by Direction de la faune et des habitats

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and

Direction de la gestion de l'habitat du poisson

FISH EVACUATION TECHNIQUE EXPERIMENTED IN SEPTEMBER 1993 IN THE ILE DUPAS DALE DITCH SYSTEM ENHANCED BY DUCKS UNLIMITED CANADA

by

Suzanne Lepage Nathalie Gélinas Ducks Unlimited Canada

for

Ministère de l'Environnement et de la Faune du Québec and

Fisheries and Oceans Canada

Québec, March 1995

Technical Report

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Reference:

LEPAGE, S. and N. GÉLINAS. 1994. Fish evacuation technique experimented in September 1993 in the Ile Dupas dale ditch system enhanced by Ducks Unlimited Canada. Ducks Unlimited Canada (Québec Region) for the Ministère de l'Environnement et de la Faune du Québec and Fisheries and Oceans Canada, Québec.

79 p.

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SUMMARY

The purpose of this study was to find a means of preventing the winterkill of fish in an impoundment on Ile Dupas, an Island near Berthierville. More specifically, study objectives were: 1) to characterize the summer fish population in the dale ditches; 2) to define and corroborate the conditions for the maximum effectiveness of drawdown as a fish evacuation technique; 3) to verify the effects of drawdown on the behaviour of organisms, including fish, in the study area; 4) to estimate the number of fish trapped inside the impoundment after drawdown; and 5) to approximate the cost of using drawdown as a fish management technique. The fish community comprised 16 species of adults and juveniles over one year, and 9 species of fry. The distribution of the main species, Yellow Perch, Brown Bullhead, Golden Shiner, Northern Pike and Pumpkinseed, was relatively homogeneous in the collector canal. Population estimates for the canal sector indicate the presence of 1407 adults (9 species) and 3094 young-of-the-year (4 species). The total dale ditch population was estimated at 19 539 adults (9 species) and 74 400 fry (4 species). During the drawdown operation, which ran 192 hours, 87 981 fish representing 11 species were evacuated. Of this total, 16 000 captures, comprising individuals measuring 50 mm or more and representing 9 different species, were evacuated from an estimated community of 93 939. Some 139 kg of fish were reportedly evacuated from the impoundment, i.e. 21% of the total mass calculated for the 8 most abundant species present in it. Data collected in the two experimentai ditches aise shows that most of the species in this sector moved downstream after drawdown began. Several bird species, including Great Blue Heron, Greater Yellowlegs and Belted Kingfisher, came to feed on fish and small organisms throughout the drawdown. Fish carcasses and tracks in the dale ditches indicate the presence of raccoons. Post-drawdown fishing in the basin upstream of the control structure gave rise to an estimate of 15 912 fish (12 species) in this sector.

Added to previous captures, this brings the total number of fish evacuated from the

impoundment to 103 893. Over 21 000 of these individuals represented species marked

prier te drawdown (23% of the estimated total community). Study objective 5 is net dealt

with in this report since corrective measures were carried out in an enhancement in

summer 1994, and new experiments are required before the cost of the management

technique to be used can be assessed.

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TABLE OF CONTENTS

Paae

SUMMARY iii

TABLE OF CONTENTS

LIST OF TABLES vii

LIST OF FIGURES ix

LIST OF APPENDICES xi

1. INTRODUCTION 1

1.1 Purpose of experiments 2

2. MATERIALS AND METHODS 5

2.1 Study area 5

2.2 Sampling methods 5

2.2.1 Preliminary sampling in the dale ditches 8

2.2.2 Fish sampling before drawdown 8

2.2.3 Fish sampling during drawdown 8

2.2.4 Fish sampling after drawdown 8

2.3 Characterization of the fish community 8

2.3.1 Initial inventory in the dale ditches 8 2.3.2 Fish community structure and distribution 12

2.3.3 Fish community estimates 13

2.4 Drawdown experiments 13

2.4.1 Draining 13

2.4.2 Number and mass of fish evacuated 15 2.4.3 Fish movement in the dale ditches during drawdown . . . 15 2.4.4 Evaluation of the number of fish trapped in the dale ditches

after drawdown 17

2.4.5 Behaviour of other wildlife species during drawdown 17

2.4.6 Refilling of the impoundment 17

2.5 Statistical analyses 17

3. RESULTS AND DISCUSSION 19

3.1 Characterization of the fish community 19 3.1.1 Initial inventory in the dale ditches 19 3.1.2 Fish community structure and distribution 19

3.1.2.1 Fish community structure in the collector

canal and basin 19

3.1.2.2 Fish community distribution in the collector

canal 21

3.1.2.2.1 Adults and juveniles over one year . . 21

3.1.2.2.2 Young-of-the-year 23

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3.1.3 Fish community estimates 23 3.1.3.1 Fish community estimates based on catch per

surface unit 23

3.1.3.2 Fish community estimates based on the mark-

release-recapture method 26

3.2 Drawdown experiments 28

3.2.1 Draining 28

3.2.2 Number and mass of fish evacuated 30

3.2.2.1 Number of fish evacuated 30

3.2.2.2 Mass of fish evacuated 39

3.2.3 Fish movement in the dale ditches during drawdown . . . 39 3.2.4 Evaluation of the number of fish trapped in the dale ditches

after drawdown 50

3.2.5 Behaviour of other wildlife species during drawdown . . . 51

3.2.6 Refilling of the impoundment 51

4. CONCLUSION 53

5. RECOMMENDATIONS 57

ACKNOWLEDGEMENTS 59

REFERENCES 61

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LIST OF TABLES

Paae Table 1. Species captured during fish inventories in

the Ile Dupas dale ditches, summer 1991 and 1993 20 Table 2. Comparison of mean pre-drawdown adult and

juvenile-over-one-year (TI 50 mm) CPUE in the callector canal, based on proximity to the control

structure, Ile Dupas, summer 1993 22

Table 3. Comparison of mean pre-drawdown young-of-the-year (TI > 50 mm) CPUE in the collector canal, based on proximity to the control structure, lie Dupas,

summer 1993 24

Table 4. Fish community estimates and density (TI 50 mm) for the collector canai in the Ile Dupas

dale ditch system, summer 1993 25

Table 5. Fish community estimates and density (TI k 50 mm) in the lie Dupas dale ditches, based on

recaptures downstream of the control

structure, summer 1993 27

Table 6. Fish community estimates and density (Tl 50 mm) in the Ile Dupas dale ditches, based on

recaptures downstream of the control structure

and in the basin (post-drawdown), summer 1993 29 Table 7. Number of adults and juveniles over one year captured

at the Ile Dupas dale ditch outlet, and population estimates for the impoundment as a whole,

summer 1993 32

Table 8. Number of young-of-the-year captured at the Ile Dupas dale ditch outlet, and population estimates for the

