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Changes in fish populations affected by the construction of the La Grande complex (Phase I), James Bay region, Quebec Can. J. Zool. Downloaded from www.nrcresearchpress.com by Canadian Science Publishing on 08/12/15 For personal use only. Jean-Claude DesLandes, Sylvie Guenette, Yves Prairie, Dominique Roy, Richard Verdon, and Rejean Fortin Abstract: Catches per unit of effort (CPUE) with experimental gill nets, recruitment, growth, and condition were monitored between 1977 and 1992 to evaluate the impact of impoundment on the main fish species of La Grande 2, Opinaca, and Caniapiscau reservoirs and the Boyd-Sakami diversion. CPUE and recruitment of northern pike (Esox lucius) and lake whitefish (Coregonus clupeaformis) increased markedly at most stations after impoundment and decreased at the end of the series. The lake whitefish and cisco (Coregonus artedii) showed their most striking rise in CPUE at two bay stations of La Grande 2 and Opinaca reservoirs. CPUE and recruitment of the longnose sucker (Catostomus catostomus), white sucker (Catostomus commersoni), and lake trout (Salvelinus namaycush) (Caniapiscau) showed a general decrease following impoundment. CPUE for the walleye (Stizostedion vitreum) also decreased at several stations; however, the two most southerly stations in La Grande 2 reservoir and the Boyd - Sakami station showed high CPUE during the series. Concentration- redistribution phenomena explain part of the observed variations in CPUE. Correlation analyses showed that walleyes and white suckers were attracted to the warmer, more turbid stations, and that the high primary and secondary productivity of bay stations attracted the coregonines. Growth and condition of the main species increased during variable time intervals after impoundment and decreased at the end of the series. RCsumC : Les captures par unite d'effort (CPUE) au filet maillant experimental, le recrutement, la croissance et la condition ont CtC suivis entre 1977 et 1992 afin d'kvaluer l'impact de la mise en eau sur les principales espkces de poissons des reservoirs La Grande 2, Opinaca et Caniapiscau et du dktournement Boyd-Sakami. Les CPUE et le recrutement du Grand Brochet (Esox lucius) et du Grand CorCgone (Coregonus clupeaformis) ont fortement augment6 h la suite de la mise en eau, pour diminuer en fin de sCrie. Le Grand CorCgone et le Cisco (Coregonus artedii) de lac ont connu leurs plus fortes hausses de CPUE aux deux stations de baie des rCservoirs La Grande 2 et Opinaca. Les CPUE et le recrutement du Meunier rouge (Catostomus catostomus), du Meunier noir (Catostomus commersoni) et du Touladi (Salvelinus namaycush) (Caniapiscau) ont connu une baisse gCnCralisCe h la suite de la mise en eau. Les CPUE du DorC jaune (Stizostedion vitreum) ont aussi diminuC h plusieurs stations; toutefois, les deux stations situCes le plus au sud dans le rCservoir La Grande 2 et la station du dktournement Boyd-Sakami ont affichC des CPUE ClevCes pendant la sCrie. Des phCnomknes de concentration-redistribution expliquent en partie les variations de CPUE observCes. Des analyses de corrClation ont rCvC1C que le DorC jaune et le Meunier noir se concentrent aux stations les plus chaudes et les plus turbides, alors que la productivitC primaire et secondaire ClevCe des stations de baie attire les corCgoninCs. La condition et la croissance des espkces principales a augment6 pendant une pCriode de temps variable h la suite de la mise en eau, pour diminuer en fin de sCrie. Introduction Several studies have addressed the problem of the effects of impoundment and river diversion on fish populations. The Received March 2 1, 1995. Accepted June 28, 1995. J.-C. DesLandes, S. GuCnette, Y. Prairie, and R. Fortin. DCpartement des Sciences biologiques, UniversitC du Quebec h MontrCal, C.P. 8888, succursale Centre Ville, Montreal, PQ H3C 3P8, Canada. D. Roy. SociCtC d'Energie de la Baie James, 500 Boulevard RenC-LCvesque Ouest, Montreal, PQ H2Z 1Z9, Canada. R. Verdon. Hydro-QuCbec, vice-prCsidence Environnement, 75 Boulevard RenC-LCvesque Ouest, Montreal, PQ H2Z lA4, Canada. Can. J. Zool. 73: 1860- 1877 (1995). Printed in Canada / IrnprirnC au Canada Southern Indian Lake impoundment and Churchill River diversion (northern Manitoba) were the object of several major publications in the late 1970s and early 1980s. Volume 4 1, No. 4, of the Canadian Journal of Fisheries and Aquatic Sciences, published in 1984, presents a series of papers on the impacts of this hydroelectric project, some of which are cited below. International symposia have focused on reservoir ecology and management (see, among others, Campbell et al. 1982). The present study differs from previous ones on northern reservoir fisheries in several key features related to general environmental characteristics, composition of the fish communities, and objectives of the research. Contrary to the Southern Indian Lake system, there were no major shoreline erosion and sedimentation problems in the La Grande DesLandes et al. Can. J. Zool. Downloaded from www.nrcresearchpress.com by Canadian Science Publishing on 08/12/15 For personal use only. complex (James Bay region, Quebec). Also, the rise in water levels and the area of flooded terrestrial habitat were much more important in the La Grande complex. Moreover, the fish communities differ in composition between the eastern and western part of the complex; they also differ from the fish communities in other northern reservoirs. Therefore, the impact of impoundment on the fish communities in environments affected by the development of the La Grande complex could not readily have been predicted from the results of previous studies. Most fisheries studies related to reservoirs in Canada have focused on the effects of impoundment on major commercial and sport fish species (lake whitefish, Coregonus clupeaformis; walleye, Stizostedion vitreum; northem pike, Esox lucius; lake trout, Salvelinus namaycush) . The present study differs by addressing changes in populations of all major species vulnerable to experimental gill nets over a prolonged time series covering 1-2 years before and 7 14 years after impoundment. Phase I of the La Grande complex is one of the largest hydroelectric projects constructed in Canada. It involves a series of reservoirs and modified-flow environments covering a total area of 13 520 km2. To quantify changes in the various compartments of the modified ecosystems, an environmental monitoring program was initiated in 1977- 1978 and is still ongoing for some compartments. Our objective is to report on the changes of the fish populations in three reservoirs (La Grande 2, Opinaca, and Caniapiscau) and in one increased-flow environment (the Boyd - Sakami diversion). Changes in relative abundance (catches per unit of effort, CPUE) , year-class strength, growth, and condition of the main fish species will be reviewed. The covariations between changes in CPUE, water quality, chlorophyll a concentration, and zooplankton biomass will also be presented. Interpretation of results will focus on distinguishing between fish concentration-redistribution phenomena in response to environmental changes and variations in recruitment in explaining the changes in relative abundance of the more important species. General description of the La Grande complex (Phase I) Three generating stations were built along La Grande Rivikre (Fig. 1): La Grande 2, La Grande 3, and La Grande 4. Each has a corresponding reservoir, the oldest being La Grande 2 (maximum area 2835 km2; mean annual drawdown 3.3 m; mean depth 22 m; impounded November 1978 - December 1979), located downstream from the other two. To increase flows in the La Grande Rivikre, three rivers were diverted. To the south, the Eastmain and Opinaca rivers were cut off in 1980 to create the