Western Lake Michigan Research Articles

Crossing boundaries between science and policy: Two case studies illustrate the importance of boundary organizations in the Great Lakes Basin

Boundary organizations are institutions that interface between science and policy by facilitating interactions between scientists, policy specialists, and other stakeholders to inform collaborative decision-making. Natural resource management in the Great Lakes Basin is complex and a shared exercise among two federal governments, eight states, two provinces, and over 200 sovereign Tribes, First Nations, and Métis. Many governmental agencies have recognized a need to effectively engage with other jurisdictions in order to bridge the gaps between scientific knowledge and policy decisions. As a result, boundary organizations have emerged to facilitate planning and implementation of collaborative governance frameworks. This commentary highlights how decades of shared governance of the world’s largest freshwater surface water system is augmented and assisted by boundary organizations in addressing two key Great Lakes management issues – Western Lake Erie Basin nutrient levels and Lake Michigan fisheries – which are complex, broad in scale, and pose challenges that must be addressed collaboratively across jurisdictions. While there are many governmental and non-governmental entities that engage in boundary organization-like behaviors, this commentary will be centered on three key institutions: The Great Lakes Executive Committee’s Annex 4 (Nutrients) Subcommittee, the Great Lakes Commission, and the Great Lakes Fishery Commission. We illustrate how each organization procedurally engages stakeholders, especially within state and provincial jurisdictions, to produce information and products that add breadth and capacity to manage the ecosystems of the Great Lakes. We also highlight areas of success and opportunities for improvement in collaborative governance frameworks now and into the future.

Survival, fidelity, and dispersal of Double-crested Cormorants on two Lake Michigan islands

Abstract Colony fidelity and dispersal can have important consequences on the population dynamics of colonial-nesting birds. We studied survival and inter-colony movements of Double-crested Cormorants (Phalacrocorax auritus; cormorants) nesting at Spider and Pilot islands, located 9 km apart in western Lake Michigan, during 2008–2014. We used live resighting and dead recovery data from both colonies, plus dead recoveries from throughout North America, in a multistate live and dead encounter model to estimate annual survival, inter-colony movements, plus temporary and permanent emigration to unmonitored sites. Annual survival averaged 0.37 (annual process variation, σ⌢=0.07) for hatch-year, 0.78 (σ⌢=0.08 ) for second-year, and 0.89 (σ⌢=0.04 ) for after-second year birds. The best approximating model recognized only 2 age classes for transition probabilities, indicating little difference in fidelity and movement probabilities after the natal year. Annual fidelity to Spider and Pilot islands averaged 0.53 (σ⌢=0.17 ) and 0.48 (σ⌢=0.24) for second-year and 0.55 (σ⌢=0.23 ) and 0.62 (σ⌢=0.16 ) for after-second year cormorants, respectively, indicating substantial emigration for both age classes. For birds that dispersed, emigration was approximately equally divided among neighboring colonies, temporary emigration sites from which surviving birds subsequently returned, or permanent emigration sites from which birds never returned (but were still encountered through dead recoveries). Our results indicate that Double-crested Cormorants in the Great Lakes have tremendous potential to disperse, which may help to explain their rapid recolonization following historically low populations in the early 1970s.

Spatial variability of lake trout diets in Lakes Huron and Michigan revealed by stomach content and fatty acid profiles

Despite long-term efforts to restore lake trout (Salvelinus namaycush) populations in the Great Lakes, they continue to experience insufficient recruitment and rely on hatchery programs to sustain stocks. As lake trout reproductive success has been linked to diets, spatial heterogeneity in diet compositions is of interest. To assess spatial components of adult lake trout diets, we analyzed stomach contents and fatty acid profiles of dorsal muscle collected throughout Lake Michigan and along Lake Huron’s Michigan shoreline. Lake trout from Lake Huron were generally larger in both length and mass than those from Lake Michigan. However, lake trout from Lake Michigan varied more in size based on depth of capture with smaller fish being caught more in deeper set nets. Fatty acids and stomach contents indicated that alewife (Alosa pseudoharengus) were consumed more in western Lake Michigan in contrast with round goby (Neogobius melanostomus) along the eastern shoreline. Conversely, in Lake Huron, lake trout primarily consumed rainbow smelt (Osmerus mordax). These results indicate that diet compositions of lake trout populations are relatively plastic and offer new insights into within-basin heterogeneity of Great Lakes food webs.

