Lake Trout Eggs Research Articles

Thiamine status of lake trout in lake Ontario and its relation to diet after the colonization of round goby, 2005–2006

A predominance of alewives (Alosa pseudoharengus), a species having high thiaminase activity, in Lake Ontario lake trout (Salvelinus namaycush) diets, has been related to thiamine deficiency in lake trout eggs during 1994–2004. The late 1990s invasion by round goby (Neogobius melanostomus), that appear to have thiaminase activity of low biological activity, represented a potential to reduce the dietary importance of alewife and, as a result, increase lake trout thiamine levels if they became sufficiently important in lake trout diets. To evaluate whether lake trout thiamine levels increased as alewives were displaced by round gobies in lake trout diets, we collected 199 lake trout ranging from 305 to 893 mm in 2005–2006 and measured their muscle thiamine levels and diet composition. Diet composition (percent by weight) was estimated from MixSIR based on stable isotopes (δ15N and δ13C) measured from lake trout and their prey. Overall, alewife and goby dominated lake trout diet (78%), with round goby dominating the diet (55–57%) of smaller individuals (<600 mm), and alewife dominating the diet (59–73%) of larger, reproductively active individuals. Lake trout muscle thiamine declined with increases in lake trout length and the proportion of alewife eaten (p < 0.01). The proportion of lake trout below 500 pmol/g thiamine also declined; this threshold is associated with a loss of equilibrium in adults. Despite the increasing albeit size-related consumption of round goby, it remains inadequate as muscle thiamine levels in mature lake trout (i.e., >600 mm) during 2005–2006 appear unchanged from levels observed in 1996.

Thiamine concentrations in lake trout and Atlantic salmon eggs during 14 years following the invasion of alewife in Lake Champlain

Thiamine (vitamin B1) deficiency in Great Lakes salmonines has been linked to consumption of alewife Alosa pseudoharengus. Thiamine deficiency has been recognized as a possible impediment to lake trout Salvelinus namaycush recruitment in the Great Lakes and Atlantic salmon Salmo salar recruitment in the Finger Lakes and Baltic Sea. Alewife invaded Lake Champlain in 2003 which provided an opportunity to investigate changes in thiamine concentrations in salmonine predators during an alewife invasion. We monitored egg unphosphorylated and total thiamine concentrations in lake trout and Atlantic salmon in 2004 and 2007–2019, assessed whether concentrations were associated with mortality, and examined thiaminase activity in alewife. Total thiamine concentrations in lake trout and Atlantic salmon were significantly lower than in 2004 for seven of the ten collection years for lake trout and for nine of the 12 collection years for Atlantic salmon. Mortality and signs of thiamine deficiency were observed in laboratory-reared Atlantic salmon free embryos but not in lake trout. Average thiaminase activity in adult alewife declined from 5200 pmol/g/min in 2006 to 1500 pmol/g/min in 2012. Our results provide further evidence that a diet that includes alewife reduces egg thiamine concentrations in salmonines. This effect was observed within four years of the invasion of alewife.

Comprehensive assessment of legacy organic contaminants and trends in lake trout from Cayuga Lake, New York: 2011–2017

A seven-year assessment of lake trout contaminants and statistical trend modeling was conducted on Cayuga Lake, New York as the reference monitoring site for the USEPA Great Lakes Fish Monitoring and Surveillance Program (GLFMSP). In all, over 200 individual compounds including polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), organochlorine pesticides (OCPs), polychlorinated napthalenes (PCNs), polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) were monitored for the 2011–2017 study period. Legacy contaminants such as PCBs, OCPs, and PBDEs dominate the whole-fish lake trout contaminant profile in the 24–147 ng/g mean concentration range. Total dioxin-like contaminants monitored included PCDD/Fs, CP-PCBs, and PCNs with a mean concentration range of 2.0–516 pg/g. TEQ concentrations for the dioxin-like compounds ranged between 0.3 and 5.3 pg-TEQ/g, and total TEQ averaged 7.2 pg-TEQ/g for whole-fish lake trout. Contaminant trends were modeled using log-linear correlations for both whole-fish lake trout and lake trout eggs. All select compounds modeled for whole-fish lake trout showed significant decreases (average −53.4%) over the study period. Tissue residue guidelines for the protection of wildlife were uniformly exceeded for both mammals and birds based on mean 2011–2017 total TEQ concentrations measured. All lake trout skin-on fillets from Cayuga Lake analyzed exceed the USEPA human health screening value of 0.15 pg-TEQ/g-ww. Whole-fish lake trout from Cayuga Lake have significantly lower mean total TEQs (6.5 pg-TEQ/g) as compared with the average total TEQ in the Great Lakes (range: 21.9–50.8 pg-TEQ/g) (U = 10.000, p < 0.001), although rates of contaminant decline, contaminant ratios, and half-life (t1/2) are comparable.

