Downstream migration of juvenile European sturgeon

Estuaries

Vol. 24, No. 1, p. 108–115

February 2001

Downstream Migration of Juvenile European Sturgeon Acipenser

sturio L. in the Gironde Estuary ERIC ROCHARD1,*, MARIO LEPAGE2, PIERRE DUMONT3, SERGE TREMBLAY 4, and CHRISTINE GAZEAU1 Cemagref (French Institute of Agricultural and Environmental Engineering Research), 50 avenue de Verdun, B.P. 3, 33612 Cestas cedex, France 2 Association Girondine pour l’Expe ´rimentation et le Developpement des Ressources Aquatiques, 805 Chemin de Reden 33240 Saint Andre´ de Cubzac, France 3 Faune et Parcs Que ´bec, 201 Place Charles-Lemoyne, Longueuil, (Que´bec), J4K 2T5, Canada 4 Faune et Parcs Que ´bec, 675 Boul. Rene´-Le´vesque est, Que´bec, (Que´bec), G1R 5V7, Canada 1

ABSTRACT: The European sturgeon (Acipenser sturio) is an endangered diadromous fish species that spawns in the rivers in late spring and early summer. The juveniles spend their first years in the brackish waters (5‰ to 25‰) of the estuary zone before moving out to sea. This study describes the downstream migration pattern of juvenile sturgeon, belonging to the 1994 cohort, the only one born naturally in the Gironde basin, France since the end of the 1980s. During October 1994 to December 1996 the inland section of the Gironde estuary was sampled monthly by trawl (n ! 818 tows) and all European sturgeon caught (n ! 381) were marked and released. The first sturgeon of the 1994 cohort (TL ! 27 cm) were caught in early March 1995 in the zones furthest upstream. During their second fall of life, juveniles gradually acclimatized, and spread over a wide range of salinity conditions. A first incursion into marine water was also observed (at least for a few fish) by the end of the second winter. During this second period, sturgeon showed preference for two particular zones situated at 18 and 38 km, respectively, from the mouth of the estuary. These zones, belonging to two different salinity sectors of the estuary, did not appear to be any different to their neighbors with regards to depth and type of substrate. There were no significant size differences among estuarine zones. Seasonal movements of sturgeon seem to be motivated by a search for warmer temperatures. After a period of early acclimatization of 15 months, juvenile European sturgeon appear to be highly tolerant of salinity variations.

Introduction The European sturgeon, Acipenser sturio L. is one of nine diadromous species of sturgeon in the world (Rochard et al. 1990; Birstein 1993). Acipenser sturio is considered as endangered in France, where it has been fully protected since 1982 (Lepage and Rochard 1995), as well as in its entire distribution area, where it is now very rare (Williot et al. 1997). The European sturgeon is known to reproduce only in the Gironde-Garonne-Dordogne river basin, South West France, in late spring and early summer. In 1994, natural reproduction took place in this basin, following a 5–6 yr absence of juvenile production. The Gironde estuary is the only route between the spawning grounds and the open sea where juveniles and adults spend most of their life. The Gironde population can be found from the Bay of Biscay to Scandinavia (Rochard et al. 1997). In the Gironde estuary, the spatial distribution of the young sturgeon has been known roughly since the work of Magnin (1962). He described

their longitudinal distribution according to salinity zones of the estuary. Castelnaud and Trouvery (1984), after the 1982 fishing ban, distinguished fish ! 3 yr old as resident to the estuary and fish " 3 yr as estuarine-marine stage sturgeon. Rochard (1992) indicated that the first marine captures of sturgeon of the 1988 cohort occurred during their second winter of life (TL # 57 cm). Magnin (1962) considered that the young European sturgeon used the Gironde estuary as a downstream migration route rather than as a particular habitat. No concentration areas of juveniles have ever been identified for this species. Juveniles of Atlantic sturgeon Acipenser oxyrinchus, a closely related anadromous sturgeon (Magnin 1964; Birstein et al. 1997; Wirgin et al. 1997) were observed to concentrate at the limit between fresh and brackish water in the Hudson River (Dovel and Berggren 1983). These authors identified some winter concentration of immature fish in zones deeper than 7.5 m in fresh and brackish water. In fall or spring, Atlantic sturgeon were recorded in shallower areas than at other periods. Their summer distribution was not clearly delineated. From his experience and a synthesis of the lit-