impoundment as a whole, summer 1993 34

Table 9. Comparison by species of the mean number of adults and juveniles over one year exiting at different hours of the day during drawdown

of the Ile Dupas dale ditches, summer 1933 42

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Table 10. Comparison by species of the mean number of young-of-the-year exiting at different hours of the day during drawdown of the Ile Dupas

dale ditches, summer 1933 43

Table 11. Water temperatures and DO concentrations measured in the basin during drawdown of the lie Dupas dale ditches,

summer 1993 43

Table 12. Total mass (kg) of fish (Tl 50 mm) in the

Ile Dupas dale ditches before and after drawdown,

summer 1993 44

Table 13. Comparison of mean CPUE in Ditch III based on

fish movement during drawdown of the Ile Dupas dale

ditches, summer 1993 45

Table 14. Comparison of mean CPUE in Ditch V based on

fish movement during drawdown of the Ile Dupas dale

Table 15. Comparison of mean CPUE for the two expérimental ditches based on fish movement during drawdown of

the Ile Dupas dale ditches, summer 1993 48

Table 16. Downstream movement of fish in ditches III and V toward the basin and downstream of the control

structure 49

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LIST OF FIGURES

Page Figure 1. Location of the Ile Dupas dale ditches 6 Figure 2. DUC-enhanced dale ditches on

Ile Dupas 7

Figure 3. Fish inventory sampling plan (species and

developmental stage) conducted in Ile Dupas dale

Figure 4. Sampling plan before drawdown of the lie Dupas

Figure 5. Sannpling plan during drawdown of the Ile Dupas

Figure 6. Modifications to the control structure for use in drawdown of the Ile

Dupas dale ditches, summer 1993 14

Figure 7. Installation of fishing gear during

fish evacuation, Ile Dupas dale ditches,

summer 1993 16

Figure 8. Drop in water level during drawdown of the Ile Dupas

Figure 9. Total number of fish which exited during drawdown

of the Ile Dupas dale ditches, summer 1993 35 Figure 10. Fish species exiting the Ile Dupas dale ditches

throughout the drawdown operation, summer 1993 36 Figure 11. Fish species exiting early or late in the drawdown

of the Ile Dupas dale ditches, summer 1993 40

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L1ST OF APPENDICES

Page Appendix 1. Surface area sampled at stations in the collector

canal (C), basin (B) and certain ditches (D) of the Ile Dupas dale ditch enhancement,

summer 1993 63

Appendix 2. List of common names, scientific names and codes for the fish species discussed in

this report 64

Appendix 3. Pre-drawdown CPUE of adults and juveniles over one year (Ti z 50 mm) using a seine in the collector canal and basin,

Ile Dupas, summer 1993 65

Appendix 4. Pre-drawdown CPUE of young-of-the-year (TI 50 mm) using a seine

in the collector canal and basin,

Ile Dupas, summer 1993 66

Appendix 5. Mortality due to marking (Golden Shiner and

Yellow Perch) during the two pre-drawdown sampling programs conducted in the Ile Dupas dale ditches,

summer 1993 67

Appendix 6. Total number of marked fish (Ti 50 mm) in the collector canal, basin and ditches during the second sampling program, before drawdown of

the lie Dupas dale ditches, summer 1993 68 Appendix 7. Number of fish recaptured downstream of the

control structure during drawdown, and in the basin after drawdown of the Ile Dupas dale

Appendix 8. Number of fish captured downstream of the control structure during drawdown of the Ile

Dupas dale ditches, summer 1993 70

Appendix 9. Number of fish less than 50 mm in length per

cup 71

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Appendix 10. Maximum and minimum Iength, mean Iength and mean weight of species (TI 50 mm) captured during pre-drawdown sampling, Ile Dupas claie

Appendix 11. Length-weight ratios used ta determine the mass of fish in the dale ditches based on the equation log P = a + b log L (P = weight in grams; L = Iength in mm; a = constant;

b = slope) 73

Appendix 12. CPUE for downstream migrants in ditches III and V during drawdown of the Ile Dupas dale ditches,

summer 1993 74

Appendix 13. CPUE for upstream migrants in ditches III and V during drawdown of the Ile Dupas dale ditches,

summer 1993 76

Appendix 14. Number of seine captures in the basin after drawdown of the Ile Dupas dale ditches,

summer 1993 78

Appendix 15. Mass of fish captured in the basin after drawdown of the Ile Dupas dale ditches,

summer 1993 79

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1. INTRODUCTION

Various research projects conducted in recent years have targeted a better understanding of the value of wetlands in order to enhance them. In 1988, the Ministère du Loisir, de la Chasse et de la Pêche du Québec (now the Ministère de l'Environnement et de la Faune-MEF) and Ducks Unlimited Canada (DUC) began a five- year study to evaluate the needs of wildlife species other than waterfowl in DUC enhancement projects. This study was an integral part of an agreement, signed in 1987, constituting a five-year plan to protect and enhance wildlife habitats. Other signatories included the Fondation de la faune du Québec, Wildlife Habitat Canada and the Office de planification et de développement du Québec, now the Secrétariat aux affaires régionales. The Canadian Wildlife Service joined in 1989.

Phase 2 of the study, which began in May 1993 and is being carried out jointly by MEF, Fisheries and Oceans Canada (DFO) and DUC, is aimed at rounding out the expertise gained on multispecies floodplain enhancement during the first five years of research.

One of the objectives of Phase 2 is to improve the drawdown technique for use in fish evacuation. This report presents the results of drawdown experiments conducted in DUC-enhanced date ditches on Ile Dupas.

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Phase I of this project raised two major problems related to the utilization of impounded marshes by aquatic faune (Lepage et al. 1994): 1) the winterkill of fish trapped inside the impoundments, and 2) the inability of certain fish species to access the impoundments in spring.

The purpose of this study was to find a solution to the problem of winterkill. Fish access impounded areas with spring flood waters; however, once these waters begin to recede and water levels stabilize inside the impoundment, the retaining structure can become an obstacle to downstream migration. Adults and fry become trapped, resulting in high fish mortality when the water freezes over in winter.

The dale ditches on Ile Dupas were selected as a test site for several reasons. First, a site characterization and study of site utilization by fish had already been carried out by Grondin et al. in 1994. While the results of this work showed that fish can easily access the site in spring and that a large number of fish of various species were using the site in 1990 and 1991, the problem of winterkill persists. Second, drawdown experiments conducted in 1991 were virtually unsuccessful. Grondin et al. (1994) suspect that Iowering water levels tao quickly, coupled with the presence of a basin upstream of the control structure, may have hindered the process. Third, the impoundment is easily accessible, is relatively small compared to other impoundments, and is equipped with a pumping station, facilitating post-drawdown refilling.

