Bloater, Coregonus hoyi, are deepwater planktivores native to the Laurentian Great Lakes and Lake Nipigon. Interpretations of commercial fishery time series suggest they were common in Lake Ontario through the early 1900s but by the 1950s were no longer captured by commercial fishers. Annual bottom trawl surveys that began in 1978 and sampled extensively across putative bloater habitat only yielded one individual (1983), suggesting that the species had been locally extirpated. In 2012, a multiagency restoration program stocked bloater into Lake Ontario from gametes collected in Lake Michigan. From 2012 to 2020, 1,028,191 bloater were stocked into Lake Ontario. Bottom trawl surveys first detected stocked fish in 2015, and through 2020 ten bloater have been caught (total length mean = 129 mm, s.d. = 44 mm, range: 96-240 mm). Hatchery applied marks and genetic analyses confirmed the species identification and identified stocking location for some individuals. Trawl capture locations and acoustic telemetry suggested that stocked fish dispersed throughout the main lake within months or sooner, and the depth distribution of recaptured bloater was similar to historic distributions in Lake Ontario and other Great Lakes. Predicted bloater trawl catches, based on modeled population abundance and trawl survey efficiency, were similar to observed catches, suggesting that post-stocking survival is less than 20% and contemporary bottom trawl surveys can quantify bloater abundance at low densities and track restoration. (C) US Geological Survey and The Author(s). Published by International Association for Great Lakes Research on behalf of Elsevier B.V. All rights reserved.

Strain performance evaluations are vital for developing successful fishery management and restoration strategies. Here, we utilized genotypes from 36 microsatellites to investigate hatchery strain contribution to collections of naturally produced lake trout (Salvelinus namaycush) sampled across Lake Michigan. Strain composition varied by area, with recoveries of Seneca Lake strain exceeding expectations based on stocking records in northern Lake Michigan but performing similarly to other strains in southern Lake Michigan. Interstrain hybrids were present at moderate frequencies similar to expectations based on simulations, suggesting that strains are interbreeding randomly. We hypothesize that the superior performance of the Seneca Lake strain in northern Lake Michigan is partially due to adaptive advantages that facilitate increased survival in areas with high mortality from sea lamprey (Petromyzon marinus) predation, such as northern Lake Michigan. However, when this selective pressure is lessened, the Seneca Lake strain performs similarly to other strains. Our study demonstrates that strain performance can vary across small spatial scales and illustrates the importance of conducting thorough strain evaluations to inform management and conservation.

Under certain conditions, polychlorinated biphenyl (PCB) concentration in individuals of one sex of an adult fish population may exceed that of the other sex by more than a factor of two. This phenomenon, known as the PCB hot spot effect, has been postulated to be contingent upon the following two conditions: (1) presence of a PCB hot spot in the bottom sediments of the aquatic ecosystem, such that prey PCB concentrations in the hot spot region are substantially higher than prey PCB concentrations in locations distant from the hot spot, and (2) habitat use varying between the sexes, such that individuals of one sex inhabit the hot spot region to a considerably greater degree than individuals of the other sex. To test whether PCB concentrations in walleye Sander vitreus from lower Green Bay of Lake Michigan displayed a PCB hot spot effect, whole-fish PCB concentrations were determined in ten female and ten male adult walleye from the population spawning in the Fox River, the main tributary to lower Green Bay. In addition, mark-recapture data for the Fox River walleye population were analyzed to determine differences in spatial distributions between the sexes. Results revealed that the ratio of mean PCB concentration in males to mean PCB concentration in females was only 1.13, indicating the absence of a PCB hot spot effect. This result was corroborated by the mark-recapture data analysis, which showed no significant difference in habitat use between the sexes. Thus, although condition 1 was met, condition 2 was not met. Consequently, the PCB hot spot effect was not observed in the Fox River walleye population. Lack of a significant difference in PCB congener distributions between the sexes further corroborated our conclusions.

