Resurgence of Cisco (Coregonus artedi) in Lake Michigan

In recent decades, many factors that were linked with the decline of Great Lakes cisco (Coregonus artedi) populations have subsided. The goal of this study was to investigate where cisco exist in Lake Michigan and evaluate evidence for recovery including when, where, and to what extent it is occurring. We evaluated datasets from several independent monitoring efforts that did and did not target cisco. We also evaluated trends in commercial and recreational catches of cisco. Across these datasets, there was strong evidence of a sustained recovery of cisco stocks that began in Lake Michigan in the mid-2000s. Fall gill net surveys and commercial fisheries provided reasonable indications of a population recovery in the northeast by 2011. Further south, Ludington Pump Storage barrier net monitoring also recorded increasing numbers of cisco starting in 2011. Recreational harvest estimates were valuable in evaluating spatial distributions but were less valuable as an early signal of abundance shifts. Measures of the recreational harvest of cisco most notably increased in 2014. The highest catch rates and harvest occurred in Grand Traverse Bay and northern Lake Michigan as evidenced by recreational, commercial, and fall netting surveys. Observations of cisco are expanding and have increased in intensity along the eastern shore of Lake Michigan south to Muskegon in both fishery dependent and independent surveys. The similarity in trends from all data sources indicate that cisco abundance has increased, and their range within the basin continues to expand.     

Evidence of a remnant self-sustaining strain of lake trout in the Lake Michigan basin

Managers have long embraced the need to maintain diversity as a requisite condition for population and community sustainability. In the case of Great Lakes lake trout, diversity has been severely compromised. The identification of new gamete sources may be beneficial to lake trout reintroduction efforts, particularly in situations where native stocks have been completely extirpated such as in Lake Michigan. Lake trout from Elk Lake, Michigan, are genetically distinct from domestic hatchery strains and historical forms of lake trout from Lake Michigan. Importantly, Elk Lake fish were genetically distinct from Marquette strain lake trout which were previously stocked into Elk Lake. Elk Lake fish were most similar to Lake Michigan basin-derived Lewis Lake (LLW) and Green Lake (GLW) hatchery strains and to historical Lake Michigan populations from the Charlevoix, Michigan area. While all individuals exhibited characteristics of lean form lake trout, the body shape of lake trout from Elk Lake, stocked lean fish from Lake Michigan and Lake Superior wild lean strains from near Isle Royale differed. Elk Lake fish were more fusiform, elongate, and streamlined with a narrower caudal peduncle compared to hatchery lean strains and wild lean forms from the Isle Royale region of Lake Superior. The lake trout population in Elk Lake is a remnant of a now extirpated native Lake Michigan population that was established either by natural colonization or stocking from historical Lake Michigan populations. Elk Lake lake trout is as genetically diverse as other strains used in Great Lakes reintroduction efforts and likely represent a viable gamete source representing genetic diversity lost from Lake Michigan.     

Survival of Lake Trout Eggs and Fry Reared in Water from the Upper Great Lakes

As part of continuing studies of the reproductive failure of lake trout (Salvelinus namaycush) in Lake Michigan, we measured the survival of lake trout eggs and fry of different origins and reared in different environments. Eggs and milt were stripped from spawning lake trout collected in the fall of 1980 from southeastern Lake Michigan, northwestern Lake Huron, south central Lake Superior, and from hatchery brood stock. Eggs from all sources were incubated, and the newly hatched fry were reared for 139 days in lake water from each of the three upper Great Lakes and in well water. Survival of eggs to hatching at all sites was lowest for those from Lake Michigan (70% of fertilized eggs) and highest for eggs from Lake Superior (96%). Comparisons of incubation water from the different lakes indicated that hatching success of eggs from all sources was highest in Lake Huron water, and lowest in Lake Michigan water. The most notable finding was the nearly total mortality of fry from eggs of southeastern Lake Michigan lake trout. At all sites, the mean survival of Lake Michigan fry through 139 days after hatching was only 4% compared to near 50% for fry from the other three sources. In a comparison of the rearing sites, little influence of water quality on fry survival was found. Thus, the poor survival was associated with the source of eggs and sperm, not the water in which the fry were reared.     