The purpose of the experiments was to improve the drawdown technique so it couid be used to evacuate fish from the impoundment and prevent winterkill, and to evaluate the potentiel impact of the technique on other organisms in the impoundment.

The five study objectives were:

1. to characterize the summer fish population in the dale ditches;

2. to define and corroborate the conditions required for maximum effectiveness of drawdown as a fish evacuation technique;

3. to verify the effects of drawdown on the behaviour of organisms, including fish, in the study area;

4. ta estimate the number of fish trapped inside the impoundment after drawdown and identify the causes; and

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5. ta approximate the cost of using drawdown as a fish management technique.

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2. MATERIALS AND METHODS 2.1 Studv area

The Ile Dupas dale ditches (46°07'N, 73°06'W) constitute one of three DUC enhancement projects located in the island's community pasture (Shooner et al.

1988). Part of the Berthier-Sorel archipelago, Ile Dupas is located near Berthierville, a village on St. Lawrence's north shore approximately 70 km east of Montréal (Figure 1)

The island is flat, like much of the St. Lawrence lowlands, and is located in the two- year flood zone. The flow regime around it shows considerable seasonal variations, with high water levels during spring floods and zero-to-low flow rates during summer, depending on the overall water level (Shooner et al. 1988).

The soit in the community pasture is composed of recent alluvial deposits on a base of polder clay from the Champlain Sea (Pilon et al. 1981). Its fertility nnakes the area prime farmland, and the island has been used for grazing since the 1940s, although today's herd is much smaller.

The dale ditches became operational in fall 1988 (Figure 2). An estimated 14.72 ha are submersed at the normal operating level of 5.65 m. The impounded area consists of 25 ditches averaging 6.5 m in width and 60 cm in depth, and covering a surface area of 14.38 ha. The material excavated to dig the ditches was used to make furrows for summer use by livestock. All of the ditches flow into a collector canal (length: 800 m; width: 3.9 m; mean depth: 1.2 m). A 284-m2 basin was dug between the control structure and the collector canal in the southern portion of the impoundment to enhance winter survival of fish. Water levels are regulated by a stoplog control structure located downstream of the basin. A pumping system (2879 L/min capacity) was installed northwest of the impounded area and can be activated automatically or manually whenever the ditches require filling. The system is driven by water from the St. Lawrence River.

2.2 Sampling methods

Various sampling methods were used during summer 1993, all based on the results obtained by Grondin et al. (1994) in studies on the saure site.

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Saint-Barthélemy •

Berthierville

Pierreville

0 5 10 km ^çà, ⁶ ^Saet,

— ^{^ro",}

SOFiEL

Figure 1 Location of the 11e Dupas date ditches.

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Figure 2. DUC-enhanced daie ditches on lie Dupas.

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2.2.1 Preliminary sampling in the claie ditches

Preliminary sampling (species and developmental stage) was conducted on August 3, 4 and 5, 1993 (Figure 3). A sampling station located in Ditch I and another in Ditch Il, near the collector canal, used seines. This method was also used at stations located in the collector canal and the fish basin. Three other sampling stations used fyke nets installed at the entrance to ditches IV and V and in the collector canal between ditches V and VI.

2.2.2 Fish sampling before drawdown

The second sampling program ran from August 30 to September 3. In all, 19 seined sampling stations were set up in the collector canal sector, and 1 in the basin, so as to cover the entire basin area (Figure 4). Only 4 sampling stations were set up in the ditches due to extremely dense aquatic vegetation and the low number of fish captured.

2.2.3 Fish sampling during drawdown

Drawdown ran from September 7 to 15. Fish movement during drawdown was observed in two ditches (III and IV) using fyke nets (Figure 5). In addition, all species exiting via the control structure were captured in a fyke net downstream of the structure (Figure 5).

2.2.4 Fish sampling after drawdown

September 16 was devoted to sampling fish trapped in the basin upstream of the control structure. The basin was seined seven times that day.

2.3 Characterization of the fish communitv 2.3.1 Initial inventorv in the dale ditches

The initial inventory conducted during prelinninary sampling was designed to collect qualitative data (species and developmental stage) on the new fish community in the dale ditches: the community changes annually due to winterkill and the problem of impoundment accessibility during spring flooding.

Fish were captured with a beach seine (10 m long, 1.8 m high, 7 mm mesh) inside closed stations. Each station in the canal and ditches was blocked at both ends with nets (5 m long, 1.8 m high, 3 mm mesh), which were placed so as to form an enclosure with the banks.

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IV V

VI

IX X XI

XII XII/

XIV XV

XVI XVII

XV III XIX

XX XXI XXI I

XXIII XXIV

XXV

Collector canal

■

PUMPING STATION D SEINE STATION CONTROL STRUCTURE > FYKE NET STATION

Figure 3. Fish inventory sampling plan (species and developmental stage)

conducted in the Ile Dupas dale ditches, summer 1993.

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Xi XII

I

fi

XI Ir XIV

XV

xvi xvi

XVIII

xix

XX XXI

xxii xxur

xxrv xxv

R I

R3

I

C8 /

C9 CIO%

..je Du D

^Collector

C12 CI4 f6f C13 C19

1 1 1 t canal

CI I

CI5 O17

• PUMPING STATION à CONTROL STATION

13 SEINE STATION

Sampling plan before drawdown of the lie Dupas date ditches, summer 1993.

Figure 4.

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V

X _XI XII

XIII X I V

X V VI

VII VI II

IX

X V I X VII

XV 1 I XI X

X X X X I XXII

XX I I 1 X XI V

X XV IV

I

Collector canal

■

PUMPING STATION > FYKE NET STATION

à CONTROL STATION • PHYSICOCHEMICAL STATION Figure 5. Sampling plan during drawdown of the Ile Dupas dale ditches, summer

1993.

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Prior to seining, aIl aquatic vegetation was raked away. Enclosures measured 130.00 m2 in the ditch sector, 91.26 m2 in the collector canal, and 388.80 m2 in the basin (including the portion of the canal adjacent to the basin).

Fish were also captured with a fyke net (wings: 4 m long, 1 m high; house: 1.6 m long, 0.8 m high; mesh: 3 mm) in the collector canal between ditches V and VI (Figure 3). The wings of the net pointed toward the basin. Two other fyke nets (wings: 3 m long, 0.7 m high; house: 2 m long, 0.4 m high; mesh: 10 mm) were placed at the entrance to ditches IV and V.

Fish captured were identified by species and classified by size, i.e. adults and juveniles over one year, or young-of-the-year (0 + ), then returned to the water (Scott and Crossman 1974; Grondin et al. 1994). Species which could not be identified in the field were preserved in a 95% alcohol solution for identification in the laboratory using identification keys taken from Scott and Crossman (1974), Legendre (1966) and Aver (1982).

