Round Whitefish Research Articles

Effects of Fixed and Fluctuating Temperature on Hatch of Round Whitefish and Lake Whitefish Eggs

AbstractTemperature‐response information for use in evaluating thermal discharges is often over 30 years old and in the nonpeer‐reviewed literature, especially for Round Whitefish Prosopium cylindraceum and Lake Whitefish Coregonus clupeaformis exposed to nonlethal, elevated, and variable temperatures. Egg incubation experiments on Round Whitefish collected in Lake Ontario and Lake Whitefish collected in Lake Huron were carried out from December 13, 2011, to April 7, 2012. Experimental treatments included ambient baseline control conditions as well as fixed and fluctuating (variable) temperature increases of 1, 2, 3, and 5°C above ambient baseline conditions. For both species, the window for hatching for all experimental temperature treatments was variable (range, 10–38 d for Round Whitefish and 11–44 d for Lake Whitefish), and the hatching windows tended to be greater as temperatures increased. Our results indicated that both fixed and variable incremental increases in temperature above ambient baseline conditions have a statistically significant effect on 50% hatch, and hatch occurs earlier with higher incremental temperature increases. The ecological significance of advanced hatch, such as indirect mortality and food source availability, was evaluated.Received February 19, 2013; accepted July 8, 2013

Biotic interactions in temporal trends (1992–2010) of organochlorine contaminants in the aquatic food web of Lake Laberge, Yukon Territory

Declines in 6 organochlorine (OC) contaminant groups; chlordane (CHL), DDT, HCH, toxaphene (CHB), PCB and chlorinated benzenes (CBz) were measured in biota of a sub-Arctic lake (Lake Laberge, YT) following the closure of a commercial fishery in 1991. This study examined morphological (length, weight, age), biochemical (lipid content, δ13C, δ15N), population and OC data for 9 fishes and zooplankton between 1993 and 2003 (2010 for lake trout) to investigate causes for the OC declines. Growth dilution was a major factor influencing the decrease of OCs in lake trout, round whitefish and possibly zooplankton most notably in the early 2000s. A decline in lipids of most fish species also contributed to OC declines, although no such change was evident for zooplankton. It is suspected that increases in fish populations or climate variations over the 1990s, may have contributed towards a shift in plankton community composition. From 1991 to 1999, CPUE increased for 7 of the fish species and declined for 2 others. Concurrently, the zooplankton community shifted from an abundance of C. scutifer in 1993 to dominance by D. pribilofensis in 2001. Nitrogen and carbon stable isotope data suggested that food web interactions for most fish species have not changed over time. Although concentrations of OCs have declined in many fishes, the “rate” of OC transfer (using slopes of log OC vs. nitrogen isotope ratios) through the food web was greater in 2001 than in 1993. Overall, the declines in OC concentrations in the fish from Lake Laberge occurred concurrently with changes in their growth, lipid, and abundance, suggesting that ecosystem responses to the closure of the fishery were in part responsible for the lower contaminants in these fishes. As a result of this study, the Yukon government rescinded the health advisory for limiting the consumption of fish from Lake Laberge.

Development and characterization of twenty-two novel microsatellite markers for the mountain whitefish, Prosopium williamsoni and cross-amplification in the round whitefish, P. cylindraceum, using paired-end Illumina shotgun sequencing

We isolated and characterized a total of 22 microsatellite loci in Prosopium williamsoni. Loci were screened in 24 individuals from across a portion of its range in the Peace River area of northeastern British Columbia. The number of alleles per locus ranged from 3 to 19, observed heterozygosity ranged from 0.111 to 0.950, and the probability of identity values ranged from 0.013 to 0.604. These new loci will be used for conducting investigations into the genetic structure and diversity of extant populations of this important forage fish. The loci were also screened in the congener P. cylindraceum, which is of interest as an indicator species in the Great Lakes region of North America; 15 of the 22 loci produced scorable PCR products in the round whitefish.