Size segregation and seasonal patterns in rusty crayfish Faxonius rusticus distribution and abundance on northern Lake Michigan spawning reefs

Non-native rusty crayfish are abundant egg predators on spawning reef habitats for lake trout and coregonines in northern Lake Michigan. To better understand rusty crayfish life-history on these unique habitats, we conducted monitoring in 2012 and 2013 at four locations previously identified as spawning areas for native fish. With the aid of a graphical causal model, we conducted an exploratory statistical analysis using a Bayesian multilevel modeling approach with model selection based on information criteria to identify important environmental variables for predicting rusty crayfish distribution and abundance on spawning reefs. We also compared seasonal trends in relative abundance, inferred from catch-per-unit-effort calculations from trapping, to previously reported accounts from a smaller inland lake. The results from our modeling provide evidence of size-class segregation across subtle changes in habitat characteristics of spawning reefs. Specifically, we found evidence that the distribution of >30 mm rusty crayfish was only weakly related to rock density (#/m²) relative to juveniles and smaller size classes. We also observed highest relative abundances from minnow trap monitoring in mid-October when water temperatures averaged 13.9 °C, which is later in the year and at cooler temperatures than similar monitoring from smaller inland lakes has reported. We hypothesize that unique environmental conditions elicit novel life-history responses from rusty crayfish on Lake Michigan spawning reefs and discuss our findings in the context of native fish restoration in the Laurentian Great Lakes.     

Otolith microchemistry and isotopic composition as potential indicators of fish movement between the Illinois River drainage and Lake Michigan

Naturally occurring chemical markers in otoliths offer a potential but untested means to identify source environment for fishes in the upper Illinois River system and Lake Michigan, including individuals that may breach or circumvent electrical barriers in the Chicago Sanitary and Ship Canal or be transferred via bait buckets between these formerly isolated drainages. The objectives of this study were to determine whether water and fish otolith stable isotopic and elemental compositions differ among Lake Michigan, the upper Illinois River, and three tributaries of the upper Illinois River (Fox, Des Plaines and DuPage Rivers) and to determine whether otolith isotopic and elemental signatures could be used to identify the water body from which individual fish were collected. Water and fish otolith samples were obtained from each site during 2007 and analyzed for δ¹⁸O and a suite of trace element concentrations; otoliths also were analyzed for δ¹³C. Otolith δ¹³C values for Lake Michigan fish were distinct from individuals collected in the Illinois River and tributaries. Fish collected in the Fox and Des Plaines Rivers could be distinguished from one another and from fish captured in the Illinois and DuPage Rivers using otolith Sr:Ca and Ba:Ca ratios. Otolith isotopic and elemental compositions may enable identification of source environment for fishes that move or are transferred between the Illinois River drainage and Lake Michigan; however, temporal variation in otolith chemical signatures should be assessed.     

巢湖表层沉积物与环棱螺重金属分布特征及生态风险评价

以巢湖优势种淡水贝类——环棱螺与表层沉积物为研究对象,采用HNO_3-HF-HClO_4全消解法消解,然后使用电感耦合等离子体发射光谱仪(ICP-AES)对二者重金属(Cd,Cr,Cu和Pb)的总量分布进行研究,同时运用潜在生态风险指数法及单因子污染指数法与均方根综合指数评价二者重金属风险。结果表明:巢湖沉积物均已受到了一定程度的重金属Cd,Cr,Cu,Pb污染,其中以Cd的潜在生态风险最大,而环棱螺中则以Pb和Cr为高风险元素;重金属的分布特征和相关性分析显示,巢湖沉积物中的重金属来源具有相似性,而环棱螺对重金属的积累因多种环境因素的综合作用,使得环棱螺体内的重金属来源不同。因此,在未来的巢湖环境治理过程中,可着重考虑对重金属Cd,Cr,Pb的治理,同时应加大对贝类等生物监测措施的应用。     

Differential habitat use patterns of yellow perch Perca flavescens in eastern Lake Michigan and connected drowned river mouth lakes

Understanding stock structure and habitat use is important for sustainable fisheries management and conservation of genetic and phenotypic diversity. In eastern Lake Michigan, yellow perch Perca flavescens is found in both the nearshore region of Lake Michigan proper and drowned river mouth (DRM) lakes, small estuary-like systems directly connected to Lake Michigan. Recent genetic analyses suggest complex stock structure between these habitats and the potential migration of Lake Michigan yellow perch into the profundal zone of DRM lakes. We quantified carbon (δ¹³C) and oxygen (δ¹⁸O) stable isotope ratios of yellow perch otolith cores to index natal origins, and measured muscle δ¹³C, nitrogen (δ¹⁵N), δ¹⁸O, and hydrogen (δ²H) isotope ratios to reflect recent diet and habitat use. Stable isotope ratios of otolith cores and muscle samples support the existence of resident populations in nearshore Lake Michigan and DRM lakes, as well as Lake Michigan migrants using DRM lakes. Most fish caught in DRM lakes that had natal and recent stable isotope values similar to Lake Michigan fish were collected during fall in DRM lake profundal zones. Comparison of otolith core and muscle stable isotope ratios of individual yellow perch suggest that individuals that recently migrated to DRM lakes also spent early life in Lake Michigan. Differential habitat use patterns of yellow perch in eastern Lake Michigan may have important implications for harvest estimates and fishing regulations. Migration by Lake Michigan fish into DRM lakes does not appear to be related to reproduction, and the underlying benefits of these migrations remain unclear.     

