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Geology topics

Charles P. Madenjian

Publications and source records attributed to Charles P. Madenjian.

At least 127 records · Page 7Linked to original sources
USGS2009

Status and trends of prey fish populations in Lake Michigan, 2008

The Great Lakes Science Center (GLSC) has conducted lake-wide surveys of the fish community in Lake Michigan each fall since 1973 using standard 12-m bottom trawls towed along contour at depths of 9 to 110 m at each of seven index transects. The resulting data on relative abundance, size structure, and condition of individual fishes are used to estimate various population parameters that are in turn used by state and tribal agencies in managing Lake Michigan fish stocks. All seven established index transects of the survey were completed in 2008. The survey provides relative abundance and biomass estimates between the 5-m and 114-m depth contours of the lake (herein, lake-wide) for prey fish populations, as well as burbot, yellow perch, and the introduced dreissenid mussels. Lake-wide biomass of alewives in 2008 was estimated at 8.27 kilotonnes (kt) (1 kt = 1000 metric tons), which was the smallest biomass estimate in the entire time series and 29% lower than the 2007 estimate. Lake-wide biomass of bloater in 2008 was estimated at 3.33 kt, which was the lowest estimate since 1977 and 38% lower than the 2007 estimate. Rainbow smelt lake-wide biomass equaled 0.89 kt, which was only 0.01 kt higher than 2007, which is the lowest estimate in the time series. Deepwater sculpin lake-wide biomass equaled 5.23 kt, which is the fourth straight year of declining biomass. The 2008 estimate is the second smallest in the time series, and 39% lower than the 2007 estimate. Slimy sculpin lake-wide biomass remained relatively high in 2008 (2.75 kt), increasing 25% over 2007. Ninespine stickleback lake-wide biomass equaled only 0.50 kt in 2008, which was 79% lower than the 2007 estimate. The final prey fish, exotic round goby, increased two orders of magnitude between 2007 and 2008, from 0.02 to 4.65 kt. Round gobies now represent 18% of the prey fish biomass. Burbot lake-wide biomass (0.91 kt in 2008) has remained fairly constant since 2002. Numeric density of age-0 yellow perch (i.e., < 100 mm) equaled 0.7 fish per ha, which is indicative of a relatively poor year-class. Lake-wide biomass of dreissenid mussels dropped precipitously in 2008, down to 9.47 kt, and a 96% decline from the 2007 biomass estimate. Overall, the total lake-wide prey fish biomass estimate (sum of alewife, bloater, rainbow smelt, deepwater sculpin, slimy sculpin, round goby, and ninespine stickleback) in 2008 was 25.62 kt, which was the lowest observed since the survey began in 1973.

Lake Michigan↗
USGS2006

Long-term trends of bloater ( Coregonus hoyi ) recruitment in Lake Michigan: evidence for the effect of sex ratio

Long-term population trends are generally explained by factors extrinsic (e.g., climate, predation) rather than intrinsic (e.g., genetics, maternal effects) to the population. We sought to understand the long-term population dynamics of an important native Lake Michigan prey fish, the bloater Coregonus hoyi . Over a 38-year time series, three 10- to 15-year phases occurred (poor, excellent, and then poor recruitment) without high interannual variability within a particular phase. We used dynamic linear models to determine whether extrinsic (winter and spring temperature, alewife predator densities) or intrinsic factors (population egg production, adult condition, adult sex ratio) explained variation in recruitment. Models that included population egg production, sex ratio, winter and spring temperature, and adult bloater condition explained the most variation. Of these variables, sex ratio, which ranged from 47% to 97% female across the time series, consistently had the greatest effect: recruitment declined with female predominance. Including biomass of adult alewife predators in the models did not explain additional variation. Overall our results indicated that bloater recruitment is linked to its sex ratio, but understanding the underlying mechanisms will require additional efforts.