Assessing feeding competition between lake whitefish Coregonus clupeaformis and round whitefish Prosopium cylindraceum

Abstract We collected lake whitefish Coregonus clupeaformis and round whitefish Prosopium cylindraceum from the main basin of Lake Huron and Georgian Bay in the Laurentian Great Lakes of North America to investigate details of diet and feeding tactics of these species in different seasons. Lake whitefish supports important commercial fisheries in Lake Huron and both species make use of habitats near the Bruce Nuclear Power Development, on the eastern shore of Lake Huron. Most fish of both species showed generalist feeding behavior, but some lake whitefish appeared to show specialist prey selection. The invasive spiny water flea Bythotrephes longimanus was an important component of the diet of both species. There was considerable dietary overlap between the whitefish species, but the ecological implications of these dietary overlaps are mitigated by the fact that dominant prey species differed in most seasons. We conclude that the potential for ecologically significant interactions between lake whitefish and round whitefish resulting from competition for similar benthic food resources in the main basin of Lake Huron is probably low.

Discrimination of the Endangered Atlantic Whitefish from Lake Whitefish and Round Whitefish by Use of External Characters

AbstractThe scope of phenetic variation exhibited by populations of lake whitefish Coregonus clupeaformis in the Canadian Maritime Provinces is not known, which complicates the task of accurately identifying specimens of the endangered Atlantic whitefish C. huntsmani. We developed criteria for accurate (≥95% correct classification) species discrimination using morphological data acquired from archival and recent coregonid collections from the Maritime Provinces. We used discriminant function analyses to assess (1) the usefulness of Atlantic whitefish external typological characters in accurately identifying species, (2) the temporal stability of reportedly diagnostic characters, and (3) which morphological characters could be used to accurately identify these coregonids to species. Not all external typological characters were robust in the discrimination of recent Atlantic whitefish and lake whitefish collections. Several previously identified diagnostic characters exhibited significant temporal variation but remained robust in species discrimination. Species‐specific differences for the number of lateral line scales, mouth position, and proportional pectoral fin length could be used concomitantly for accurate (≥95% correct classification) identification of coregonid species in the Canadian Maritime Provinces. Atlantic whitefish possessed a significantly (P < 0.001) greater number of lateral line scales (range = 90–102; mean ± SD = 93.79 ± 2.70) than lake whitefish (range = 66–91; mean = 77.18 ± 4.21), a significantly (P < 0.01) shorter pectoral fin than lake whitefish (11% and 16% of fork length, respectively), and a significantly (P < 0.001) more terminal mouth than lake whitefish (ratio of upper jaw to lower jaw: mean ± SD = 0.98 ± 0.07 and 0.82 ± 0.06, respectively). These characters can also be used to distinguish between Atlantic whitefish and round whitefish Prosopium cylindraceum collected in the Maritime Provinces.

Nearshore Habitat and Fish Community Associations of Coaster Brook Trout in Isle Royale, Lake Superior

AbstractWe characterized the nearshore habitat and fish community composition of approximately 300 km of shoreline within and adjacent to the major embayments of Isle Royale, Lake Superior. Sampling yielded 17 species, of which 12 were widespread and represented a common element of the Lake Superior fish community, including cisco Coregonus artedi, lake whitefish C. clupeaformis, round whitefish Prosopium cylindraceum, lake trout Salvelinus namaycush, rainbow smelt Osmerus mordax, lake chub Couesius plumbeus, longnose sucker Catostomus catostomus, white sucker C. commersonii, trout‐perch Percopsis omiscomaycus, ninespine stickleback Pungitius pungitius, burbot Lota lota, and slimy sculpin Cottus cognatus. The presence of brook trout S. fontinalis in an embayment was associated with the common species of the Isle Royale nearshore fish community, particularly cisco, longnose sucker, and round whitefish. However, brook trout were present in only five embayments and were common only in Tobin Harbor. Most Isle Royale embayments had broadly overlapping ranges of nearshore habitats. Within embayments, fish were distributed along a habitat gradient from less‐protected rocky habitat near the mouth to highly protected habitat with mixed and finer substrates at the head. Embayments with brook trout had greater mean protection from the open lake, greater variation in depth, greater mean cover, and higher mean frequencies of large substrates (cobble, boulder, and bedrock). Within those embayments, brook trout were associated with habitat patches with higher mean frequencies of small substrates (particularly sand and coarse gravel). Within Tobin Harbor, brook trout were associated with midembayment habitat and species assemblages, especially those locations with a mixture of sand, gravel, and cobble substrates, an absence of bedrock, and the presence of round whitefish, white sucker, and trout‐perch. Comparison of embayments with the model, Tobin Harbor, showed that six embayments without brook trout had very similar arrays of habitat. However, four embayments with brook trout had relatively different arrays of habitat from Tobin Harbor. These results suggest that there is potential for further recovery of brook trout populations across Isle Royale nearshore habitats.