Finally, a number of Yellow Perch (Perce flavescens) and Golden Shiner (Notemigonus crvsoleucas) measuring 50 mm or more were marked by clipping a portion of the right pelvic fin. The fish were subsequently placed in a holding cage (1 m2 with bottom, 3 mm mesh) for approximately 15 hours. The purpose of the ablation was to verify the resulting mortality rate among Yellow Perch and Golden Shiner, believed to be less resistant than other species, in order to determine whether this would be an effective technique during the second sampling program. The specimens were then returned to the dale ditches.

2.3.2 Fish community structure and distribution

The structure of the fish community in the collector canal and basin was determined in the second sampling program. Fish captured were identified by species except for Cyprinidae other than Golden Shiner. Total length (TI) and developmental stage (adult and juvenile over one year, or young-of-the-year) were also noted for several specimens of each species except Central Mudminnow (Umbra limi), Tadpole Madtom (Noturus gvrinus), Burbot (Lots Iota), Banded Killifish (Fundulus diaohanus), and Cyprinidae other than Golden Shiner. Specimens of Northern Pike (Esox lucius), Yellow Perch, Brown Bullhead (Ictalurus nebulosus) and Bowfin (Amin calva) were weighed ta establish a length-weight ratio.

In addition ta structure, distribution of the fish community (marked species only) in the collector canal was determined during the second sampling program (cf. Section

2.3.3).

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2.3.3 Fish communitv estimates

The second sampling program was also aimed at quantifying the population of marked species using two estimation methods. First, population estimates were calculated for each of the marked species in the collector canal, excluding the basin, based on the number of captures per enclosure using the procedure described by Scherrer (1984) and employed in previous studies (Pépin et al. 1991; Gélinas et al. 1993; Grondin et al. 1994). The population of these senne species was then calculated for the entire dale ditch system using the Petersen mark-release-recapture method. Calculations were based on the number of fish marked during the second sampling program and recaptured at the exit during drawdown. A second estimate was made using the number of fish recaptured downstream of the structure plus the number recaptured in the fish basin after drawdown.

Sampling was conducted inside closed stations covering the entire collector canal (Figure 4). Two nets (4 m long, 1.8 m high, 7 mm mesh) were installed in the canal, near the entrance te two ditches, to create a barrier. A beach seine (10 m long, 1.8 m high, 7 mm mesh) was used to capture fish during three different sampling sessions. Some stations were also set up in the dale ditches. Enclosures in the dale ditches measured 130.00 m2/station, white those in the collector canal varied between 91.26 m2 and 138.06 m2, depending on the distance between ditches (Appendix 1). Finally, the basin was sampled several times using seines, this time without creating an enclosure, in order to increase the number of marked fish captured.

Only individuels measuring 50 mm or more (TI) were identified and marked by clipping a portion of the left pelvic fin, except for Central Mudminnow, Tadpole Mudtom, Banded Killifish, Cyprinidae other than Golden Shiner, and the fry of Pumpkinseed (Leoornis gibbosus) and Black Crappie (Pomoxis niqromaculatus). Yellow Perch and Golden Shiner specimens were kept in a holding cage for approximately 20 hours to evaluate mortality due to ablation.

2.4 Drawdown exoeriments 2.4.1 Draining

Soli was spread over the floor of the basin located upstream of the control structure to make it shallower and prevent fish from aggregating during drawdown, a problem which had been noted in this sector in 1991 (Huet 1970; Grondin et al. 1994).

Water levels were lowered using the control structure, which was adjusted to obtain a suitable rate of 5 - 20 cm/d (Gélinas et al. 1993). All stoplogs except for the bottom one were removed and replaced with a reinforced wood panel with a 500-cm2 opening (Figure 6). A removable plank was used to adjust the size of the opening as needed.

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REMOVABLE PLANK

CONTROL STRUCTURE

WATER LEVEL 5.65 m

BASE OF OPENING 4.25 m

APRON 4.00 m

STOPLOG (height 15 cm)

Figure 6. Modifications to the control structure for use in drawdown of the lie Dupas dale ditches, summer 1993.

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The drawdown operation was to completely drain the dale ditches but not the collector canal, since the water level in the canal stabilizes when it reaches the same level as the St. Lawrence (approximately 4.50 m at that time of year). The drop in water level was recorded throughout the drawdown using a gauge on the control structure.

2.4.2 Number and mass of fish evacuated

Fish exiting the marsh via the control structure during the drawdown were intercepted using a fyke net (wings: 4 m long, 1 m high; house: 1.6 m long, 0.8 m high; mesh:

3 mm) installed downstream (figures 5 and 7). The net was raised every six hours, and more frequently, i.e. every two or three hours, during peak exit periods. When captures were high, the fish were kept in a holding cage white awaiting handling. At the end of the drawdown, the fish exit was blocked with a net. The sector between the control structure and the fyke net was seined three times to capture individuels

which had not entered the fyke net.

Ail specimens 50 mm or more were identified and counted. Several individuels of Northern Pike, Yellow Perch, Brown Bullhead and Bowfin were measured (TI) and weighed. Marked fish were also noted. Fish measuring Iess than 50 mm (TI) were identified and counted (volumetric analysis): cupfuls of small fish were taken at various intervals and counted, evaluating the proportion of panfish, Cyprinidae and other species in each cup. Thereafter, only the number of cups and the proportion of each species captured were noted during net-raisings. Captured fish were returned to the water downstream of the fishing gear.

The mass of fish 50 mm or more evacuated during drawdown was estimated using length-weight ratios.

Finally, dissolved oxygen concentrations and water temperature were measured in the basin throughout the drawdown using a YSI mode! 57 oxygen meter.

2.4.3 Fish movement in the dale ditches during drawdown

Fish behaviour during drawdown was documented for ditches III and V using two fyke nets (wings: 3 m long, 0.7 m high; house: 2 m long, 0.4 m high; mesh: 10 mm) per ditch. The nets were installed opposite each other so as to intercept any fish moving either upstream or downstream in the ditches (Figure 5). The nets were raised five times a day between 6:00 a.m. and 7:30 p.m. Following each 7:30 p.m. raising, the entrance to the nets was blocked to prevent captures during the night.

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LEADING TO THE ST. LAWRENCE

Figure 7. Installation of fishing gear during fish evacuation, Ile Dupas date ditches, summer 1993.

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Fish measuring 50 mm or more (TI) captured in the downstream fyke nets were identified and then marked by ciipping the left pectoral fin. Captures measuring less than 50 mm were identified and counted (volumetric analysis). Ail of the fish were then returned to the water downstream of the nets to verify whether they would swim toward the exit.