In general, fish residing in rivers differ from fish residing in lakes in their mercury (Hg) isotope ratios. Specifically, fish residing in lakes typically show enriched values for the isotope ratios of delta Hg-202 (mass-dependent fractionation of isotope Hg-202) and Delta Hg-199 (mass-independent fractionation of isotope Hg-199) compared with fish residing in rivers, because photochemical effects acting on Hg isotope ratios are stronger in lakes than in rivers. Whole-fish determinations of Hg isotope ratios in age-0 and adult (ages 4-11) walleye (Sander vitreus) caught in the Fox River, the main tributary to lower Green Bay of Lake Michigan, were dissimilar. Age-0 fish exhibited a river signature for (delta Hg-202 and Delta Hg-199, with means equal to 0.00 and 0.26 parts per thousand, respectively. Significantly elevated levels of delta Hg-202 and Delta Hg-199 were observed in adult fish, indicating that adult fish primarily resided in the bay. Our results implied that the Fox River serves as a nursery area for juvenile walleye in the Fox River-lower Green Bay ecosystem. Moreover, corrections for photochemical fractionation of delta Hg-202 revealed that age-0 and adult walleye shared the same source of Hg in this ecosystem. In addition, Hg isotope ratios did not significantly differ between the sexes of adult walleye, suggesting that these ratios did not fractionate during the Hg elimination process.

Since the 1950s, fishery agencies on Lake Michigan have pursued Lake Trout Salvelinus namaycush rehabilitation through Sea Lamprey Petromyzon marinus control, harvest regulations, and stocking millions of fish annually. Stocking was prioritized at four historically important spawning locations beginning in 1985, and coded wire tags (CWTs) were used to help evaluate performance. We used data from coded-wire-tagged fish captured in fishery-independent surveys from 1998 to 2014 to evaluate relative postrelease survival of Lake Trout, estimated by CPUE and corrected for the number of fish stocked, across 173 CWT lots of the 1994-2003 year-classes stocked at the four locations. Boosted regression tree models were used to assess the relative influence of four variables on Lake Trout CPUE in two age-groups (ages 4-5 and ages 6-10) and were paired with ANOVAs to test for statistical significance. Genetic strain (29.1%), stocking location (27.8%), mortality at release (23.1%), and predator density (19.9%) had similar influences on the relative survival of younger fish, whereas the relative survival of older fish was heavily influenced by stocking location (79.8%). Survival of both age-groups was lowest for fish stocked in the Northern Refuge, where the age structure was truncated due to fishery harvest and Sea Lamprey predation. Survival of stocked fish was higher at the Southern Refuge, Clay Banks, and Julian's Reef, where mortality from Sea Lamprey and harvest was lower and where increases in wild Lake Trout have been observed in recent years. Stocked Lake Michigan remnant genetic strains also appeared to survive better than strains from other lakes at these three locations, but strain effects could not be fully disentangled from the effects of stocking location, and the continued stocking of multiple genetic strains may provide resiliency to future selection pressures. Continued progress toward rehabilitation will require reducing fishing and Sea Lamprey-induced mortality in northern Lake Michigan to build parental stocks of advanced ages as well as balancing efforts among competing management goals.

Lake Trout Salvelinus namaycush were extirpated from Lake Michigan by the early 1950s, and as part of an effort to restore naturally reproducing populations, hatchery-reared fish have been stocked since the early 1960s. Stocked fish are marked with a fin clip to differentiate them from wild, lake-produced Lake Trout; marking error for the 20072010 year-classes of Lake Trout stocked by federal hatcheries averaged 3.0%. Egg deposition, emergent fry, and wild juvenile Lake Trout have previously been observed, but no sustained wild recruitment has been measured in assessment surveys or in sport and commercial fishery catches. In 2011 and 2012, we caught juvenile Lake Trout in gill-net and bottom trawl catches that were targeting Bloater Coregonus hoyi in water depths greater than 80m. Unclipped, wild Lake Trout represented 20% of all Lake Trout caught in a southern offshore region of Lake Michigan. In northwestern Lake Michigan wild recruits represented from 10% to 27% of the 20072009 year-classes, and we recovered a small number of wild Lake Trout from the 2010 year-class. This is the first evidence for consecutive year-classes of naturally produced Lake Trout surviving beyond the fry stage in Lake Michigan. Received July 13, 2012; accepted November 26, 2012