Opinaca reservoir (maximum area 1040 krn2; mean annual drawdown 3.6 m; mean depth 8.2 m; impounded April-September 1980), which flows into the La Grande 2 reservoir via the Boyd - Sakami diversion. The increase in level relative to natural conditions in the diversion was approximately 2 m. During the years following impoundment, a 2°C increase in mean water temperature and a 2-m decrease in transparency (from 3.5 m in 1978 to 1.5 m in 1981) were observed in the diversion. To the east, the northward-flowing Caniapiscau River was cut off in October 1981 to create Caniapiscau reservoir (maximum area 4275 km2; mean annual drawdown 2.1 m; mean depth 16.8 m; impounded October 1981 - September 1984), the largest in the complex, which drains into La Grande 4 reservoir via the Laforge diversion. Three reservoirs (La Grande 2, Opinaca, Caniapiscau), two increased-flow environments (Boyd - Sakami diversion, La Grande Rivikre), and two decreased-flow environments (Eastmain and Caniapiscau rivers) were targeted for the monitoring program (Bachand and Fournier 1977). Stations that were to be sampled over the years were established on a reasoned choice basis, near shore (0 - 5 m), in contrasted habitats, and in each of the selected environments. A detailed physical description of the stations can be found in Dussault and Boudreault (1981). The present paper focuses on fish population changes in the three reservoirs and in the BoydSakami diversion. Five stations were sampled from 1978 to 1992 in La Grande 2 reservoir. Before impoundment, three of these stations (G2 403, G2 404, G2 405; Fig. 1) were located in lakes and two in rivers (G2 400, G2 406). Two stations in La Grande 2 reservoir (G2 404 and G2 406) were eventually affected by major modified tributaries (the Boyd-Sakami diversion and the outlet from La Grande 3 reservoir). Station G2 405 is located in a protected bay, while the other two stations are located close to the main body of the reservoir. One site in nearby Detcheverry lake (SB 400; Fig. 1) was also sampled as a reference station. Waters in the main body of Opinaca reservoir flow north, whereas there is no measureable flow in protected bays. The four stations in Opinaca reservoir are compared with one reference station in Rondde-Poele Lake (EA 302). Station EM 401 (lentic before impoundment) is located in a shallow, protected bay, while the other three (EM 402, lentic before impoundment; EM 400 and EM 403, lotic; Fig. 1) are located in the main body of the reservoir along the flow gradient. Only three stations were sampled throughout the series in Caniapiscau reservoir: one in the southwestern branch (CA 4.13; Fig. 1) and two in the northeastern branch (CA 4 11 and CA 4 16); all three were located in lakes before impoundment. Lakes Nouveau and Hazeur served as reference stations. Only one station, located outside the main flow channel, was sampled in the BoydSakami diversion (SK 400; Fig. 1). Materials and methods Data collection In the western section of the La Grande complex (La Grande 2 and Opinaca reservoirs, Boyd-Sakami diversion), the fish communities were sampled annually from 1977 or 1978 to 1984 and in 1988 and 1992. The eastern section (Caniapiscau region) was sampled in 1980, 1981, 1982, 1987, and 1991. Sampling occurred monthly from June to SeptemberOctober, five times per season in the western section of the complex and four times per season in the eastern section. All samplings were performed using four multifilament gill nets 45.7 m in length x 2.4 m in depth, set in pairs, a net of uniform 7.6- or 10.2-cm stretched mesh attached to an experimental gill net (stretched mesh ranging from 2.5 to 10.2 cm). Each pair of nets was set perpendicular to and near the shore. During impoundment of La Grande 2 reservoir (1979), nets were set at an angle from surface to bottom to avoid submerged trees. Each sampling period lasted 48 h until 1982 Can. J. Zool. Downloaded from www.nrcresearchpress.com by Canadian Science Publishing on 08/12/15 For personal use only. 1862 Can. J. Zool. Vol. 73, 1995 DesLandes et al. Can. J. Zool. Downloaded from www.nrcresearchpress.com by Canadian Science Publishing on 08/12/15 For personal use only. and 24 h from 1983 on. When the effort lasted 48 h, the nets were nevertheless visited every 24 h. The fish caught were counted, measured, and weighed. For a given day and station, catches of the four nets were averaged and expressed as numbers of fish per net-day , i .e., catches per unit of effort (CPUE). For each species and station, the four or five monthly CPUE values were averaged to obtain the mean seasonal CPUE for a given year. Fish aged as part of a mercury-monitoring program in the same environments (Brouard et al. 1990) were used to construct age-length keys and to study variations in growth. Changes in water quality and chlorophyll a concentration in the photic zone (0- 10 m) were monitored at the same stations starting in 1978, along with those of zooplankton biomass in the water column (0 -25 m maximum) from 1978 to 1984. Details of the methodology used in the collection and analysis of these data were presented by FrCchette (1978, 1980). Data analyses CPUE For two reservoirs (La Grande 2 and Opinaca), changes in CPUE for each of the principal fish species were first studied using univariate repeated-measures analysis of variance (Freund et al. 1986), which provides an overall indication of variations over time and between stations. ANOVAs and Kruskal-Wallis tests were used to detect possible interannual differences in CPUE at the various stations. To better understand variations in relative abundance for a given species and environment, CPUE were also computed for two size groups, each representing 50% of all specimens caught over the time series. We would expect an increase in recruitment to show up first in CPUE of small specimens and later in those of larger specimens. Phenomena associated with fish concentration -redistribution would likely affect CPUE of both size groups simultaneously. These results were presented in detail by DesLandes (1994). Only the main features are highlighted here. Parametric correlation analyses were used to determine relationships between CPUE of the main fish species and environmental variables (biomass of the main zooplankton taxa, chlorophyll a concentration, and various physicochemical parameters: dissolved oxygen concentration, temperature, transparency, total phosphorus level, conductivity, pH). The analyses were performed on yearly averages of the variables to avoid interference from seasonal phenomena and thus emphasize annual and interstation variations. Year-class strength, growth, and condition Johnson's (1957) year class strength index (YCSI) was used to quantify recruitment. The index is obtained by dividing the sum of the annual contributions (percentage of catches) for a given year class by the sum of the annual contributions for an average year class at the corresponding ages. Calculations were made for the longnose sucker (Catostomus catostomus) lake whitefish, northern pike, and walleye when sample sizes over the series in a given environment were important enough to justify them. To generate the indices, we used the agefrequency data from the mercury program (Brouard et al. 1990) to generate age-length keys that were applied to the length-frequency histogram of catches made in the corresponding year by the environmental monitoring program. The age-frequency distributions thus generated were used to calculate the YCSI. Trends in recruitment were also inferred from the percentage of small specimens in yearly lengthcomposition data ( < 250 mm for whitefish, catostomids, and walleyes; < 150 mm for ciscoes (Coregonus artedii); <350 mm for northern pike). Samples from the mercury program were also used to study interannual variations in growth in length