Limnetic Larval Fish in the Nearshore Zone of the Western Basin of Lake Erie

Gizzard shad/alewife, Dorosoma cepedianum/Alosa pseudoharengus, emerald shiners, Notropis atherinoides, white bass/white perch, Morone chrysops/Morone americana, and yellow perch, Perca flavescens, constituted over 97% of the larval fish collected in Ohio and Michigan waters of the western basin of Lake Erie during 1977. Significantly greater numbers of gizzard shad/alewife and spottail shiner, Notropis hudsonius, larvae were captured immediately adjacent to the shore than at a depth of 5 m offshore while greater numbers of smelt, Osmerus mordax, larvae were captured at points further offshore at a depth of 5 m than at points immediately adjacent to the shore. Significantly greater numbers of walleye, Stizostedion vitreum, larvae were collected along the Ohio shoreline portion of the study area than in Maumee Bay or along the Michigan shoreline. Significantly greater numbers of freshwater drum, Aplodinotus grunniens, larvae were collected in Maumee Bay.     

Distribution and Abundance of Rainbow Smelt in Western Lake Superior as Determined from Acoustic Sampling

Distribution and abundance of rainbow smelt (Osmerus mordax, Mitchill) in the western arm of Lake Superior were described through an acoustic sampling program conducted from June 1978 through early June 1980. Density and abundance were highest in shallow water (bottom depth <50 m), where rainbow smelt underwent vertical movements from the bottom and concentrated in warmer (11 to 16° C) near-surface water (2- to 30-m depth) at night. Rainbow smelt densities and abundance were extremely low in the deeper waters (>50 m) along the north shore and in the open lake. Abundance of rainbow smelt in the western area of the lake was estimated to be 286 X 10⁶ fish (4.35 X 10⁶ kg) in 1978. Abundance declined to 164 X 10⁶ fish (2.5 X 10⁶ kg) in 1979. Comparison of rainbow smelt abundance estimates with commercial yields suggested that annual fishing mortality is 15 to 27%. Predation by salmonid populations is considered a potentially important factor influencing smelt abundance levels. Rainbow smelt distribution and changes in abundance described by this study indicate smelt production is not high enough to support the fisheries or predator populations previously supported by native lake herring (Coregonus artedii, Lesueur) stocks.     

Native rainbow smelt and nonnative alewife distribution related to temperature and light gradients in Lake Champlain

Alewife (Alosa pseudoharengus) recently became established in Lake Champlain and may compete with native rainbow smelt (Osmerus mordax) for food or consume larval rainbow smelt. The strength of this effect depends partly on the spatial and temporal overlap of different age groups of the two species; therefore, we need a better understanding of factors affecting alewife and rainbow smelt distributions in Lake Champlain. We used hydroacoustics, trawls, and gill nets to document vertical fish distribution, and recorded environmental data during 16 day–night surveys over two years. Temperature, temperature change, and light were all predictors of adult and age-0 rainbow smelt distribution, and temperature and light were predictors of age-0 alewives' distribution (based on GAMM models evaluated with AIC). Adult alewives were 5–30 m shallower and age-0 alewives were 2–15 m shallower than their rainbow smelt counterparts. Adult rainbow smelt distribution overlapped with age-0 rainbow smelt and age-0 alewives near the thermocline (10–25 m), whereas adult alewives were shallower (0–6 m) and overlapped with age-0 alewives and rainbow smelt in the epilimnion. Adult rainbow smelt were in water < 10–12 °C, whereas age-0 rainbow smelt were in 10–20 °C, and adult and age-0 alewives were in 15–22 °C water. Predicting these species distributions is necessary for quantifying the strength of predatory and competitive interactions between alewife and rainbow smelt, as well as between alewife and other fish species in Lake Champlain.     