Canadian Journal of Fisheries and Aquatic Sciences↗
USGS2006

Changes in seasonal energy dynamics of alewife ( Alosa pseudoharengus ) in Lake Michigan after invasion of dreissenid mussels

The dreissenid mussel invasion of Lake Michigan during the 1990s has been linked to a concomitant decrease in the abundance of the amphipod Diporeia . We tracked the seasonal energy dynamics of alewife ( Alosa pseudoharengus ) in Lake Michigan during 2002&ndash;2004 and compared our findings with previously published results for years 1979&ndash;1981. Adult alewife energy density exhibited a pronounced seasonal cycle during both the pre-invasion and post-invasion periods, with energy density in October or November nearly twice as high as that in early summer. However, on average, adult alewife energy density was 23% lower during the post-invasion period compared with the pre-invasion period. This significant decline in energy density was attributable to decreased importance of Diporeia in adult alewife diet. In contrast, energy density of juvenile alewives did not significantly differ between the pre-invasion and post-invasion periods. To attain a weight of 8 kg by age 4, bioenergetics modeling indicated that a Chinook salmon ( Oncorhynchus tshawytscha ) in Lake Michigan would have to consume 22.1% more alewives during the post-invasion period compared with the pre-invasion period.

Canadian Journal of Fisheries and Aquatic Sciences↗
USGS2006

Evaluation of a lake whitefish bioenergetics model

We evaluated the Wisconsin bioenergetics model for lake whitefish Coregonus clupeaformis in the laboratory and in the field. For the laboratory evaluation, lake whitefish were fed rainbow smelt Osmerus mordax in four laboratory tanks during a 133-d experiment. Based on a comparison of bioenergetics model predictions of lake whitefish food consumption and growth with observed consumption and growth, we concluded that the bioenergetics model furnished significantly biased estimates of both food consumption and growth. On average, the model overestimated consumption by 61% and underestimated growth by 16%. The source of the bias was probably an overestimation of the respiration rate. We therefore adjusted the respiration component of the bioenergetics model to obtain a good fit of the model to the observed consumption and growth in our laboratory tanks. Based on the adjusted model, predictions of food consumption over the 133-d period fell within 5% of observed consumption in three of the four tanks and within 9% of observed consumption in the remaining tank. We used polychlorinated biphenyls (PCBs) as a tracer to evaluate model performance in the field. Based on our laboratory experiment, the efficiency with which lake whitefish retained PCBs from their food (I?) was estimated at 0.45. We applied the bioenergetics model to Lake Michigan lake whitefish and then used PCB determinations of both lake whitefish and their prey from Lake Michigan to estimate p in the field. Application of the original model to Lake Michigan lake whitefish yielded a field estimate of 0.28, implying that the original formulation of the model overestimated consumption in Lake Michigan by 61%. Application of the bioenergetics model with the adjusted respiration component resulted in a field I? estimate of 0.56, implying that this revised model underestimated consumption by 20%.

Transactions of the American Fisheries Society↗
USGS2005

Lake whitefish and Diporeia spp. in the Great lakes: an overview

Because of growing concern in the Great Lakes over declines in abundance and growth of lake whitefish (Coregonus clupeaformis) and declines in abundance of the benthic amphipod Diporeia spp., a workshop was held to examine past and current trends, to explore trophic links, and to discuss the latest research results and needs. The workshop was divided into sessions on the status of populations in each of the lakes, bioenergetics and trophic dynamics, and exploitation and management. Abundance, growth, and condition of whitefish populations in Lakes Superior and Erie are stable and within the range of historical means, but these variables are declining in Lakes Michigan and Ontario and parts of Lake Huron. The loss of Diporeia spp., a major food item of whitefish, has been a factor in observed declines, particularly in Lake Ontario, but density-dependent factors also likely played a role in Lakes Michigan and Huron. The loss of Diporeia spp. is temporally linked to the introduction and proliferation of dreissenid mussels, but a direct cause for the negative response of Diporeia spp. has not been established. Given changes in whitefish populations, age-structured models need to be re-evaluated. Other whitefish research needs to include a better understanding of what environmental conditions lead to strong year-classes, improved aging techniques, and better information on individual population (stock) structure. Further collaborations between assessment biologists and researchers studying the lower food web would enhance an understanding of links between trophic levels.

Technical Report↗
USGS2004

Status of lake trout rehabilitation on Six Fathom Bank and Yankee Reef in Lake Huron