of the four species already mentioned, by comparing average sizes at age of capture. Changes in the Fulton condition index in modified and reference environments were studied using only individuals of the most represented length classes for each species, to eliminate the effects of interannual variations in length distribution. Length classes used in the calculations were 100 -200 mm for ciscoes, 400 - 600 mm for walleyes and lake trout, 300 - 500 mm for longnose suckers, white suckers (Catostomus commersoni), and lake whitefish, and 500-700 mm for northern pike. For the various species, ANOVAs and Student - Neuman - Keuls tests were used to compare annual means for a given environment and between modified and reference environments. Results Trends in CPUE for the main species La Grande 2 and Opinaca reservoirs and Boyd-Sakami diversion Under natural conditions, catches in the lentic and lotic environments incorporated in La Grande 2 (Fig. 2) and Opinaca reservoirs (Fig. 3), in the Boyd- Sakami Diversion (Fig. 4) and catches in reference lakes in the western part of the La Grande complex (not illustrated) were generally dominated by walleyes, followed by lake whitefish, longnose and white suckers, northern pike, and ciscoes. The order of importance of the last five species varied somewhat between the various environments. Burbot (Lota lota), lake trout, yellow perch (Perca flavescens), lake sturgeon (Acipenser jidvescens), round whitefish (Prosopium cylindraceum), spottail shiners (Notropis hudsonius), trout-perch (Percopsis omiscomaycus), and lake chub (Couesius plumbeus) were also gill-netted in relatively small numbers in the three water bodies. Other secondary species include the pearl dace (Semotilus margarita; La Grande 2 and Opinaca reservoirs), brook charr (Salvelinus fontinalis; La Grande 2 reservoir), and fallfish (Semotilus corporalis; Opinaca reservoir). Globally, the secondary species, including burbot and lake trout, represented less than 6 % of the catches under natural conditions and their contribution became negligible over the years. For each of the two western reservoirs, repeated-measures analyses of variance performed on mean CPUE over the five (La Grande 2) and four stations (Opinaca) sampled throughout the series revealed significant changes over time ( p < 0.05) for the six main species (walleye, both coregonines, both catostomids, and northern pike). However, trends over time varied significantly ( p < 0.05) between stations for these species, except for the cisco in La Grande 2 reservoir and the two catostomids in Opinaca reservoir. 1864 Can. J. Zool. Vol. 73, 1995 Fig. 2. (A-F) Trends in mean annual CPUE of the main species at the various stations in La Grande 2 reservoir, including global mean CPUE (stations regrouped), from 1977 to 1992; * and k: interannual differences ( p < 0.05) in mean CPUE detected by ANOVAs (*) and Kruskal-Wallis tests (k). (G and H) Trends in YCSI (1976- 1986) and percentage of small specimens in total catches (1978 - 1992). The arrow below the abscissa indicates the year of impoundment. Longnose sucker 12 n > - --------- -- 10 Can. J. Zool. Downloaded from www.nrcresearchpress.com by Canadian Science Publishing on 08/12/15 For personal use only. 03 '73 &a G2400 *k - G2403 *k -----= Mean 6 0 ----- G2404*k G2405*k - w 5 White sucker G2400 G2403 *k G2404*k G2405*k - 9 * * Mean V LU 3 4 a 0 2 0 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 t 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 t Year El -------- -- Cisco -------- -- -- - G2400 G2403 *k Lake whitefish G2400 *k G2403 ---G2404 G2404 - G2405 G2406 Mean Year * G2405*k a a a G2406 *k Mean ; ; * 0- ' ' -0 -0 -0 0 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 t El t Year EI Northern pike 12 Year Walleye - \ h > 10- -- A 1 \ \ \ -------- M 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 t Year 0 - G2400 k G2403 k G2404 k G2405 e a -- * n * 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 t Year DesLandes et al. Fig. 2 (concluded). Year class strength index "I -- 3,o Can. J. Zool. Downloaded from www.nrcresearchpress.com by Canadian Science Publishing on 08/12/15 For personal use only. % small specimens 16 % 76 77 78 79 80 81 82 ------ --- 14 % Longnose sucker White sucker (1) Walleye (I) Lake whitefish Northern pike 83 84 85 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 86 t Year class Year Table 1. Parametric correlations (Pearson's correlation, r, and and probability level) computed between mean summer CPUE of the principal species and mean summer values of environmental variables at the various stations in La Grande 2 and Opinaca reservoirs and at the Boyd -Sakami station. Longnose sucker r P White sucker r P Cisco r Lake whitefish P r P Northern pike r P Walleye r P Transparency Temperature pH Phosphorus level Conductivity Chlorophyll a concn. Dissolved oxygen concn. Copepods Cladocerans Rotifers Total zooplankton Longnose suckers showed significant ( p < 0.05) interannual differences in mean CPUE at the five stations in La Grande 2 reservoir, at three of the four stations in Opinaca reservoir, and at the Boyd - Sakami station (Figs. 2, 3, and 4). Similarly, white suckers showed significant ( p < 0.05) interannual differences in mean CPUE at four of the five stations in La Grande 2 reservoir, at three of the four stations in Opinaca reservoir, and at the Boyd-Sakami station. Because the 1980 increase in CPUE of both catostomids at station G2 404 in La Grande 2 reservoir (Fig. 2) and at station EM 402 in Opinaca reservoir (Fig. 3) involved both large and small specimens (DesLandes 1994), it is interpreted as a change in concentration in response to environmental changes (beginning of operation of the Boyd - Sakami diversion in the case of G2 404). However, the decrease in CPUE of both catostomids at most stations of the two reservoirs at the end of the series to values globally lower than initial ones indicates major recruitment problems. Longnose sucker YCSI values in La Grande 2 reservoir were higher in 1978- 1979, before and during impoundment; they declined afterwards (Fig. 2). The proportion of <250 mm white suckers in the catches has been very low since 1979 (Figs. 2 and 3). At the Boyd - Sakami station, the sharp rise in longnose sucker CPUE began coincidentally with the diversion and continued in the following year. Catches then fell but remained well above prediversion levels (Fig. 4). However, the proportion of small longnose suckers ( <250 mm) after the diversion is generally low (Fig. 4) despite the substantial CPUE observed. Catches of both catostomids were higher in less transparent waters, as suggested by the significant negative Pearson's correlations, r, ( p < 0.05), observed between mean seasonal CPUE values at the various stations of the three modified environments and mean seasonal water transparency (r = -0.26 for longnose suckers and -0.23 for white suckers; Table 1); however, r values are low and the relationship cannot be generalized to the Caniapiscau region, where both water transparency and longnose sucker CPUE are high. Of the two catostomids, only white suckers showed a positive significant correlation with water temperature (r = 0.32), suggesting that this species concentrates at the warmer stations. White sucker CPUE values were also positively correlated with conductivity (r = 0.28, p < 0.01 ; Table 1). Lake whitefish CPUE increased at several stations in Can. J. Zool. Downloaded from www.nrcresearchpress.com by Canadian Science Publishing on 08/12/15 For personal use only. DesLandes et al. Fig. 3 (concluded). EI Year class strength index % small specimens 80% ------- -- Whae sucker (1) 170% Longnose sucker (1) Cisco (2) -- 0 0- Can. J. Zool. Downloaded from www.nrcresearchpress.com by Canadian Science Publishing on 08/12/15 For personal use only. 0 0 0- 0' 0 ' - '0 '0' 0 ' 0 ' I 76 77 78 79 80 83 82 81 85 84 78 86 79 t Year class I I I I 80 81 82 83 84 85 86 87 88 89 90 91 92 Year Fig. 4. (A) Trends in mean annual CPUE of the main species at the Boyd-Sakami station, including global mean CPUE (stations regrouped), from 1978 to 1992. * and k: interannual differences ( p < 0.05) in mean CPUE detected by ANOVAs (*) and Kruskal-Wallis tests (k). (B and C) Trends in YCSI (1976- 1986) and percentage of small specimens in total catches (1978- 1992). The arrow below the abscissa indicates the year of diversion. El CPUE -- --- 12 A 78 79 80 81 t 82 -------- Longnose sucker +k Cisw + -No&ernpiketk 83 84 85 86 87 88 White sucker ycsl -- +k + Lake whitefish k Lake whitefish Northern pike WaIleye+k 89 90 91 92 n 76 78 79 Year 80 81 Year t % small specimens --- 78 79 80 81 t 82 83 84 85 Year Longn- sucker Walbye 86 87 88 89 90 91 92 82 m 84 as 86 Can. J. Zool. Vol. 73, 1995 Can. J. Zool. Downloaded from www.nrcresearchpress.com by Canadian Science Publishing on 08/12/15 For personal use only. La Grande 2 and Opinaca reservoirs and at the BoydSakami station following impoundment (Figs. 2, 3, and 4). Interannual variations are statistically significant ( p < 0.05) at four of the five stations in La Grande 2 reservoir, at three of the four stations in Opinaca reservoir, and at the BoydSakami station (Figs. 2, 3, and 4). However, the pattern of variation was quite variable between stations and the increase was greater at the two bay stations (G2 405, Fig. 2; EM 401, Fig. 3). Lake whitefish CPUE showed significant ( p < 0.05) positive Pearson's correlations with total phosphorus (r = 0.7; Table I), chlorophyll a concentration (r = 0.51), and biomass of the various zooplankton taxa (r values varying between 0.21 and 0.46; Table 1); they also showed negative significant correlations with water transparency (r = -0.25) and dissolved oxygen concentration (r = -0.3 1). The high productivity of the bay stations, resulting from high rates of decomposition of terrestrial vegetation, seems to attract lake whitefish and is probably responsible for the significant correlations observed. Concentration phenomena were also observed at station G2 406 during filling of La Grande 3 reservoir (in 1981 and 1982). At all stations of the three environments, lake whitefish CPUE values at the end of the series were equivalent to or higher than initial values. Increased recruitment also explains part of the rise in whitefish CPUE values over the series. Lake whitefish YCSI increased in the year of impoundment and gradually declined over time in both reservoirs and at the Boyd - Sakami station (Figs. 2, 3, and 4). In Opinaca reservoir, whitefish YCSI showed significant positive Pearson's correlations (r = 0.9; p < 0.01) with mean summer cladoceran biomass and with the mean total zooplankton biomass. Similar correlations were not observed in La Grande 2 reservoir or at the Boyd Sakami station. Interannual variations in cisco CPUE were significant ( p < 0.05) at one station in La Grande 2 reservoir (G2 403, Fig. 2), at three stations in Opinaca reservoir (Fig. 3), and at the Boyd-Sakami station (Fig. 4). An important rise in cisco CPUE during the years following impoundment was observed at the bay stations of both reservoirs (G2 405, Fig. 2; EM 401, Fig. 3). Cisco CPUE showed significant positive ( p < 0.05) correlations with total phosphorus level (r = 0.73), chlorophyll a concentration (r = 0.38), and biomass of various zooplankton taxa (r values between 0.26 and 0.33), and significant negative ( p < 0.05) correlations with water transparency (r = -0.25) and dissolved oxygen concentration (r = -0.35), as was the case for lake whitefish, suggesting that the high productivity of the bay stations attracts the coregonines. The proportion of < 150-mm ciscoes was higher from 1978 to 1981 than from 1982 to 1984 (Figs. 2 and 3), suggesting that recruitment may have been higher at the time of impoundment and declined somewhat since then. The spectacular increase in the proportion of small specimens in the 1988 and 1992 catches in both reservoirs (Figs. 2 and 3) is chiefly due to a major decrease in the abundance of large specimens, possibly resulting from concentration redistribution phenomena. At most stations in the three modified environments, cisco CPUE values at the end of the series were roughly equivalent to initial values. The northern pike is the only species to show statistically significant ( p < 0.05) variations over time at all stations in the three environments. Most stations in the two reser- voirs showed a general and prolonged rise in CPUE after impoundment (Figs. 2 and 3). Peak CPUE values, however, did not occur during the same year at all stations. The increase in northern pike CPUE was more gradual and less important at the Boyd -Sakami station (Fig. 4), the peak occurring in 1988. Unlike ,those of most species, northern pike catches were generally less abundant at ,the two reservoir bay stations during the first years after impoundment (G2 405, Fig. 2; EM 401, Fig. 3). The breakdown of CPUE values into two size groups (DesLandes 1994) suggests that part of the increase is due to concentration phenomena, particularly at station G2 404 in 1981 (the first year of operation of the Boyd - Sakami diversion) and at station G2 406 during filling of La Grande 3 reservoir (198 1, 1982; Fig. 2). Northern pike CPUE values showed a positive, significant correlation with water temperature (r = 0.22, p < 0.05; Table 1) and chlorophyll a concentration (r = 0.26, p < 0.05) and a negative, significant correlation with conductivity (r = -0.25, p < 0.05; Table 1). As was the case for lake whitefish, the rise in northern pike CPUE during the series also involved increased recruitment. Northern pike YCSI increased in the year of impoundment in both reservoirs and gradually declined over time (Figs. 2 and 3). YCSI peaked in 1982 at the Boyd-Sakami station (Fig. 4) and declined thereafter. Northern pike CPUE values were decreasing at most stations in the three environments at the end of the series, but they were globally equivalent to (reservoirs) or higher than (diversion) initial values. Interannual variations in walleye CPUE were statistically significant ( p < 0.05) at four stations in La Grande 2 reservoir (Fig. 2), at three stations in Opinaca reservoir (Fig. 3), and at the Boyd -Sakami station (Fig. 4). From 1981 to 1988, more than 94% of walleye catches in La Grande 2 reservoir occurred at the two warmer, less transparent, and most productive stations, G2 404 (near ,the Boyd - Sakami diversion) and G2 405 (bay) (Fig. 2), where the fish seem to concentrate. This partly explains the negative significant correlation observed between walleye CPUE and water transparency (r = -0.45, p < 0.001 ; Table 1) and the positive correlation observed with water temperature (r = 0.34, p < 0.001). Trends in CPUE at stations G2 404 and G2 405 are quite variable, showing a drastic decline in 1979 (year of impoundment, possible dilution effect) or 1980, followed by values equivalent to initial ones from 1980- 1981 to 1988, and by a drastic decline in 1992 (Fig. 2). Three of the four Opinaca reservoir stations showed a major decline after impoundment; this nearly instantaneous decrease involved both small and large individuals, suggesting a redistribution phenomenon. CPUE values then increased slightly from 1988 to 1992, but were still inferior to initial ones at three of the four stations in 1992 (Fig. 3). At the Boyd - Sakami station, interannual variations in CPUE were very pronounced for walleyes, with a peak of 13 fish per net-day occurring soon after the diversion (Fig. 4). This abrupt rise in CPUE regardless of size (DesLandes 1994) suggests a sustained concentration phenomenon at the station. In La Grande 2 reservoir, the proportion of <250-mm walleyes was high