Distribution and Abundance of Larval Fish in the Nearshore Waters of Western Lake Huron

Ichthyoplankton was collected at 17 nearshore (bottom depth ≥5 m but ≤10 m) sites in western Lake Huron during 1973–75 with a 0.5-m net of 351-micron mesh towed at 99 m/min. Larvae of rainbow smelt (Osmerus mordax) dominated late spring and early summer catches and larvae of alewives (Alosa pseudoharengus) the midsummer catches. Larval yellow perch (Perca flavescens) were caught in early summer but were rarely the dominant species. The time of spawning and hatching, and thus occurrence of larvae, differed between areas but was less variable for alewives than for yellow perch. The appearance of larvae in Saginaw Bay was followed successively by their appearance in southern, central, and northern Lake Huron. Rainbow smelt were most abundant in northern Lake Huron and yellow perch and alewives in inner Saginaw Bay. Densities of either rainbow smelt or alewives occasionally exceeded 1/m³, whereas those of yellow perch never exceeded 0.1/m³. Abundance of alewives was usually highest 1 to 3 m beneath the surface and that of rainbow smelt 2 to at least 6 m beneath the surface. Important nursery areas of rainbow smelt were in bays and off irregular coastlines and those of yellow perch were in bays. All nearshore waters seemed equally important as nursery areas of alewives.     

Inshore-offshore Distribution of Larval Fishes in Lake Superior off the Western Coast of the Keweenaw Peninsula,Michigan

We surveyed the larval fish community in Lake Superior off the western coast of the Keweenaw Peninsula, Michigan, as a first component in understanding how the Keweenaw Current affects larval fish distribution and survivorship. On transects at Ontonagon, Houghton, and Eagle Harbor, we collected larval fishes with a 1-m diameter plankton net towed through surface and deep (below metalimnion) waters at an inshore location (1 km from shore) and an offshore location (5–9 km from shore) during day and night in 1998 and 1999. The most abundant larvae caught were lake herring (Coregonus artedii), rainbow smelt (Osmerus mordax), burbot (Lota lota), deepwater sculpin (Myoxocephalus thompsoni), and spoonhead sculpin (Cottus ricei). Lake herring was generally most abundant at the surface during the day, while the other four species avoided the surface by day but not at night. Overall, larval fish density was greater inshore than offshore, with exceptions for particular locations and seasonal periods (1.24x for lake herring, 12.93x for rainbow smelt, 1.27x for burbot, 1.25x for deepwater sculpin, and 4.26x for spoonhead sculpin). Differences in the sizes of larvae between inshore and offshore locations, in conjunction with density patterns, suggest a seasonal inshore to offshore movement. Despite the presence of the Keweenaw Current, the overall distribution patterns of larval fishes follow those of previous studies conducted in the Great Lakes, but with lower densities.     

Catches of Larval Rainbow Smelt (Osmerus Mordax) and Alewife (Alosa Pseudoharengus) in Plankton Nets of Different Mesh Sizes

Four 0.5-m plankton nets (one each of 0.355-, 0.450-, 0.560-, and 0.750-mm mesh) were used to collect larvae of the alewife (Alosa pseudoharengus) and rainbow smelt (Osmerus mordax) in northwestern Lake Huron and the St. Marys River. The number of larvae collected varied inversely with mesh size and the average length of larvae varied directly with mesh size. Numbers of each species caught in 0.355- and 0.450-mm mesh nets were significantly greater (P < 0.01) than the numbers caught in 0.560- and 0.750-mm mesh nets. Numbers of alewives caught were significantly greater (P < 0.01) in the 0.355- than in the 0.450-mm mesh, but numbers of rainbow smelt caught in the two meshes were about equal. Between the two larger-mesh nets, numbers of each species caught did not differ significantly. Nets with mesh larger than 0.355 mm were less effective at catching recently hatched larvae, and this reduced effectiveness accounted wholly for the smaller numbers caught in the 0.450-mm mesh net. Smaller numbers were caught in the 0.560- and 0.750-mm mesh nets not only because they caught fewer recently hatched larvae but also because they failed to retain older, larger larvae. Information on the effect of mesh size and towing speed on catches of larvae is important for making accurate estimates of larval densities.     

Response of Yellow Perch Diet to a Changing Forage Base in Southern Lake Michigan, 1984-2002

This study evaluated yellow perch (Perca flavescens) diet in southern Lake Michigan to determine whether prey consumed fluctuated with abundance of zooplankton, benthic invertebrates, and fish species during the period 1984 to 2002. Some change in benthic community abundance was evident from samples collected in the region during the period, including the naturalization of the round goby and the zebra mussel between 1993 and 2002. In addition, changes in fish abundance were evident from 1984 to 2002, when spottail shiner (Notropis hudsonius) and alewife (Alosa pseudoharengus) increased, while yellow perch, and rainbow smelt (Osmerus mordax) declined. Non-indigenous species eaten by yellow perch in 2002 included spiny water fleas (Bythotrephes longimanus), round gobies, and alewives with the latter two species dominating the diet by volume. Yellow perch did exhibit prey preferences, although they consumed a variety of different organisms over the period of study. This euryphagous characteristic of yellow perch is expected to promote its persistence in southern Lake Michigan, despite a changing prey base.     