Early feeding of round goby (Neogobius melanostomus) fry

Abstract Larval and early juvenile stages have been shown to strongly influence recruitment variability for fishes, with successful foraging during these stages improving growth and survival. This study provides the first targeted study of the early juvenile (“fry”) diet of the round goby ( Neogobius melanostomus ), a prominent Great Lakes invasive fish. We sampled diurnally in diverse benthic environments of nearshore western Lake Michigan and two coastal tributaries. Three major prey types numerically dominated the diurnal benthic diet: harpacticoid copepods, chydorid cladocerans and chironomid larvae. Pelagic prey, of the types observed in nocturnal, pelagic studies, were rare. It was typical for the local diet of round goby fry to feature a primary prey type which contributed > 50% numerically to the total diet, which may be associated with patchy distributions of benthic prey. An ontogenetic diet shift from harpacticoids to chydorids was suggested between 6–26 mm SL. The dominance of benthic meiofauna in the diet of round goby fry expands upon the prior narrative of nocturnal, pelagic foraging, and demonstrates the flexibility of this species to thrive in diverse nursery habitats.

Polarimetric Radar Observations from a Waterspout-Producing Thunderstorm

Abstract A family of four waterspouts was produced by a convective cell over western Lake Michigan on 12 September 2013. This storm initiated along a boundary north of a mesolow in a low-level cold-air advection regime, and developed supercell characteristics once the second waterspout was in progress. Polarimetric characteristics of the storm, and of the development of supercell character, are presented. These observations represent the first documented polarimetric radar observations of waterspout-producing convection in the Great Lakes region. Unusually high differential reflectivity values accompanied this storm and its initiating boundary. The high values along the boundary are partially explained by a high density of dragonflies. High differential reflectivity values were present through much of the storm of interest despite very low aerosol concentration at low levels in the lake-influenced air mass. Finally, this case illustrates the importance of environmental awareness on waterspout-favorable days, especially when boundaries are nearby to serve as a potential source of enhanced environmental vertical vorticity.

Pressure from top and bottom: Lower food web responses to changes in nutrient cycling and invasive species in western Lake Michigan

Abstract Over recent decades, Lake Michigan phytoplankton and zooplankton communities have experienced dramatic changes driven both from the bottom (e.g. P loading reduction) and top (e.g. predatory cladoceran invasions) of the food web. We used two data sets from nearly identical sampling at an offshore station (100 m depth) in western Lake Michigan (1988–92 and 2007–09), to test for bottom-up effects (i.e. declines in chlorophyll a (chl a ) or increases in particulate C:P, leading to declines in P-rich cladocerans versus copepods), and top-down effects of invasive predatory cladocerans (i.e. declines in native zooplankton from predation or competition). Between the two periods, total P and particulate C declined, while nitrate and silicate increased. While chl a in the largest cells (> 53 μm and 10–53 μm fractions) decreased, particulate C:P ratios were unchanged. Total zooplankton abundance and biomass declined significantly between sampling periods, notably cyclopoid copepods, but not Bosmina or Daphnia species, nor the invasive cladoceran, Bythotrephes longimanus . Bottom-up effects, usually associated with ‘benthification’ attributed to Dreissena grazing, are more consistent with the changes observed than are effects of invasive predatory cladocerans. Differences in observations from those in eastern Lake Michigan or other lake-wide surveys are difficult to reconcile but seem more likely due to temporal differences in sampling rather than spatial ones. Discerning the trajectory of Lake Michigan will require better accounting for zooplankton life histories, more sophisticated understanding of nutritional quality and diet for zooplankton, and clearer coupling of pelagic–benthic cycles of elements, including Si and N.

