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.     

Depth and temperature selection of lake charr (Salvelinus namaycush) ecotypes in Lake Superior revealed by popup satellite archival tags

Lake charr exhibit morphological diversity in large North American lakes, largely attributed to habitat partitioning. Bathythermal habitats of lean lake charr have been assessed but remain largely unknown for other lake charr ecotypes. Popup satellite archival tags (PSATs) were used to determine depth and temperature profiles of lean (n = 15), siscowet (n = 16) and redfin (n = 3) lake charr ecotypes in Lake Superior during segments of the year. Monthly median depths of leans were < 20 m throughout the year while the median depth of redfins was 24–32 m (June-September). Monthly median depth of siscowets ranged from 103 to 204 m but they exhibited four distinctive patterns: deep (>80 m), shallow (<10 m), extreme vertical movements (>80–0 m), and high frequency vertical movements between 100 and 125 m. Siscowets were the most stenothermal with habitat temperatures generally 4–5 °C but not exceeding 12 °C. Leans were found as low as 0 °C (January-March) and up to 15 °C (July-September). Median monthly habitat temperatures for redfins were 1–3 °C colder than leans during July-September. PSATs confirmed that siscowets exhibit extreme vertical movements and may feed near the surface. The timing and duration of the extreme vertical movements exhibited by siscowets did not appear to be associated with time of day (i.e., not diel vertical movements) and may be described as opportunistic movements presumably related to foraging. Data obtained from the PSATs reinforces the view that lake charr are highly adaptive and can thrive in cold, oligotrophic lakes by optimizing the use of the entire water column.     

Vertical oscillations of the thermocline caused by internal waves modify coldwater pelagic fish distribution: Results from a large stratified lake

Fishes typically occupy a species-specific temperature range, with their occupied depth being related to the lake’s temperature profile. When a fish’s preferred temperature range coincides with the thermocline, the location of their preferred thermal habitat is influenced by the rise and fall of internal waves, leading to possible changes in fish depth. These internal waves are common in large, stratified lakes, yet we do not know how they affect the spatial distribution and behavior of freshwater fishes. We conducted nighttime hydroacoustic surveys in a large, deep embayment of a large thermally stratified lake to observe whether pelagic fish respond to vertical oscillations of the thermocline caused by internal waves. The coldwater pelagic fish in our study (primarily cisco, Coregonus artedi) typically occupied a narrow vertical band approximately 5–8 m thick and temperatures between 10.8 ± 0.8–13.6 ± 1.6 °C (fishes sized 106–500 mm), just below the thermocline (centered around 15–17 °C). Importantly, the upper bound of fish depth varied in response to vertical thermocline movements associated with internal waves, suggesting fish respond to changes in their physical environment on timescales commensurate with basin-scale internal wave periods (hours to days), to remain within their preferred thermal habitat. Dissolved-oxygen levels were typically above avoidance thresholds of these fish, thus not likely exerting a strong influence on fish location. Our findings emphasize the need to account for internal waves when designing hydroacoustic and netting surveys, as thermocline movements can influence where fish are located.     

Dreissenid mussels are not a “dead end” in Great Lakes food webs

Dreissenid mussels have been regarded as a “dead end” in Great Lakes food webs because the degree of predation on dreissenid mussels, on a lakewide basis, is believed to be low. Waterfowl predation on dreissenid mussels in the Great Lakes has primarily been confined to bays, and therefore its effects on the dreissenid mussel population have been localized rather than operating on a lakewide level. Based on results from a previous study, annual consumption of dreissenid mussels by the round goby (Neogobius melanostomus) population in central Lake Erie averaged only 6 kilotonnes (kt; 1 kt = one thousand metric tons) during 1995–2002. In contrast, our coupling of lake whitefish (Coregonus clupeaformis) population models with a lake whitefish bioenergetics model revealed that lake whitefish populations in Lakes Michigan and Huron consumed 109 and 820 kt, respectively, of dreissenid mussels each year. Our results indicated that lake whitefish can be an important predator on dreissenid mussels in the Great Lakes, and that dreissenid mussels do not represent a “dead end” in Great Lakes food webs. The Lake Michigan dreissenid mussel population has been estimated to be growing more than three times faster than the Lake Huron dreissenid mussel population during the 2000s. One plausible explanation for the higher population growth rate in Lake Michigan would be the substantially higher predation rate by lake whitefish on dreissenid mussels in Lake Huron.     