in 1978- 1979 and low or null from 1980 to 1983. The slight increase in this proportion observed from 1984 on could indicate better recruitment, possibly limited to the southern portion of the reservoir. In Opinaca reservoir, despite the decline in CPUE following impoundment, DesLandes et al. voirs, the proportion of small individuals ( < 350 mm) was practically null at the three stations throughout the series (Fig. 5). Interannual variations in CPUE for lake trout were significant ( p < 0.05) at the two stations in the northeastern part of the reservoir (CA 4 11 and CA 41 6; Fig. 5), where CPUE decreased from 3-4 specimens per net-day in 1980 to 0.2 specimen per net-day in 1991. CPUE remained relatively constant at station CA 413. However, the analysis of CPUE broken down into two size groups (DesLandes 1994) shows that the apparent stability at station CA 4.13 actually reveals a decrease in 1620-mm specimens coupled with an increase in large specimens. Recruitment problems are thus evident in 1991, small specimens being practically absent from catches at the three stations. Caniapiscau Reservoir The fish community in Caniapiscau reservoir differs from those in La Grande 2 and Opinaca reservoirs by the absence of walleyes and ciscoes. Five main species were captured: lake whitefish, longnose sucker, lake trout, northern pike, and white sucker. Secondary species include lake chub, burbot, round whitefish, brook charr, and landlocked salmon (Salrno salar) . Station CA416 showed a significant decrease in longnose sucker CPUE over time, whereas relative stability ( p > 0.05) was observed at the other two stations. YCSI and the proportion of specimens < 250 mm were higher in 1981 than in 1982- 1987, suggesting that recruitment decreased after impoundment (Fig. 5). However, by 1991 the proportion of small longnose sucker had returned to its 1980 level. The white sucker was always less abundant than its congener (Fig. 5). At the two stations where significant interannual variations were observed, white sucker CPUE peaked in 1981- 1982 (the start of impoundment), before declining markedly to lower than initial values in 1991. Interannual variations in CPUE for lake whitefish were significant ( p < 0.05) at two of the three stations (Fig. 5). At these two stations, CPUE decreased during the first year of impoundment (1982), remained low in 1987, and increased at the end of the series in 1991. The YCSI increased in 1980, peaked in 1981, and declined following impoundment (1982- 1985; Fig. 5). The decrease in the proportion of <250-mm specimens observed in 1987 also suggests less efficient recruitment. The generalized increase in CPUE observed in 1991, coupled with the very high proportion of small specimens, suggests a restoration of recruitment in the late 1980s. However, this increase in the proportion of small specimens is partly due to a significant rise in the proportion of dwarf individuals in the 1991 catches. Dwarf lake whitefish were present in the natural lakes that were incorporated in Caniapiscau reservoir, but their contribution to experimental catches increased significantly between the early 1980s and 1991. Trends in northern pike CPUE were similar at the three stations in Caniapiscau reservoir. CPUE increased significantly ( p < 0.0001) after impoundment, peaking in 1987, and decreasing to initial values in 1991 (Fig. 5). The general increase in CPUE in 1987 was observed mainly in specimens 5680 mm (DesLandes 1994) and was probably related to increased recruitment at the beginning of impoundment. However, contrary to what was observed in the other reser- Growth and condition In spite of the low annual sample sizes available for the study of growth, interannual differences were observed between mean sizes at several ages for the main species in the various environments, generally between years showing extreme values. Variations in growth rate following impoundment are fairly clear in La Grande 2 reservoir (Fig. 6). Growth rates of longnose suckers, whitefish, pike, and walleyes rose sharply after impoundment. Thereafter, growth rates declined, mean sizes at early ages (7 and under) at the end of the series (1988 and 1992) being lower than, or equivalent to, those observed at the start of the series (Fig. 6). The oldest fish, which benefited from faster growth rates during their early years, were still longer, on average, at a given age at the end of the series. Generally, all species showed a significant increase ( p < 0.05) in condition factor after reservoir impoundment, with the exception of whitefish and pike in the Boyd-Sakami diversion and white sucker and pike in Caniapiscau reservoir (Fig. 7). The increase in condition factor for the various species, expressed as percentage of initial values, was practically identical in La Grande 2 and Opinaca reservoirs: longnose sucker, 20.0 vs. 17.9%; white sucker, 13.2 vs. 11.2%; whitefish, 15.7 vs. 15.7%; cisco, 18.8 vs. 18.5%; northern pike, 17.5 vs. 20.4% ; walleye, 2 1.3 vs. 24.4 % . However, the improvement in condition was generally lower in the Boyd - Sakami diversion: longnose sucker, 5.6 % ; white sucker, 6.0%; walleye, 6.6 % ; whitefish, no significant increase. The condition of northern pike in the diversion did not improve significantly after the increase in flow, but it declined at the end of the series (1988 - 1992; Fig. 7). In Caniapiscau reservoir, the improvement in condition was equivalent to that observed in the other two reservoirs for whitefish (14.9 %) but lower for longnose suckers (14.4 %). Lake trout condition increased by 14.3 % . Trends in condition varied from one species to another for a given environment and from one environment to another for a given species (Fig. 7). However, at the end of the series, most species in the four modified environments showed mean condition values higher than or equivalent to the initial ones. Can. J. Zool. Downloaded from www.nrcresearchpress.com by Canadian Science Publishing on 08/12/15 For personal use only. walleye YCSI was higher from 1981 to 1983 than in preceding years (Fig. 3). Variations in CPUE were considerably more pronounced in La Grande 2 and Opinaca reservoirs than at the reference stations in Detcheverry and Rond-de-Poele lakes (not illustrated). Only walleyes showed significant interannual variations ( p < 0.05) in Detcheverry Lake, as a result of the increase in catches observed in 1988; however, CPUE for this species fell sharply in 1992, as in the reservoir. None of the major species showed significant interannual variations in CPUE in Rond-de-Poele Lake. In Detcheverry Lake, CPUE values for the various species were much lower than in La Grande 2 reservoir throughout the series, whereas the contrary was observed for several species in Rond-de-Poele Lake relative to Opinaca reservoir. Discussion The small number of stations sampled throughout the series in each environment, coupled to the fact that stations were located in contrasted nearshore habitats, on a reasoned 1870 Can. J. Zool. Vol. 73, 1995 Fig. 5. (A-E) Trends in mean annual CPUE of the main species at the various stations in Caniapiscau reservoir, including global mean CPUE (stations regrouped), from 1980 to 1991. * and k: interannual differences ( p < 0.05) in mean CPUE detected by ANOVAs (*) and Kruskal-Wallis tests (k). (F and G) Trends in YCSI (1976- 1985) and percentage of small specimens in total catches (1980- 1991). The arrow below the abscissa indicates the year of impoundment. 