Six Fathom Bank, an offshore reef in the central region of Lake Huron's main basin, was stocked annually with hatchery-reared lake trout Salvelinus namaycush during 1985&ndash;1998, and nearby Yankee Reef was stocked with hatchery-reared lake trout in 1992, 1997, and annually during 1999&ndash;2001. We conducted gill-net surveys during spring and fall to evaluate performances of each of the various strains of lake trout, as well as the performance of the entire lake trout population (all strains pooled), on these two offshore reefs during 1992&ndash;2000. Criteria to evaluate performance included the proportion of &ldquo;wild&rdquo; fish within the population, spawner density, adult survival, growth, maturity, and wounding rate by sea lamprey Petromyzon marinus . Although naturally reproduced age-0 lake trout fry were caught on Six Fathom Bank and Yankee Reef, wild lake trout did not recruit to the adult population to any detectable degree. The density of spawning lake trout on Six Fathom Bank (>100 fish/305 m of gill net) during 1995&ndash;1998 appeared to be sufficiently high to initiate a self-sustaining population. However, annual mortality estimates for all lake trout strains pooled from catch curve analyses ranged from 0.48 to 0.62, well exceeding the target level of 0.40 suggested for lake trout rehabilitation. Annual mortality rate for the Seneca Lake strain (0.34) was significantly lower than that for the Superior&ndash;Marquette (0.69) and Lewis Lake (0.69) strains. This disparity in survival among strains was probably attributable to the lower sea-lamprey-induced mortality experienced by the Seneca Lake strain. The relatively high mortality experienced by adult lake trout partly contributed to the lack of successful natural recruitment to the adult population on these offshore reefs, but other factors were probably also involved. We recommend that both stocking of the Seneca Lake strain and enhanced efforts to reduce sea lamprey abundance in Lake Huron be continued.

North American Journal of Fisheries Management↗
USGS2002

Measurement error associated with surveys of fish abundance in Lake Michigan

In fisheries, imprecise measurements in catch data from surveys add uncertainty to the results of fishery stock assessments. The USGS Great Lakes Science Center (GLSC) began to survey the fall fish community of Lake Michigan in 1962 with bottom trawls. The measurement error was evaluated at the level of individual tows for nine fish species collected in this survey by applying a measurementerror regression model to replicated trawl data. It was found that the estimates of measurement-error variance ranged from 0.37 (deepwater sculpin, Myoxocephalus thompsoni ) to 1.23 (alewife, Alosa pseudoharengus ) on a logarithmic scale corresponding to a coefficient of variation = 66 to 156%. The estimates appeared to increase with the range of temperature occupied by the fish species. This association may be a result of the variability in the fall thermal structure of the lake. The estimates may also be influenced by other factors, such as pelagic behavior and schooling. Measurement error might be reduced by surveying the fish community during other seasons and/or by using additional technologies, such as acoustics. Measurement-error estimates should be considered when interpreting results of assessments that use abundance information from USGS-GLSC surveys of Lake Michigan and could be used if the survey design was altered. This study is the first to report estimates of measurement-error variance associated with this survey.

Lake Michigan↗
USGS2002

Dynamics of the Lake Michigan food web, 1970-2000

Herein, we document changes in the Lake Michigan food web between 1970 and 2000 and identify the factors responsible for these changes. Control of sea lamprey ( Petromyzon marinus ) and alewife ( Alosa pseudoharengus ) populations in Lake Michigan, beginning in the 1950s and 1960s, had profound effects on the food web. Recoveries of lake whitefish ( Coregonus clupeaformis ) and burbot ( Lota lota ) populations, as well as the buildup of salmonine populations, were attributable, at least in part, to sea lamprey control. Based on our analyses, predation by salmonines was primarily responsible for the reduction in alewife abundance during the 1970s and early 1980s. In turn, the decrease in alewife abundance likely contributed to recoveries of deepwater sculpin ( Myoxocephalus thompsoni ), yellow perch ( Perca flavescens ), and burbot populations during the 1970s and 1980s. Decrease in the abundance of all three dominant benthic macroinvertebrate groups, including Diporeia , oligochaetes, and sphaeriids, during the 1980s in nearshore waters ( 50 m deep) of Lake Michigan, was attributable to a decrease in primary production linked to a decline in phosphorus loadings. Continued decrease in Diporeia abundance during the 1990s was associated with the zebra mussel ( Dreissena polymorpha ) invasion, but specific mechanisms for zebra mussels affecting Diporeia abundance remain unidentified.