Changes in age-0 yellow perch habitat and prey selection across a round goby invasion front

Abstract Round gobies (Neogobius melanostomus) are reported to have negative impacts on benthic invertebrate prey and are now highly abundant throughout most of the Laurentian Great Lakes. This study used the initial spread of round gobies north and south from the Milwaukee Harbor as a natural experiment to assess the competitive interactions between age-0 yellow perch and round gobies in western Lake Michigan. Habitat selection and diet of age-0 yellow perch in relation to round goby abundance were analyzed using fish captured in micro-mesh gillnets in 2006 and 2007 at six study locations from Sheboygan to Wind Point, Wisconsin. An age-0 yellow perch shift in habitat preference from rock to sand was associated with relatively high round goby abundance at rock sites. An increase in round goby catch per unit effort was also correlated with a decrease in benthic prey (chironomid larvae + amphipods) abundance at rock sites and occurrence of these prey items in age-0 yellow perch stomach contents. Our results suggest that age-0 yellow perch may have undergone local niche shifts as a consequence of competition with round gobies. Further study may help elucidate the causal mechanism behind our observations and determine what effect round gobies are having on the yellow perch population of western Lake Michigan, as a whole.

Effects of future urban and biofuel crop expansions on the riverine export of phosphorus to the Laurentian Great Lakes

Increased phosphorus (P) loadings threaten the health of the world's largest freshwater resource, the Laurentian Great Lakes (GL). To understand the linkages between land use and P delivery, we coupled two spatially explicit models, the landscape-scale SPARROW P fate and transport watershed model and the Land Transformation Model (LTM) land use change model, to predict future P export from nonpoint and point sources caused by changes in land use. According to LTM predictions over the period 2010–2040, the GL region of the U.S. may experience a doubling of urbanized areas and agricultural areas may increase by 10%, due to biofuel feedstock cultivation. These land use changes are predicted to increase P loadings from the U.S. side of the GL basin by 3.5–9.5%, depending on the Lake watershed and development scenario. The exception is Lake Ontario, where loading is predicted to decrease by 1.8% for one scenario, due to population losses in the drainage area. Overall, urban expansion is estimated to increase P loadings by 3.4%. Agricultural expansion associated with predicted biofuel feedstock cultivation is predicted to increase P loadings by an additional 2.4%. Watersheds that export P most efficiently and thus are the most vulnerable to increases in P sources tend to be found along southern Lake Ontario, southeastern Lake Erie, western Lake Michigan, and southwestern Lake Superior where watershed areas are concentrated along the coastline with shorter flow paths. In contrast, watersheds with high soil permeabilities, fractions of land underlain by tile drains, and long distances to the GL are less vulnerable.

Evidence of Wild Juvenile Lake Trout Recruitment in Western Lake Michigan

AbstractLake Trout Salvelinus namaycush were extirpated from Lake Michigan by the early 1950s, and as part of an effort to restore naturally reproducing populations, hatchery‐reared fish have been stocked since the early 1960s. Stocked fish are marked with a fin clip to differentiate them from wild, lake‐produced Lake Trout; marking error for the 2007–2010 year‐classes of Lake Trout stocked by federal hatcheries averaged 3.0%. Egg deposition, emergent fry, and wild juvenile Lake Trout have previously been observed, but no sustained wild recruitment has been measured in assessment surveys or in sport and commercial fishery catches. In 2011 and 2012, we caught juvenile Lake Trout in gill‐net and bottom trawl catches that were targeting Bloater Coregonus hoyi in water depths greater than 80 m. Unclipped, wild Lake Trout represented 20% of all Lake Trout caught in a southern offshore region of Lake Michigan. In northwestern Lake Michigan wild recruits represented from 10% to 27% of the 2007–2009 year‐classes, and we recovered a small number of wild Lake Trout from the 2010 year‐class. This is the first evidence for consecutive year‐classes of naturally produced Lake Trout surviving beyond the fry stage in Lake Michigan.Received July 13, 2012; accepted November 26, 2012