Lake trout demographics in relation to burbot and coregonine populations in the Algonquin Highlands, Ontario

The objective of the study was to test the hypothesis that lake trout populations change in relation to cisco, lake whitefish, round whitefish and burbot populations in lakes in the Algonquin Highlands region of Ontario. Lake trout population change is greatest where cisco and lake whitefish are present. Lake trout populations in lakes without either coregonine tend to have small adults and many juveniles. Where cisco or lake whitefish are present, adult lake trout are large, juvenile abundance is low, and the stock-recruit relationship appears to be uncoupled likely due to a larval bottleneck. Lake trout populations in these lakes may be sensitive to overfishing and recruitment failure. Lake trout populations do not appear to change in relation to round whitefish. There appears to be an indirect positive change on juvenile lake trout abundance through reductions in the density of benthic coregonines in the presence of large, hypolimnetic burbot.

Otolith Trace Element Chemistry as an Indicator of Anadromy in Yukon River Drainage Coregonine Fishes

AbstractEight coregonine species have been documented in the Yukon River drainage. They include inconnuStenodus leucichthys, broad whitefishCoregonus nasus, humpback whitefishC. pidschian, Alaska whitefishC. nelsonii, least ciscoC. sardinella, Bering ciscoC. laurettae, round whitefishProsopium cylindracium, and pygmy whitefishP. coulterii. Personal use, sport, and commercial fisheries within the drainage target several of these species. Some species are capable of anadromous life histories, as evidenced by their presence in estuaries, yet few studies have investigated the upstream migrations of anadromous components of these populations. Only inconnu migrations have been previously examined in the Yukon River drainage. We investigated the distribution of anadromous coregonine fish in the Yukon River drainage in Alaska using sampling and otolith chemistry procedures. Six species were identified in sample collections from eight regions of the drainage between 1,200 and 2,000 km upstream from the Bering Sea. Gonadosomatic indices indicated that most sampled fish of all species were mature and preparing to spawn. Anadromous inconnu, broad whitefish, and humpback whitefish were distributed in the Yukon River and its tributaries to a maximum distance of 1,700 km from the sea. Anadromous least cisco were distributed in the Yukon River and its tributaries up to 1,600 km from the sea, whereas anadromous Bering cisco were present in the Yukon River main stem as far upstream as 2,000 km. No anadromous round whitefish were detected. Few coregonine spawning areas have been identified, so the actual migration distances of anadromous species may be greater than presented here.

Lake Whitefish and Lake Herring Population Structure and Niche in Ten South-Central Ontario Lakes

This study compares simple fish communities of ten oligotrophic lakes in south-central Ontario. Species densities and population size structure vary significantly among these lake communities depending on fish species present beyond the littoral zone. Lake whitefish are fewer and larger in the presence of lake herring than in their absence. Diet analysis indicates that lake whitefish shift from feeding on both plankton and benthic prey when lake herring are absent to a primarily benthic feeding niche in the presence of lake herring. When benthic round whitefish are present, lake whitefish size and density decline and they move lower in the lake compared to round whitefish. Burbot are also fewer and larger in lakes with lake herring than in lakes without herring. Burbot, in turn, appear to influence the population structure of benthic coregonine species. Lower densities of benthic lake whitefish and round whitefish are found in lakes containing large benthic burbot than in lakes with either small burbot or where burbot are absent. Predation on the pelagic larvae of burbot and lake whitefish by planktivorous lake herring alters the size and age structure of these populations. As life history theory predicts, those species with poor larval survival appear to adopt a bet-hedging life history strategy of long-lived individuals as a reproductive reserve.