4 Longnose sucker - White sucker --- Can. J. Zool. Downloaded from www.nrcresearchpress.com by Canadian Science Publishing on 08/12/15 For personal use only. CA411 h h - - -- -- -- Mean *k Mean *k 8 0 8 1 8 2 8 3 8 4 8 5 8 6 8 7 8 8 8 0 9 0 0 1 t 8 0 8 1 Year El Lake whitefish ---- -- Northern pike CA411 *k CA413 CA416*k 25- \ -.,\ - 8 2 8 3 8 4 8 5 8 6 8 7 8 8 8 0 9 0 9 1 t Year 30- C M l l *k CA413 CA416 k I 8 0 8 1 I 0 I I I l I I I I l I 8 2 8 3 8 4 8 5 8 6 8 7 8 8 8 9 9 0 9 1 t 8 0 8 1 8 2 8 3 8 4 8 5 8 6 8 7 8 8 8 0 9 0 0 1 t Year 4 Year Lake trout - --- -- 0 0 CA416*k CA413 CA411 *k Mean *k - - - - 0 - - - - - 0 80 l l r l l l f l l l l 81 82 83 84 85 86 87 88 89 90 91 t Year DesLandes et al. Fig. 5 (concluded). El Year class strength index - 70 Lake whitefish *- - % small specimens - Longnow sucker 50- Longnosesucker --- Lake whitefish Northern pike Can. J. Zool. Downloaded from www.nrcresearchpress.com by Canadian Science Publishing on 08/12/15 For personal use only. a, 4- 40- C 2 30a I 76 77 1 1 I I 1 I I 1 78 79 80 81 82 83 84 85 Year * choice basis, precludes any quantitative extrapolation of our results to the fish communities as a whole. However, such was not the initial objective of the environmental monitoring network. Given the size of the modified environments, the cost of applying a sampling strategy that would have provided adequate coverage of all the major habitats in each reservoir would have been prohibitive. Instead, the importance and generality of the changes in CPUE and recruitment observed in this study were inferred qualitatively from the comparison of trends between stations in a given environment and between modified environments showing the same general characteristics (La Grande 2 and Opinaca reservoirs, the Boyd - Sakami diversion). Moreover, by being located in relatively shallow nearshore areas, our sampling stations address most of the major components of the fish community because benthic and fish production in these northern oligotrophic environments is concentrated in relatively shallow habitats (Dumont 1977; Johnson 1975). The fact that the major changes in CPUE and recruitment observed in the modified environments did not occur in nearby reference lakes supports the interpretation that they were related to impoundment. Although the impoundment of the various environments studied occurred 12 - 16 years ago, the dynamics of the various species is still changing. Important variations in relative abundance were still being observed in 1991 for lake whitefish and northern pike in Caniapiscau reservoir. We may expect further changes in the various environments, related to short- and long-term water level manipulations, which may affect recruitment success of the various species, predator prey relationships, availability of proper spawning grounds for species using nearshore hard substrates, etc. The environmental monitoring network is still ongoing to follow these changes, although with longer intervals between samplings (2 -4 years). CPUE and recruitment of the main species Catostomids As already mentioned, the negative correlation between longnose and white sucker CPUE and water transparency should 20 - 10 - ---- o - b - - - - - - - 80 81 82 t 83 84 85 86 87 v 88 - m 89 . 90 1 91 Year be interpreted with caution in view of the high longnose sucker CPUE observed in the transparent waters of Caniapiscau reservoir. However, the positive correlation between white sucker CPUE and water temperature probably reflects a preference for warmer waters (bays, shallow areas of lakes and reservoirs) at these latitudes (Boucher and Roy 1985). The white sucker showed CPUE values higher than those of its congener only in warmer environments (Opinaca reservoir at the beginning of the series, Rond-de-Poele Lake). The white sucker's difficulties in Caniapiscau reservoir are possibly related to the marked decline in mean temperatures over the summer after impoundment. The lack of adequate spawning grounds for catostomids cannot explain the difficulties experienced by both species at most stations in La Grande 2 and Opinaca reservoirs, because several good-quality spawning tributaries are available (Skguin et al. 1978a, 1978b; Boucher and Roy 1985). Their decline could be due to the marked increase in abundance of whitefish and northern pike, leading to competition among benthivores and (or) to increased predation by northern pike. Competition among benthivores would be more likely to involve the younger stages, because older juveniles and adult longnose suckers and whitefish showed sharp increases in growth rate and condition factor following reservoir impoundment. Increased predation following the rise in northern pike abundance could lead to decreased sucker abundance: the introduction of northern pike was indeed effective in controlling the abundance of longnose and white suckers in Eleven Mile Lake reservoir (Cook and Bergersen 1988). The competition -predation hypotheses do not explain the fact that the longnose sucker maintained relatively high CPUE after the impoundment of Caniapiscau reservoir. The general difficulties experienced by both catostomids in several reservoirs in Quebec need to be investigated further. Lake whitejish Bay stations of La Grande 2 and Opinaca reservoirs concentrated lake whitefish over most of the series, probably because of their high general productivity. In Great Bear Lake (Northwest Territories, Canada), lake whitefish are restricted to bays, where they are rarely captured deeper than 1872 Can. J. Zool. Vol. 73. 1995 Fig. 7. Interannual variations in Fulton's condition index (K) for the various species in the four modified ecosystems and in the reference lakes during the years indicated. The arrow below the abscissa indicates the year of impoundment. For the various species, "R" denotes samples from a reference lake. Fig. 6. Interannual variations in growth of longnose suckers, lake whitefish, northern pike, and walleyes from La Grande 2 reservoir (n is the sample size), during the years indicated. *, interannual differences ( p < 0.05) in mean total length at capture at a given age. Only the initial and final sampling years and one or two intermediate ones are illustrated. Can. J. Zool. Downloaded from www.nrcresearchpress.com by Canadian Science Publishing on 08/12/15 For personal use only. Longnose sucker Age (years) Northern pike Age (years) 20 m; the distribution of species in this oligotrophic lake is related to the density of benthic organisms (Johnson 1975). A similar situation was observed in Detcheverry Lake (reference lake for La Grande 2 reservoir), where lake whitefish were rarely captured deeper than 5 m (Dumont 1977). The fact that lake whitefish CPUE values were much higher at the bay stations of La Grande 2 and Opinaca reservoirs explains the rather unusual negative correlation with water transparency that was observed. However, the slightly lower values of this variable at the bay stations (ca. 2.25 m at G2 405 and ca. 1.7 m at EM 40 1) than at the other stations sampled are not considered limiting for this species. The relatively high transparency values observed at most of the Lake whitefish Age (years) Walleye Age (years) reservoir stations and in the Boyd -Sakami diversion, which for the most part are located outside of the zone of glaciomarine and glacio-lacustrine silts and clays, are probably partly responsible for the success of the species in these systems. Differential movements in response to changes in local conditions after impoundment explain part of the variations in whitefish CPUE observed in this study. However, increased recruitment was also involved. The rapid but short-term increase in recruitment was probably due to the general increase in primary and secondary production following impoundment, as suggested by the positive correlation between YCSI and zooplankton biomass in Opinaca reser- BesLandes et al. La Grande 2 reservoir Can. J. Zool. Downloaded from www.nrcresearchpress.com by Canadian Science Publishing on 08/12/15 For personal use only. Longnose sucker (n = 1693) Longnose sucker (n = 1881,R White sucker (n = 1413) Whitefish (n = 3500) Cisco (n = 989) Cisco (n = 1 07),R Northern pike (n = 2392) Walleye (n = 1823) WaDleye (n = 383),R 5 0.9 78 80 82 84 86 88 90 92 0.5 78 Year 4 86 82 84 86 88 90 92 Year 4 Opinaca reservoir Longnose sucker (n = 482) - - - White sucker (n = 1251) - - - Whitefish (n = 2997) -Whitefish (n = 978),R --- Cisco (n = 1060) Cisco (n = 388),R N o b e r n pike (n = 1562) Northern pike (n = 516),R Walleye (n = 2228) Walleye (n = 701),R - * - - -- -. -- -- -5 4 t 78 0.5 78 80 82 80 82 84 86 88 90 92 84 86 88 98 92 Year 4 Year Boyd-Sakami diversion Longnose sucker (n = 912) = 246) = 249) sucker (n -- --- --- White White sucker (n - Y - - .