Exploring potential drivers of spatiotemporal variation in length-at-age and condition of two common nearshore fishes in southwestern Lake Michigan

Understanding factors like fish size and condition can be crucial to fisheries management. In the Great Lakes, long-term observations of these factors are used to describe the effects of ecosystem dynamics on fish ecology. Considering the diversity of available benthic habitat and dynamic environmental conditions in the nearshore zone of Lake Michigan, we sought to understand the variability in length-at-age and condition of yellow perch and round goby, two important members of the nearshore fish community. We measured these metrics from both species captured in gillnets across three locations in Illinois waters of southwest Lake Michigan from June through October 2008–2012 and related the spatiotemporal differences seen to observed variation in temperature, prey abundance, and competitor abundance. Yellow perch length-at-age varied annually for all three age-classes, with differences relating to thermal regime (age-0 and age-2) and alewife abundance (age-1). Yellow perch condition, measured as residual weight, varied spatially and annually, showing a negative relationship with round goby abundance. Round goby length-at-age was greater at the southern location, likely due to warmer temperatures, while condition was greater at the rockier location. In general, model selection analysis revealed several potential relationships between the relative abundances of competitor species and length and condition of yellow perch and round goby, while prey abundance may be related to round goby condition. These results demonstrate the importance of understanding the diversity in nearshore habitats and the influence of this diversity on factors affecting fish production, including size and condition, in southwest Lake Michigan.     

Cold and wet: Diatoms dominate the phytoplankton community during a year of anomalous weather in a Great Lakes estuary

As sentinels of climate change and other anthropogenic forces, freshwater lakes are experiencing ecosystem disruptions at every level of the food web, beginning with the phytoplankton, a highly responsive group of organisms. Most studies regarding the effects of climate change on phytoplankton focus on a potential scenario in which temperatures continuously increase and droughts intersperse heavy precipitation events. Like much of the conterminous United States in 2019, the Muskegon River watershed (Michigan, USA) experienced record-breaking rainfall accompanied by unusually cool temperatures, affording an opportunity to explore how an alternate potential climate scenario may affect phytoplankton. We conducted biweekly sampling of environmental variables and phytoplankton in Muskegon Lake, a Great Lakes Area of Concern that connects to Lake Michigan. We compared environmental variables in 2019 to the previous eight years using long-term data from the Muskegon Lake Observatory buoy, and annual monitoring excursions provided historical phytoplankton data. Under cold and wet conditions, diatoms were the single dominant division throughout the entire growth season – an unprecedented scenario in Muskegon Lake. In 10 of the 13 biweekly sampling days in 2019, diatoms comprised over 75% of the phytoplankton community in the lake by count, indicating that the spring diatom bloom persisted through the fall. Additionally, phytoplankton seasonal succession and abundance patterns typically seen in this lake were absent. In a world experiencing reduced predictability, increased variability, and regional climate anomalies, studying periods of extreme weather events may offer insight into how natural systems will be affected and respond under future climate scenarios.     

Spatial patterns in dry weight of nearshore Lake Michigan prey fishes

The ability for individuals to adapt to local food sources likely allows for regional persistence of aquatic populations. In the Laurentian Great Lakes, invasive species and changing physical conditions have vastly altered local and regional food webs, leading to the potential for variation in energy flow up to large predators. To assess the potential that prey fish condition varies spatially, we examined patterns in proportion dry weight of prey fishes collected from Lake Michigan nearshore areas (<16 m) in 2010 and 2011. Absolute pairwise differences in dry weight of fishes were as high as 9% between some of our sampling sites, but sites exhibiting relatively high or low values were patchily spread across the lake. The range of proportion dry weight values observed in our study do encompass previously-reported mean values; however, the values reported in our study are slightly lower than those previously reported for Lake Michigan. In contrast to previous studies, our analyses do not suggest that variation in prey fish condition exhibits a consistent spatial pattern around nearshore Lake Michigan, and high within-site variation of prey fish condition prevents strong conclusions about specific sites providing generally more (or less) energy-rich prey to predators.     