The epiphytic microbiota of the globally widespread macroalga Cladophora glomerata (Chlorophyta, Cladophorales)

The filamentous chlorophyte Cladophora produces abundant nearshore populations in marine and freshwaters worldwide, often dominating periphyton communities and producing nuisance growths under eutrophic conditions. High surface area and environmental persistence foster such high functional and taxonomic diversity of epiphytic microfauna and microalgae that Cladophora has been labeled an ecological engineer. We tested the hypotheses that (1) Cladophora supports a structurally and functionally diverse epiphytic prokaryotic microbiota that influences materials cycling and (2) mutualistic host-microbe interactions occur. Because previous molecular sequencing-based analyses of the microbiota of C. glomerata found as western Lake Michigan beach drift had identified pathogenic associates such as Escherichia coli, we also asked if actively growing lentic C. glomerata harbors known pathogens. We used 16S rRNA gene amplicon pyrosequencing to examine the microbiota of C. glomerata of Lake Mendota, Dane, Wisconsin, United States, during the growing season of 2011, at the genus- or species-level to infer functional phenotypes. We used correlative scanning electron and fluorescence microscopy to describe major prokaryotic morphotypes. We found microscopic evidence for diverse bacterial morphotypes, and molecular evidence for ca. 100 distinct sequence types classifiable to genus at the 80% confidence level or species at the 96-97% level within nine bacterial phyla, but not E. coli or related human pathogens. We inferred that bacterial epiphytes of lentic C. glomerata have diverse functions in materials cycling, with traits that indicate the occurrence of mutualistic interactions with the algal host.

Implementation and Validation of Dynamical Downscaling in a Microscale Simulation of a Lake Michigan Land Breeze

A nested numerical simulation of a land breeze observed over the western Lake Michigan shoreline was completed using the University of Wisconsin Nonhydrostatic Modeling System. This simulation was compared against observations obtained using a variety of instrumentation, most notably a scanning lidar system. The simulated land breeze is shown to closely represent the observations. Most notably, the influence of synoptic scale evolution on the strength and extent of the land breeze is illustrated to be a critical factor in accurately simulating the lifetime of the land breeze. Additionally, model resolution is shown to impact the ability of the simulation to accurately portray the fine scale features of the land breeze circulation, including the depth of the land breeze front and interactions between the land breeze and prevailing synoptic scale flow.

Tracing Social Interaction: Perspectives on Archaic Copper Exchange from the Upper Great Lakes

AbstractCompositional analysis of copper is used to explore interaction between the Late Archaic occupants of the Northern Lakes region and their contemporaries in eastern and east-central Wisconsin. Copper artifacts from Burnt Rollways phase sites in northern Wisconsin and the Upper Peninsula are compared with the Middle Archaic Reigh and Late Archaic Riverside sites. Laser ablation inductively coupled plasma mass spectrometry analysis of copper demonstrates differential acquisition of resources between Burnt Rollways and their contemporaries and suggests barriers to social interaction between the Northern Lakes and societies in the Lake Winnebago and western Lake Michigan regions.

A short-term look at potential changes in Lake Michigan slimy sculpin diets

Diporeia hoyi and Mysis relicta are the most important prey items of slimy sculpins ( Cottus cognatus) in the Great Lakes. Slimy sculpins were collected from dreissenid-infested bottoms off seven Lake Michigan ports at depths of 27–73 m in fall 2003 to study their lake-wide diets. Relatively large dreissenid biomass occurred at depths of 37- and 46-m. Quagga mussels ( Dreissena bugnesis) composed at least 50% of dreissenid biomass at Manistique, Saugatuck, and Sturgeon Bay. Mysis accounted for 82% of the sculpin diet by dry weight at eastern Lake Michigan while Diporeia composed 54–69% of the diet at western Lake Michigan and dominated the diets of slimy sculpins at all sites deeper than 46 m. In northern Lake Michigan, this diet study in new sites showed that slimy sculpin consumed more prey with low energy contents, especially chironomids, than Mysis and Diporeia in shallow sites (depth <55 m). We recommend diet studies on sedentary benthic fishes to be conducted along perimeters of the Great Lakes to observe changes in their diets that may be impacted by changing benthic macroinvertebrate communities.