Monitoring shallow benthic fish assemblages in the Laurentian Great Lakes using baited photoquadrats: Enhancing traditional fisheries monitoring methods

Abstract Photoquadrats and underwater video surveys are standard non-destructive monitoring methods in marine ecosystems and are becoming more common in freshwater systems. The high water clarity found in most of the Laurentian Great Lakes make them ideally suited for photoquadrat sampling. We compared the effectiveness of baited photoquadrats to monitor benthic fish communities of shallow, littoral habitats of northern Lake Michigan. We compared our results with baited minnow traps, a technique commonly used to monitor benthic fish communities in freshwater ecosystems. Photoquadrats baited with lake trout (Salvelinus namaycush) eggs, the most effective attractant, proved to be an efficient tool for sampling round goby (Neogobius melanostomus) relative abundance and resulted in higher round goby catches than in photoquadrats baited with a commercially available fish attractant or unbaited photoqudrats. This method allowed sites to be surveyed rapidly (requiring

Evaluation of the thiamine dose-response relationship for lake trout (Salvelinus namaycush) fry using an individual based model

Substantial natural reproduction of lake trout (Salvelinus namaycush) has not been achieved in the Great Lakes, except for Lake Superior and a few areas in Lake Huron, despite continued stocking efforts. Low thiamine levels in lake trout eggs, which can result in lethal and sublethal impacts (thiamine deficiency complex, TDC) on fry, may contribute to widespread recruitment failure in lake trout populations. We hypothesized that incorporation of sublethal impacts into dose-response curves would result in estimates of EC50s (median lethal concentrations) for fry greater than the estimates that rely only on acute mortality and that predation would exacerbate thiamine effects. To investigate the sublethal effects of TDC (prey capture success and predation mortality) on cohort growth and survival, we developed an individual-based model for lake trout fry. The model tracks daily activities, including consumption, respiration, growth, and mortality, of lake trout from hatch until fry reach a length of 33 mm when we assume fry feed naturally and thiamine effects are minimized. Model output with sublethal impacts resulted in an EC50 (7.3 nmol/g) that was greater than published studies that are limited to acute mortality (1.5 nmol/g). Furthermore, when we included interstitial and pelagic predation, the impact of sublethal effects shifted the EC50 values even higher (7.4–10 nmol/g). Simulation results indicate that low thiamine levels, in combination with moderate to high predation, can eliminate lake trout cohorts. Our simulations suggest that the sublethal effects of low thiamine can contribute to poor lake trout recruitment more than previously suspected.

Evidence of lake trout (Salvelinus namaycush) spawning and spawning habitat use in the Dog River, Lake Superior

Abstract Lake trout spawn primarily in lakes, and the few river-spawning populations that were known in Lake Superior were believed to be extirpated. We confirmed spawning by lake trout in the Dog River, Ontario, during 2013–2016 by the collection of and genetic identification of eggs, and we describe spawning meso- and microhabitat use by spawning fish. Between 2013 and 2016, a total of 277 lake trout eggs were collected from 39 of 137 sampling locations in the river. The majority of eggs (220) were collected at the transition between the estuary and the river channel crossing the beach. Lake trout eggs were most often located near the downstream end of pools in areas characterized by rapid changes in depth or slope, coarse substrates, and increased water velocities, where interstitial flows may occur. Depths in wadeable areas where eggs were found averaged 0.9 m (range: 0.4 to 1.3 m) and substrate sizes consisted of large gravel, cobble, and boulder; comparable to spawning characteristics noted in lakes. Water velocities averaged 0.66 m·s−1 (range: 0.33 to 1.7 m3·s−1) at mid-depth. This information on spawning habitat could be used to help locate other remnant river-spawning populations and to restore river-spawning lake trout and their habitat in rivers that previously supported lake trout in Lake Superior. The Dog River population offers a unique opportunity to understand the ecology of a river spawning lake trout population.