Effect of landscape factors on fish distribution in arctic Alaskan lakes

Summary1. The distribution of species is affected by many factors operating at a variety of temporal and spatial scales in a heterogeneous landscape. In lakes, fish communities are dynamic, influenced by landscape‐level factors that control colonisation and extinction.2. We used classification and regression tree (CART) analyses to quantify the importance of landscape‐level factors in determining the distribution of fish species in 168 arctic Alaskan lakes. Factors including lake size, depth, outflow gradient, distance to other lakes, lake order, altitude, river drainage and age of glacial surface were analysed. These factors could affect either access of fish to a lake (colonisation variables), or their survival in a lake that already had been colonised (extinction variables).3. The presence of a species was predicted accurately in 78.4% ± 10.5% (mean ± SD) of cases, and absence in 75.0% ± 6.1% of cases. The relative importance of extinction versus colonisation variables varied with species. Extinction variables were most important for lake trout (Salvelinus namaycush) and slimy sculpin (Cottus cognatus), a mixture of extinction and colonisation variables was important for arctic char (Salvelinus alpinus), and colonisation variables were most important for arctic grayling (Thymallus arcticus) and round whitefish (Prosopium cylindraceum).4. Ecological differences among species account for much of the difference in relative importance of colonisation versus extinction variables. In addition, stream piracy events have occurred over geologic time scales, which have resulted in lakes that are currently inaccessible but support relict fish populations.5. Climate warming, currently occurring in the arctic, is likely to alter further the stream network, which could have dramatic effects on fish distributions by affecting access to isolated lakes or isolating lakes that are currently accessible.

Prevalence of a Testicular Anomaly in Lake Trout ( Salvelinus namaycush) of the Great Lakes Basin

This report examines the occurrence of a testicular anomaly in lake trout ( Salvelinus namaycush) from the Great Lakes basin. The anomaly was uni- and to a much lesser extent bilateral constriction along varying lengths of either testis. Prevalence throughout the Great Lakes was similar ranging from 28% in Lake Michigan to 44% in Lake Erie, and was unrelated to rearing history, strain, age, or year for the period 1979 to 1985. The anomaly has been present in the Great Lakes drainage basin since at least 1927. Prevalence of testicular constrictions in lake trout from inland lakes that ranged from 2.8 to 13.7% was lower than in the Great Lakes. All salmonids in Lake Ontario were affected in the order lake trout > brown trout and rainbow trout > coho salmon and chinook salmon although a morphologically similar anomaly was also found in lake whitefish and round whitefish, lake herring, alewives, and white suckers. Lake trout from one inland lake had a smaller portion of the testis affected than lake trout from Lake Ontario where affected fish showed compensatory growth in the contralateral normal testis. Constrictions were equally prevalent in self-sustaining and non self-sustaining stocks, they affected a relatively small portion of the testis for which there was compensatory growth of the normal testis, and they did not block the sperm duct. The potential for testicular constrictions to affect lake trout reproduction in the Great Lakes while unclear seems low.

Abundance of Selected Fish Species in Relation to Temperature and Salinity Patterns in the Sagavanirktot Delta, Alaska, Following Construction of the Endicott Causeway

Hydrography data from 1985 to 1993 in the vicinity of the Endicott Causeway near Prudhoe Bay, Alaska, were analyzed to determine if the trend in yearly post-causeway construction temperature and salinity was toward colder and more saline conditions. No significant (P<=0.05) relationship was found for temperature; however, salinity levels significantly decreased with time following construction. Intra-year variations in hydrographic conditions appeared to be due to coast-wide meteorological and oceanographic events rather than causeway-induced. We analyzed abundance indices, as log e, transformed catch-per-unit-effort (CPUE), of four marine species (arctic cod, Boreogadus saida; fourhorn sculpin, Myoxocephalus quadricornis; arctic flounder, Pleuronectes glacialis; saffron cod, Eleginus gracilis); two freshwater species (round whitefish, Prosopium cylindraceum; arctic grayling, Thymallus arcticus); and the anadromous rainbow smelt, Osmerus mordax, to determine whether there were increases in the abundance of marine species and declines in anadromous and freshwater species. We developed the "best-fit" regression models, based on the criterion of maximized regression F-ratio (i.e., minimized regression p-values), using year, temperature, salinity, and all possible interactions. Results suggested that all species exhibited changes in abundance levels over the period of record. In particular, arctic flounder and rainbow smelt both showed sharp increases in abundance five years after construction. For all species, the observed changes in abundance levels appears to be due to naturally occurring events in the species life history or changes in coast-wide meteorological conditions rather than to causeway-induced alterations in local hydrographic conditions.