Recolonization of lake whitefish river spawning ecotypes and estimates of riverine larval production in Green Bay,Lake Michigan

Lake whitefish (Coregonus clupeaformis) within the waters of Green Bay, Lake Michigan have recently shown a substantial increase in abundance. Furthermore, after over 100 years of extirpation, adult lake whitefish are found spawning within major Wisconsin tributaries to Green Bay. Many knowledge gaps still exist with respect to the chronology of adult river migrations, including the physical characteristics of upstream habitats selected for reproduction and the extent of larval production by these riverine ecotypes. Here, we use hydroacoustic imaging along with egg and larval surveys to evaluate this novel riverine spawning in 2017 and 2018. Highest abundance of adults was observed in the month of November as temperatures declined below 8 °C. Spawning areas consisted of cobble substrates, and site-specific fish densities were primarily correlated with river flows between 0.3 and 1.0 m/s, with specific values varying by tributary and year. Locations of egg deposition mirrored areas of high observed fish densities. Larval production was documented on each tributary using active trawl ichthyoplankton sampling, and larvae were observed outmigrating to open water environments. We estimated tributaries produced 452,000 larvae in 2017 and 721,000 larvae in 2018. To our knowledge, this represents the first documentation of successful lake whitefish larval production from Green Bay tributaries and suggests tributary spawning populations contribute to the greater abundance of lake whitefish observed in recent years.     

Measures of larval lake whitefish length and abundance as early predictors of year-class strength in Lake Michigan

Many lake whitefish stocks in Lake Michigan have experienced substantial declines in growth and condition since the 1990s. Reduced growth and condition could result in reduced quality or quantity of eggs produced by spawning females, which in turn could negatively impact recruitment. We evaluated the potential for reduced recruitment by measuring early life stage density and length, and we discuss the utility of these measures as early indicators of lake whitefish year-class strength. Overall, mean larval density (number per 1000 m³ ± SE) in Lake Michigan was greater in 2006 (373.7 ± 28.3) than in 2005 (16.6 ± 24.8); whereas, mean length (mm ± SE) of larval lake whitefish was smaller in 2006 (12.87 ± 0.07) than in 2005 (14.38 ± 0.13). The ratio of zooplankton to fish density did not show an expected relationship with larval fish density or length. Rather, variation in larval density was best explained by a multiple-regression model that included larval length, spring wind intensity, and adult stock density as predictor variables. Our results suggest that the density of larval lake whitefish is not directly regulated by temperature or zooplankton density at the time of emergence, but that a potential for density-dependent regulation exists when larval emergence rates are high. We conclude that the observed declines in growth and condition of adult lake whitefish are not resulting in substantial reductions in recruitment.     

Predation on Lake Whitefish Eggs by Longnose Suckers

In November 1981, we observed intense predation on lake whitefish (Coregonus clupeaformis) eggs by longnose suckers (Catostomus catostomus) on lake whitefish spawning grounds in northwestern Lake Huron. Since longnose suckers commonly frequent the same habitat used by spawning lake whitefish, there exists the potential for high losses of eggs due to sucker predation.     

Determining the efficacy of microsatellite DNA-based mixed stock analysis of Lake Michigan's Lake Whitefish commercial Fishery

Management of commercially exploited fish should be conducted at the stock level. If a mixed stock fishery exists, a comprehensive mixed stock analysis is required for stock-based management. The lake whitefish Coregonus clupeaformis comprises the primary commercial fishery across the Great Lakes. Recent research resolved that six genetic stocks of lake whitefish were present in Lake Michigan, and long-term tagging data indicate that Lake Michigan's lake whitefish commercial fishery is a mixed stock fishery. The objective of this research was to determine the usefulness of microsatellite data for conducting comprehensive mixed stock analyses of the Lake Michigan lake whitefish commercial fishery. We used the individual assignment method as implemented in the program ONCOR to determine the accuracy level at which microsatellite data can reliably identify component populations or stocks. Self-assignment of lake whitefish to their population and stock of origin ranged from > 96% to 100%. Evaluation of genetic stock discreteness indicated a moderately high degree of correct assignment (average = 75%); simulations indicated supplementing baseline data by ～ 50 to 100 individuals could increase accuracy by up to 4.5%. Simulated mixed stock commercial harvests with known stock composition showed a high degree of correct proportional assignment between observed and predicted harvest values. These data suggest that a comprehensive mixed stock analysis of Lake Michigan's lake whitefish commercial fishery is viable and would provide valuable information for improving management.     