Identification of lake trout Salvelinus namaycush spawning habitat in northern Lake Huron using high-resolution satellite imagery

The availability and quality of spawning habitat may limit lake trout recovery in the Great Lakes, but comprehensive maps of current spawning habitats are unavailable at the basin scale. Current methods used to identify lake trout spawning locations are time- and labor-intensive and spatially limited. Due to the observation that some lake trout spawning sites are relatively clean of overlaying algae compared to adjacent areas not used for spawning, we hypothesized that spawning sites could be identified using satellite imagery. Satellite imagery collected just before and after the spawning season in 2013 was used to assess whether lake trout spawning habitat could be identified based on its spectral characteristics. Results indicated that Pleiades high-resolution multispectral satellite imagery can be successfully used to estimate the vegetative cover of substrates and temporal changes in vegetative coverage, and that models developed from processed imagery can be used to identify potential shallow water lake trout spawning sites based on comparison of sites where lake trout eggs were and were not observed after spawning. Satellite imagery is a new potential tool for identifying nearshore lake trout spawning habitat across broad areas of the Great Lakes.

Sculpin Predation on Lake Trout Eggs in Interstices: Skull Compression as a Novel Foraging Mechanism

Lake Trout (Salvelinus namaycush) spawn in fall and hatch in spring; to protect their eggs over the winter they spawn on rocky reefs, where the eggs settle deeply into interstices. Uptake of water swells the eggs, further protecting them by lodging in crevices. Freshwater sculpin (Cottidae), the primary predator of salmonid eggs, are small, benthic fishes with malleable bodies and large heads. To acquire the lipid-rich resource contained in eggs, sculpin must penetrate deeply into interstices, and have a method to extract lodged eggs. We placed Slimy Sculpin (Cottus cognatus) into a tunnel containing a sequence of funnels with progressively smaller openings to determine the smallest hole they could enter. Forty-three of 138 Slimy Sculpin (31%) moved through openings up to 19.7% smaller than the maximum width of their head, indicating that they compressed their skull to move through the opening. We examined egg acquisition behavior by placing Lake Trout eggs into holes drilled in a Plexiglass plate. Holes ...

Lake trout in northern Lake Huron spawn on submerged drumlins

Recent observations of spawning lake trout Salvelinus namaycush near Drummond Island in northern Lake Huron indicate that lake trout use drumlins, landforms created in subglacial environments by the action of ice sheets, as a primary spawning habitat. From these observations, we generated a hypothesis that may in part explain locations chosen by lake trout for spawning. Most salmonines spawn in streams where they rely on streamflows to sort and clean sediments to create good spawning habitat. Flows sufficient to sort larger sediment sizes are generally lacking in lakes, but some glacial bedforms contain large pockets of sorted sediments that can provide the interstitial spaces necessary for lake trout egg incubation, particularly if these bedforms are situated such that lake currents can penetrate these sediments. We hypothesize that sediment inclusions from glacial scavenging and sediment sorting that occurred during the creation of bedforms such as drumlins, end moraines, and eskers create suitable conditions for lake trout egg incubation, particularly where these bedforms interact with lake currents to remove fine sediments. Further, these bedforms may provide high-quality lake trout spawning habitat at many locations in the Great Lakes and may be especially important along the southern edge of the range of the species. A better understanding of the role of glacially-derived bedforms in the creation of lake trout spawning habitat may help develop powerful predictors of lake trout spawning locations, provide insight into the evolution of unique spawning behaviors by lake trout, and aid in lake trout restoration in the Great Lakes.

Perfluoroalkyl acids in the Canadian environment: Multi-media assessment of current status and trends