Radium-226 in water, sediments, and fish from lakes near the city of Elliot Lake, Ontario, Canada

Ra-226 was measured by α-emission spectroscopy in water, sediments, and fish (tissues and gut contents), from five lakes in a watershed containing U mining and milling operations at Elliot Lake, Ontario, and from control lakes in an adjacent non-industrialized watershed. Ra-226 transfer parameters from lake water and sediments to fish tissues, and annual intakes by humans consuming fish, were estimated. Mean dissolved 226Ra levels ranged from ∼76 mBq litre −1 in water of the most affected lake, to < 10 mBq litre −1 in control lakes. Levels in summer were consistently higher than in fall or winter; no consistent variation with depth was noted. Sediment levels ranged from ∼3000 mBq g −1 dry wt in one study lake to < 100 mBq g −1 dry wt of sediment in control lakes. Bone 226Ra concentrations were higher than in muscle. The lake trout ( Salvelinus namaycush), a predatory secondary consumer, had bone 226Ra levels (< 20 mBq g −1 dry wt) that did not show significant site variation. In contrast, bottom feeding whitefish had significantly more 226Ra in bone tissue (to 38 mBq g −1 dry wt in the lake whitefish, Coregonus clupeaformis, and 76 mBq g −1 in round whitefish, Prosopium cylindraceum) in study lakes than in controls (< 20 mBq g −1 dry wt). Ra-226 levels in lake trout muscle were low and showed erratic variation among lakes whereas levels in whitefish muscle did not vary significantly among study and control sites. Lake herring (= cisco, Coregonus artedii), a planktivorous fish taken only from Quirke Lake, had mean 226Ra levels of 18 and 1.4 mBq g −1 dry wt in bone and muscle, respectively. Gut 226Ra levels, highest in lake trout from McCabe and Quirke Lakes (126 ± 53, 64 ± 44 mBq g −1 dry wt, respectively), and just detectable in McCabe and Elliot Lake whitefish (24 ± 2, 36 ± 14 mBq g −1 dry wt, respectively), were below detection in lake trout and whitefish from other lakes. Concentration ratios (CRs) of 226Ra from water to muscle ranged from 8 to 14 in lake trout, 7 to 14 in whitefish, and 4 to 6 in lake herring. The water to bone CRs varied from 81 to 142, 314 to 548, and 126 to 272 in the same species. CRs were always < 1 between sediments and fish tissues. Annual intake of 226Ra by human consumers of these fish was estimated to provide an effective dose of 0.003 mSv year −1. This represents a small fraction of both natural background (> 2 mSv year −1) and the public dose limit (5 mSv year −1).

Radionuclides (lead-210, polonium-210, thorium-230, and -232) and thorium and uranium in water, sediments, and fish from lakes near the city of Elliot Lake, Ontario, Canada