Movements of Rainbow Steelhead Trout (Salmo Gairdneri) in Lake Ontario and a Hypothesis for the Influence of Spring Thermal Structure

To examine movements of rainbow/steelhead trout (Salmo gairdneri) and associated environmental influences, 28 fish were radiotagged in and near a tributary of Lake Ontario during spring spawning runs in 1981 and 1982. Trout initially entering the lake from the tributary generally exhibited east-west reversals of movement along the southern shore of Lake Ontario before dispersing off shore. Seasonal movement rates averaged 3.2 ± 1.6km/d over periods of 6–94 d; mean short term rates were 0.50 ± 0.46 km/h. Temperatures occupied in the lake were 9.1 ± 1.5°C; temperatures at which fish were last observed were 9.8 ± 3.8° C. Movements offshore and ultimate disappearances occurred from April to July, but were most pronounced when temperatures near shore exceeded 10° C. By linking trout movements to seasonal thermal structure in Lake Ontario, a testable hypothesis was established to explain the distribution of rainbow trout in spring and early summer. Based on tracking data, information provided by south shore anglers, and literature on the physical limnology of Lake Ontario, we hypothesize that rainbow trout disperse off shore in spring with thermal fronts, particularly the 6–8° C zone known as the spring thermocline.     

Genome-wide genetic diversity may help identify fine-scale genetic structure among lake whitefish spawning groups in Lake Erie

In Lake Erie, lake whitefish Coregonus clupeaformis supported lucrative fisheries before populations were decimated by overfishing and water quality degradation. In recent years, there has been a renewed interest in lake whitefish and management of the fishery they support. Lake whitefish spawn on several reefs throughout Lake Erie, but the relative recruitment dynamics and contributions of spawning groups to the fishery are not well understood. Modern high-throughput sequencing approaches offer new opportunities to census population diversity and to identify subtle differences among closely related populations. We used high-throughput sequencing data to evaluate the genetic structure and diversity of lake whitefish collected opportunistically across broad spatial scales in Lake Erie. Using RAD-capture (Rapture), we sequenced and genotyped individuals (N = 88) from the west, central, and east basin of Lake Erie at 120,268 single nucleotide polymorphisms (SNPs). Lake whitefish from Niagara and Crib Reefs (west basin) diverged from the three collections. Interestingly, these were the only lake whitefish collected during the act of spawning (late November), and all other fish were collected pre-spawn (August-early November). These results suggest that some lake whitefish spawning reefs may be reproductively isolated, though definition of these groups into stocks will require more intentional sampling during the act of spawning.     

Diet and trophic ecology of introduced salmonines at two south shore ports of Lake Superior, 2019

Introduced salmonines (Oncorhynchus spp. and Salmo spp.) are important components of recreational fisheries in the Great Lakes. These fishes were stocked heavily following the fishery collapse of lake trout (Salvelinus namaycush) due to sea lamprey (Petromyzon marinus) predation and commercial overharvest. While salmonine introductions occurred in all five of the Great Lakes, Lake Superior is less productive than the lower Great Lakes restricting most introduced salmonines to a small percentage of available habitat. This suggests the potential for high resource overlap between salmonines, yet little information has been published related to the trophic ecology of introduced salmonines in Lake Superior. Furthermore, hybrid splake (S. namaycush × S. fontinalis) are also stocked by various state agencies while little information on their trophic ecology exists. To address uncertainties associated with introduced salmonines, we assessed the diet composition and isotopic niche overlap of fish collected from two south shore ports of Lake Superior. Introduced salmonines consumed similar prey items including rainbow smelt (Osmerus mordax), coregonines, and three-spined stickleback (Gasterosteus aculeatus) in varying amounts, with a notable percentage of diet comprising terrestrial invertebrates. These results are further supported by stable isotope analysis which indicated high isotopic niche overlap among introduced salmonines suggesting a mixed diet attributed to benthic, pelagic, and terrestrial sources. Our characterization of the salmonine community provides important information that will inform fisheries research and management in Lake Superior.     