Fish measuring 50 mm or more (TI) captured in the upstream fyke nets were identified and then marked by ciipping the anal fin. Captures measuring less than 50 mm were identified and counted (volumetric analysis). All of the fish were then returned to the water upstream of the nets to determine whether they would continue their upstream migration. The tip of the left pectoral fin was cut off marked individuels recaptured in the downstream net. They were then returned to the water downstream of the nets.

2.4.4 Evaluation of the number of fish trapped in the dale ditches after drawdown

Several field visits were made to the claie ditch sector and a few ta the collector canal during drawdown to verify whether any fish remained trapped.

Once the drawdown was complete, the basin was isolated using seines. Initial seining was performed in the basin (net 20 m long, 1.8 m high, 7 mm mesh). Subsequentiy, a second seine was attached ta the first to cover more of the sampiing area. Captured species were identified and counted in the first seven net tows, although only the lest six were used to evaluate the total number of fish in the basin using the Zippin method. The fish were then released downstream of the control structure. The basin was seined three more times to free the remaining fish.

2.4.5 Behaviour of other wildlife species during drawdown

The field team aiso observed species other than fish during drawdown to determine their presence and any behayioural changes caused by the evacuation of the fish.

2.4.6 Refilling of the impoundment

The pumping system was activated at noon on September 15, as soon as the stoplogs were repiaced in the control structure.

2.5 Statistical analyses

The pre-drawdown distribution of fish in the collector canal, and upstream/

downstream movement data collected during drawdown of the two experimental ditches were analyzed using a t-test (p < 0.05). Data not conforming to normality and homogeneity of variance requirements were converted using a log (x + 1) and

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reanalyzed with the t-test.

Analysis of variance (ANOVA) for a classification criterion and a Student-Newman- Keules (SNK) multiple range test were used to determine whether the number of fish exiting during drawdown varied with the time of day. Data not conforming to normality and homogeneity of variance requirements were converted using a log (x + 1) and reanalyzed using the Kruskal-Wallis test. Tukey's variance test was applied wherever data still did not conform to normality and homogeneity of variance requirements.

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3. RESULTS AND DISCUSSION

3.1 Characterization of the fish community 3.1.1 Initial inventory in the dale ditchess

More than 15 species of fish were inventoried in the impoundment in early August (Table 1). Excluding Cyprinidae (_Notropis spp.) other than Golden Shiner, 11 of these species were in the adule stage; 4 of them, i.e. Northern Pike, Yellow Perch, Pumpkinseed and Brown Bullhead, were also captured in the fry2 stage. Fry only of Black Crappie, Burbot and Bowfin were inventoried during preliminary sampling.

3.1.2 Fish community structure and distribution

3.1.2.1 Fish communitv structure in the collector canal and basin

In late August, over 14 species of adult fish were identified in the canal and basin sector (Table 1), the saure species inventoried during preliminary sampling, with the exception of Fallfish (Semotilus corporalis). Three new species of adult fish were captured during the second sampling program, i.e. Black Crappie, Bowfin and one Darter (Etheostoma sp.).

Fry inventoried during the second sampling program were of the same species as those inventoried during preliminary sampling, with the exception of Burbot (Table 1). One new species was captured, i.e. Golden Shiner.

The species identified during the two summer 1993 sampling programs were virtually the same as those inventoried in the same impoundment in summer 1991 (Grondin et al.

1994), with a few exceptions. For instance, Logperch (Percina caprodes), Carp (Cyprinus carpio), Redfin Pickerel (Esox americanus americanus) and Largemouth Bass (Micropterus salmoides) were captured in 1991 but not in 1993, while White Sucker (Catostomus commersoni), Fallfish and Darter were captured in 1993 but not in 1991.

1 The terni "adult" is used to designate both adult fish and juveniles over one year.

2 The terni "fry" is used to designate young-of-the-year.

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Table 1. Species captured in fish inventories in the Ile Dupas dale ditches, summer 1991 and 1993.

SPECIESa SUMMER 1991' 1st SAMPLING

PROGRAM (1993)

2nd SAMPLING PROGRAM

(1993) Fry Adult Fry Adult Fry Adult

Northern Pike x x x x x x

Yellow Perch x x x x x x

Pumpkinseed x x x x x x

Brown Bullhead x x x x x x

Rock Bass x x x

Black Crappie White Sucker Fallfish

x x x

x x

x

Tadpole Madtom x x x x

Central Mudminnow Burbot

x x

x

x x

Bowfin x x x x x

Banded Killifish x x x x

Golden Shiner x x x x x

Cyprinidae Darter Logperch Carp

Redfin Pickerel Largemouth Bass

x x

x x x x

x

Total 12 16 8 12 8 14

a See Appendix 2 for scientific narines

b Taken from Grondin et al. 1994 (Summer inventories before, during and after drawdown)

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3.1.2.2 Fish community distribution in the collecter canal 3.1.2.2.1 Adults and juveniles over one year

Catch per unit effort (CPUE) in the collector canal reveals discrepancies between stations (Appendix 3), with the number of fish captured per station varying between 12 and 1 03. While the standard deviation is higher than the mean for all species except Yellow Perch, total mean CPUE (44.21) exceeds the standard deviation (19.76).

The three main species comprising the 1181 captures, including those in the basin, were Yellow Perch (45%), Golden Shiner (34%) and Black Crappie (8%).

Although most species captured were the same as in 1991, the number of captures per species in the collector canal differed markedly. Brown Bullhead, the most abundant species (mean CPUE = 25.40) captured in this sector in 1991 along with Golden Shiner, ranked fourth in 1993, with a mean CPUE of 2.74. Sampling conducted in another impoundment, i.e. Massettes Marsh, reported mean CPUEs of 42.35 in 1989 (Pépin et al. 1991) and 26.59 in 1990 (Gélinas et al. 1993) for this same species. Mean CPUE for Yellow Perch, on the other hand, rose from 1.85 in 1991 to 26.47 in 1993. Mean CPUE recorded for Massettes Marsh was 3.94 in 1989 and 3.06 in 1990.

These differences may stem from the fishing method used. The total area sampied in 1993 (1863 m2) was at least twice that sampled using seine traps in 1990 (756 m2) and 1991 (900 m2), facilitating the capture of more fish, especially gregarious species such as Yellow Perch. Although this hypothesis does net explain the lower number of Brown Bullhead captured, the denser submergent vegetation in the canal sector compared with summer 1991 may have made it easier for these bottom-dwellers to escape capture, despite the fact that each sampling station was seined three times.

Finally, a correlation between the spatial distribution of fish in the collector canal and the proximity of the control structure was determined (Table 2). 0f the nine species compared, only Golden Shiners were captured in much greater numbers in stations Iocated near the control structure. The same phenomenon was observed in 1991 for both Golden Shiner and Blacknose Shiner (Grondin et al. 1994). A significant discrepancy was aise noted in 1991 for total mean CPUE, which was higher for the sector near the control structure (Grondin et al. 1994).