In Canada, perfluoroalkyl acids (PFAAs) have been the focus of several monitoring programs and research and surveillance studies. Here, we integrate recent data and perform a multi-media assessment to examine the current status and ongoing trends of PFAAs in Canada. Concentrations of perfluorooctane sulfonate (PFOS), perfluorooctanoate (PFOA), and other long-chain perfluorocarboxylates (PFCAs) in air, water, sediment, fish, and birds across Canada are generally related to urbanization, with elevated concentrations observed around cities, especially in southern Ontario. PFOS levels in water, fish tissue, and bird eggs were below their respective Draft Federal Environmental Quality Guidelines, suggesting there is low potential for adverse effects to the environment/organisms examined. However, PFOS in fish and bird eggs tended to exceed guidelines for the protection of mammalian and avian consumers, suggesting a potential risk to their wildlife predators, although wildlife population health assessments are needed to determine whether negative impacts are actually occurring. Long-term temporal trends of PFOS in suspended sediment, sediment cores, Lake Trout (Salvelinus namaycush), and Herring Gull (Larus argentatus) eggs collected from Lake Ontario increased consistently from the start of data collection until the 1990s. However, after this time, the trends varied by media, with concentrations stabilizing in Lake Trout and Herring Gull eggs, and decreasing and increasing in suspended sediment and the sediment cores, respectively. For PFCAs, concentrations in suspended sediment, sediment cores, and Herring Gulls generally increased from the start of data collection until present and concentrations in Lake Trout increased until the late 1990s and subsequently stabilized. A multimedia comparison of PFAA profiles provided evidence that unexpected patterns in biota of some of the lakes were due to unique source patterns rather than internal lake processes. High concentrations of PFAAs in the leachate and air of landfill sites, in the wastewater influent/effluent, biosolids, and air at wastewater treatment plants, and in indoor air and dust highlight the waste sector and current-use products (used primarily indoors) as ongoing sources of PFAAs to the Canadian environment. The results of this study demonstrate the utility of integrating data from different media. Simultaneous evaluation of spatial and temporal trends in multiple media allows inferences that would be impossible with data on only one medium. As such, more co-ordination among monitoring sites for different media is suggested for future sampling, especially at the northern sites. We emphasize the importance of continued monitoring of multiple-media for determining future responses of environmental PFAA concentrations to voluntary and regulatory actions.

Microhabitat association of Hemimysis anomala on fish spawning reefs in Grand Traverse Bay, Lake Michigan

In 2006 the bloody-red mysid (Hemimysis anomala), a new invasive species to the Great Lakes, was discovered in the Muskegon channel which flows into Lake Michigan. As predicted at the time of introduction, it quickly expanded its range in Lake Michigan and has recently been documented in Grand Traverse Bay near Elk Rapids, Michigan. Its effects on Great Lakes ecosystems, however, remain unclear owing to a lack of information on its abundance. Using a gear intended to sample lake trout and lake whitefish eggs from cobble substrate during spawning, we found variable densities of Hemimysis at three near shore reefs at Elk Rapids, Lake Michigan over a two year period. The highest densities of Hemimysis were found at the reef with the highest quality fish (i.e., lake trout and lake whitefish) spawning habitat based on the proportion of rounded cobble and rubble substrates, and the amount of interstitial space. Abundance of Hemimysis on all reefs was highly seasonally dependent, ranging from 0 to 31.4 per m². The highest numbers were seen during the fall when water temperatures were between 6 and 12°C. Based on the association of Hemimysis with cobble substrates and the abundance of this type of habitat in northeastern Lake Michigan, we predict substantial expansion of Hemimysis in this area of the lake. Additional evaluation of Hemimysis on fish spawning habitat is needed to determine potential interactions with other Great Lakes biota, particularly larval fish with which they may compete.

Increasing Thiamine Concentrations in Lake Trout Eggs from Lakes Huron and Michigan Coincide with Low Alewife Abundance

AbstractLake trout Salvelinus namaycush in the Laurentian Great Lakes suffer from thiamine deficiency as a result of adult lake trout consuming prey containing thiaminase, a thiamine‐degrading enzyme. Sufficiently low egg thiamine concentrations result in direct mortality of or sublethal effects on newly hatched lake trout fry. To determine the prevalence and severity of low thiamine in lake trout eggs, we monitored thiamine concentrations in lake trout eggs from 15 sites in Lakes Huron and Michigan from 2001 to 2009. Lake trout egg thiamine concentrations at most sites in both lakes were initially low and increased over time at 11 of 15 sites, and the proportion of females with egg thiamine concentrations lower than the recommended management objective of 4 nmol/g decreased over time at eight sites. Egg thiamine concentrations at five of six sites in Lakes Huron and Michigan were significantly inversely related to site‐specific estimates of mean abundance of alewives Alosa pseudoharengus, and successful natural reproduction of lake trout has been observed in Lake Huron since the alewife population crashed. These results support the hypothesis that low egg thiamine in Great Lakes lake trout is associated with increased alewife abundance and that low alewife abundance may currently be a prerequisite for successful reproduction by lake trout in the Great Lakes.Received January 4, 2011; accepted July 20, 2011