Feeding ecology of age-0 lake whitefish in Saginaw Bay,Lake Huron

Age-0 lake whitefish Coregonus clupeaformis (11–160 mm total length) were collected from Saginaw Bay, Lake Huron during April–November 2009 and 2010 for diet analysis and for the evaluation of ontogenetic changes in feeding ecology. Lake whitefish ≤ 50 mm ate mainly zooplankton, after which their diets switched mainly to benthic macroinvertebrates. Cyclopoida were the dominant prey consumed by very small lake whitefish (< 17 mm) and the most frequently selected zooplankton type for individual small fish. Once lake whitefish reached 18–19 mm, Cyclopoida in the diet declined and cladocerans emerged as an important diet item. Daphnia were the most common cladoceran in the diets, but for fish 31–50 mm Bosminidae were also relatively important. Although the shift to Daphnia could represent an ontogenetic point when lake whitefish were large enough to effectively handle this prey, it also took place when the relative availability of Daphnia increased. Once lake whitefish were > 50 mm, Chironomidae larvae became a dominant prey item and this shift to benthivory coincided with a 55% increase in length-adjusted energy content between June and July. However, as fish grew (around 110–120 mm), Sphaeriidae and the benthic zooplankton Chydoridae became increasingly important in the diet. As these less energetically rich prey were incorporated into the diet, there were corresponding 21 and 15% decreases in length-adjusted energy content from July to August and September, respectively.     

Evidence of Lake Whitefish Spawning in the Detroit River: Implications for Habitat and Population Recovery

Historic reports imply that the lower Detroit River was once a prolific spawning area for lake whitefish (Coregonus clupeaformis) prior to the construction of the Livingstone shipping channel in 1911. Large numbers of lake whitefish migrated into the river in fall where they spawned on expansive limestone bedrock and gravel bars. Lake whitefish were harvested in the river during this time by commercial fisheries and for fish culture operations. The last reported landing of lake whitefish from the Detroit River was in 1925. Loss of suitable spawning habitat during the construction of the shipping channels as well as the effects of over-fishing, sea lamprey (Petromyzon marinus) predation, loss of riparian wetlands, and other perturbations to riverine habitat are associated with the disappearance of lake whitefish spawning runs. Because lake whitefish are recovering in Lake Erie with substantial spawning occurring in the western basin, we suspected they may once again be using the Detroit River to spawn. We sampled in the Detroit River for lake whitefish adults and eggs in late fall of 2005 and for lake whitefish eggs and fish larvae in 2006 to assess the extent of reproduction in the river. A spawning-ready male lake whitefish was collected in gillnets and several dozen viable lake whitefish eggs were collected with a pump in the Detroit River in November and December 2005. No lake whitefish eggs were found at lower river sites in March of 2006, but viable lake whitefish eggs were found at Belle Isle in the upper river in early April. Several hundred lake whitefish larvae were collected in the river during March through early May 2006. Peak larval densities (30 fish/1,000 m³ of water) were observed during the week of 3 April. Because high numbers of lake whitefish larvae were collected from mid-and downstream sample sites in the river, we believe that production of lake whitefish in the Detroit River may be a substantial contribution to the lake whitefish population in Lake Erie.     

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.     