* Corresponding author: e-mail: [email protected]. ! 2001 Estuarine Research Federation

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Migration of European Sturgeon

erature, Magnin (1962) thought that older (" 2 yr) European sturgeon juveniles migrated from the ocean to the estuary, between late June and the end of September, but that concerned fish had already left the estuary. Rochard (1992) presented a pattern of downstream migration of young juvenile sturgeon over a period of 2 yr for the 1988 cohort. He used a salinity gradient to split the estuary into two reaches, from 0.5‰ to 5‰ and from 5‰ to 30‰, and defined three stages of behavior during their stay in the estuary: the first, from July to February in the reaches of low salinity, the second from March to the following November in the whole estuary, and the third one, after December, in the reaches of higher salinity. In their work on A. oxyrinchus Dovel and Berggren (1983) used salinity to distinguish several zones in the Hudson estuary but also considered temperature to be an important factor in understanding the movements of the young sturgeon. They identified a downstream migration of juveniles as water temperature in the river dropped below 20$C. This migration occurs in October and, at this time, some sturgeon leave the estuary. When the water temperature rises, juveniles initiate an upstream movement from their winter concentration areas. The aims of this study were to precisely describe the distribution of young European sturgeon in the Gironde estuary, to identify and characterize their concentration areas, and to analyze their estuarine movement patterns in relation to environmental conditions in the estuary. Material and Methods STUDY AREA The study was conducted in the lower part of the Gironde estuary (Fig. 1). Formed by the Garonne and Dordogne Rivers, it is the largest in Western Europe (635 km2 at high tide). It drains 81,000 km2 with a flow of about 1,000 m3 s%1 (Allen 1972), the outflow mainly following the east bank of the estuary; tidal marine intrusion varies from 1.1 to 2.0 & 109 m3 (Bonnefille 1970), with the inflow mainly following the west bank. The study area covers 45 of the 76 km of the estuary and 75% of the trawlable surface (excluding shoals, wrecks, and navigation channels). Water depth ranges from 3 m, close to the shelf in the upper estuary, to 26 m in the lower estuary; it is generally between 4 and 10 m. The bottom of the estuary is mainly a mixture of sand and mud, with the sandiest part in the lower sectors and the muddiest part in the upper sectors, where turbidity can be on average as high as 1 g l%1 near the surface and 10 g l%1 over the bottom (Latouche and Jouanneau 1994). Temperature in the estuary follows a gradient between

Fig. 1.

109

Study area in the Gironde estuary (France).

the ocean and the river. This slight gradient (1$C) inverts twice a year, in spring and fall. Coastal water is warmer than the river in winter, while the river is warmer than coastal water in summer (Centre National pour l’Exploitation des Oceáns 1977). In summer the highest temperatures (18$C to 26$C) are recorded in the upper part of the estuary and in winter they are observed in the lower part (6$C to 12$C) (Centre National pour l’Exploitation des Oceáns 1977; Maurice 1994). Salinity also follows a gradient, highly variable according to the strength of the tide and the river flow. Except in summer at lowest flows when salinity increases by about 10‰, the lower estuary is about 15‰, the mid estuary is about 10‰, and the upper estuary is about 4‰. SAMPLING METHOD AND EQUIPMENT Samples were taken over a period of 27 mo from October 1994 to December 1996. Except for August 1995 and September to November 1996, two sampling campaigns, spread over 7 d on average, were performed monthly. Because of the low num-