Spatial and temporal distributions of lake whitefish spawning stocks in Northern lakes Michigan and Huron, 2003–2008

Adult lake whitefish were tagged and released from the Big Bay de Noc (BBN) and Naubinway (NAB) stocks in northern Lake Michigan, and the Detour (DET) and Cheboygan (CHB) stocks in northern Lake Huron during 2003–2006 to describe their spatial and temporal distributions. The contemporary spatial distributions were compared with past distributions of the BBN and NAB stocks. Sixty-two percent of BBN tag recoveries occurred in Wisconsin waters during winter, spring and summer, but 83% of fall tag recoveries were made near the tagging site. Eighty-eight percent of the NAB tag recoveries were made in the management unit of tagging and 7% occurred into northern Lake Huron. Over 90% of the DET stock remained in the vicinity of the tagging sites regardless of the season, while 75% of the CHB tag recoveries were made in northwestern Lake Huron and 17% were made in Ontario. Based on regression tree analysis, there were strong stock, season, and year effects on movement distances, with weaker effects due to sex and length at tagging. Spatial distribution of the BBN stock changed from 1978–1982 to 2003–2008, but spatial distribution of the NAB stock did not. Substantial differences in movement and distribution existed among the four stocks, large seasonal differences in spatial distribution were found within some stocks, and lake whitefish exhibited strong spawning site fidelity. Present management unit boundaries are inappropriate for managing three of our four stocks, and agencies should consider developing single harvest limits for both northern Lake Huron and western Lake Michigan.

Phylogeography and post‐glacial colonization patterns of the rainbow darter, Etheostoma caeruleum (Teleostei: Percidae)

AbstractAim To examine the effects of historical climate change and drainage isolation on the distribution of mitochondrial DNA cytochrome b genetic variation within the rainbow darter, Etheostoma caeruleum (Percidae: Etheostomatinae).Location Eastern North American streams including tributaries to the Mississippi River, Great Lakes, Potomac River and Hudson Bay drainages.Methods Parsimony analyses, Bayesian analyses and haplotype networks of mitochondrial DNA sequences.Results Four major clades were recovered from sampled populations of E. caeruleum. Three of four clades are distributed in the western portion of the species’ range (primarily west of the Mississippi River). Samples from this region do not form a monophyletic group, and sequences often vary greatly between samples from adjacent stream systems (up to 7.2% divergence). A basal clade includes samples from the White River system in the Ozark Highlands. The northern Ozarks–upper Midwest clade includes samples from Missouri River tributaries and the upper Midwest (Hudson Bay, upper Mississippi River, and western Lake Michigan drainage). The eastern clade is composed of individuals from the Ohio River, Great Lakes and Potomac River. The Mississippi River corridor clade includes samples from middle and lower Mississippi River tributaries.Main conclusions The four major clades of E. caeruleum are deep allopatric lineages with well‐defined boundaries and have additional phylogeographical structure within each clade. The Ozark Highlands have the greatest levels of diversity relative to distributional area, with marked cytochrome b subdivisions between adjacent stream systems. Samples from previously glaciated areas do not have a subset of the cytochrome b diversity found in unglaciated areas, but four separate source areas are identified based on phylogenetic analyses. Dispersal into previously glaciated areas followed several known glacial outlets and, based on sequence divergence between populations, may have occurred during different glacial or interglacial stages. The disjunct distribution and cytochrome b pattern of E. caeruleum in the Mississippi River corridor clade is consistent with late Pleistocene and Recent changes in the course and characteristics of the middle and lower Mississippi River. Phylogeographical boundaries between clades of E. caeruleum correspond to independent sources of biogeographical information and provide insight into historical stream drainage relationships, post‐glacial colonization and drainage isolation patterns.