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Table 2. Comparison of mean pre-drawdown adult and juvenile-over-one-year (TI 50 mm) CPUE in the collector canal based on proximity to the control structure, Ile Dupas, summer 1993.

SPECIES MEAN CPUE

NEAR FAR

Northern Pike 0.56 0.30 0.454 0.438

(0.73) (0.68)

Yellow Perch 26.22 26.70 0.300 0.932

(9.52) (13.97)

Pumpkinseed 1.89 6.00 0.057 0.297

(2.26) (11.23)

Brown Bullhead 3.11 2.40 0.728 0.662

(3.92) (3.03)

Rock Bass 0.22 0.10 0.166 0.49

(0.44) (0.32) 3

Black Crappie 1.67 2.40 0.855 0.588

(2.35) (3.31)

White Sucker 0.11 0.10 0.883 0.941

(0.33) (0.32)

Bowfin 0.22 0.00 0.000 0.169

(0.44)

Golden Shiner 12.56 4.10 0.035 0.042*

(9.86) (5.61)

Total 46.78 42.20 0.234 0.627

(11.40) (25.46) NEAR: Stations Cl to C9

FAR: Stations C10 to C19 ( ) Standard deviation

* Significant discrepancy with unconverted values

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3.1.2.2.2 Young-of-the-year

Young-of-the-year CPUE also varied considerably in the collector canai, ranging from 17 to 209 captures per station (Appendix 4). The standard deviation, however, was lower than the mean for two of the four species, i.e. Brown Bullhead and Northern Pike. The standard deviation (45.33) for total mean CPUE (97.21) was also lower.

A total of 1864 fry were captured in the canal and basin, with Brown Bullhead clominating the catch (97%). Yellow Perch accounted for nearly 2% of the total catch, Northern Pike 1% and Bowfin less than 1 %.

Mean Brown Bullhead CPUE (94.05) for the sector in summer 1993 was very close to that of 1991 (80.35) (Grondin et al. 1994). Mean CPUE in Massettes Marsh was 182.35 in 1989 (Pépin et al. 1991) and 75.06 in 1990 (Gélinas et al. 1993). Mean Bowfin CPUE in 1993 (0.26) was also similar to the 1991 figure (0.10). However, the mean catch of Yellow Perch (1.74) and Northern Pike (1.16) in summer 1993 was lower than in 1991, when it was 7.10 for Yellow Perch and 10.75 for Northern Pike (Grondin et al. 1994).

Table 3 shows the spatial distribution of young-of-the-year based on proximity to the control structure. Northern Pike was the only species captured in greater numbers in stations near the structure. This behaviour had not been noted for this species in 1991, but had been observed for Pumpkinseed and Central Mudminnow. Total mean CPUE was also higher near the control structure (Grondin et al. 1994).

3.1.3 Fish community estimates

3.1.3.1 Fish communitv estimates based on catch per surface unit Table 4 shows fish community estimates for the collector canal, exciuding the basin.

The total community of the nine adult species inventoried in the collector canal sector was estimated at 1407 (1219 - 1603) in 1993, compared with 4497 individuels (3051 - 5954) in 1991 (Grondin et al. 1994). There were apparently eight times more adult Yellow Perch in the collector canal in 1993 (an estimated 843 individuels) than in 1991 (an estimated 103 individuels). The opposite is true for Brown Bullhead, however, with an estimated 1415 individuels in 1991 compared with 87 in 1993. A similar trend was noted for Golden Shiner, Northern Pike and Pumpkinseed, for which estimates showed five times fewer individuels in 1993. Two hypotheses may explain this situation. First, various factors of change may influence the composition of the

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Table 3. Comparison of mean pre-drawdown young-of-the-year (TI 50 mm) CPUE in the collector canal based on proximity to the control structure, Ile Dupas, summer 1993.

SPECIES MEAN CPUE F p

NEAR FAR

Northern Pike 1.67 0.70 0.414 0.045*

(1.12) (0.82)

Yellow Perch 0.22 3.10 0.065 0.156

(0.44) (5.78)

Brown Bullhead 104.56 84.60 0.125 0.338

(25.38) (55.62)

Bowfin 0.22 0.30 0.706 0.804

(0.67) (0.68)

Total 106.78 88.70 0.102 0.401

(26.31) (57.71) NEAR: Stations Cl to C9

FAR: Stations C10 to C19 ( ) Standard deviation

* Significant discrepancy with unconverted values

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Table 4. Fish community estimates and density (Tl 50 mm) for the collector canal in the Ile Dupas dale ditch system, summer 1993.

SPECIES ESTIMATE DENSITY (fish/ha)

Adults Fry Adults Fry

Northern Pike 13 37 42 119

(10-24) (30-51)

Yellow Perch 843 55 2702 172

(732-960) (17-101)

Pumpkinseed 129 414

(52-214)

Brown Bullhead 87 2993 279 9593

(58-124) (2570-3424)

Rock Bass 5 16

(4-13)

Black Crappie 65 208

(41-97)

White Sucker 3 10

(3-11)

Bowfin 3 8 10 26

(3-11) (5-19)

Golden Shiner 258 827

(176-347)

TOTAL 1407 3094 4510 9917

(1219-1603) (2658-3538) ( ) Confidence interval (95%)

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marsh fish community from year to year, i.e. duration of spring flood and height of flood waters, water temperature, presence of obstacles hindering marsh access, etc.

Second, more fish may use the dale ditch sector than the canal, or vice versa depending on the species, for a number of reasons.

An estimated 3094 (2658 - 3538) fry representing four species were inventoried in this sector. Brown Bullhead was the most abundant species, with 2993 individuels, significantly exceeding Yellow Perch, Northern Pike and Bowfin. A total of 5470 individuels was estimated in 1991 for the same four species (Grondin et al. 1994).

Brown Bullhead was also dominant in 1991 (4471 individuels).

Logically, based on the greater abundance of adult Yellow Perch in 1993, there should have been more young-of-the-year than in 1991. However, estimates indicate that only 55 (17 - 101) Yellow Perch aged 0 + frequented the collector canal, compared with an estimated 395 (220 - 582) in summer 1991 (Grondin et al. 1994). Although the aquatic vegetation in the impoundment was denser than in 1991, ecological conditions remained conducive to the production of Yellow Perch fry (Scott and Crossman 1974). It is therefore possible that a relatively large number of Yellow Perch aged 0 + , as well as other species, may have used the dale ditches in summer 1993, as testified by various factors. Maintaining the system at its normal operating level during summer 1993 favoured the distribution of Yellow Perch aged 0+ in the ditch sector, unlike in 1991, when the water level in the impoundment dropped approximately 30 cm due to a drought (Grondin et al. 1994). In addition, 117 Yellow Perch aged 0 + were captured as they moved downstream in the two experimental ditches during the drawdown operation. In 1991, only five Yellow Perch fry were captured at the 88 fyke-net stations in the ditch sector during pre-drawdown sampling (Grondin et al. 1994). Another factor may have been the timing and duration of the spring flood, whose waters enable fish to circulate freely between the impoundment and adjacent watercourses.