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ber of sturgeon caught, the data were grouped into four seasonal bins. A 12-m long catamaran-type trawling research ship deployed a bottom trawl, 13 m wide and 3.5 m in height with a cod end stretch mesh of 70 mm. Trawls were conducted during the daytime according to a sampling plan stratified for estuarine zones and tidal cycle. The mean tow duration was 28 min (6 to 67 min), at an average speed of 4.5 knots for trawls occurring in the same direction as the current and at an average speed of 1.5 knots for trawls occurring in the opposite direction of the current. According to the benthic habits of this species, we calculated sampling effort based upon both area sampled (m2) and the sampling speed (m s%1) (CPUE # n m%3 s). Sampling speed was integrated into the unit of effort because mean number of sturgeon per tow was higher (p ! 0.001, K-W ANOVA) for faster tows (0.74 sturgeon per trawl for speeds over 4.2 knots; n # 400) than for slower tows (0.18 sturgeon per trawl; n # 418). In 1996, the study area was divided into 10 regularly sampled main zones (no. 1 to 10) and three irregularly covered supplementary zones (no. 0, 11, and 12) (Fig. 1). Trawling in these supplementary zones required particular meteorological and hydrological conditions. The trawling effort of 1994 and 1995 was not sampled according to these zones and were reclassified based on the 1996 sampling grid. The distance covered by tows overlapping two (or, rarely, three) zones, and the corresponding sturgeon catch, were evenly distributed among these zones. This reclassification of tows artificially increased the total number of tows from 687 to 818 and reduced their mean duration by 5 min; the 818 trawls totaled 376 h of fishing. The mean surface sampled was approximately 4.4 ha (0.6 to 13.0 ha). The mean distance covered by a trawl was slightly over 3,400 m long at a mean depth of 7.1 m (2.5 to 27.0 m), which is approximately the average depth found in the estuary at mid-tide. All the trawls were done between 0900 and 2300: 375 during flood and 443 during ebb; 461 were directed upstream, 351 downstream, and 6 had varying heading directions. Mean duration and area covered were similar during the flow and the ebb (p " 0.38, K-W ANOVA) as well as for the heading direction of fishing (p " 0.15). Captured sturgeon were handled carefully, mostly in a tank supplied with air or pure oxygen. Their total and fork length were measured to the nearest centimeter. They were weighed with a precision of 50 g and tagged with a specially designed sturgeon tag (Castelnaud et al. 1991). With the exception of the spring and summer 1995 period, for each sturgeon capture, a sample of bottom water was taken with a sampling bottle, for temperature and salinity

Fig. 2. Daily evolution of the Gironde estuary temperature and the river flow during the study (from Blaye Nuclear Power Plant and Port Autonome de Bordeaux unpublished data).

control. A thin slice of the first pectoral fin ray was removed from the live fish for age estimation (Rochard and Jatteau 1991). All fish were then released. Age was determined according to the method of Rochard and Jatteau (1991); we were focusing our attention on the 1994 cohort. The recovery of tagged sturgeon was used to highlight movement between sampling zones. Statistical analysis was produced with SYSTAT 8.0 software; non parametric Kruskal-Wallis (K-W) ANOVA, '2 tests, and Pearson correlation coefficients were used. Results SAMPLING ACTIVITY Of the 818 trawls, 123 were successful, cumulating 381 captures of 349 individuals: 26 fish were captured twice and 3 fish were captured three times during the study. The mean catch was 0.47 sturgeon per trawl. Capture rates were similar (p # 0.055, K-W ANOVA) during ebb and flood tides. Of the successful trawls, 53% resulted in a single capture, and 35% between two and five captures; the maximum recorded was 36 sturgeon in one trawl. Multiple captures were more frequent in 1996 than in 1995 (p # 0.01, '2 test). Ranges of temperature recorded for each sturgeon capture site varied from 5$C to 25$C depending on the season (Fig. 2); salinity varied from 0‰ to 25.5‰. STURGEON CAPTURE FEATURES Individual aging indicated that most of the sturgeon were from the 1994 cohort born in either the Dordogne or Garonne Rivers. Eight specimens captured in 1996 were assigned to the 1995 cohort. Only two specimens from our sampling were larger fish: a mature male of 170 cm and a 138-cm specimen of unknown sex.


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occurred at sea (see Rochard et al. 1997) juveniles from the Gironde estuary may have initiated coastal migration at this time. One tagged fish, 57 cm TL, 640 g was declared captured at sea by commercial fishermen along the Bay of Biscay Coast on March 11, 90 km from the mouth of the Gironde estuary. In spring 1996, CPUE continued to increase in the upper and lower estuary but were at a lower level in the median estuary (p # 0.003, K-W ANOVA). The maximum seasonal CPUE in the whole estuary was recorded in summer 1996 (9.76 & 10%6 n m%3 s) with a lower abundance in the median part (p ! 0.001) than in the upper and lower part (Table 1). The fall captures were reduced and concentrated in the median and lower parts; no sturgeon were caught in the upper part and the overall seasonal CPUE was low. Mean total length more than doubled in 21 mo from 31.3 to 72.7 cm (Table 2). Growth for this estuary phase, using a von Bertalanffy growth function, was expressed by TL # 66.91(1 % e%0.0038(t%81.07)) (r2 # 0.80) (Fig. 4). The lengthweight relationship was expressed by W # 0.07 TL3.23 (n # 329; r2 # 0.88) . The minimum length recorded was smaller in the upper and median estuary (TL 25 and 27 cm, respectively) than in the lower estuary (TL 48 cm) or at sea (TL 57 cm). Maximum lengths did not vary with salinity, but minimum lengths were smaller (TL 27 cm) in the lower salinity class (0–2.5‰) than in the median classes (TL 46 and 48 cm for 2.5–10‰ and 10– 20‰, respectively) and in the higher salinity class (TL 55 cm; 20–26‰).