- -- -- 0.8 Cisco (n = 75) Cisco (n = 257),R Northern pike (n = 234) Nortern pike (n = 143,R Walleye (n = 1273) Y - Whitefish (n = 570) Whitefish (n = 718),R 0.7 -- - Walleye (n = 4811,R 0.6 ----0.8 -I $8 8Q 82 84 86 88 98 92 5 0.5 78 80 82 84 86 Year 88 90 92 Year Caniapiscau reservoir - --- Longnose sucker (n = 1415) White sucker (n = 304) Whitefish (n = 2489) Whitefish (n = 281),R - --- 0.8 80 82 84 86 Year 88 90 78 82 84 86 Year 88 9092 Northern pike (n = 318) - - Lake trout (n = 1 TI),R Lake trout (n = 362) Can. J. Zool. Vol. 73, 1995 Can. J. Zool. Downloaded from www.nrcresearchpress.com by Canadian Science Publishing on 08/12/15 For personal use only. voir. Although several authors use the same explanation for the increase in whitefish recruitment in reservoirs (Machniak 1975a), few present quantitative data relating year-class strength and indices of secondary production. Lake whitefish tend to concentrate at the mouth of tributaries during the fall, and also immediately downstream of La Grande 3 power station (Boucher and Roy 1985), in the lower part of the Coutaceau River rapids (station G2 404), and along dikes (Lalumikre and Dussault 1992), probably in preparation for spawning. In Outardes-2 reservoir, the only identified spawning ground is located immediately downstream of Outardes-3 power station (Gendron 1988; Fortin and Gendron 1990), in relatively swift waters. The eggs either remain in the deeper part of the canal or drift over long distances in deeper water, which may prevent the negative effects of a winter drawdown. The fact that whitefish in La Grande 2 reservoir did not show a marked decrease in recruitment following the very substantial winter 1982 drawdown (8 m), as well as the lack of correlation, for the available series, between year-class strength and winter drawdown, suggest that eggs are deposited at sites where they are not affected by fluctuations in water level. Gendron (1990, 1991) found no significant relationship between winter drawdown and whitefish year-class strength in Outardes-4, Pipmuacan, and Baskatong reservoirs. However, in Cross Lake (Churchill-Nelson complex), a rather shallow lake, where ciscoes and whitefish spawn on reefs and off rocky points in the lake (Ayles 1976), year-class strength was negatively correlated with winter drawdown (Bodaly et al. 1984; Gaboury and Patalas 1984). Another factor that may explain the increase in whitefish recruitment after impoundment in the La Grande complex is the rise in grow.th rates and condition, which may have been responsible for a general increase in potential fecundity of the populations (Healey 1978). However, higher growth rates may have introduced a positive bias in the evaluation of year-class strength, the fish becoming vulnerable to the fishing gear at younger ages. The late increase in lake whitefish CPUE at the three stations in Caniapiscau reservoir (1991) compared with La Grande 2 and Opinaca reservoirs may have resulted from interactions with northern pike. The minimal proportion of <250-mm specimens in 1987 coincides with the highest northern pike CPUE of the series, and the maximal 1991 proportion with the lowest northern pike CPUE. Predation by northern pike may have suppressed whitefish abundance during the first years after impoundment. The presence of cisco in the western reservoirs may have tempered the impact of northern pike predation on whitefish. As mentioned earlier, the rise in the proportion of dwarf lake whitefish in the 1991 catches (ca. 50%) also contributed to the increase in the proportion of small specimens at the end of the series. Cisco As already mentioned for lake whitefish, the important catches of ciscoes at the two highly productive bay stations are largely responsible for the negative correlations observed with water transparency and dissolved oxygen concentration, and for the positive correlations with total phosphorus level, chlorophyll a concentration, and biomass of the various zooplankton taxa. A negative correlation between cisco CPUE and water transparency was also mentioned by Beauvais (1977)' for lakes in Quebec's James Bay region. Given the proximity to shore of the stations sampled in this study, and the lack of stations sampling the pelagic zone, the cisco is probably the species for which our CPUE results are least representative. The importance of the cisco in the diet of northern pike at the end of the series in La Grande 2 reservoir (R. Verdon, unpublished data) suggests that CPUE underestimates the abundance of this important species. More studies are required to better understand the dynamics of the cisco in the La Grande complex. Northern pike A general increase in northern pike CPUE was observed after impoundment at practically all stations sampled in this study. While at the start of the series, the northern pike often ranked second among predators, behind the walleye (westernsector reservoirs) or lake trout (Caniapiscau reservoir), it became the dominant predator only a few years after impoundment. The lesser occurrence of northern pike in reservoir bay stations in the western sector remains unexplained. It may have been associated with the lower water transparency in these sectors. Several authors mention that water transparency may affect the choice of habitat by this species (Moreau and Legendre 1979; Chapman and Mackay 1984; Cook and Bergersen 1988). However, our study did not reveal a significant relationship between northern pike CPUE and water transparency, and this factor did not prove critical for the survival of the species within the range of values encountered in our study. Although major concentration -redistribution phenomena were involved, the increase in northern pike CPUE at reservoir stations and the Boyd -Sakami station was also dependent on a widespread increase in recruitment, as revealed by the analysis of YCSI. As has often been mentioned in the literature (Machniak 1975b), the improvement in recruitment was probably due to the increase in available spawning and nursery habitat related to the rise in water levels. The general increase in zooplankton production following impoundment observed in this study may also have been an important factor affecting recruitment. Submerged trees, which in certain areas develop an abundant growth of periphyton, offer habitat conditions similar to macrophyte beds, and possibly contribute to increased survival of juveniles by providing food and cover. High recruitment values were observed for at least 3 years in the two western reservoirs and the BoydSakami diversion, i.e., for a somewhat longer period than at Wupaw Bay, Southern Indian Lake, where they lasted only 1 year (Bodaly and Lesack 1984; Strange et al. 1991) following rapid degradation of the submerged vegetation. Northern pike CPUE decreased in most of the environments at the end of the series, particularly in Caniapiscau reservoir, where water levels fell regularly between 1986 and 1990. The sequence of increased pike abundance and recruitment after reservoir impoundment, followed, after a variable number of years of operation, by decreased abunA. Beauvais. 