Differentiation between lake whitefish and cisco eggs based on diameter

Cisco (Coregonus artedi) and lake whitefish (Coregonus clupeaformis) are native fish species of management concern in the Laurentian Great Lakes that often overlap in spawning locations and timing. Thus, species-level inference from in situ sampling requires methods to differentiate their eggs. Genetic barcoding and hatching eggs to visually identify larvae are used but can be time and cost intensive. Observations in published literature indicate that lake whitefish eggs may be larger than cisco eggs in the Great Lakes, but this has not yet been substantiated. Samples from shared spawning grounds are unlikely to contain similarly sized or colored eggs from other species. Thus, we assessed whether lake whitefish and cisco eggs could be separated based on size alone. Fertilized, hardened eggs were collected in situ during spawning at Elk Rapids, Lake Michigan and Chaumont Bay, Lake Ontario and preserved in ethanol. Individual eggs were measured and genetically identified. Mean diameter for cisco (2.45 mm, SD = 0.22, n = 444) was smaller than for lake whitefish (3.21 mm, SD = 0.20, n = 99). We used classification trees to identify a species-separating size threshold of 2.88 mm (95% bootstrap CI = [2.877, 2.976]), which classified eggs with an accuracy rate of 96%. Differences between species across other samples from the same locations were mostly consistent with the threshold size, but we suggest validation if applying this method to other populations. Separation of cisco and lake whitefish eggs by diameter can be accurate, efficient, and especially suitable for large sample sizes.     

Movement patterns of smallmouth and largemouth bass in and around a Lake Michigan harbor: The importance of water temperature

Smallmouth bass Micropterus dolomieu and largemouth bass M. salmoides in southwestern Lake Michigan use shallow, warm harbors for spawning during spring. After the reproductive period ends in early summer, catch rates from standardized sampling of smallmouth bass in harbors decrease. Fishes are presumed to use the main lake during summer but little is known about how black basses use main-lake habitat in Lake Michigan or mechanisms driving this transition. We tracked smallmouth bass (N = 26) and largemouth bass (N = 8) using radio and acoustic telemetry during 2005–2006 near North Point Marina, IL. A temperature difference persisted between inside and outside the harbor for much of May–October where harbor temperatures were generally warmer than those outside the harbor. Smallmouth bass responded to water temperature changes, inhabiting the harbor until temperatures outside the harbor approached 18.5 °C, at which time they left the harbor. Frequent temperature fluctuations of > 3 °C occurred outside the harbor within 24-hour periods. Sudden reductions in water temperature were associated with smallmouth bass temporarily returning to the harbor until the temperature outside the harbor again approached 18.5 °C. As water cooled during fall, smallmouth bass again returned to the harbor. Largemouth bass exhibited comparatively restricted movements during this time and rarely ranged outside the harbor. Thus, home range estimates for smallmouth bass (142.6 ha) were an order of magnitude greater than those of largemouth bass (12.9 ha). Both water temperature and species played a role in determining the degree of movement outside the harbor but both black basses used the harbor as a thermal refuge.     

Spatio-temporal trends in the food habits of age-0 lake whitefish

We compared diets of juvenile lake whitefish among six sites in Lake Michigan and one in Lake Superior during 2005 and 2006 to assess spatial and temporal patterns in food habits and evaluate if ontogenetic diet shifts occur that may influence growth and survival. A total of 262 and 496 juveniles were captured in 2005 and 2006, the majority of which were captured during June and July. Sites in southern Lake Michigan tended to have larger juveniles, and the smallest juveniles were observed at Naubinway, northern Lake Michigan, and Whitefish Point, Lake Superior. The mean number of prey items per stomach differed among sampling sites and years. Copepods were the most prevalent prey item, and were present in greater than 70% of juvenile stomachs from most sites. However, the percent by number of copepods decreased during July as chironomids and other benthic macroinvertebrates increased in number. There was a significant positive relationship between percent of benthic prey items and mean length of juvenile lake whitefish. A substantial increase in the percent of benthic prey consumed after 40 mm (total length) was observed and likely resulted from juvenile lake whitefish crossing a size threshold for benthic feeding relating to morphological changes (i.e., transition of mouth opening from terminal to sub-terminal) in addition to a potential increase in the availability of emergent macroinvertebrates. Timing of the transition to benthic feeding is likely regulated by the number of prey per juvenile and the overlap with peak emergence of important benthic aquatic invertebrates such as chironomids. A better understanding of these factors will increase our understanding of juvenile lake whitefish growth and survival, which are necessary for improving year-class strength predictions.     