A Comparison of Egg Funnel and Egg Bag Estimates of Egg Deposition in Grand Traverse Bay, Lake Michigan

AbstractReefs are used for spawning by Great Lakes fishes such as the lake trout Salvelinus namaycush, lake whitefish Coregonus clupeaformis, and cisco C. artedi. We designed egg funnels as a new type of sampling gear for quantifying egg deposition while minimizing losses of eggs to physical disturbance and predation. Egg funnels were compared with traditionally used egg bags to quantify egg deposition by spawning fish. In addition, the efficiencies of the egg funnels and egg bags were measured by seeding each gear type with artificial lake trout and lake whitefish eggs in situ. Egg funnels were as efficient as or more efficient than egg bags for capturing artificial eggs and naturally deposited lake trout eggs in both 2008 and 2009. Egg funnels had lower efficiency than egg bags for capture of natural coregonid eggs in 2008, potentially due to avoidance of the gear by spawners; gear avoidance was reduced by burying the egg funnels more deeply in 2009. Accurate estimates of egg deposition will assist fisheries managers in making more‐informed decisions for management actions, such as stocking strategies and habitat protection.Received November 29, 2010; accepted April 26, 2011

Predation of lake trout and lake whitefish embryos by crayfish: Implications of shifts in crayfish dominance in Lake Simcoe

We compared predation rates of two Lake Simcoe crayfish, Orconectes virilis and Orconectes propinquus, on lake trout and lake whitefish eggs and alevins in laboratory experiments and assessed the potential impact of predation on egg survival in the lake. Experiments were conducted at 4 and 8 °C on gravel and one or three layers of cobble substrate. Predation rates on lake trout eggs at 8 °C were 2.3 and 3.6 eggs/crayfish/day on gravel, 1.3 and 1.4 on one layer of rock, and 1.2 and 0.2 on three layers of rock for O. propinquus and O. virilis, respectively. Similar rates were observed for lake whitefish eggs but the effect of substrate depth was greatly reduced. Predation rates were significantly lower at 4 °C. Overall O. propinquus was a more effective predator of eggs than the larger O. virilis, even after adjustment to a common body size. Lake trout alevins were eaten by both species, but at about one third the rate of lake trout eggs. Vulnerability of alevins declined in deeper substrate, especially for older alevins. In the lake O. propinquus was abundant on rocky substrates to the near exclusion of O. virilis. Based on observed densities of O. propinquus in Lake Simcoe, estimated consumption of lake trout eggs ranged from 5.6 to 16.6 eggs/m 2/day depending on temperature and substrate depth. Shifting crayfish dominance from O. virilis to O. propinquus, or vice versa, has the potential to significantly influence egg survival and recruitment success of lake trout and to a lesser degree lake whitefish in Lake Simcoe.

Development of Two New Sampling Techniques for Assessing Lake Trout Reproduction in Deep Water

AbstractEfforts to restore lake trout Salvelinus namaycush at shallow sites have yielded minimal success in the lower four Great Lakes, and management agencies are examining offshore deepwater reefs as possible sites for restoration efforts. Previous techniques developed for assessing egg deposition and fry relative abundance in shallow waters are not viable for use on deep offshore reefs. To assess lake trout reproduction at deep reefs, two new methods (deepwater egg traps and remote electrofishing) were tested in Lake Champlain, Vermont. Deepwater egg traps were paired with buried egg bags to compare egg collections between the two types of gear at shallow sites in Lake Champlain. After testing, the new egg traps were deployed in Lake Michigan to quantify egg deposition at two sites within the Mid‐Lake Reef Complex. There was a significant positive correlation between estimates of egg density in the paired deepwater traps and buried egg bags and no significant difference between egg densities in the two types of gear. In Lake Michigan, eggs were collected at depths greater than 35 m; the egg densities within the Mid‐Lake Reef Complex were above or similar to egg densities at shallow sites in Lake Michigan. To assess fry relative abundance, a remotely operated electrofisher with video was tested on different densities and life stages of lake trout fry in the laboratory and was used to detect fry in Lake Champlain. The remote electrofisher detected fry in 90% of laboratory trials and 34–56% of field trials. Fry were vulnerable to the electrofisher at all developmental stages, and fry detection from the remote electrofisher was proportional to fry density in the laboratory and fry catch per unit effort from traditional emergent fry traps. These methods provide the potential to detect and quantify lake trout egg deposition and detect fry at deep offshore sites and may improve evaluation of the contribution of deep reefs to lake trout restoration.