Wild juvenile salmonid abundance in Wisconsin tributaries indicates limited contributions to Lake Michigan fisheries

Natural reproduction of salmonids occurs in many Lake Michigan tributaries, yet little is known about abundance and the potential contribution of wild fish hatching in Wisconsin tributaries. The objectives of our study were to determine if: 1) abundance of wild juvenile salmonids (primarily adfluvial rainbow trout, Oncorhynchus mykiss, referred to as steelhead) varied among selected Wisconsin streams based on available spawning and age-0 habitat; 2) stream temperature regimes could limit survival of juvenile salmonids, and 3) wild juvenile salmonids outmigrate from Wisconsin tributaries into Lake Michigan or larger tributaries. In 2016 and 2017, juvenile salmonid abundance was estimated in six Wisconsin tributaries to Lake Michigan by multiple-pass depletion sampling using backpack electrofishing. Habitat assessments included steelhead redd surveys, age-0 habitat surveys, and stream temperatures were monitored using in-stream loggers. Passive integrated transponder (PIT) tagging and PIT antennas were used to detect outmigration from three streams (Willow, Stony and Hibbard creeks). Population estimates for individual streams ranged from 75 to 2276 for juvenile steelhead and from 0 to 243 for juvenile coho salmon, Oncorhynchus kisutch. No correlation was detected between juvenile steelhead abundance and quality age-0 habitat. Stream temperatures rarely exceeded the thermal limit for steelhead (27 °C). Outmigration rates for three streams ranged from 0.6% to 3.1%, but these estimates were considered minimum values. Low abundance of wild juvenile steelhead and coho salmon alone suggest that the contributions of these tributaries to Lake Michigan fisheries are likely small. Furthermore, relying on returns of wild steelhead produced in these streams is probably insufficient to maintain stream fisheries.     

White and longnose sucker diet composition in Lake Michigan and Saginaw Bay,Lake Huron

Sucker species (Catostomidae) are a common benthic invertivore in North American waterbodies yet are understudied in the Laurentian Great Lakes. Despite large biomass and potential competition with more economically valuable fish species, the diets of Great Lakes suckers are poorly described. We explored the gut contents of adult white suckers (Catostomus commersonii) in Lake Michigan and Saginaw Bay, Lake Huron, and longnose suckers (Catostomus catostomus) in Lake Michigan. Chironomidae was a primary prey for white suckers in both lakes, along with Amphipoda in Saginaw Bay. For longnose suckers in Lake Michigan, gut contents were dominated by Amphipoda and Isopoda. Relative to other benthic invertebrate taxa, white suckers positively selected Chironomidae as preferred prey (indexed via Chesson’s alpha). Moreover, the average length of consumed Chironomidae and Amphipoda increased with white sucker length, suggesting preferential size selection of prey as suckers grow. Although dreissenid mussels are overwhelmingly abundant in the benthos of Lake Michigan and Lake Huron, relatively few dreissenid mussels were consumed by either sucker species. Thus, suckers are unlikely to contribute to the control of these invasive invertebrates.     

Are Pulp and Paper Mills Sources of Toxaphene to Lake Superior and Northern Lake Michigan?

Toxaphene was a broad-spectrum pesticide consisting of a mixture of highly chlorinated bornanes and bornenes. After its ban in 1982, toxaphene concentrations have shown a general decline in the environment as a whole and in most of the Great Lakes specifically. Recent work, however, shows that toxaphene concentrations are not decreasing in fishes from Lake Superior and northern Lake Michigan. Non-atmospheric, relatively localized sources are a possible explanation for these observations. For example, toxaphene could be inadvertently produced and released by pulp and paper mills, which could be synthesizing toxaphene-like compounds as a byproduct of bleached paper production. Reported here is a study of surficial river sediment collected upstream and downstream from seven pulp and paper mills, from five areas of previous toxaphene use, and from two presumed pristine locations. Concentrations of toxaphene found downstream are similar to those found upstream from each of the pulp and paper mills. Concentrations in sediment from rivers near previous toxaphene use locations were higher than concentrations in the background samples. These data suggest that pulp and paper mills are not now sources of toxaphene but that toxaphene used as a pesticide in the Great Lakes basin (although these uses were very small compared to its use in the southern US) could be a potential source.     