High Resolution Bathymetry and Lakebed Characterization in the Nearshore of Western Lake Michigan

The nearshore zone of western Lake Michigan is poorly characterized both in terms of its fine scale bathymetry and its lakebed characteristics. Difficulties in characterizing the lakebed of this region arise in part because of its patchiness as well as the fact that much of the lakebed is not amenable to conventional sediment sampling techniques. With this in mind, high precision bathymetry and the lakebed characteristics of ∼17.5 km2 of the nearshore of western Lake Michigan (near Oak Creek, WI) were mapped using multi-beam and single beam sonar in conjunction with lakebed characterization and surface mapping software. Lakebed features showing nearshore bluff erosion material, glacial scouring, and isolated ridges were revealed in a 1.0 × 1.5 meter resolution map of the survey area. A map of the bottom characteristics, meanwhile, showed a patchwork of four distinct bottom types (area %): mostly rock (12.8%), cobble and sand (21.0%), mostly sand (60.3%), and clay outcrops (5.9%). All predicted bottom types were shown to be accurately characterized by direct observation with an ROV.

Prey Preferences of Eurasian Ruffe and Yellow Perch: Comparison of Laboratory Results with Composition of Great Lakes Benthos

The consumption of benthic macroinvertebrates by ruffe ( Gymnocephalus cernuus) and yellow perch ( Perca flavescens), two potential competitors in the Great Lakes, was investigated. Laboratory experiments were conducted to determine the food preferences of ruffe and yellow perch and to compare their feeding rates on two types of substrate (sand and cobble). For comparison with natural communities, we sampled benthic macroinvertebrates from western Lake Michigan and compiled published data on invertebrate community structure from all of the Great Lakes. Ruffe and yellow perch both preferentially consumed soft-bodied taxa (e.g., chironomid midge larvae, mayflies, and non-cased caddisflies) and avoided hard-bodied taxa (e.g., cased caddisflies, snails, and clams) in laboratory studies. Prey preferences offish in mixed-fish species treatments were more diverse than those in single-fish species treatments. Ruffe and yellow perch of similar sizes consumed approximately 5% of their body mass per 24 hours at 20°C on sand, whereas their feeding rates were reduced by over 50% on cobble, where prey were likely able to escape predation by hiding. Results from our laboratory experiments, field survey, and review of published studies indicate that oligochaetes and chironomids, the two most numerous macroin-vertebrate taxa in each of the Great Lakes, are vulnerable to ruffe predation. Less abundant taxa range from vulnerable (amphipods, flatworms, and caddisflies) to invulnerable (sphaeriid clams, gastropods, and zebra mussels). Our study suggests that (1) the composition of benthic macroinvertebrate fauna in each of the Great Lakes is suitable for ruffe, and (2) ruffe and yellow perch will likely prefer similar food resources where they co-occur.

Diet of Burbot in Green Bay and Western Lake Michigan With Comparison to Other Waters

