from an exploited freshwater fish

We used a field experiment, population modeling, and an analysis of 30 years of data from walleye (Sander vitreus; a freshwater fish) in Lake Erie to show that maternal influences on offspring survival can affect population dynamics. We first demonstrate experimentally that the survival of juvenile walleye increases with egg size (and, to a lesser degree, female energy reserves). Because egg size in this species tends to increase with maternal age, we then model these maternal influences on offspring survival as a function of maternal age to show that adult age structure can affect the maximum rate at which a population can produce new adults. Consistent with this hypothesis, we present empirical evidence that the maximum reproductive rate of an exploited population of walleye was approximately twice as high when older females were abundant as compared to when they were relatively scarce. Taken together, these results indicate that age- or size-based maternal influences on offspring survival can be an important mechanism driving population dynamics and that exploited populations could benefit from management strategies that protect, rather than target, reproductively valuable individuals.

Assessing Length‐Related Bias and the Need for Data Standardization in the Development of Standard Weight Equations

AbstractThe recently developed empirical percentile (EmP) method, a technique for deriving standard weight (Ws) equations, putatively reduces the length‐related biases that often plague such equations. To determine whether the EmP method is superior to the regression line–percentile (RLP) method in reducing length‐related biases, we developed new Ws equations by applying both methods to two morphologically distinct species, walleye Sander vitreus and black crappie Pomoxis nigromaculatus. We also investigated diagnostic approaches to provide quality control for weight–length data. We evaluated the new Ws equations with filtered independent data to determine which equation reduced length bias the most. We suggest a protocol for evaluating length‐related bias using an independent data set. Our results showed that for randomly selected walleye populations, the RLP method did not have any length‐related biases when relative weight (Wr) was plotted as a function of length. However, the Wr values calculated from the EmP Ws equations were length biased when the latter were applied to those same populations. Both methods generated Ws equations that were length biased when Wr was plotted as a function of length for black crappies. Further, the absolute difference in Wr between the RLP and EmP methods indicates that there is little difference between the methods as far as their relevance to management is concerned. Based on these results, we believe that revising existing Ws equations using the EmP method is unnecessary and that the RLP technique should remain the standard for developing Ws equations pending the development of an approach that clearly eliminates methodological length bias.

Evaluation of harassment of migrating double-crested cormorants to limit depredation on selected sport fisheries in Michigan

Diverse management techniques have been used to mitigate conflicts between humans and double-crested cormorants ( Phalacrocorax auritus) including harassment methods supplemented by lethal take. In this study we evaluated impacts of programs to harass spring migrating cormorants on the walleye ( Sander vitreus) fishery in Brevoort Lake and the yellow perch ( Perca flavescens) and walleye fisheries at Drummond Island. Cormorant foraging declined significantly ( p < 0.05) at both locations subsequent to initiation of harassment programs. Overall harassment deterred 90% of cormorant foraging attempts while taking less than 6% lethally on average at each site. Yellow perch were a predominate prey item in number and biomass at both locations. Walleye made up a small proportion of the diet at both locations. However, both walleye and yellow perch abundance increased significantly ( p < 0.05) at Drummond Island. Walleye abundance at age 3 increased to record levels in 2008 following 3 years of cormorant management at Brevoort Lake. The estimated cormorant consumption of age-1 walleye in the absence of management at Brevoort Lake during 2005 would account for 55% of the record 2006 age-1 walleye population. These results support the hypothesis that cormorant predation on spawning aggregations of sportfish was a significant mortality factor and cormorant management reduced sportfish mortality and increased abundance at both locations. Continuation of harassment programs and fishery assessments will determine whether improvement of targeted sport fisheries through control of spring migrating cormorants is sustainable.