Fatty acids in ten species of fish commonly consumed by the Anishinaabe of the upper Great Lakes

The Chippewa Ottawa Resource Authority (CORA) in Sault Ste. Marie, Michigan, has been monitoring contaminant concentrations in the fillet portions of fish from the 1836 treaty-ceded waters of lakes Superior, Huron, and Michigan since 1991. The goal is to provide up to date consumption advice for their CORA member tribes. For the first time since the program started, CORA has included fatty acid analysis in 2016 monitoring of fish in Lake Superior. Ten species were targeted by CORA based on 25 years of experience and regular discussions with Anishinaabe fish consumers. This paper reports these results and presents some preliminary discussion of the consequences for consumption advice for the CORA member tribes who inhabit the Great Lakes region. Six of the species were sampled from Lake Huron and Lake Superior and four were sampled from supermarkets. Wild caught fish are an important link to the culture of Great Lakes Native American tribes and important sources of food and omega-3 polyunsaturated fatty acids (PUFA N-3). While some PUFA N-3 data from the Great Lakes is available, this dataset provides an important supplement and is specific to the 1836-treaty ceded waters of CORA. This paper confirms the presence of PUFA N-3s in Great Lakes fish traditionally harvested by the CORA tribes.     

Lake-wide Distribution of Dreissena in Lake Michigan, 1999

The Great Lakes Science Center has conducted lake-wide bottom trawl surveys of the fish community in Lake Michigan each fall since 1973. These systematic surveys are performed at depths of 9 to 110 m at each of seven index sites around Lake Michigan. Zebra mussel (Dreissena polymorpha) populations have expanded to all survey locations and at a level to sufficiently contribute to the bottom trawl catches. The quagga (Dreissena bugensis), recently reported in Lake Michigan, was likely in the catches though not recognized. Dreissena spp. biomass ranged from about 0.6 to 15 kg/ha at the various sites in 1999. Dreissenid mussels were found at depths of 9 to 82 m, with their peak biomass at 27 to 46 m. The colonization of these exotic mussels has ecological implications as well as potential ramifications on the ability to sample fish consistently and effectively with bottom trawls in Lake Michigan.     

A Comparison of Lake Trout Spawning,Fry Emergence,and Habitat Use in Lakes Michigan,Huron, and Champlain

Restoration of self-sustaining populations of lake trout is underway in all of the Great Lakes and Lake Champlain, but restoration has only been achieved in Lake Superior and in Parry Sound, Lake Huron. We evaluated progress toward restoration by comparing spawning habitat availability, spawner abundance, egg and fry density, and egg survival in Parry Sound in Lake Huron, in Lake Michigan, and in Lake Champlain in 2000–2003. Divers surveyed and assessed abundance of spawners at 5 to 15 sites in each lake. Spawning adults were sampled using standardized gill nets, eggs were sampled using egg bags, and fry were sampled using emergent fry traps and egg bags left on spawning reefs overwinter. Spawning habitat was abundant in each lake. Adult lake trout abundance was low in Lake Michigan and Parry Sound, and very high at one site in Lake Champlain. Egg deposition was lowest in Lake Michigan (0.4–154.5 eggs•m⁻², median = 1.7), intermediate in Parry Sound (39–1,027 eggs•m⁻², median = 278), and highest in Lake Champlain (0.001–9,623 eggs•m⁻², median = 652). Fry collections in fry traps followed the same trend: no fry in Lake Michigan, 0.005–0.06 fry•trap⁻¹ day⁻¹ in Parry Sound, and 0.08–3.6 fry•trap⁻¹ in Lake Champlain. Egg survival to hatch in overwinter egg bags was similar in Lake Michigan (7.6%) and Parry Sound (2.3–8.9%) in 2001–02, and varied in Lake Champlain (0.4–1.1% in 2001–02, and 1.8–18.2 in 2002–03). Lake trout restoration appears unlikely in northern Lake Michigan at current adult densities, and failure of restoration in Lake Champlain suggests that there are sources of high mortality that occur after fry emergence.     

Simultaneous Process-Response Study on the East and West Coasts of Lake Michigan

A 15-day time-series study was conducted simultaneously on both the east and west sides of Lake Michigan during the summer of 1974 at Zion, Illinois and South Haven, Michigan. The primary objective of the study was to determine the nature of process-response activities as a single weather system moves offshore and onshore as it crosses the lake. Observed patterns in the weather, waves and currents are quite similar at both sites. Changes in processes and the topographic response occurred simultaneously on both sides of the lake, at least within the limits of sampling intervals. The conceptual model developed by the authors for Lake Michigan coastal processes, was found to be applicable to this study.     