We compared the relative abundance of lake trout Salvelinus namaycush spawners in gill nets during fall 1999-2001 in Lake Michigan at 19 stocked spawning sites with that at 25 unstocked sites to evaluate how effective site-specific stocking was in recolonizing historically important spawning reefs. The abundance of adult fish was higher at stocked onshore and offshore sites than at unstocked sites. This suggests that site-specific stocking is more effective at establishing spawning aggregations than relying on the ability of hatchery-reared lake trout to find spawning reefs, especially those offshore. Spawner densities were generally too low and too young at most sites to expect significant natural reproduction. However, densities were sufficiently high at some sites for reproduction to occur and therefore the lack of recruitment was attributable to other factors. Less than 3% of all spawners could have been wild fish, which indicates that little natural reproduction occurred in past years. Wounding by sea lamprey Petromyzon marinus was generally lower for Seneca Lake strain fish and highest for strains from Lake Superior. Fish captured at offshore sites in southern Lake Michigan had the lowest probability of wounding, while fish at onshore sites in northern Lake Michigan had the highest probability. The relative survival of the Seneca Lake strain was higher than that of the Lewis Lake or the Marquette strains for the older year-classes examined. Survival differences among strains were less evident for younger year-classes. Recaptures of coded-wire-tagged fish of five strains indicated that most fish returned to their stocking site or to a nearby site and that dispersal from stocking sites during spawning was about 100 km. Restoration strategies should rely on site-specific stocking of lake trout strains with good survival at selected historically important offshore spawning sites to increase egg deposition and the probability of natural reproduction in Lake Michigan.

Beginning in 1995, the size of yearling lake trout (Salvelinus namaycush) stocked into the upper Great Lakes was increased based on the assumption that these fish would be healthier and survive better since natural mortality is thought to be inversely proportional to body size. We compared the postrelease, relative survival of paired stockings of lake trout reared to 44 fish/kg (standard size) with larger fish reared to 22–26 fish/kg (enhanced size) in Lake Michigan. About 60,000 lake trout each of the standard and enhanced sizes for the 1994–1997 year classes were released as yearlings in spring near Clay Banks Reef, Wisconsin, and identified with coded-wire tags and an adipose fin-clip. Recaptures were made from 1997 to 2003 in four gill net surveys conducted in spring and fall near the release location. Comparisons of catch-per-unit of effort corrected for numbers stocked generally indicated no significant differences in relative survival of standard and enhanced lake trout. An autopsy-based assessment of overall fish health and condition indicated few measures where significantly different between standard and enhanced lake trout prior to stocking. Size differences between standard and enhanced fish remained statistically significant at all observed ages at recapture; however growth rates were the same for the two groups. Stocking numbers at certain sites on Lake Michigan were reduced concurrent with the change to larger yearlings with the expectation of increased survival, which did not occur; hence recruitment was essentially reduced in these areas for the restoration program.

Adult walleye were collected from several locations in the Lower Fox River and Green Bay, Wisconsin (the assessment area) and two relatively uncontaminated reference locations (Lake Winnebago and Patten Lake, Wisconsin) between July and October in 1996 and 1997. Whole body and liver samples collected in 1996 were analyzed for total PCBs, PCB congeners, and liver histological lesions. Follow-up sampling in 1997 included examination of liver histopathology, PCBs in liver samples, measurement of ethoxyresorufin-O-deethylase (EROD) activity, immunological evaluation of kidney and blood samples, measurement of plasma vitellogenin, and examination of tissues for parasites as well as bacterial and viral infections. Mean PCB concentrations in whole body and liver samples were elevated in assessment area walleye (4.6 to 8.6 and 3.6 to 6.4 mg/kg wet weight, respectively) compared to PCB concentrations in reference areas (0.04 mg/kg in walleye fillets from Lake Winnebago). A significant (p<0.01) elevation was observed in the prevalence (26%) of hepatic preneoplastic foci of cellular alteration (FCA) and neoplasms in 5 to 8 year old walleye collected from the assessment area, compared to reference area fish (6% prevalence). Walleye from the assessment area also contained multiple FCA and hepatic tumors per liver sample, whereas no tumors and a reduced prevalence of FCA were observed in reference area walleye. Both tumors and FCA were more prevalent in female fish than in male fish within the 5 to 8 year age classes. There were no remarkable effects on immunological parameters in assessment area walleye, although hematocrit was elevated and blood monocyte counts were 40% lower than those of reference area fish. The data did not show any clear distinctions in the prevalence of disease between reference and assessment area walleye. EROD activity was similar in assessment area and reference area walleye. Plasma vitellogenin was elevated in female walleye from eastern Green Bay, but was not detected in male fish from this location. The results of this investigation demonstrate significant elevation in hepatic preneoplastic lesions and hepatocellular adenomas and carcinomas in assessment area walleye exposed to elevated concentrations of PCBs. These histopathological lesions are consistent with long-term exposure to tumor promoters such as PCBs, although quantitative association between tumors and PCBs was not observed at the level of the individual fish. Additional research would be needed to elucidate the causal mechanisms underlying tumorigenesis.