Increase in lake trout reproduction in Lake Huron following the collapse of alewife: Relief from thiamine deficiency or larval predation?

In the Great Lakes there is still uncertainty as to the population level effects of a thiamine deficiency on salmonines caused by high consumption of alewives Alosa pseudoharengus. A resurgence of lake trout Salvelinus namaycush reproduction in Lake Huron following the crash of alewife stocks between 2002 and 2004 provided an opportunity to evaluate the relative effects of this crash on reproduction through relief from either alewife mediated thiamine deficiency or alewife predation on larval lake trout relative to possible changes in the size of the lake trout spawning stock. Changes in mean lake trout egg thiamine concentration post crash at one spawning reef in Parry Sound, where mean thiamine concentration increased by almost two-fold, were consistent with diet switching from alewives to rainbow smelt Osmerus mordax, the next most abundant prey fish in Lake Huron. Although thiamine levels for lake trout collected at a second reef in Parry Sound did not change post-crash, levels both pre- and post-crash were consistent with a rainbow smelt diet. A reef specific fry emergence index was found to be positively related to reef specific egg thiamine concentration but negatively related to reef specific occurrence of EMS, a thiamine deficiency related mortality syndrome. We found little evidence for overlap between the timing of spring shoreward migration of alewives and lake trout emergence, suggesting that relief from alewife predation effects had relatively little effect on the observed increase in lake trout recruitment. Numbers of spawners in the north, north-central, and southern zones of the lake increased from 2000 onwards. Overall the abundance post-2003 was higher than from pre-2004, suggesting that spawner abundance may also have contributed to increased lake trout reproduction. However, predicted numbers of spawners and measured abundance of wild recruits in assessment gear were poorly correlated suggesting that the increase in reproduction was not totally spawner dependent and hence relief from thiamine deficiency was also likely involved. We conclude from this that eliminating the effects of an alewife diet mediated thiamine deficiency can have positive effects on lake trout reproduction but more research is required to understand the effect of spawner number and the role of spawning habitat availability.

Egg Fatty Acid Composition from Lake Trout Fed Two Lake Michigan Prey Fish Species

We previously demonstrated that there were significant differences in the egg thiamine content in lake trout Salvelinus namaycush fed two Lake Michigan prey fish (alewife Alosa pseudoharengus and bloater Coregonus hoyi). Lake trout fed alewives produced eggs low in thiamine, but it was unknown whether the consumption of alewives affected other nutritionally important components. In this study we investigated the fatty acid composition of lake trout eggs when females were fed diets that resulted in different egg thiamine concentrations. For 2 years, adult lake trout were fed diets consisting of four combinations of captured alewives and bloaters (100% alewives; 65% alewives, 35% bloaters; 35% alewives, 65% bloaters; and 100% bloaters). The alewife fatty acid profile had higher concentrations of arachidonic acid and total omega-6 fatty acids than the bloater profile. The concentrations of four fatty acids (cis-13, 16-docosadienoic, eicosapentaenoic, docosapentaenoic, and docosahexaenoic acids) were higher in bloaters than in alewives. Although six fatty acid components were higher in lake trout eggs in 2001 than in 2000 and eight fatty acids were lower, diet had no effect on any fatty acid concentration measured in lake trout eggs in this study. Based on these results, it appears that egg fatty acid concentrations differ between years but that the egg fatty acid profile does not reflect the alewife-bloater mix in the diet of adults. The essential fatty acid content of lake trout eggs from females fed alewives and bloaters appears to be physiologically regulated and adequate to meet the requirements of developing embryos.

Linking Egg Thiamine and Fatty Acid Concentrations of Lake Michigan Lake Trout with Early Life Stage Mortality

