Feeding ecology of lake whitefish larvae in eastern Lake Ontario

We examined the feeding ecology of larval lake whitefish (Coregonus clupeaformis) in Chaumont Bay, Lake Ontario, during April and May 2004–2006. Larvae were collected with towed ichthyoplankton nets offshore and with larval seines along the shoreline. Larval feeding periodicity was examined from collections made at 4-h intervals over one 24-h period in 2005. Inter-annual variation in diet composition (% dry weight) was low, as was spatial variation among collection sites within the bay. Copepods (81.4%), primarily cyclopoids (59.1%), were the primary prey of larvae over the 3-year period. Cladocerans (8.1%; mainly daphnids, 6.7%) and chironomids (7.3%) were the other major prey consumed. Larvae did not exhibit a preference for any specific prey taxa. Food consumption of lake whitefish larvae was significantly lower at night (i.e., 2400 and 0400 h). Substantial variation in diet composition occurred over the 24-h diel study. For the 24-h period, copepods were the major prey consumed (50.4%) and their contribution in the diet ranged from 29.3% (0400 h) to 85.9% (1200 h). Chironomids made up 33.4% of the diel diet, ranging from 8.0% (0800 h) to 69.9% (0400 h). Diel variation in the diet composition of lake whitefish larvae may require samples taken at several intervals over a 24-h period to gain adequate representation of their feeding ecology.     

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.     

Thiamine concentrations in lake whitefish eggs from the upper Great Lakes are related to maternal diet

Thiamine deficiency is responsible for reproductive impairment in several species of salmonines in the Great lakes, and is thought to be caused by the consumption of prey containing thiaminase, a thiamine-degrading enzyme. Because thiaminase levels are extremely high in dreissenid mussels, fish that prey on them may be susceptible to thiamine deficiency. We determined thiamine concentrations in lake whitefish Coregonus clupeaformis eggs from the upper Laurentian Great Lakes to assess the potential for thiamine deficiency and to determine if thiamine concentrations in lake whitefish eggs were related to maternal diet. Mean thiamine concentrations in lake whitefish eggs were highest in Lake Huron, intermediate in Lake Superior, and lowest in Lake Michigan. Some fish had thiamine concentrations below putative thresholds for lethal and sublethal effects in salmonines, suggesting that some larval lake whitefish may currently be at risk of at least sublethal effects of low thiamine concentrations, although thiamine thresholds are unknown for lake whitefish. Egg thiamine concentrations in lake whitefish eggs were statistically significantly related to isotopic carbon signatures, suggesting that egg thiamine levels were related to maternal diet, but low egg thiamine concentrations did not appear to be associated with a diet of dreissenids. Egg thiamine concentrations were not statistically significantly related to multifunction oxidase induction, suggesting that lower egg thiamine concentrations in lake whitefish were not related to contaminant exposure.     

Determinants of temperature in small coastal embayments of Lake Ontario

Along 25 km of the Lake Ontario shoreline near Toronto, Ontario, small coastal embayments (0.4–32 ha) have been constructed or modified by lake-infilling to restore warmwater fish habitat. The variation in thermal habitat quality for warmwater fishes among these embayments is very high; temperatures range from those found within a small pond to those of much cooler Lake Ontario. Since meteorological conditions and surface heat fluxes are almost identical, the temperature variation among embayments must be caused by differences in bathymetry or exchange with Lake Ontario. However, a previous study on these embayments found paradoxically that temperatures were not strongly associated with channel size or embayment bathymetry. This paper resolves the paradox by showing that flushing times for almost all of the constructed embayments were less than 1 day, and often less than 12 h. With so little time to warm within the embayments, water temperatures of almost all embayments remained very close to the temperatures of the adjacent lake waters. The coldest embayments connected directly to open Lake Ontario and warmer embayments connected to Lake Ontario through other embayments or protected harbors, where the inflowing water from Lake Ontario had already substantially warmed. To allow embayments along the exposed shoreline of Toronto to reach acceptable temperatures for warmwater fish, we use heat budgets to calculate that average summer flushing times must be increased from their current length of 1.5 to 5.5 h to approximately 30 h. Such changes could be achieved through large reductions in the channel cross section.     