Canadian Journal of Fisheries and Aquatic Sciences↗
USGS2001

The fish community of a small impoundment in upstate New York

Moe Pond is a dimictic impoundment with surface area of 15.6 ha, a mean depth of 1.8 m, and an unexploited fish community of only two species: brown bullhead ( Ameiurus nebulosus ) and golden shiner ( Notemigonus crysoleucas ). The age-1 and older brown bullhead population was estimated to be 4,057 individuals, based on the Schnabel capture-recapture method of population estimation. Density and biomass were respectively estimated at 260 individuals/ha and 13 kg/ha. Annual survival rate of age-2 through age-5 brown bullheads was estimated at 48%. The golden shiner length-frequency distribution was unimodal with modal length of 80 mm and maximum total length of 115 mm. The golden shiner population estimate was 7,154 individuals, based on seven beach seine haul replicate samples; the density and biomass were 686 shiners/ha and 5 kg/ha, respectively. This study provides an information baseline that may be useful in understanding food web interactions and whole-pond nutrient flux.

New York↗
USGS2000

Lipid concentrations in Lake Michigan fishes: Seasonal, spatial, ontogenetic, and long-term trends

Lipid concentrations were measured in seven species of fish from several locations in Lake Michigan during spring, summer, and fall in 1994 to 1995. Adult alewife ( Alosa pseudoharengus ) and age-2 coho salmon ( Oncorhynchus kisutch ) exhibited pronounced seasonal changes in lipid content. Adult alewives averaged 7.4% lipid, on a wet weight basis, during spring (May), 2.6% in summer (July), and 12.2% in fall (late September through October). Spring lipid concentration was low in age-2 coho salmon, averaging only 1.9%, then increased to 7.8% during summer and decreased to 4.5% by fall. In contrast, lipid content in adult bloater ( Coregonus hoyi ) was relatively constant with respect to season, ranging between 10.6% and 12.4% during the year. Lipid concentration increased with fish size for all species except rainbow smelt ( Osmerus mordax ). Although deepwater sculpin ( Myoxocephalus thompsoni ) were considerably larger than slimy sculpin ( Cottus cognatus ) (mean total length of 117 mm vs 68 mm), mean lipid content of deepwater sculpin (7.6%) was only slightly higher than that for slimy sculpin (6.6%). Comparison of lipid concentrations from this study with previous studies indicated that lipid concentration in lake trout ( Salvelinus namaycush ) and alewives in Lake Michigan did not change significantly from 1969&ndash;1971 to 1994&ndash;1995. Lipid concentration in large (about 250 mm total length) adult bloaters near Saugatuck (along the southeastern shore of the lake) decreased from 23.3% in 1980 to 11.9% in 1986, but showed no significant change between 1986 and 1994&ndash;1995.

Journal of Great Lakes Research↗
USGS2000

A new approach toward evaluation of fish bioenergetics models

A new approach was used to evaluate the Wisconsin bioenergetics model for lake trout ( Salvelinus namaycush ). Lake trout in laboratory tanks were fed alewife ( Alosa pseudoharengus ) and rainbow smelt ( Osmerus mordax ), prey typical of lake trout in Lake Michigan. Food consumption and growth by lake trout during the experiment were measured. Polychlorinated biphenyl (PCB) concentrations of the alewife and rainbow smelt, as well as of the lake trout at the beginning and end of the experiment, were determined. From these data, we calculated that lake trout retained 81% of the PCBs contained within their food. In an earlier study, application of the Wisconsin lake trout bioenergetics model to growth and diet data for lake trout in Lake Michigan, in conjunction with PCB data for lake trout and prey fish from Lake Michigan, yielded an estimate of PCB assimilation efficiency from food of 81%. This close agreement in the estimates of efficiency with which lake trout retain PCBs from their food indicated that the bioenergetics model was furnishing accurate predictions of food consumption by lake trout in Lake Michigan.

Canadian Journal of Fisheries and Aquatic Sciences↗
USGS2000

Reduction in recruitment of white bass in Lake Erie after invasion of white perch