Spatial Patterns in PCB Concentrations of Lake Michigan Lake Trout

Most of the PCB body burden in lake trout (Salvelinus namaycush) of the Great Lakes is from their food. PCB concentrations were determined in lake trout from three different locations in Lake Michigan during 1994–1995, and lake trout diets were analyzed at all three locations. The PCB concentrations were also determined in alewife (Alosa pseudoharengus), rainbow smelt (Osmerus mordax), bloater (Coregonus hoyi), slimy sculpin (Cottus cognatus), and deepwater sculpin (Myoxocephalus thompsoni), five species of prey fish eaten by lake trout in Lake Michigan, at three nearshore sites in the lake. Despite the lack of significant differences in the PCB concentrations of alewife, rainbow smelt, bloater, slimy sculpin, and deepwater sculpin from the southeastern nearshore site near Saugatuck (Michigan) compared with the corresponding PCB concentrations from the northwestern nearshore site near Sturgeon Bay (Wisconsin), PCB concentrations in lake trout at Saugatuck were significantly higher than those at Sturgeon Bay. The difference in the lake trout PCB concentrations between Saugatuck and Sturgeon Bay could be explained by diet differences. The diet of lake trout at Saugatuck was more concentrated in PCBs than the diet of Sturgeon Bay lake trout, and therefore lake trout at Saugatuck were more contaminated in PCBs than Sturgeon Bay lake trout. These findings were useful in interpreting the long-term monitoring series for contaminants in lake trout at both Saugatuck and the Wisconsin side of the lake.     

Feeding ecology of emerald shiners and rainbow smelt in central Lake Erie

To better understand the feeding ecology of two important Laurentian Great Lakes prey species, rainbow smelt Osmerus mordax and emerald shiners Notropis atherinoides, we quantified the diet composition, selectivity, daily ration, and diet overlap of both species in offshore central Lake Erie during May through October 2005, which spanned a period of severe hypolimnetic hypoxia (< 2 mg O₂/L). Rainbow smelt fed upon a variety of prey taxa, including zooplankton, chironomid pupae and larvae, and fish, whereas emerald shiners primarily consumed cladocerans, if available. In turn, diet overlap between rainbow smelt and emerald shiners was low except during September when hypolimnetic hypoxia reduced rainbow smelt access to benthic prey. Rainbow smelt most frequently selected chironomid pupae, while emerald shiners generally selected pupae or large predatory cladocerans (Leptodora or Bythotrephes). Daily ration and individual consumption by rainbow smelt were 54–68% less during hypoxia than at the same site during stratified pre-hypoxic or mixed post-hypoxic conditions. Although emerald shiner daily ration and individual consumption decreased between pre-hypoxic and hypoxic periods, it continued to decrease during the post-hypoxic period, suggesting that reduced consumption may not have been linked to hypoxic conditions. Ultimately, our findings suggest that emerald shiners are as important regulator of zooplankton abundance in the Great Lakes as rainbow smelt, given their potentially high mass-specific consumption rates, selectivity and diet patterns, and current high abundance.     

Changes in a Population of Exotic Rainbow Smelt in Lake Superior: Boom to Bust, 1974–2005

Changes in a population of rainbow smelt (Osmerus mordax) in the Apostle Islands region of Lake Superior were chronicled over a 32-yr time series, 1974-2005. At the beginning of the time series, rainbow smelt was the predominant prey species, abundance of lake herring (Coregonis artedi) was very low, and the dominant predator was stocked lake trout (Salvelinus namaycush). Following a period of successful lake trout stocking in the 1970s, the rainbow smelt population declined sharply in 1980, largely through mortality of adult fish and subsequent poor recruitment. In the succeeding 4 years, rainbow smelt populations reached historic low levels, resulting in reduced food resources for both wild and stocked lake trout. During 1985–1990 lake herring stocks began a spectacular recovery following the appearance of a very strong 1984 year class and subsequent 1988, 1989, and 1990 year classes. Rainbow smelt benefited from the high abundance of young lake herring as an alternate prey source for lake trout and showed a partial recovery in the late 1980s. However, a growing lake trout population coupled with an 8-yr period of low herring reproduction after 1990 resulted in a diminished rainbow smelt population dominated by age-1 and 2 fish and showing a pattern of alternating recruitment attributed to cannibalism. Low productivity of rainbow smelt and intermittent production of herring over the past decade has left lake trout populations with a diminished prey base. Although lake trout recovery benefited from the presence of rainbow smelt as a prey resource, the Lake Superior fish community was fundamentally altered by the introduction of rainbow smelt.     