Comparison of precision and bias of scale, fin ray, and otolith age estimates for lake whitefish (Coregonus clupeaformis) in Lake Champlain

We compared the precision, bias, and reader uncertainty of scales, dorsal fin rays, and otolith age estimates from 151 lake whitefish (Coregonus clupeaformis) from Lake Champlain, 2009. Mean and systematic differences in age estimates were compared among structures using consensus ages from two readers; precision of age structures was quantified through the use of age-bias plots, coefficient of variation, and percent agreement; reader confidence was indexed as a measure of overall reader uncertainty for each individual fish by structure. Mean age estimates based on otoliths were systematically higher (7.8 years) than based on scales (6.0 years) or fin rays (5.6 years). Ages determined using otoliths generated a wider range of ages and greater number of age classes (1-23 years, 20 age classes) when compared with scales (1-16 years, 15 age classes) and fin rays (1-14 years, 13 age classes). Otoliths were the most precise of the structures (CV = 4.7, compared with 7.4 for scales and 12.1 for fin rays). Percent agreement between readers indicated high precision and reproducibility of age estimates using otoliths. Percent reader uncertainty was lowest when using otoliths (7.6%) in comparison with fin rays (21.2%) or scales (26.8%). This study is the first evaluation of precision and bias of age structures for Lake Champlain's unexploited lake whitefish population and suggests that otoliths are the most appropriate structure for age estimation. However, the differences in age estimates from the three structures in this study emphasize the importance of validating aging structures to provide accurate age estimates for Lake Champlain.     

Contemporary diets of Lake Superior lake whitefish off the Keweenaw Peninsula and changes in condition from the 1980s to 2010s

Over the last two decades, declines in lake whitefish (Coregonus clupeaformis) recruitment and growth in many areas of the Laurentian Great Lakes have raised concerns about the status of this important species. Although Lake Superior populations have been less affected than those in other Great Lakes, these populations still face multiple threats. We characterized lake whitefish diets collected off the Keweenaw Peninsula between 2015 and 2017 and compared results to previous Lake Superior studies. We additionally estimated length-weight relationships to determine whether lake whitefish body condition (i.e., expected weight-at-length) had changed since the 1980s. Diet diversity was low, although individual specialization was moderate to high. Fish transitioned from consuming Diporeia in the spring to Mysis and fish eggs during fall and winter; sphaeriids composed 20–30% of diets across all seasons. Compared to findings for other Lake Superior regions, lake whitefish diets comprised lower percentages of high energy items (e.g., Diporeia, Mysis) and higher percentages of low energy items (e.g., sphaeriids). Expected weights in the 2000s and 2010s were lower in the 400- and 500-mm length groups but similar in larger lengths groups compared to the 1980s; condition was highest across all lengths in the 1990s. The observed decline in condition since the 1990s in the 400- and 500-mm length groups, in combination with possibly greater consumption of less energetically profitable items, suggests that lake whitefish <600 mm or preferred prey resources in this lake region may be experiencing stressors leading to condition declines, although what these stressors are remain unknown.     

Sea lamprey wounding in Canadian waters of Lake Huron from 2000 to 2009: Temporal changes differ among regions

Lake Huron has undergone a number of substantial changes in recent years, including changes to management of the parasitic sea lamprey, Petromyzon marinus. While control strategies of lamprey involving lampricides have had some success, lamprey spawning in St. Marys River has been a major and persistent problem and has led to intensified treatment beginning in 1998. The objective of our study was to broadly examine lamprey spatial wounding dynamics of lake whitefish (Coregonus clupeaformis) and lake trout (Salvelinus namaycush) within the Canadian waters of Lake Huron from 2000 to 2009. Temporal trends were evident and these differed among regions (North Channel, northern Main Basin, southern Main Basin, northern Georgian Bay, and southern Georgian Bay). There was a monotonic annual increase in probability of wounding for both lake trout and lake whitefish in three of the five regions, with high increases seen in both northern and southern Georgian Bay. The increases in three of the five regions are unexpected given the ongoing treatment of St. Marys River.     

