Cynthia M. Tomcko

Exploitation of Northern Pike in Seven Small North-Central Minnesota Lakes

Abstract Exploitation of native northern pike Esox lucius was studied in seven small ( 500 mm total length) were approximately 2 to 9 times greater than for smaller fish (≤500 mm). Relatively high total annual mortality rates (mean = 0.48) compared to annual exploitation ...

Angler Noncompliance with Slot Length Limits for Northern Pike in Five Small Minnesota Lakes

Abstract Successful application of special regulations, such as slot length limits for northern pike Esox lucius, hinges upon angler compliance with the regulations. Yet, noncompliance was greater than expected under 508–762-mm (two lakes) or 559–762-mm (three lakes) protected slot length ranges in five north-central Minnesota lakes. Illegal fish averaged 13% of the harvested northern pike measured by creel clerks and 19% of the tag returns. Exploitation of similar-sized northern pike was greater in two reference lakes that had no size limits. In reference lakes, an average of 57% of creeled fish and 70% of tag returns were fish of sizes that would have been protected by slot length limits. In spite of extensive promotional efforts, the high (up to 29% on one lake) voluntary tag returns from illegal-size fish implies a simple lack of awareness of special regulations among anglers. Levels of noncompliance reported here show that individual lake management will require a fishing public that is more aware of...

The Relationship of Bluegill Growth, Lake Morphometry, and Water Quality in Minnesota

Abstract We evaluated the relationships between the growth rate of bluegill Lepomis macrochirus and lake morphometry and water quality at 1,146 Minnesota lakes to determine the potential strength of the latter two variables in a model of bluegill growth. Secchi depth and maximum depth were negatively correlated with bluegill length at ages 1–6, whereas total alkalinity, percent of littoral area, and mean maximum July air temperature were positively correlated with bluegill length at those ages. Secchi depth, maximum depth, and total alkalinity explained 16–33% of the variation in the growth of bluegill through their first 5 years. The lake morphometry and water quality variables did not explain more of the variation in bluegill growth among lakes because other variables, such as food resources, species interactions, and exploitation also affect bluegill growth. However, these two variables can be used to preliminarily sort candidate lakes for regulation of recreational angling for bluegill and should be c...

Bluegill Recruitment, Growth, Population Size Structure, and Associated Factors in Minnesota Lakes

Abstract To better understand the differences among populations of bluegill Lepomis macrochirus, we analyzed the relationships between bluegill recruitment, growth, population size structure, and associated factors from approximately 2,600 Minnesota lakes. Potential explanatory variables for our models included bluegill year-class strength, growth, population size structure, the relative abundance and mean weight of predator species, physical and chemical characteristics of lakes, summer air temperature, and season. Bluegill year-class strength, growth, and population size structure were more strongly related to each other than to predator and lake characteristics, temperature, or season. Growth of age-6 bluegills was positively associated with population size structure and inversely related to year-class strength, suggesting density-dependent growth effects for adult bluegills. Growth of age-3 bluegills was inversely related to Secchi depth, so early growth and productivity may be linked. Bluegill popula...

Management Briefs: Tag Loss and Handling Mortality for Northern Pike Marked with Plastic Anchor Tags

Abstract Two assumptions of mark-recapture experiments are that marks are not lost and that mortality does not differ between marked and unmarked fish. To evaluate if these assumptions were met on northern pike Esox lucius marked with plastic anchor tags, we estimated tag loss from and handling mortality of northern pike trapped and tagged during the spring in a lake in Minnesota. Annual tag loss was 1.8% for Floy tags and 0% for modified Dennison tags. Proportions of tags observed to have been lost from creeled fish in six other lakes in Minnesota were 1.2% for Floy tags and 2.5% for Dennison tags. Short-term handling mortality of northern pike trapped, tagged, and held in net pens was 2.4%.