3.1.3.2 Fish communitv estimates usine the mark-release-recaoture method

Fish mortality due to marking is very low among Yellow Perch (Appendix 5), and varies between 5% and 32% for Golden Shiner. Bath species were therefore used for estimates, despite the possible loss of a certain number of marked Golden Shiner.

Table 5 shows population estimates for each species marked prior to drawdown (Appendix 6) and recaptured downstream of the control structure (Appendix 7). Based on these estimates, a total of 112 781 (89 384 - 136 178) fish frequented the dale ditches.

However, estimates could not be derived for several species given the absence or low number of recaptures.

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Table 5. Population estimates and density for fish (TI 50 mm) in the Ile Dupas dale ditches, based on recaptures downstream of the control structure, summer 1993.

SPECIES ESTIMATE DENSITY

(fish/ha)

Northern Pike N/A 224 N/A 15

(50-397)

Yellow Perch 5756 N/A 108 N/A

(4782-6730)

Pumpkinseed 1376 93

(951-1802)

Brown Bullhead 858 83 278 58 5658

(36-1680) (73 748-92 808)

Rock Bass N/A N/A

Black Crappie N/A N/A

White Sucker NIA N/A

Bowfin 9 N/A 0.6 N/A

(1-17)

Golden Shiner 21 280 1446

(9816-32 744)

TOTAL 29 279 83 502 1706 5673

(15 586-42 973) (73 798-93 205) N/A Nat applicable since zero or very few fish were recaptured ( ) Confidence interval (95%)

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Estimates in Table 6 are more precise as they include fish recaptured downstream of the control structure during drawdown and in the basin after drawdown (Appendix 7), bringing the total number of fish present in the impoundment te 93 939 (78 104 - 109 798) individuels. Only the number of Yellow Perch fry could not be estimated since none were recaptured either downstream of the control structure or in the basin.

Other elements support the hypothesis regarding distribution of Yellow Perch fry in the ditch sector. The late downstream movement noted among young-of-the-year of this species, as observed in the experimental ditches, probably led to a toss of fish in the water pockets forming in the collector canal towards the end of the drawdown.

This would explain the absence of Yellow Perch fry in the basin.

Predation is another plausible cause of the absence of recaptures of marked young Yellow Perch in the collecter canal and basin (Scott and Crossman 1974; Gélinas and Lepage 1994).

A comparison of 1991 and 1993 estimates, excluding young-of-the-year Brown Bullhead as was done in 1991, results in an estimated 21 474 (12 896 - 30 258) fish measuring 50 mm or more in the dale ditches in 1993 compared with 379 969 (248 790 - 511 149) individuels in 1991 (Grondin et al. 1994). This difference may be due, in part, to an overestimation of the community in 1991 since very few marked fish in the entire impoundment (2726 individuels) were recaptured (30 specimens) during drawdown and in the basin (Grondin et al. 1994). By the same token, 1993 estimates are probably low, since ail marked fish (2920 individuels) were located in the collecter canal, i.e. the sector relatively close ta the exit and having less dense aquatic vegetation than the ditch sector, thereby facilitating the downstream movement of fish. In addition, the much lower density of fish in the canal in 1993 may aise indicate significant use of the dale ditches prier te drawdown. It is thus possible that some individuels in this sector were not able to reach the exit before the end of the drawdown. It is also possible that fewer individuels entered the impoundment in 1993, or that they had more time te exit the impoundment once flood waters began to recede.

3.2 Drawdown experinnents 3.2.1 Draininq

Drawdown of the dale ditches ran 192 hours and resulted in a 99-cm reduction in the water level from 14.72 ha to 0.14 ha, exposing the entire ditch sector. In terms of volume, over 56 040 m3 of water were evacuated; the remaining 530 m3 was in the collecter canal and basin.

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Table 6. Population estimates and density for fish (Ti 50 mm) in the Ile Dupas dale ditches, based on recaptures downstream of the control structure and in the basin (post-drawdown), summer 1993.

SPECIES ESTIMATE DENSITY

(fish/ha)

Northern Pike 221 528 15 36

(0-466) (1 1 1 -945)

Yellow Perch 5966 N/A 1 12 N/A

(5176-6756)

Pumpkinseed 1280 87

(929-1631)

Brown Bullhead 2161 72 465 147 4923

(447-3874) (65 390-79 540)

Rock Bass 16 1

(2-30)

Black Crappie 1900 129

(270-3530)

White Sucker 6 0.4

(0-12)

Bowfin 29 1407 2 96

(0-59) (0-2996)

Golden Shiner 7960 541

(5961-9959)

TOTAL 19 539 74 400 1034 5055

(12 785-26 317) (65 319-83 481) NIA Nat applicable since zero fish were recaptured

( ) Confidence interval (95%)

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The average drop in water level throughout the drawdown was 12 cm/d (Figure 8), although the actual rate varied: it was quite rapid (21 cm/d) from hour 0 to hour 60.

Heavy precipitation between hour 54 and hour 66 slowed the process somewhat Lira hour 72. The water level inside the impoundment restabilized at hour 132 after dropping 79 cm to 4.86 m, just above the 4.21-m level of the river. The evacuation channel downstream of the control structure was blocked by a wooden panel which was subsequently removed in order to continue the drawdown to the full 192 hours.

Although the drawdown experiment could not be carried out under the anticipated conditions, i.e. lowering the water level in the impoundment to the same level as the St. Lawrence, the proposed rates of water level reduction for this type of management (5 cm/d to 20 cm/d) were achieved, unlike in 1991, when the mean drawdown rate was 41 cm/d (Gélinas et aI. 1993; Grondin et al. 1 994).

Finally, despite pre-drawdown backfilling, the basin remained approximately 50 cm deeper than the apron of the control structure.

3.2.2 Number and mass of fish evacuated 3.2.2.1 Number of fish evacuated

With the exception of White Sucker, Fallfish and Burbot, all species inventoried during the first two sampling programs were captured during the drawdown operation. In all, 87 981 fish were captured downstream of the control structure (Appendix 8). Of these, over 71 312 were mainly Cyprinidae and Pumpkinseed aged 0 +, with some Central Mudminnows, Banded Killifish, and Black Crappie fry (Appendix 9). The drawdown appears to have resulted in the evacuation of just over 18% of marked species, or 16 493 fish of an estimated community of 93 939 individuals. In 1991, 9783 fish representing 16 species were evacuated during drawdown experiments (Grondin et aI. 1994). Over 5570 of these were young-of-the-year, the dominant species (36%) being Brown Bullhead aged 0 + . Only 1% of an estimated resident community of 863 566 individuals was reportedly evacuated during the 1991 drawdown experiments.