Diver and Underwater Television Observations of Fish Behavior in a Great Lakes Commercial Trap Net

Behavior of lake whitefish (Coregonus clupeaformis) and lake trout (Salvelinus namaycush) in commercial trap-net pots was observed by scuba divers and by a low-light sensitive, underwater television (UTV) system. During the day, lake whitefish schooled and swam in circular patterns within the pot, keeping at least 1 m off the lake bottom. Occasionally, lake whitefish that were outside the net were observed swimming in concert with fish inside the pot. Whitefish schools disbanded at dawn and dusk, presumably because of reduced visual acuity during periods of low light. Lake trout were most often in groups of two or three individuals which were generally segregated from whitefish schools. Aggressive, escapement, and feeding behavior were not observed for either species. As nets were lifted, fish became very excited and darted about wildly. Net leads may have elicited schooling responses from fish, causing them to enter the pot. Fish stayed 3 to 5 m away from divers, making observations of behavior difficult. However, divers were able to make some observations which were not possible with UTV. Physical presence of UTV equipment did not alter fish behavior as did diver presence. Compared with divers, UTV could be deployed longer and at lower light levels. UTV also provided a permanent record (video cassette) that could subsequently be reviewed. An integration of UTV and diver observations gave the best understanding offish behavior in trap-net pots.     

Can otolith microchemistry identify the natal origin of larval lake whitefish Coregonus clupeaformis in the waters of Green Bay?

Lake whitefish Coregonus clupeaformis play a key role in the socioeconomics of the Laurentian Great Lakes region and serve as an important conduit of energy among trophic levels. Lake wide declines in adult abundance creates the need for improved understanding of population dynamics, currently hampered by a lack of early life history information and resolution of subpopulation structure. We predicted that otolith microchemistry had the potential to broaden our understanding of lake whitefish ecology by determining the natal origin of larvae at a resolution (e.g., spawning location) unachievable with other techniques. To test this prediction, in 2017 and 2018, recently hatched, pelagically-drifting lake whitefish larvae were collected from the open waters of Green Bay, Lake Michigan and four large tributaries (Menominee, Fox, Oconto and Peshtigo Rivers), where spawning activity has been recently documented. Linear discriminant function analyses conducted at the broadest level of resolution (i.e., river vs. open-water origin) correctly assigned 80.5% and 75.0% of individuals to their natal location in 2017 and 2018, respectively. Reclassification success at a finer resolution (i.e., specific rivers) ranged from 75.9% in 2017 to 42.1% in 2018. We discuss these promising results with respect to underlying variation in geological features, water chemistry and interannual river conditions that eggs and larvae experience during incubation and before prolonged drift.     

Lake Whitefish Relative Abundance,Length-at-Age,and Condition in Lake Michigan as Indicated by Fishery-independent Surveys

In the mid 1990s, growth and condition of lake whitefish (Coregonus clupeaformis) declined within commercial catches in Lake Michigan. However, underlying mechanisms responsible for the declines have not been thoroughly explored. Using fishery-independent survey data, we examined growth and relative abundance of adult whitefish over historical (1980–1990) and recent (1996–2005) time periods in three regions of Lake Michigan: north, mid, and south. Relative abundance was assessed from catch-per-unit-effort (CPUE) of independent surveys, and changes in growth conditions were evaluated using size-at-age estimates. Relative abundance increased in the mid and south regions between the two time periods and decreased in the north region. Length-at-age significantly declined between the two time periods in the north, mid, and south regions; the north region consistently had the lowest length-at-age. Condition also declined between the two time periods in each region. The decline in growth and condition coupled with increases in relative abundance suggest density-dependent mechanisms are contributing to the observed population changes in the south region. The north region does not appear to be regulated by density, suggesting density-independent mechanisms, such as food web changes, are influencing stocks. Changes in the mid region are likely from a mixture of increased lake whitefish abundance and food web changes. Using fishery-independent population data, our results suggest that multiple factors are potentially contributing differentially within three Lake Michigan regions to cause similar declines in length-at-age and condition of whitefish. These factors (e.g., food web changes, lake whitefish density) should be considered when managing the commercial fishery.     