Recruitment to the adult population of white bass Morone chrysops in Lake Erie sharply declined during the early 1980s. To explain this phenomenon, we formulated the following four hypotheses: (1) the biological characteristics of adult spawners changed during the early 1980s, so that the ability to produce eggs decreased; (2) the decrease in phosphorus loadings to Lake Erie during the 1970s resulted in a lower abundance of crustacean zooplankton and thus in reduced survival of age-0 white bass; (3) the increase in the population of adult walleyes Stizostedion vitreum in Lake Erie during the 1970s and 1980s led to reduced survival of age-0 white bass; and (4) establishment of the white perch Morone americana population in Lake Erie during the early 1980s led to reduced survival of the early life stages of white bass. The growth, maturity, and fecundity of adults during the period 1981–1997 were compared with the same characteristics found by earlier studies. The mean length, weight, and condition factors that we calculated were higher than those reported for Lake Erie in 1927–1929 for all age groups examined, and white bass in Lake Erie matured at an earlier age during 1981–1997 than during 1927–1929. Fecundity estimates ranged from 128,897 to 1,049,207 eggs/female and were similar to estimates from other populations. Therefore, the first hypothesis was rejected. With respect to the second hypothesis, zooplankton surveys conducted during 1970 and 1983–1987 indicated that the abundance of crustacean zooplankton in Lake Erie did not change between the two time periods. However, these results were not conclusive because only a single-year survey was conducted before 1980. Based on walleye diet studies and estimates of walleye population size, walleye predation pressure on age-0 white bass in Lake Erie during 1986–1988 was just slightly higher than that during 1979–1981. Thus, such pressure can explain only a minor portion of the reduction in white bass recruitment. To test the fourth hypothesis, intervention analysis was applied to the long-term abundance series for white bass. The abundance of age-0 white bass in Lake Erie between 1982 and 1997 was significantly lower than that between 1969 and 1981. The catch per unit effort of adult white bass in commercial trap nets between 1987 and 1997 was significantly lower than it was before 1987. Moreover, the period of reduced recruitment for white bass in Oneida Lake, New York, which extends from 1955 to the present, coincides with occupation of the lake by white perch. Thus, of the four hypotheses entertained, the most plausible explanation for the reduction in white bass recruitment in Lake Erie is that white perch reduced the survival of white bass during its early life history.

Michigan, New York, Ohio, Ontario, Pennsylvania↗
USGS1999

Laboratory evaluation of a lake trout bioenergetics model

Lake trout Salvelinus namaycush , aged 3 and 6 years and with average weights of 700 and 2,000 g, were grown in laboratory tanks for up to 407 d under a thermal regime similar to that experienced by lake trout in nearshore Lake Michigan. Lake trout were fed alewife Alosa pseudoharengus and rainbow smelt Osmerus mordax , prey typical of lake trout in Lake Michigan. Of the 120 lake trout used in the experiment, 40 were fed a low ration (0.25% of their body weight per day), 40 were fed a medium ration (0.5% of their body weight per day), and 40 were fed a high ration (ad libitum). We measured consumption and growth, and we compared observed consumption with that predicted by the Wisconsin bioenergetics model. For lake trout fed the medium ration, model predictions for monthly consumption were unbiased. Moreover, predicted cumulative consumption by medium-ration lake trout for the entire experiment (320 d for smaller lake trout and 407 d for larger lake trout) agreed quite well with observed cumulative consumption; predictions were as close as within 0.1 to 5.2% of observed cumulative consumption. Even so, the model consistently overestimated consumption by low-ration fish and underestimated consumption by high-ration fish. The bias was significant in both cases, but was more severe for the low-ration trout. Because the low-ration and high-ration regimes were probably unrealistic for lake trout residing in Lake Michigan and because the model fit our laboratory data rather well for medium-ration trout, we conclude that applying the Wisconsin bioenergetics model to the Lake Michigan lake trout population in order to estimate the amount of prey fish consumed by lake trout each year is appropriate.

Transactions of the American Fisheries Society↗
USGS1999

Status of lake trout rehabilitation in the Northern Refuge of Lake Michigan

The Northern Refuge in Lake Michigan was established in 1985 as part of a rehabilitation program to stock yearling lake trout Salvelinus namaycush in areas with the best potential for success. Stocking of hatchery-reared lake trout within the refuge began in 1986 at three reefs: Boulder Reef, Gull Island Reef, and Richards Reef. On each reef from 1991 to 1997 we conducted gill-net surveys during the fall spawning season to evaluate performance of adult lake trout, and we conducted beam trawl surveys for naturally reproduced age-0 lake trout in the spring. Criteria to evaluate performance included spawner density, growth, maturity, and mortality. We found no evidence of natural reproduction by lake trout from our surveys. Nevertheless, density of spawning lake trout on Boulder Reef (69 fish/305 m of gill net/night) and Gull Island Reef (34 fish/305 m of gill net/night) appeared to be sufficiently high to initiate a self-sustaining population. Growth and maturity rates of lake trout in the Northern Refuge were similar to those for lake trout stocked in the nearshore region of Lake Michigan. In the Northern Refuge, growth rate for the Marquette strain of lake trout was slightly higher than for the Lewis Lake strain. Annual mortality estimates from catch curve analyses ranged from 0.46 to 0.41, and therefore, these estimates approached a level that was considered to be sufficiently low to allow for a self-sustaining population. Thus, it appeared that the lack of evidence for natural reproduction by lake trout in the Northern Refuge should not be attributed to inability of the population to attain a sufficiently large stock of spawners.