Resurgence of Emerald Shiners Notropis atherinoides in Lake Huron's Main Basin

Emerald shiners Notropis atherinoides were formerly common in Lakes Huron and Michigan, but declined during the 1960s as the exotic alewife Alosa pseudoharengus proliferated. The Lake Huron emerald shiner population was chronically depressed through 2004; however, we detected resurgence in emerald shiner density and biomass in Lake Huron during acoustic and midwater trawl surveys conducted during 2004–2006. Emerald shiners were not found during 2004, but by 2006 main basin density exceeded 500 fish/ha, biomass estimates exceeded 0.5 kg/ha, and emerald shiners contributed more to pelagic biomass than alewives or rainbow smelt Osmerus mordax. Length frequency distributions suggested that increased density was the result of two consecutive strong year classes in 2005 and 2006. Emerald shiner distributions also expanded from a focus in western Lake Huron in 2005 to a lakewide distribution in 2006. Emerald shiners occurred offshore, but were nearly always associated with epilimnetic surface waters warmer than 19°C. Resurgence of emerald shiners was likely a consequence of reduced alewife abundance, as they declined concurrently with alewife proliferation during the early 1960s. Return of this species may benefit native nearshore piscivores; however, benefits to Pacific salmonids Oncorhynchus spp. are uncertain because emerald shiners are smaller and still less abundant than historically important prey species, and they may be thermally segregated from salmonines.     

Hydroacoustic Estimates of Abundance and Spatial Distribution of Pelagic Prey Fishes in Western Lake Superior

Lake herring (Coregonus artedi) and rainbow smelt (Osmerus mordax) are a valuable prey resource for the recovering lake trout (Salvelinus namaycush) in Lake Superior. However, prey biomass may be insufficient to support the current predator demand. In August 1997, we assessed the abundance and spatial distribution of pelagic coregonines and rainbow smelt in western Lake Superior by combining a 120 kHz split beam acoustics system with midwater trawls. Coregonines comprised the majority of the midwater trawl catches and the length distributions for trawl caught fish coincided with estimated sizes of acoustic targets. Overall mean pelagic prey fish biomass was 15.56 kg ha⁻¹ with the greatest fish biomass occurring in the Apostle Islands region (27.98 kg ha⁻¹), followed by the Duluth Minnesota region (20.22 kg ha⁻¹), and with the lowest biomass occurring in the open waters of western Lake Superior (9.46 kg ha⁻¹). Biomass estimates from hydroacoustics were typically 2–134 times greater than estimates derived from spring bottom trawl surveys. Prey fish biomass for Lake Superior is about order of magnitude less than acoustic estimates for Lakes Michigan and Ontario. Discrepancies observed between bioenergetics-based estimates of predator consumption of coregonines and earlier coregonine biomass estimates may be accounted for by our hydroacoustic estimates.     

Food of Trout and Salmon in Lake Ontario

Stomachs of trout and salmon (n = 1,904) were collected from fish registered at fishing tournaments held in New York State waters of Lake Ontario between April and September 1983 and 1984. Numbers of adult-sized fish containing identifiable food items were 323 lake trout (Salvelinus namaycush), 289 brown trout (Salmo trutta), 24 rainbow trout (S. gairdneri), 164 coho salmon (Oncorhynchus kisutch), and 63 chinook salmon (O. tschawytscha) Proportional similarity in diet between pairs of species was high and normally exceeded 0.70; diet composition of individual species was similar between years. Alewives (Alosa pseudoharengus) were the main prey of all species during all months and were normally 110–149 mm in standard length. Rainbow smelt (Osmerus mordax) was the second most common prey eaten but was generally found in fewer than 20% of the stomachs examined during any month. Diet diversity was generally higher during April-May than during July-September for coho salmon, lake trout, and brown trout. Larger brown trout ate larger alewife in 1983 but not in 1984. Results suggest that the five trout and salmon species in Lake Ontario are potential competitors.     