Silica Depletion in Lake Michigan: Verification Using Lake Superior as an Environmental Reference Standard

A mixing model based on the regression of silica concentrations on specific conductance was used to predict silica concentrations in the outflowing waters of Lake Michigan and Lake Superior on 21 cruises conducted from 1954 to 1980 in northern Lake Huron where chemical characteristics are determined by mixtures of the two source waters. Specific conductance @ 25°C can be used in the mixing model because it varies from 95 μS?cm^?1 in Lake Superior waters to 267 μS?cm^?1 in Lake Michigan waters. Results show that silica concentrations in the outflow of Lake Michigan in 1954 were comparable to the Lake Superior concentrations (ca. 2.0 mg?L^?1) but were reduced to <0.4 mg?L^?1 in summer cruises beginning in 1968. These results provide strong evidence that silica concentrations in the outflowing waters of Lake Michigan decreased at least 2.0 mg?L^?1 from 195 to 1971. Confidence can be given to this postulated decrease in concentration because the accuracy of data used in these analyses was verified by comparisons with the relatively invariate silica concentration in Lake Superior.     

Characterization of Growth and Winter Survival of Age-0 Yellow Perch in Southeastern Lake Michigan

Reduced growth and increased winter mortality are possible mechanisms for poor recruitment of age-0 yellow perch (Perca flavescens) in southeastern Lake Michigan. To assess the role of growth variability on recruitment of age-0 yellow perch, we quantified length-by-date and abundance during 1996–2002. Analyses revealed significant differences in length among years and sites. To resolve spatial trends, we contrasted the 1998 cohort from southeastern Lake Michigan with conspecifics from populations located within and beyond the Lake Michigan watershed. These analyses identified significant differences in length; trends suggested that the capacity for growth in the cohort from southeastern Lake Michigan was similar to these other populations. To characterize winter survival for the 1998 age-0 cohort, otoliths from age-1 perch in 1999 were used to back-calculate length (BCL) from the previous autumn. The BCL distribution was significantly larger than the 1998 distribution, but analyses led to the inference that this age-0 cohort experienced little mortality during the winter of 1998–99. We used a regression model to quantify risk of winter starvation for the 1998 cohort, using 1981–2001 winter water temperatures from southeastern Lake Michigan. This analysis suggested that risk of starvation for the 1998 cohort across this period was low (1.0–4.4%). Back calculation and regression analyses allow us to reject the hypothesis that reduced growth led to a high rate of winter mortality for the 1998 cohort. If we extrapolate our findings to past cohorts, the results would be similar unless these fish were significantly smaller than the 1998 cohort. Variation in age-0 length, growth, or winter mortality failed to fully explain yellow perch recruitment patterns in southeastern Lake Michigan. We hypothesize that factors acting before the juvenile stage is sampled during summer are shaping recruitment; resolving this requires further, stage-specific studies.     

Diet of lake trout and burbot in Northern Lake Michigan during spring: Evidence of ecological interaction

We used analyses of burbot (Lota lota) and lake trout (Salvelinus namaycush) diets taken during spring gill-net surveys in northern Lake Michigan in 2006–2008 to investigate the potential for competition and predator–prey interactions between these two species. We also compared our results to historical data from 1932. During 2006–2008, lake trout diet consisted mainly of alewives (Alosa pseudoharengus) and rainbow smelt (Osmerus mordax), whereas burbot utilized a much wider prey base including round goby (Neogobius melanostomus), rainbow smelt, alewives, and sculpins. Using the Schoener's diet overlap index, we found a higher potential for interspecific competition in 1932 than in 2006–2008, though diet overlap was not significant in either time period. No evidence of cannibalism by lake trout or lake trout predation on burbot was found in either time period. In 2006–2008, however, lake trout composed 5.4% (by weight) of burbot diet. To determine whether this predation could be having an impact on lake trout rehabilitation efforts in northern Lake Michigan, we developed a bioenergetic-based consumption estimate for burbot on Boulder Reef (a representative reef within the Northern Refuge) and found that burbot alone can consume a considerable proportion of the yearling lake trout stocked annually, depending on burbot density. Overall, we conclude that predation, rather than competition, is the more important ecological interaction between burbot and lake trout, and burbot predation may be contributing to the failed lake trout rehabilitation efforts in Lake Michigan.