The natural reproduction of lake trout Salvelinus namaycush in Lake Michigan is thought to be compromised by nutritional deficiency associated with inadequate levels of thiamine (vitamin B1) in their eggs. However, mortality driven by thiamine deficiency (commonly referred to as early mortality syndrome [EMS]) is not the only significant cause of low lake trout survival at early life stages. In this study, we sought to better understand the combined effects of variable levels of thiamine and fatty acids in lake trout eggs on prehatch, posthatch, and swim-up-stage mortality. We sampled the eggs of 29 lake trout females from southwestern Lake Michigan. The concentrations of free thiamine and its vitamers (e.g., thiamine monophosphate [TMP] and thiamine pyrophosphate [TPP]) as well as fatty acid profiles were determined in sampled eggs. Fertilized eggs and embryos were monitored through the advanced swim-up stage (1,000 degree-days). Three distinct periods of mortality were identified: prehatch (0-400 degree-days), immediately posthatch (401-600 degree-days), and swim-up (601-1,000 degree-days). Stepwise multiple regression analysis revealed (1) that cis-7-hexadecenoic acid in both neutral lipids (NL) and phospholipids (PL) correlated with prehatch mortality, (2) that docosapentaenoic acid in PL and docosahexaenoic acid in NL correlated with posthatch mortality, and (3) that total lipids, TPP, and palmitoleic acid in NL, linoleic acid, and palmitic acid in PL correlated with the frequency of EMS. These results indicate the complexity of early life stage mortality in lake trout and suggest that inadequate levels of key fatty acids in eggs, along with variable thiamine content, contribute to the low survival of lake trout progeny in Lake Michigan.

Addition of round gobies to the prey community of Lake Ontario and potential implications to thiamine status and reproductive success of Lake Trout

The addition of round goby to the Lake Ontario prey fish community can, because it appears to lack thiaminase, ameliorate the effects of a diet high in thiaminase-rich alewives associated with a thiamine deficiency in Lake Trout. However, the effects of round goby predation may negate such effects. We conducted studies to assess the proportion, by weight, of round gobies in the diet of contemporary Lake Ontario Lake Trout and predicted its effect on egg thiamine concentration, and early mortality syndrome (EMS) and growth retardation, two indicators of thiamine deficiency. We compared these parameters to that for the historic prey community, as well as possible future diet scenarios with progressively greater proportions of round goby (e.g. 50, 75 and 100%). To assess the combined and separate effects of the thiamine deficiency we used a modeling approach to determine the effect of EMS (70%) typical of Lake Ontario Lake Trout, growth retardation, density of Lake Trout egg (e.g. 100, 500, 1000, and 5000 eggs m−2) and round goby (e.g. 1, 9, 18, 35, 75, 150 gobies m−2), three separate durations of round goby residence on Lake Trout spawning habitat (e.g. 30, 60 and 213 days), and at either a shallow water or deepwater temperature regime. Eight years after first invading Lake Ontario, the proportion of round gobies in Lake Trout diets remains low, although the highest observed (25%) would result in significant amelioration of the thiamine deficiency from past conditions. Modeling indicated that the negative effects associated with round goby predation on eggs and fry could easily exceed any positive effects resulting from amelioration of the thiamine deficiency by a diet containing round goby. The effects of goby predation were found to be dependent on density of both Lake Trout eggs and round gobies, and while predation effects were minimally affected by temperature regime, they were highly dependent on the period of round goby residence on Lake Trout spawning habitat.

Effects of Temperature and Density on Consumption of Trout Eggs by Orconectes propinquus and O. rusticus

Using laboratory experiments, we assessed the effect of temperature and predator density on consumption of trout eggs by crayfish. We quantified the effect of four temperature ranges (2–3, 4–5, 7–8, and 10–12°C) on consumption of lake trout ( Salvelinus namaycush) and rainbow trout ( Oncorhynchus mykiss) eggs by a native ( Orconectes propinquus) and an exotic (Orconectes rusticus) crayfish found in the Great Lakes. Mean individual consumption ranged from 0.1 to 1.5 lake trout eggs per day and from 1.0 to 5.5 rainbow trout eggs per day, respectively. Temperature influenced consumption for O. rusticus feeding on rainbow trout eggs but not O. propinquus. The effect of conspecific predator competition on rainbow trout egg consumption was evaluated using three densities of crayfish (1, 5, 10 per tank). For O. rusticus, mean individual consumption rate was higher at the lowest crayfish density (1 crayfish/tank) than at medium (5 crayfish/tank) or high (10 crayfish/tank) densities. A similar conspecific effect was not evident in O. propinquus and total consumption per unit area increased with crayfish density. The effect of competition among crayfish species on consumption of rainbow trout eggs was examined by comparing feeding rates at high crayfish abundance levels in single and mixed species treatments. Mixed species interactions did not significantly alter crayfish feeding rates. Results from these experiments provide empirical data to aid in modeling how temperature and predator density affect lake trout egg predation rates in the field.