1977. Niches et associations de poissons dans les lacs de la Radissonnie quCbCcoise. Une Ctude effectuCe pour le compte de la SociCtC d'Cnergie de la Baie James. Centre de recherche en sciences de I'environnement, UniversitC du Quebec a Montreal (PolycopiC). DesLandes et al. dance and recruitment has been documented by several authors (Machniak 1975b). Can. J. Zool. Downloaded from www.nrcresearchpress.com by Canadian Science Publishing on 08/12/15 For personal use only. Walleye At the beginning of the series, the walleye was the dominant predator in most of the environments incorporated in the La Grande 2 and Opinaca reservoirs. Its CPUE subsequently fell at the majority of stations following impoundment. Thereafter, more than 94 % of walleyes caught in La Grande 2 reservoir came from the two warmer and more turbid stations. The Boyd -Sakami station also showed high CPUE. In La Grande 2 reservoir, the decrease in the proportion of small specimens from 1980 on suggests that recruitment started declining shortly before the 1979 impoundment. In Opinaca reservoir, YCSI agrees with the proportion of small specimens, suggesting a certain increase in recruitment after impoundment. This is not borne out by CPUE values, which decreased following impoundment. However, this decline involved fish of all size classes and was too rapid to be explained by reproductive failure. Results of complementary samplings conducted in 1981 (Plante 1982) suggest that the decrease in CPCTE, in this shallower reservoir, could be due to walleyes avoiding the inshore sampling stations after impoundment and occupying former beds of lakes and rivers outside the zone of submerged trees. The negative correlation observed between walleye CPUE and water transparency and the positive correlation with temperature are due to the fish concentrating in the two less transparent and warmer stations in La Grande 2 reservoir and at the Boyd- Sakami station. The higher predation efficiency of walleyes in turbid water has frequently been mentioned in the literature (Vandenbyllaardt et al. 1991). On the other hand, temperature may be a limiting factor at these northern latitudes, the 54th parallel (northern part of La Grande 2 reservoir) being the northern limit of distribution of walleyes in Quebec (Beauvais 1977, see footnote I). Water temperature and spring warming rate have often been mentioned as major determinants of walleye year-class strength (Busch et al. 1975; Koonce et al. 1977; Forney 1980; Hokanson and Lien 1986). Machniak (1975~)points out the importance of access to good spawning sites and of substrate quality for this species' reproductive success. Brouard (1983) and Gilles Shooner Inc. (1984) showed that in La Grande 2 reservoir, some walleye spawned on the slopes of a dike or on an old road used to build it, or in tributaries. In Detcheverry Lake and in Southern Indian Lake reservoir, walleyes also spawn in tributaries (Gaudreault 1978; Bodaly 1980). Like the two catostomids, walleyes should not lack good spawning sites in the reservoirs of the La Grande complex. However, several studies have shown a relationship between reservoir age and the success of walleye populations (Jenkins 1970; Bennett and McArthur 1990). In reservoirs where nearshore spawning grounds are selected, it takes 5 or more years before ice and wave action loosens enough coarse material from the shores for recruitment to improve (Machniak 1 9 7 5 ~ ) . Lake trout Several explanations for lake trout recruitment problems in reservoirs have been suggested. Because of its homing behavior, the species suffers a considerable disadvantage during reservoir impounding because spawners show little inclination to find new spawning sites in more suitable sectors (Gendron and Bklanger 1991). Moreover, winter drawdown probably limits the survival of embryos and alevins because eggs are usually deposited in relatively shallow water. Thus, lake trout reproduction has become impossible in Bark Lake (Ontario) as a result of sizeable annual drawdowns of approximately 10 m (Wilton 1985). In Mitis reservoir (Quebec), the lake trout population has remained stable in spite of mean winter drawdowns of 3 m (Gendron 1992). The exceptional habitat available for the species and the scarcity of competitors are factors contributing to its success in this reservoir. In Caniapiscau River, lake trout did not experience recruitment problems following the river's cutoff, probably because the ancestral spawning grounds remained accessible to spawners (Belzile 1990). Secondary species Because of the type of fishing gear used, it was impossible to follow the dynamics of the small fish species in the La Grande complex. However, diet studies of piscivorous predators can give information on their abundance. For example, Sage Ltke (1983) showed that sticklebacks represented the second most important group of prey, behind coregonines, for walleyes in La Grande 2 and Opinaca reservoirs in 1981 and 1982; they also were the third most important prey of burbot in La Grande 2 reservoir. Recent studies have also demonstrated the importance of sticklebacks in the diet of ciscoes (R. Verdon, unpublished data). Growth and condition An increase in growth rate like that observed in La Grande 2 reservoir following impoundment has often been mentioned for the northern pike and walleye, but less often for the whitefish (Machniak, 1975a, 1975b, 1 9 7 5 ~ ) .In certain reservoirs, the presence of competing planktivorous species, coupled with a reduction in benthic resources after impoundment, led to decreased growth rates of young whitefish and to a possible reduction in their rate of escape from predators (Machniak 1 9 7 5 ~ ) .The general increase in primary and secondary production after impoundment favors the recruitment of several fish species used as prey by northern pike and walleyes, whose growth rates increase. The rise in fish growth rates following impoundment is generally short-lived, this parameter showing a decline with reservoir age (Nelson 1974). The lesser improvement in the different species' condition at the Boyd - Sakami station, compared with that observed in the La Grande 2 and Opinaca reservoirs could be due to the lower increase in overall productivity in this sector (DesLandes 1994). The colder thermal regime of Caniapiscau reservoir could be responsible for the lesser improvement in condition of the two catostomids, whereas the improvement for whitefish was comparable to that observed in the reservoirs in the western sector. The improvement in lake trout condition in Caniapiscau reservoir is worthy of mention. Acknowledgments We thank Martin Dufort and Roger Schetagne of HydroQuibec and Daniel Dussault of Groupe Environnement Shooner Inc., who gave us access to the La Grande complex environmental monitoring network data base. We are grate- Can. J. Zool. Vol. 7 3 , 1995 ful to Hydro-Quebec's vice-presidence Environnement for funding this study. References Can. J. Zool. Downloaded from www.nrcresearchpress.com by Canadian Science Publishing on 08/12/15 For personal use only. Ayles, H.A. 1976. Lake whitefish (Coregonus clupeaformis (Mitchill)) in Southern Indian Lake, Manitoba. Can. Fish. Mar. Sew. Tech. Rep. No. 640. Bachand, C.A., and Fournier, J. J. 1977. RCseau de surveillance Ccologique du complexe La Grande. Service Environnement, SociCtC d'Cnergie de la Baie James, MontrCal, QuCbec. Belzile, L. 1990. RCseau de suivi environnemental du complexe La Grande, Phase I (1989). Etude des rendements de p6che (Rivikre Caniapiscau) . Groupe Environnement Shooner Inc., pour le Service en environnement et santC publique, viceprCsidence Environnement Hydro-QuCbec . Bennett, D.H., and McArthur, T.J. 1990. Predicting success of walleye stocking programs in the United States and Canada. Fisheries (Bethesda) , 15(4): 19 -23. Bodaly, R.A. 1980. Pre- and post spawning movements of walleye, Stizostedion vitreum, in Southern Indian Lake, Manitoba. Can. Tech. Rep. 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