Fig. 3. Mean seasonal catch per unit of effort (no. of sturgeon caught m%3 s) in the different parts of the Gironde estuary.

DISTRIBUTION AND CHANGE OVER TIME From October 1994 to February 1995, no sturgeon were captured in the trawling samples. The first one appeared on March 7, 1995, a period of high river flow and low temperature (! 10$C) (Fig. 2), in the upper zone of the estuary. During that same season other juveniles (n # 18) were observed in the same area and one capture occurred in the median estuary. In spring 1995, CPUE decreased (Fig. 3), but sturgeon remained in the upper estuary. In summer 1995, CPUE was low (0.79 & 10%6 n m%3 s) but fish were observed in all the parts of the estuary with a lower abundance in the middle estuary (p # 0.001, K-W ANOVA) than in the other parts. In fall 1995, sturgeon continued to spread to the lower part of the estuary, there was no statistical difference between parts, but a higher abundance in zone 1 and 7 than in the others (p ! 0.001). During the winter 1995–1996, it was the same pattern. Because coincident captures

CONCENTRATION AREAS Among the twelve sampling zones, captures were strongly concentrated on zone 1 (n # 122, 32.0%) and zone 7 (n # 151, 39.6%). These zones also exhibited significantly higher CPUE (17.09 & 10%6 and 15.09 & 10%6 n m%3 s, respectively) than the

TABLE 1. Mean zonal catch per unit of effort (no. of sturgeon caught m%3 s) of the 1994 year class, from winter 1995 to fall 1996. Numbers (0–12) represent the zone number (see Fig. 1). Upper Estuary 0

1995 Winter Spring Summer Fall

22.3 4.91

1996 Winter Spring Summer Fall All seasons

6.15

1

3.34 4.36

Middle Estuary 2

4.74 0.57 1.63

3

4

5

Lower Estuary 6

7

2.39 1.86

0.75

1 6.44

2.36

1.37 15.7

1.68 25.3 45.2

3.45 2.53 2.4 4.5

1.72

1.95

11.2 1.26 1.07 5.21

3.63 2.09

19.6 33.1 14.9 8.13

17.1

1.84

0.67

0.33

3.37

0.92

15.6

8

9

10

11

12

4.23 1.01 0.80 2.87

1.29 0.41

2.47

4.73

2.35

4.46 8.05 2.68

1.2

2.96

0.94

2.12 0.66

All Zones

6.57 8.82 9.76 2.13 0

0

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TABLE 2. Change in total length and weight of the 1994 cohort trawled in the Gironde estuary between winter 1995 and fall 1996. Length (cm) Mean (n)

Weight (g) Standard deviation (min–max)


31.3 35.7 48.3 55.7

(20) (9) (6) (33)

4.3 5.6 5.5 4.3

(26–42) (25–43) (42–55) (48–66)


56.5 59.0 62.5 72.7

(70) (139) (86) (7)

3.5 3.2 3.4 2.9

(46–64) (49–68) (54–69) (69–78)

others (Table 1) (p ! 0.01, K-W ANOVA). Zones 1 and 7 are both located in the middle of the estuary; the former in the upper estuary and the latter in the lower estuary (Fig. 1). We did not find significant statistical differences (p " 0.15, K-W ANOVA) in length or weight between the juveniles caught in zones 1 and 7 when comparing the same period. MOVEMENTS OF FISH The first sturgeon was recaptured on November 11, 1995. Since then to the end of 1996, 32 recaptures were observed in the trawl catch, mostly in zones 1 (n # 18) and 7 (n # 11). Among these recaptures 19 involved movements between different parts of the estuary. Four recaptures indicated downstream movements (from upper or median estuary to a lower part) and 15 involved upstream movements (from lower or median estuary to an upper part). Discussion Young European sturgeon born in summer 1994 arrived in the Gironde estuary during their first