Mercury concentrations in commercial fish species from Lake Winnipeg, 1971–2019

Concentrations of total mercury (THg) in the axial musculature of northern pike (Esox lucius), walleye (Sander vitreus), lake whitefish (Coregonus clupeaformis), and sauger (Sander canadensis) were analyzed from several regions of Lake Winnipeg and three main inflows, focusing on Mossy Bay in the North Basin of the lake. Length-standardized means (standard means) indicated THg concentrations in pike of 550 mm fork length (0.44 ppm) and walleye measuring 400 mm (0.38 ppm) from Mossy Bay were highest in 1971 and declined to 0.13 and 0.22 ppm, respectively, by 1974. Standard means of both these species have been similar since 2010 (walleye; approximately 0.11 ppm) and 2013 (pike; approximately 0.17 ppm), except for a significant increase to 0.15 ppm in walleye in 2019, potentially related to concurrent changes in trophic interactions with invasive fish and invertebrate species. Standard means of 300 mm long sauger (0.23–0.29 ppm) did not decline significantly between 1971 and 1974, and those of 350 mm long whitefish ranged from 0.006 to 0.028 ppm from 1983 to 2016 with no clear temporal trend. In concert with data from other areas of Lake Winnipeg and the three inflows for 2000–2019, these results indicate higher contemporary concentrations in the South Basin of the lake. This latitudinal gradient in fish THg is assumed to be a result of continuous mercury inputs from the Red and the Winnipeg River, whereas a former point source of industrial mercury in the Saskatchewan River is no longer relevant. According to human consumption limits based on tolerable daily intake calculations and current THg concentrations of fish from Mossy Bay, substantial quantities of whitefish, pike, and walleye fish can be safely eaten.     

Control rule performance for intermixing lake whitefish populations in the 1836 Treaty waters of the Great Lakes: A simulation-based evaluation

We conducted a simulation-based evaluation exploring the effects of population intermixing on the current 65% total annual mortality control rule used to manage lake whitefish (Coregonus clupeaformis) fisheries in the 1836 Treaty waters of the Laurentian Great Lakes. The simulations incorporated intermixing among four populations with characteristics similar to those of lake whitefish in northern lakes Huron and Michigan. Dynamics of each population were simulated for 100 years with each stock exploited by a single fishery. An age-structured assessment of each stock was conducted every third year, with the abundance, mortality, and recruitment estimates used with the current control rule to set future harvest limits. Overall aggregate yield of the modeled system was generally not affected by intermixing, but did depend on assumed productivity levels. Mean annual yields for individual fisheries were sensitive to mixing levels, with yields from fisheries coinciding with low productivity spawning populations often similar to those coinciding with high productivity populations when intermixing occurred. Variability in yield tended to decrease slightly as mixing rates increased. Intermixing did not have a large influence on measures of spawning population sustainability, but detection of declining population sizes in low productivity populations may be difficult when intermixing occurs because of fishery yields remaining high due to immigration of fish from other spawning areas. This raises concern about the current control rule given that low productivity populations had spawning population sizes less than 20% of the unfished level in most years.     

A Saponification Procedure for the Determination of Some Chlorinated Hydrocarbons in Fish

A rapid method is described for the separation and analysis of chlorinated hydrocarbon residues in Great Lakes fishes. The method involves saponification with alcoholic KOH and chromatography on activated silica gel column with petroleum ether and diethyl ether elutions. Final analysis is achieved by gas chromatography equipped with electron-capture detection system. The recovery of the chlorinated hydrocarbon residues, using the silica gel clean-up, ranged from 82 to 94% with an average standard deviation of ± 0.8. Analysis by this method of some chlorinated hydrocarbons in Lake Huron fish gave values ranging from 0.130 to 1.89 ppm for PCB, 0.018 to 0.76 ppm for DDT and metabolites, and 0.002 to 0.040 ppm for aldrin.