Diet and Prey Selection by Lake Superior Lake Trout during Spring, 1986–2001

We describe the diet and prey selectivity of lean (Salvelinus namaycush namaycush) and siscowet lake trout (S. n. siscowet) collected during spring (April–June) from Lake Superior during 1986–2001. We estimated prey selectivity by comparing prey numerical abundance estimates from spring bottom trawl surveys and lake trout diet information in similar areas from spring gill net surveys conducted annually in Lake Superior. Rainbow smelt (Osmerus mordax) was the most common prey and was positively selected by both lean and siscowet lake trout throughout the study. Selection by lean lake trout for coregonine (Coregonus spp.) prey increased after 1991 and corresponded with a slight decrease in selection for rainbow smelt. Siscowet positively selected for rainbow smelt after 1998, a change that was coincident with the decrease in selection for this prey item by lean lake trout. However, diet overlap between lean and siscowet lake trout was not strong and did not change significantly over the study period. Rainbow smelt remains an important prey species for lake trout in Lake Superior despite declines in abundance.     

Spatial and temporal variability in lake trout diets in Lake Ontario as revealed by stomach contents and stable isotopes

Lake trout (Salvelinus namaycush) are an ecologically and economically important piscivore with reported differences in diet and feeding behaviour throughout its range. Eleven stomach content and stable isotope-based metrics were used to describe diets of 349 lake trout between two years (2013 and 2018) and among geographic zones (west, central, east, Kingston basin) in Lake Ontario. Using individual (e.g., volumetric, %V) and aggregate (e.g., index of relative importance, %IRI) diet metrics, we found an overwhelming dominance of alewife (Alosa pseudoharengus) in lake trout diets among some zones in 2013 (%V = 23.3 – 92.7; %IRI = 12.2 – 99.5) and all zones in 2018 (%V = 83.9 – 96.7; %IRI = 96.5 – 100). Round goby (Neogobius melanostomus) and rainbow smelt (Osmerus mordax) were secondary lake trout prey items with relative diet percentages only marginally reflected by spatial and temporal variation in prey abundance (round goby: %V = 1.0 – 33.3, %IRI = 0.1 – 13.2; rainbow smelt: %V = 2.5 – 54.0, %IRI = 0.1 – 54.0). Carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopic niche areas and orientations were similar across all year-zone combinations reinforcing temporal and spatial consistency in lake trout diet. The findings of this study advance the time series in describing Lake Ontario lake trout diets and can be used to complement stock assessments and management decisions associated with carrying capacity for the diverse salmonid community.     

Rainbow smelt population responses to species invasions and change in environmental condition

Invasive species can cause major disruptions in native food webs, yet the impact of species introductions and whether they will become invasive appears to be context-dependent. Rainbow smelt and alewife co-exist as invasive species in the Laurentian Great Lakes and as native species on the Atlantic coast of North America, but in Lake Champlain rainbow smelt is the dominant native forage fish and alewife are invasive. Alewife became abundant by 2007, providing an opportunity to explore the dynamics of these two species in a system where only one is invasive. We used data from a 28-year forage fish survey to compare demographics of rainbow smelt populations in three basins of Lake Champlain with different volumes, nutrient levels, and predator abundances. Rainbow smelt catch-per-unit-effort (CPUE) remained constant in the large, deep Main Lake before and after alewife invaded, but decreased in the two smaller basins. Declines were primarily a result of increased age-0 and age-1 mortality. Predation by top piscivores, system productivity, and competition for resources alone could not explain the patterns in CPUE across the basins. The mechanisms that allow alewife and rainbow smelt to co-exist could be related to system volume and oxythermal habitat availability, and may explain why the two species do not negatively affect each other in the Great Lakes. Summer hypoxia in the smaller basins could force individuals into smaller habitat volumes with higher densities of competitors and cannibalistic adult rainbow smelt. Habitat availability may mediate the impact of invasive alewife on native rainbow smelt.     