Standard deviation (min–max)

Mean (n)

na 713.5 (26) 791.5 887.1 1,104.1 1,607.1

(65) (139) (86) (7)

188.0 (450–1,300) 149.4 139.9 212.7 218.8

(350–1,100) (550–1,320) (600–1,600) (1,300–2,000)

winter (probably by the end of winter in 1995). They stayed in the upper part of the estuary during the colder period and moved upstream as the water temperature increased. From their second summer, juveniles progressively settled in the whole estuary. During their second winter, some stayed in the lower part of the estuary and some headed toward the sea, probably searching for higher temperature or salinity. From their second spring, they moved upstream and used most of the sampled zones characterized by a wide range of salinity over 5‰. They gave preference to two zones 20 km apart, both located in the middle of the estuary and sheltered from the current by longitudinal banks. Trawling, already used to study sturgeon in other rivers and estuaries (Dovel and Berggren 1983; Hasting et al. 1987; MacCabe et al. 1993; MacCabe and Tracy 1994) enabled us to sample juvenile A. sturio in the major part of the estuary. Some sections of the estuary, like the navigation channel and the intertidal zone, were not sampled and it cannot be excluded that sturgeon use these habitats. Sturgeon catchability appeared to be variable since trawling is more successful when tidal cycle and heading direction favor higher boat speeds. To reduce this variability we integrated towing speed into the CPUE formula. CHANGE OVER TIME

Fig. 4. von Bertalanffy growth function for the 1994 common sturgeon year class during their use of the Gironde estuary, from age-length scatter plot.

During spring 1995, commercial fishermen made incidental captures of young sturgeon (TL 25–40 cm) in their shrimp nets in the upper part of the estuary. They did not catch any sturgeon before the end of March. This supports the idea that, in 1995, the young juveniles of the 1994 year class did not arrive in the estuary before March. Depending on the season and their length, sturgeon used different parts of the estuary. In winter 1995–1996, sturgeon were concentrated in the lower part of the estuary. Some of them were also


caught by commercial fishermen in marine water, along the Bay of Biscay, at the end of winter and the beginning of spring. Rochard (1992), studying the cohort born in 1988 in the Gironde basin, established that growth in European sturgeon is highly seasonal. They show a large increase in length and weight from May to October and a decrease in weight with no length increase between November and April. Biometric observations of the 1994 cohort confirm these findings. The growth curve and lengthweight relationship of the 1994 year class seem similar to those of the 1988 cohort (Rochard 1992). MOVEMENTS IN RELATIONSHIP TO ENVIRONMENT At the end of winter 1995, the arrival of the yearlings in the upper estuary, where the salinity was close to zero, is interpreted as a behavior to avoid the winter conditions of the rivers (colder temperature, high river flow). During that season, the minimum water temperatures were 7.3$C in the estuary (Port Autonome de Bordeaux unpublished data), 6.2$C in the Garonne River (Boyer-Bernard 1996), and 7.1$C in the Dordogne River (Dartiguelongue 1996). This temperature pattern is quite typical, where winter temperature is higher in the estuary than in rivers, even though the difference is slight (1$C to 1.5$C) (Centre National pour l’Exploitation des Oceáns 1977; Maurice 1994). Although water temperature and high river flow may activate downstream migration of young sturgeon, salinity probably acts as a limiting factor. During their first winter, the 1994 cohort was 6 to 9 mo old (TL ! 46 cm) and used only the upper part of the estuary. Low CPUE values observed in winter, spring, and summer 1995 suggest that most of the fish were still upstream and that only a small proportion had started to move downstream (Fig. 3). We speculate that young sturgeon, born the previous summer, use the first 15 mo of life to acquire the ability to osmoregulate. We have established that no sturgeon under 46 cm TL were captured in salinity higher than 2.5‰ and the growth curve of the cohort (Fig. 4) indicates that most of them reached this size near 400 d which corresponds to summer or fall 1995. As our results and the experimental observations of Magnin (1962) suggest, sturgeon whose total length was less than 46 cm probably did not yet have the capacity for osmoregulation in brackish water and were not physiologically capable of moving farther downstream. During their second summer, we observed a progressive downstream movement into brackish waters (Fig. 3). From fall 1995 to the following summer, sturgeon were caught in the 10 zones regularly sampled. We also observed considerable