Evaluating the growth potential of sea lampreys (Petromyzon marinus) feeding on siscowet lake trout (Salvelinus namaycush) in Lake Superior

Differences in the preferred thermal habitat of Lake Superior lake trout morphotypes create alternative growth scenarios for parasitic sea lamprey (Petromyzon marinus) attached to lake trout hosts. Siscowet lake trout (Salvelinus namaycush) inhabit deep, consistently cold water (4-6 °C) and are more abundant than lean lake trout (Salvelinus namaycush) which occupy temperatures between 8 and 12 °C during summer thermal stratification. Using bioenergetics models we contrasted the growth potential of sea lampreys attached to siscowet and lean lake trout to determine how host temperature influences the growth and ultimate size of adult sea lamprey. Sea lampreys simulated under the thermal regime of siscowets are capable of reaching sizes within the range of adult sea lamprey sizes observed in Lake Superior tributaries. High lamprey wounding rates on siscowets suggest siscowets are important lamprey hosts. In addition, siscowets have higher survival rates from lamprey attacks than those observed for lean lake trout which raises the prospect that siscowets serve as a buffer to predation on more commercially desirable hosts such as lean lake trout, and could serve to subsidize lamprey growth.     

Contemporary spatial extent and environmental drivers of larval coregonine distributions across Lake Ontario

Coregonine fishes are important to Laurentian Great Lakes food webs and fisheries and are central to basin-wide conservation initiatives. In Lake Ontario, binational management objectives include conserving and restoring spawning stocks of cisco (Coregonus artedi) and lake whitefish (C. clupeaformis), but the spatial extent of contemporary coregonine spawning habitat and the environmental factors regulating early life success are not well characterized. In Spring 2018, we conducted a binational ichthyoplankton assessment to describe the spatial extent of coregonine spawning habitat across Lake Ontario. We then quantified the relative importance of a suite of biophysical variables hypothesized to influence coregonine early life success using generalized additive mixed models and multimodel inference. Between April 10 and May 14, we conducted 1,092 ichthyoplankton tows and captured 2,350+ coregonine larvae across 17 sampling areas, predominantly within embayments. Although 95% of catches were in the eastern basin, coregonine larvae were also found in historical south shore spawning areas. Most coregonine larvae were cisco; <6% were lake whitefish. Observed catches of both species across sampling areas were strongly and similarly associated with ice cover duration, but the importance of site-specific characteristics varied, such as distance to shore and site depth for cisco and lake whitefish, respectively. These results suggest that regional-scale climatic drivers and local environmental habitat characteristics interact to regulate early life stage success. Furthermore, strong regional and cross-species variation in larval distributions emphasize the importance of lake-wide assessments for monitoring both the current eastern basin populations and potential expansions into western Lake Ontario habitats.     

Depth Distribution,Diet, and Overwinter Growth of Lake Trout (Salvelinus Namaycush) in Southeastern Lake Michigan Sampled in December 1981 and March 1982

Lake trout were collected in graded-mesh gill nets and forage fishes were collected in trawls in mid December 1981 and late March 1982. The length ranges of 317 lake trout caught in December and 138 in March were 280–767 and 286–857 mm, and the age ranges I–XI and II–XIV, respectively. Three year classes (1977–79) made up almost 80% of the catches of lake trout in both sampling periods. Lake trout were most abundant at depth of 18 to 37 m in December (water temperatures, 5.5–6.8°C) and at 28–64 m in March (water temperatures, 1.0–1.3°C). Fish of the 1977–79 year classes completed 9 to 24% of their annual growth in length, and 14 to 39% of their growth in weight, between mid December and late March. Lake trout ate mainly alewives (Alosa pseudoharengus) especially young-of-the-year, in December, but primarily slimy sculpins (Cottus cognatus) in March, when alewives were mainly at depths of greater than those occupied by most lake trout. Other important food items were rainbow smelt (Osmerus mordax) and, in deeper water, deepwater sculpins (Myoxocephalus thompsoni) Bloaters (Coregonus hoyi) were eaten only sparingly, although they were abundantly available in both sampling periods. Perhaps this species, which coevolved with the lake trout in Lake Michigan and was important in the native trout's diet, is better able to avoid capture by the trout than are the exotic alewife and rainbow smelt. It may not again become a major forage species unless the other food sources become scarce.