Radionuclides ( 210Pb, 210Po, 230Th, and 232Th) and chemical Th and U were measured in water, sediments, and fish tissues (bone, muscle, and gut contents of laketrout, Salvelinus namaycush, whitefish, Coregonus clupeaformis and Prosopium cylindraceum) from four lakes in a watershed affected by U mining and milling operations at Elliot Lake, Ontario, and from control lakes in an adjacent, non-industrialized, watershed. Radionuclide concentration ratios between tissue levels and sediment and water levels were calculated. Annual radionuclide intakes and resulting doses were estimated for humans consuming fish from the watershed. Bone 210Pb levels were higher (186 mBq g −1 dry wt in laketrout and 230 mBq g −1 dry wt in one lake whitefish) than in muscle (< 50 mBq g −1 dry wt in all cases), and generally higher in fish from study lakes than from controls, but no consistent differences were observed among fish species. Similarly, 210Po levels were higher in bone (208 ± 33 mBq g −1, in laketrout) than muscle (maximum 26 ± 4 mBq g −1, in laketrout), and in study lake populations compared to controls. Laketrout 210Po bone concentrations were higher than previously reported in Canada. Levels of 230Th, 232Th, and Th were below detection limits (20 mBq g −1, 0.05 μg g −1) in body tissues in all fish species. Bone levels of U (14.6 ± 3.0 μg g −1, in lake whitefish) were higher than in muscle (most < 0.05 μg g −1, except 0.12 ± 0.04 and 0.08 ± 0.03 μg g −1 in lake whitefish) in fish from waters affected by industrial activity. In control lakes, bone and muscle levels were lower and not significantly different from each other. Muscle levels did not vary consistently with location. Concentration of 210Pb and U was seen from water and ‘gut’ material (taken as a surrogate for diet) to bone in laketrout and whitefish, and of U from water to muscle in whitefish, but in no case from sediments to tissues. Human intakes of 210Pb, 210Po, 230Th, 232Th, and U from consuming one meal of fish (375 g) per week could, in aggregate, represent an annual effective dose < 15% of the public dose limit (5 mSv). Monitoring biota living near the decommissioned Elliot Lake U operations, especially of 210Pb levels in fish muscle, with further assessment of human doses attributable to local fish and other animals in the diet, should continue. Because radionuclide effects on fish health (and on other non-human organisms) are of increasing concern, neoplasms, malformations, and reproductive anomalies in local fish deserve examination.

Description and Identification of Larval and Juvenile Stenodus leucichthys nelma (Güldenstadt) from Central Alaska

Newly hatched larvae of Stenodus leucichthys nelma ranged from 11.0-11.3 mm total length (TL). By 14 mm TL, pelvic buds developed, and seven to nine caudal-fin rays were visible. By 23 mm TL, the caudal fin had the full complement of 19 principal rays. Fin ray differentiation in the dorsal and anal fins was complete by 32 mm TL. The preanal finfold was retained in specimens at least as large as 60 mm TL. Transition to the juvenile period (loss of all finfold and acquisition of adult counts of fin rays in all fins) occurred by 70 mm TL. Stenodus leucichthys nelma larvae and juveniles can usually be differentiated from Coregonus nasus, C. clupeaformis, C. autumnalis, C. sardinella, and Prosopium cylindraceum by pigmentation patterns, the number of myomeres and anal-fin rays, mouth position and form, and snout-to-vent length.

New records of Ergasilus (Copepoda: Ergasilidae) in the Laurentian Great Lakes, including a lakewide review of records and host associations

Ergasilus nerkae was found infecting ninespine stickleback (Pungitius pungitius) in lakes Huron, Michigan, and Superior and threespine stickleback (Gasterosteus aculeatus) and round whitefish (Prosopium cylindraceum) in Lake Huron. Based upon the literature and study of archived material, we propose that E. nerkae is enzootic to the Great Lakes and that ninespine stickleback are a preferred host in Lake Huron. Prevalence of E. nerkae on ninespine stickleback increased from 17% in June to 68% in September, but mean intensity remained light. Prevalence and mean intensity increased with host length. Ergasilus luciopercarum is also reported on lake trout (Salvelinus namaycush) and largemouth bass (Micropterus salmoides) for the first time. Host–parasite records of Ergasilus spp. in North America are reviewed, biology and taxonomy are summarized, and a checklist of Great Lakes host–parasite–locality records is provided. At present, eight species of Ergasilus are known to infect 42 Great Lakes fish species.