113

movements of some tagged individuals between high salinity sectors (zone 7, 8, and 9) and a low salinity sector (zone 1) in both directions. After that, salinity does not act as a limit for sturgeon movements in the estuary. It also shows for certain the juveniles’ osmoregulation capability, at least up to 25‰. In spring and summer 1996, some 1994 cohort fish moved upstream from zone 7 to zone 1 where a group was already present. Some sturgeon probably also came to zone 1 from upper areas of the river. The incidental recapture in the Dordogne River on March 26, 1996 of a sturgeon tagged in the estuary confirms that some sturgeon went back upstream from the estuary. In the second year of life, other factors than salinity might have an influence on sturgeon movements in the estuary. The beginning of the downstream migration observed during summer 1995 coincided with the upstream movement of several Gironde estuary populations such as the white shrimp (Palaemon longirostris) and the goby (Pomatoschistus minutus) (Aurousseau 1984). The concentration of sturgeon in the lower estuary during winter 1995–1996 coincided with the downstream movement of white shrimp. Downstream movements observed during the first winter can be interpreted as a flight from colder upstream fluvial sectors. The concentration of sturgeon in the lower estuary and their presence at sea during the second winter, where bottom water temperature is slightly over 12$C (Quero 1984) would suggest a preference for warmer habitats. Similar patterns of movement were observed for juvenile Acipenser oxyrinchus oxyrinchus, in northeastern America. Many authors have found that young Atlantic sturgeon move downstream for overwintering and upstream as the water temperature increases from springtime through early summer (Dovel and Berggren 1983 and Bain 1997 in the Hudson River; Brundage and Meadows 1982 and Lazzari et al. 1986 in the Delaware River). Dovel and Berggren reported that growth rate of juvenile Atlantic sturgeon was lower if they remained in the Hudson River estuary. CONCENTRATION AREAS From fall 1995 to summer 1996, the 1994 cohort revealed a clear preference for zones 1 and 7 (Table 1), where captured sturgeon had similar characteristics of length and weight. These two zones are located downstream or upstream of sandbanks, 18 and 38 km, respectively, from the mouth of the estuary and almost equidistant from the river bank. They do not appear to be any different from other zones nearby with regard to depth and type of bed. One (zone 1) is located in a low salinity sector

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(5‰ to 10‰) while the other (zone 7) is located in a higher salinity sector (15‰ to 25‰). We hypothesize that the discriminating factor for the sturgeon presence could be the existence of sheltered areas which allow sedimentation and the development of special benthic fauna. Zone 1 is somehow sheltered by the Banc de St-Louis and the Banc de St-Este`phe and zone 7 is sheltered by the Banc des Marguerites (Fig. 1). Samples of benthic organisms and sturgeon stomach contents (using non traumatizing methods) should be taken in order to test this hypothesis. The movement pattern of the 1994 cohort adds new elements to our knowledge of the ecology of the species, particularly about their preference for two limited zones, their low sensitivity to salinity changes after their first 15 mo and the observation of a first marine migration during their second winter. The two areas (zones 1 and 7) used preferentially by young sturgeon during their initial stay in the estuary need monitoring and careful management. These results enable us to recommend particular care for these zones which constitute essential habitats for this endangered species. ACKNOWLEDGMENTS This study has received support from the European Union LIFE program, the French Ministry of Regional Planning and Environment and Ministry of Agriculture and Fisheries, the Affaires Maritimes, the Aquitaine and Poitou-Charentes Regions, the Charente Maritime and Gironde Departments, and the Agence de l’Eau Adour-Garonne. This paper has been partly produced in a joint convention between Cemagref-Conseil Supe´rieur de la Peˆche and Faune et Parcs Que´bec. We would like to thank Jean-François Bigot and Bernard Ballion, members of the Esturial crew, for their technical assistance during the trawling operations, Patrick Lambert for his advice in statistics and Jeremy Spencer for checking the English. Many thanks also to all the fishermen who collected information on captures of tagged European sturgeon.

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SOURCES

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BLAYE NUCLEAR POWER PLANT. Unpublished data. Braud et St. Louis BP 27 33820 Saint Ciers sur Gironde, France. PORT AUTONOME DE BORDEAUX. unpublished data. 152 quai de Bacalan, 33082 Bordeaux cedex, France. Received for consideration, August 17, 1999 Accepted for publication, October 10, 2000