Estimating Larval Fish Abundances from Plankton Net Catches in Long Point Bay,Lake Erie,in 1971–78

Larval fish were sampled in Long Point Bay each summer in 1971–78 by towing ½– and 1-m diameter plankton nets at about 1 m/s. We report on the analysis of 2,576 tows, giving detailed consideration to sampling problems and statistical methods. The most common larvae were rainbow smelt (Osmerus mordax), caught in about 50% of the tows, and yellow perch (perca flavescens), clupeids, and cyprinids, caught in about 10 to 20% of the tows. Most of the data contained too high proportions of zero catches to be transformed to approximately normal distribution, and were therefore analyzed using the log-linear model rather than by analysis of variance. Except for cyprinids, larval fish seemed to be scarce in surface waters during the day. Small (4 to 10 mm long) smelt and perch larvae were caught more frequently in 526– than in 782–μm mesh nets. Very few perch and cyprinid larvae longer than 10 mm were ever caught. This, and a more rapid decline of daytime than nighttime catches of larval smelt through the mid-summer, suggests that the nets were avoided by at least the larger larvae. If avoidance of sampling gear is substantial, larval fish abundances may be overestimated in years of slow growth when the larvae remain catchable longer. We saw no evidence that the recent industrial development at Nanticoke, Ontario, has affected larval fish abundances so far. As the annual mean densities of larval fish varied 15-fold or more, it would require several years of observation to detect any but drastic future change from the current levels of abundance.     

Nearshore Spatial Distribution of Fishes in Gill Net Samples,Cayuga Lake,New York

Cayuga Lake fishes were collected monthly with experimental gill nets at depths of 2 to 31.5 m. Horizontally-placed nets were arranged to allow determinations of vertical and inshore-offshore variation in fish distribution and occupied temperatures. Warmwater species concentrated in inshore and surface waters at 19-23° C during the summer. Coldwater species concentrated in hypolimnetic waters at 9-15° C during the summer. Seasonal inshore-offshore movements were evident with all species except those which were captured only close to shore (northern pike, carp, smallmouth bass, white sucker). Depth ranges in which greatest numbers of common species occurred in the summer (July-September) were: alewife, inshore and surface to 15.3 m; lake trout, 15.3 to 30.5 m or more; northern pike, inshore and surface to 15.3 m; rainbow smelt, 7.6 to 30.5 m or more; spottail shiner, inshore and 7.6 m; trout-perch, inshore and 7.6 to 15.3 m; yellow perch, inshore and 7.6 to 15.3 m. Mean occupied temperatures (° C) of common species during the summer were: alewife 18.4 to 20.9; lake trout, 12.5 to 13.6; northern pike, 19.4; rainbow smelt, 10.4-14.4; spottail shiner, 19.6 to 21.5; trout-perch, 18.7-21.3; yellow perch, 19.4 to 21.0. Depth distribution was obviously influenced by temperature preferences and also by availability of food in certain species. Catch size was directly related to water temperatures in several species.     

Survey of four essential nutrients and thiaminase activity in five Lake Ontario prey fish species

Thiamine deficiency is an impediment to salmonine reproduction in the Great Lakes, but little is known about other measures of dietary quality, such as lipid-soluble vitamins or fatty acids in prey fish. The objective of the present research was to measure selected essential nutrients and thiaminase activity in five Lake Ontario prey fish species (alewife Alosa psuedoharengus, rainbow smelt Osmerus mordax, slimy sculpin Cottus cognatus, threespine stickleback Gasterosteus aculeatus and round goby Neogobius melanostomus). Total thiamine was greater in alewife (13.6 nmol/g) than in the other species (6.2–9.0 nmol/g). In 2006, thiaminase activity was unexpectedly high in goby (12.49 nmol/g/min), sculpin (1.99 nmol/g/min) and smelt (9.24 nmol/g/min). In 2007, thiaminase activity in goby (0.99 nmol/g/min) and smelt (4.94 nmol/g/min) was low compared to 2006, whereas sculpin thiaminase activity was greatest (6.01 nmol/g/min). The causes for this variability are unknown. Thiaminase activity was within the expected range for alewife (4.31–6.31 nmol/g/min) and stickleback (0.06 nmol/g/min). Concentrations of retinoids, carotenoids, vitamin E (tocopherol) and fatty acids also differed among prey fish species. Tocopherol concentrations in goby (12.74 ng/mg), sculpin (25.29 ng/mg), and smelt (22.81 ng/mg) were greater than in alewife (1.59 ng/mg). Goby had the lowest ∑ ω-3 to ∑ ω-6 fatty acid ratio (1.44) when compared to sculpin (2.97) and smelt (2.85). Thiaminase concentrations in alewife and smelt (and possibly goby) suggest that they have the potential to adversely affect natural reproduction in salmonines. Concentrations of carotenoids, retinoids and tocopherol in prey fish appear to be lower than salmonine dietary requirements.     

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.