Phylogenetic Relationships among Members of the Coregoninae Inferred from Direct Sequencing of PCR-Amplified Mitochondrial DNA

We determined the DNA sequence of a portion of the mitochondrial cytochrome b gene for eight members of the Coregoninae (Salmonidae) from North America in an effort to elucidate phylogenetic relationships in the subfamily. DNA was prepared for sequencing by the polymerase chain reaction (PCR). Direct estimates of mitochondrial DNA (mtDNA) sequence divergence among taxa ranged from 0.0% between Arctic cisco (Coregonus autumnalis) from the Mackenzie River, Canada, and cisco (C. artedi) from the Laurentian Great Lakes to 5.8% between the inconnu (Stenodus leucichthys) and the round whitefish (Prosopium cylindraceum). As has been noted in other fish species and vertebrates in general, third position silent substitutions predominated over any other type of nucleotide change. No amino acid replacement substitutions were found among any of the eight taxa examined. Comparison of mtDNA sequence divergence estimates from this and other studies suggests that the radiation of the genera within the Coregoninae occurred relatively recently (2–6 million yr B.P.). Cladistic analysis of the mtDNA sequence data yields a hypothesis of relationships that supports previous genetic and morphological classifications of coregonines.

Salmonid diet and the size, distribution, and density of benthic invertebrates in an arctic lake

Size selective predation on molluscs was apparent for lake trout (Salvelinus namaycush) and round whitefish (Prosopium cylindraceum), but not for arctic grayling (Thymallus arcticus), in the Toolik Lake region of arctic Alaska during the summer of 1986. Lake trout consumed significantly larger molluscs of all taxa than did round whitefish, and selected larger molluscs than were available on either rocky or soft-sediment habitats. Round whitefish were not size-selective on the snail Lymnaea, but were size-selective on the snail Valvata and on clams from the soft sediments. Round whitefish consumed fewer and smaller Lymnaea compared to lake trout. Because lake trout ate more Lymnaea and also tended to select larger, reproductive-sized individuals, this fish could potentially have a more detrimental impact on the Lymnaea population. Finally, differences in Lymnaea densities and size distributions between lakes with and without lake trout suggest that these fish may be responsible for the pattern of distribution, size, and density observed for Lymnaea in Toolik Lake and other area lakes.

Size Structure of a Lake Trout (Salvelinus namaycush) Population an Arctic Lake: Influence of Angling and Implications for Fish Community Structure

The lake trout (Salvelinus namaycush) population in Toolik Lake in arctic Alaska was assessed by gill net during 1977, prior to exploitation. In 1986 we replicated the gill net study to determine the effects of recent angling pressure on the fish community. Relative composition of the catch varied between samples. Round whitefish (Prosopium cylindraceum), which are not susceptible to angling, made up 42% of the catch in 1986 and 28% in 1977. Significant reductions in the median length and weight of lake trout between the sampling periods were observed. The median size of lake trout in 1986 was below the minimum 1977 reproductive size. Differences in median length and weight of grayling (Thymallus arcticus) and of round whitefish during this period were not significantly different. The condition factor of lake trout was significantly higher in 1986. We suggest that large lake trout in Toolik Lake control salmonid recruitment and determine community structure. With continued fishing pressure further changes in the fish community and lake trophic dynamics may result.

Facteurs Influencant le Deplacement des Poissons dans la Partie Aval de la Grande Rivière à la suite de la Fermeture du Barrage LG2.

The cut-off of the La Grande Rivière at the LG2 main dam, 117 km from James Bay, resulted in saline penetrated which advanced regularly for a distance of 31 km between November 27, 1 978 and early April, 1979. After retreating to 6 km from the mouth of the river in May, the saline front rose about 20 km in July and returned to near the mouth in November 1979. The daily catch in mesh nets placed in 6 stations made it possible to follow the movements of three marine fish species (Gadus ogac, Myoxocephalus quadricornis and M. scorpius), seven freshwater species (Acipenser fulvescens, Salvelinus namaycush, Esox lucius, Couesius plumbeus, Catostomus commersoni, Percopsis omiscomaycus and Stizostedion vitreum) and six species which can tolerate quite high salinity (Salvelinus fontinalis, coregonus artedii, Coregonus clupeaformis, Prosopium cylindraceum, Catostomus catostomus and Lota Iota. Movements due to the changes in the salinity and temperature of the water have become superimposed on the natural movements observed before the river was cut off.