Abstract The restoration of lake trout Salvelinus namaycush in Lake Michigan has been an elusive goal of resource management agencies in the Great Lakes region. In this study, we estimated annual survival rates of adult lake trout from an area in northwestern Lake Michigan known as the Clay Banks refuge. We tagged and recaptured fish with gill nets during the fall spawning season (N = 12,175; 1983–1989 and 1991–1993) and with pound nets in the spring (N = 52,035; 1984–1990 and 1992–1993). We fit Cormack–Jolly–Seber models to the two sets of data. We had insufficient data to analyze annual differences in survival rates of fall-tagged fish, but we were able to estimate an overall annual survival rate of 0.67. Annual survival rates of spring-tagged fish varied between 0.53 and 0.88 and increased after 1987–1988. In addition to the mark–recapture studies, we analyzed catch rates of lake trout from gill-net and pound-net surveys to estimate survival rates using catch curve analyses; these annual rates were gen...

Progress toward lake trout restoration in Lake Michigan is described through 1993. Extinction of the native lake trout fishery by sea lamprey predation, augmented by exploitation and habitat destruction, resulted in an extensive stocking program of hatchery-reared lake trout that began in 1965. Sea lamprey abundance was effectively controlled using selective chemical toxicants. The initial stocking produced a measurable wild year class of lake trout by 1976 in Grand Traverse Bay, but failed to continue probably due to excessive exploitation. The overall lack of successful reproduction lakewide by the late 1970s led to the development and implementation in 1985 of a focused inter-agency lakewide restoration plan by a technical committee created through the Lake Committee structure of the Great Lakes Fishery Commission. Strategies implemented in 1985 by the plan included setting a 40% total mortality goal lakewide, creating two large refuges designed to encompass historically the most productive spawning habitat and protect trout stocked over their home range, evaluating several lake trout strains, and setting stocking priorities throughout the lake. Target levels for stocking in the 1985 Plan have never been reached, and are much less than the estimated lakewide recruitment of yearlings by the native lake trout stocks. Since 1985, over 90% of the available lake trout have been stocked over the best spawning habitat, and colonization of the historically productive offshore reefs has occurred. Concentrations of spawning lake trout large enough for successful reproduction, based on observations of successful hatchery and wild stocks, have developed at specific reefs. Continued lack of recruitment at these specific sites suggests that something other than stotk abundance has limited success. Poor survival of lake trout eggs, assumed to be related to contaminant burden, occurred in the late 1970s and early 1980s, but survival has since increased to equal survival in the hatchery. A recent increase in lamprey wounding rates in northern Lake Michigan appears to be related to the uncontrolled build-up of lampreys in the St. Marys River a tributary of Lake Huron. If left uncontrolled, further progress toward restoration in the Northern Refuge may be limited.

Suckers are thought to be harmful to other species of fish because of predation on eggs and fry and from competition, mainly for food. This belief has led to many sucker removal projects to enhance sport fisheries. In a review of the literature, we found that (1) although suckers eat eggs and fry of some fishes, there was no evidence that such predation is harmful to prey populations; (2) studies suggesting competition for food failed to demonstrate that food was limiting (which is necessary for competition to occur); (3) evidence is equivocal that sport fish populations fare better in the absence of suckers or after sucker removal programs. Reasons for the poor evidence that suckers are detrimental are the complexity of species interactions in aquatic communities, particularly interactive segregation (Nilsson 1967), as well as the lack of removal projects in which sucker removal alone was the sole variable. Guidelines are suggested for studies designed to elucidate the role of suckers in fish communities.