We determined foods eaten and factors influencing the diet of burbot (Lota lota) and compared our results with those of other studies to help assess the role of burbot in the fish community of Lake Michigan. We examined stomachs of 3,570 burbot of 215–845 mm total length caught by 10 methods from 5 areas of Green Bay and western Lake Michigan in depths of 1 to 160 m during September 1986 to November 1988. Fish composed 94% of the volume of the diet; percent volumes of fish identifiable to genus or species were alewife (Alosa pseudoharengus), 31; rainbow smelt (Osmerus mordax), 25; bloater (Coregonus hoyi), 13; sculpins of the genus, Cottus, 11; deepwater sculpin (Myoxocephalus thompsoni), 8; yellow perch (Perca flavescens), 8; other fish, 5. Cottus spp. were more important than their volume indicated because they were the most frequently occurring fish. Important invertebrates were Mysis relicta, found in 26% of the burbot containing food, and Pontoporeia hoyi. The diet varied with size of burbot, i.e., invertebrates were more important for the smaller burbot, and with area and depth of capture. Alewives were more important at depths of 20–39 m than at shallower and deeper depths. In waters exceeding 40 m depth the diet of burbot in Lake Michigan seemed to have changed little since the 1930s, and it was similar to the diet of burbot in Lake Superior in the 1960s. Foods commonly important for burbot in many waters in North America are sculpins, coregonids other than lake whitefish (Coregonus clupeaformis), Pontoporeia spp., M. relicta, and yellow perch, and crayfish outside of the Great Lakes. Trout and salmon ( Salmo spp., Salvelinus spp., Oncorhynchus spp.) and suckers (Catostomus spp .) were inconsequential in the diet of burbot in Lake Michigan, and they have not been found to be important in the diet of burbot in other studies.

Organochlorine concentration dynamics in Lake Michigan chinook salmon (Oncorhynchus tshawytscha)

To investigate how trophic level and physiological processes influence organochlorine (OC) chemical concentrations in chinook salmon (Oncorhynchus tshawytscha) and their offspring, data on polychlorinated biphenyl (PCB), and OC pesticides and metabolite concentrations in Lake Michigan chinook salmon muscle tissue, eggs, sac fry, and prey fish were analyzed. Total concentrations of PCBs in chinook salmon decreased exponentially from the mid-1970s to mid-1980s, but the concentrations have since shown an asymptotic response, a trend also seen in alewife (Alosa pseudoharengus), an exotic planktivore that is the principal prey of chinook salmon in western Lake Michigan, and bloater (Coregonus hoyi) an endemic planktivore. The total concentration of PCBs in chinook salmon were positively correlated with fish length. OC concentrations in chinook salmon eggs were positively correlated with the concentrations of these compounds in the muscle tissue of the gravid fish. Egg and sac fry OC concentrations were also positively correlated. Estimates of the quantities of various OCs in chinook salmon muscle tissue and eggs, reveal that between 22% and 40% of the quantities of these compounds are eliminated through spawning.

Patterns of Organochlorine Contamination in Lake Trout from Wisconsin Waters of the Great Lakes

To investigate spatial and temporal patterns of organochlorine contamination in lake trout from Wisconsin waters of the Great Lakes, we examined laboratory contaminant analysis data of muscle tissue samples from Lake Michigan (n = 317) and Lake Superior (n = 53) fish. Concentrations of polychlorinated biphenyls (PCBs), chlordane, and dieldrin, reported as mg/kg wet weight in 620 mm to 640 mm mean length Lake Michigan lake trout, decreased over time. Mean total PCB concentration declined exponentially from 9.7 in 1975 to 1.9 in 1990. Total chlordane concentration declined 63 percent from 0.48 in 1983 to 0.18 in 1990, and dieldrin declined 52 percent during this same period, from 0.21 to 0.10. The bioaccumulation rate of PCBs is significantly lower for lake trout inhabiting Lake Michigan's midlake reef complex, compared to lake trout from the nearshore waters of western Lake Michigan. Organochlorine compound concentrations were greater in Lake Michigan lake trout than Lake Superior fish. Lake Superior lean lake trout and siscowet exhibited similar rates of PCB bioaccumulation despite major differences in muscle tissue lipid content between the two subspecies. The lack of a significant difference in the PCB bioaccumulation rates of lean trout and siscowet suggests that lipid content may not be an important factor influencing PCB bioaccumulation in lake trout, within the range of lipid concentrations observed. Relative concentrations of the various organochlorine contaminants found in lake trout were highly correlated, suggesting similar mass balance processes for these compounds. Evidence presented revealing spatial and temporal patterns of organochlorine contamination may be of value in reestablishing self-sustaining populations of lake trout in Lake Michigan.