Density Dependence in Growth and Size Structure of NorthernPike Populations

Abstract The density of northern pike Esox lucius was estimated using mark–recapture techniques and compared with measures of growth and population size structure among 12 north-central Minnesota lakes. The relationships between density and either proportional stock density (PSD, a length-frequency index) or mean back-calculated length were negative and nonlinear. Regression of loge density and PSD explained 63% of the variation in PSD, and regressions of loge density and mean back-calculated length by sex for ages 2–5 explained 47% to 67% of the variation in growth rates among lakes. The relationships remained consistent when data from 17 northern Wisconsin lakes were pooled with those from the 12 Minnesota lakes (although the R 2 was reduced). The fastest growth rates and highest PSDs were observed at northern pike densities of less than 12–14 fish/ha. Inclusion of variables related to the productivity of the lakes only marginally improved the predictive capabilities of the relationship between loge den...

Interrelationships among Production, Density, Growth, and Mortality of Northern Pike in Seven North-Central Minnesota Lakes

Abstract Secondary production can be an important indicator of performance for a fish population. In this study we examined how density, growth, and mortality operated together to alter production of northern pike Esox lucius in seven north–central Minnesota lakes. Estimated densities of age-2 and older northern pike were 3–59 fish/ha and estimated production rates were 0.77–8.31 kg·ha−1·year−1 across the seven lakes. The relationship between density and production among the lakes was curvilinear, production being moderated by slow growth rates at high northern pike densities. The negative relationship between indices of individual fish growth and production was counterintuitive and demonstrated the overwhelming influence of density on northern pike population dynamics. Relationships between total mortality and either density or production were not apparent. Recreational fisheries in Minnesota appear to be harvesting much of the annual production of northern pike. Yet many northern pike populations are ma...

Variation in Gill-Net and Angling Catchability with Changing Density of Northern Pike in a Small Minnesota Lake

Abstract The variation in the catchability coefficients (q) of gill netting and angling was monitored while manipulating the density of a population of northern pike Esox lucius during a series of nettings. The nettings removed nearly one-half of the population of northern pike age 2 and older from a small Minnesota lake. Estimates of q differed among age-classes of fish, thereby corroborating the size selectivity of the multimesh experimental gill nets. The results showed that gill-net catch rates tracked the declining population size of northern pike relatively closely. However, daily estimates of q declined during the removals, indicating hyperdepletion in catch rates. The decline in q contrasts with the results of several previous studies, which showed inverse relationships between q and density, and was most likely a result of gear size selectivity and fish behavior. Changes in age and size structure stemming from our intensive nettings helped cause the declining catchability. Angling catch rates wer...

Indirect and Direct Estimates of Gill-Net Size Selectivity for Northern Pike

Abstract Size biases in gill-net sampling of populations of northern pike Esox lucius were described with indirect and direct estimates of gill-net selectivity. In deriving indirect estimates of selectivity, length distributions of northern pike retained in gill nets of five different mesh sizes were compared, and swimming performance relative to body length was used as a measure of the rate at which northern pike encounter gill nets. Selectivity was estimated directly by trapping and marking fish, and then recapturing the fish by gill netting. Indirect estimates showed that gill nets were most effective for retaining northern pike when fish length/mesh perimeter ratio was between 3.5 and 3.7. A generalized gamma function was used to model retention in the gill nets. The length structure of the northern pike population in a small lake was closely predicted when our indirect measure of selectivity was used to correct gill-net catches. Direct estimates of selectivity for all five meshes combined showed incr...

Density and Biomass of Native Northern Pike Populations in Relation to Basin-Scale Characteristics of North-Central Minnesota Lakes

Abstract Habitat and ecological constraints that influence populations of northern pike Esox lucius at the scale of the lake basin were determined for 16 north-central Minnesota lakes that varied in size, depth, and chemical productivity. The density and biomass of northern pike greater than 350 mm total length were highly variable among lakes (3.2–59.0 fish/ha, 3.6–33.6 kg/ha). Densities of large northern pike were very low; fish longer than 600 mm averaged only 1.6 fish/ ha. Northern pike populations were more closely linked to lake morphometry than to differences in other ecological factors, such as water productivity, exploitation, or the relative abundance of prey fish. For northern pike exceeding 350 mm, numbers and mass were most strongly related to littoral and optimum thermal habitat; percent littoral area was the most important variable explaining the density differences. Literature values from other geographic areas tended to support the importance of littoral habitat except when recruitment wa...