Table 7 shows the number of adult fish captured at the exit, as well as population estimates for the entire system. According to these estimates, 15% of the total fish population, excluding Central Mudminnow and Tadpole Madtom, left the impoundment during the drawdown operation. In addition, 53% of the Pumpkinseed community used the exitway, followed by Yellow Perch (25%), Northern Pike (21 %), Bowfin (17%) and Golden Shiner (8%). Only 3% of the estimated Brown Bullhead population and less than 1% of Black Crappie were evacuated.

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Drop in water levai (cm)

80 -

60 -

4 0 -

20 -

• 1 • •i -r • r. y • , r r i r •t a ( •i •

o

0 1.2 24 36 48 60 72 84 96 108 120 132 144 156 168 180 192 Duration of drawdown (cumulated hours)

Figure 8. Drop in water level during drawdown of the Ile Dupas dale ditches, summer 1993.

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Table 7. Number of adults and juveniles over one year captured at the Ile Dupas dale ditch outlet, and population estimates for the impoundment as a whole, summer 1993.

SPECIES POPULATION ESTIMATES NUMBER OF

FISH WHICH EXITED DOWNSTREAM OF

STRUCTURE & BASIN'

Northern Pike 221 47

(0-466)

VeIIow Perch 5966 1468

(5176-6756)

Pumpkinseed 1280 678

(929-1631)

Brown Bullhead 2161 65

(447-3874)

Rock Bass 16 2

(2-30)

Black Crappie 1900 15

(270-3530)

White Sucker 6 0

(0-12)

Bowfin 29 5

(0-59)

Golden Shiner 7960 671

(5961-9959)

Central Mudminnow n.d. 57

Tadpole Madtom n.d. 119

Total 19 539 3127

(12 785-26 317)

1 Based on Table 6

H

Confidence interval (95%) n.d. Estimate not determined

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The number of fry captured at the dale ditch outlet and the estimated communities for the entire system are provided in Table 8. According to these estimates, 18% of individuels measuring 50 mm or over exited the impoundment. As well, over 18% of Brown Bullhead young-of-the-year, or 13 209 individuels, were evacuated, followed by Northern Pike (16%) and Bowfin (15%) with 85 and 217 individuels respectively.

Only 31 Yellow Perch young-of-the-year were captured downstream of the structure, which is negligeable compared with the number of adults captured (1468).

Figure 9 illustrates the exit chronology for fish captured downstream of the control structure. Fish started exiting as drawdown began, and continuecito do so until hour 54, when heavy precipitation led water levels to stabilize inside the impoundment.

Downstream migrants began leaving again at hour 66, when water levels in the ditches began to drop once more. Two exit peaks were subsequently noted at hour 90 (11 719 captures) and hour 102 (12 811 captures), the reduction in water level averaging 15 cm/d. In both instances, 96% of captures were under 50 mm in length (Appendix 8). At hour 132, the stabilization of water levels due to blocking of the evacuation channel resulted in another decrease in the number of captures downstream of the control structure. Fish began exiting again at hour 138 and by the end of drawdown, i.e. after 192 hours, a total of 81 244 fish had been counted in the fyke net. This number rose to 87 981 once those fish remaining between the structure and the fyke net were counted (Appendix 8).

Figure 10 shows the exit chronology by species. The senne pattern, i.e. a steady departure of fish throughout the drawdown operation, was noted for these senne species during drawdown experiments in Massettes Marsh in summer 1990 (Gélinas et al. 1993). Half (730 individuels) of all Yellow Perch evacuated had already exited the impoundment by hour 156, i.e. after an 87-cm drop in water levet. Just over half (438 individuels) of all Pumpkinseed evacuated had exited after 108 hours of drawdown (drop in water level: 75 cm), white the same percentage of Brown Bullhead (6774 individuels) had exited alter 132 hours (drop in water level: 79 cm). Sonne 346 Golden Shiners had exited the impoundment after 150 hours (drop in water level: 82 cm), despite the fact that this species presented a heterogeneous distribution in the collector canal and had aggregated in the sector near the control structure (Table 2).

Fifty percent of fish (31 271 individuels) less than 50 mm in length (Tl) had exited the dale ditches after 96 hours of drawdown (drop in water level: 71 cm).

Figure 11 illustrates which species exited the impoundment early and which exited late. Two exit peaks were noted for Central Mudminnow within the first 36 hours of drawdown. The opposite behaviour was reported by Gélinas et al. (1993). ln summer 1990, the 8111 Central Mudminnows inventoried exited the impounded area late in the drawdown operation (Gélinas et al. 1993). Similar results were obtained for Black Crappie in the dale ditches and Massettes Marsh (Gélinas et al. 1993). Very few (15 individuels) were captured downstream of the control structure, although population estimates show the presence of 1900 individuels in the impoundment, with 65

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Table 8. Number of young-of-the-year captured at the lie Dupas dale ditch outlet, and population estimates for the impoundment as a whole, summer 1993.

SPECIES POPULATION ESTIMATES NUMBER OF

FISH WHICH EXITED DOWNSTREAM OF

STRUCTURE & BASIN'

Northern Pike 528 85

(111-945)

Yellow Perch NIA 31

Brown Buflhead 72 465 13 209

(65 390-79 540)

Bowfin 1407 217

(0-2996)

Other2 n.d. 71 312

Total 74 400 84 854

(65 319-83 481)

1 Based on Table 6

( ) Confidence interval (95%) NIA Not applicable

2 Fish measuring less than 50 mm (TI) n.d. Estimate not determined

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0 12 24 36 48 72 84 96 106 120 132 144 156 180 592

Duration of drawdown (cumulated hours)

Drop in water

leve

l (cm

)

Pumpkinseed Ad.

100

20 -

0

m.n. = 185 t.n. = 678

0 12 24 36 48 60 72 1.14 96 108 120 132 144 156 168 180 192

50

25 200

75 125 100 175 150

Number of fish captured with fyke net

Brown Bullhead Ad. & 0+

100

m.n. = 2896 t.n. = 13 274

60 -

40 -

20 - fI

r • T

72 84 96 105 120 132 144 156

Yellow Perch Ad. & 0 +

100

80

50

40

20

0

300 F 275 - 250 F 225 - 200 r 175 - 150 - 125

• 100 75 F 50 - 25

o

0 12 24 36 45, 60

3000 2750 2500 - 2250 - 2000

• 1750 - 1500

• 1250 - 1000 - 750

500 - 250 , r •-• 0 168 160 192

Figure 10. Fish species exiting the He Dupas claie ditches throughout the drawdown operation, summer 1993.