North American Journal of Fisheries Management↗
USGS1998

Net trophic transfer efficiency of PCBs to Lake Michigan coho salmon from their prey

Most of the polychlorinated biphenyl (PCB) body burden accumulated by coho salmon ( Oncorhynchus kisutch ) from the Laurentian Great Lakes is from their food. We used diet information, PCB determinations in both coho salmon and their prey, and bioenergetics modeling to estimate the efficiency with which Lake Michigan coho salmon retain PCBs from their food. Our estimate was the most reliable estimate to date because (a) the coho salmon and prey fish sampled during our study were sampled in spring, summer, and fall from various locations throughout the lake, (b) detailed measurements were made on the PCB concentrations of both coho salmon and prey fish over wide ranges in fish size, and (c) coho salmon diet was analyzed in detail from April through November over a wide range of salmon size from numerous locations throughout the lake. We estimated that coho salmon from Lake Michigan retain 50% of the PCBs that are contained within their food.

Environmental Science & Technology↗
USGS1998

Maturity schedules of lake trout in Lake Michigan

We determined maturity schedules of male and female lake trout (Salvelinus namaycush) in Lake Michigan from nearshore populations and from an offshore population on Sheboygan Reef, which is located in midlake. Gill nets and bottom trawls were used to catch lake trout in fall 1994 and 1995 from two nearshore sites and Sheboygan Reef. Each lake trout was judged immature or mature, based on visual examination of gonads. Probit analysis, coupled with relative potency testing, revealed that age-at-maturity and length-at-maturity were similar at the two nearshore sites, but that lake trout from the nearshore sites matured at a significantly earlier age than lake trout from Sheboygan Reef. However, length at maturity for the nearshore populations was nearly identical to that for the offshore population, suggesting that rate of lake trout maturation in Lake Michigan was governed by growth rather than age. Half of the lake trout males reached maturity at a total length of 580 mm, whereas half of the females were mature at a length of 640 mm. Over half of nearshore males were mature by age 5, and over half the nearshore females matured by age 6. Due to a slower growth rate, maturity was delayed by 2 years on Sheboygan Reef compared with the nearshore populations. Documentation of this delay in maturation may be useful in deciding stocking allocations for lake trout rehabilitation in Lake Michigan.

Journal of Great Lakes Research↗
USGS1998

Ontogenic and spatial patterns in diet and growth of lake trout in Lake Michigan

Lake trout Salvelinus namaycush in nearshore waters of Lake Michigan grow faster than lake trout residing offshore on Sheboygan Reef, which is in midlake. We examined the stomachs of lake trout, spanning ages 1 through 16, caught in both nearshore and offshore environments of Lake Michigan during 1994 and 1995 to determine whether diet differences may be responsible for the difference in growth rate. A comparison of the diets, coupled with bioenergetics modeling, indicated that juvenile lake trout on Sheboygan Reef experienced slow growth due to low food availability rather than to cold water temperatures. The availability of appropriate‐size prey appeared to regulate lake trout growth. Small prey fish were probably not readily available to small (200‐ to 399‐mm total length) lake trout on Sheboygan Reef, a substantial portion of whose diet consisted of invertebrates; in contrast, nearshore juveniles had a nearly 100% fish diet. Growth rate on the reef remained slow through intermediate lake trout sizes (400–599 mm total length), presumably due to low availability of rainbow smelt Osmerus mordax on the reef. Once lake trout achieved total lengths of approximately 600 mm, they grew slightly faster on Sheboygan Reef than near shore, indicating that large (>170‐mm total length) prey fish were readily available to lake trout in the reef area. On a wet‐weight basis, alewife Alosa pseudoharengus dominated the diet of large (≥600 mm total length) lake trout from both the nearshore and offshore regions of the lake, although bloater Coregonus hoyi composed over 30% of the diet on Sheboygan Reef and in southeastern nearshore Lake Michigan. Size of alewife prey increased with lake trout size. The bloater population currently represents the bulk of the biomass of the adult prey fish community, so our diet analysis suggests that large lake trout are continuing to select alewives.

Illinois, Indiana, Michigan, Wisconsin↗