© 2005 International Council for the Exploration of the Sea
Upstream migration and reproductive patterns of a population of allis shad in a small river (L'Aulne, Brittany, France)
a UMR INRA-Agrocampus Ecobiologie et Qualité des Hydrosystèmes Continentaux 65 rue de Saint Brieuc, CS 84215, F-35042 Rennes Cedex, France
b Institut d'aménagement de la Vilaine Boulevard de Bretagne, BP 11, 56130 La Roche-Bernard, France
c Centre de Recherche sur les Ecosystèmes Marins et Aquacoles, UMR CNRS/Ifremer BP 5, F-17137 L'Houmeau, France
*Correspondence to M. L. Bégout: tel: +33 546 500695; fax: +33 546 500600. e-mail: mlbegout{at}ifremer.fr.
The characteristics and activity of adult allis shad [Alosa alosa (L.)] were analysed during the last part of their upstream migration in the L'Aulne, a small river in Brittany, and during reproduction on a unique spawning ground downstream of an insurmountable dam. The age of the spawners ranged from three to seven years, females being larger and older than males. Population-level migration and reproduction were studied by counting the number of migrating fish, by estimating the sex ratio, and by counting the number of nocturnal spawning acts for three consecutive years starting in 2000. The influence of the environment, especially water temperature and discharge, was highlighted: temperature during migration may supplant the influence of water flow, although high flow could allow passage over the dam. Such factors partly explain the annual pattern of migration and reproduction during the spawning season. The study showed that the biological features and characteristics of this population of allis shad in a small river were similar to those of western Atlantic stocks in large rivers.
Keywords: allis shad, anadromous migration, biological features, reproduction, small river
Received 4 November 2004; accepted 30 May 2005.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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Originally, allis shad [Alosa alosa (L.)] were distributed along the Atlantic coast from Norway to Morocco (Baglinière, 2000). The species has a pelagic marine existence, but migrates to the higher middle watercourse of rivers to spawn. Currently, it is classified as vulnerable in Europe because of the reduction in its distribution and the threats to its freshwater habitat as a consequence of the construction of dams, pollution, and deterioration of the spawning grounds (Baglinière et al., 2003). The actual northern limit is the River Loire in France (Baglinière, 2000) and the southern limit the rivers Mondego and Vouga in Portugal (Baglinière et al., 2003), following from the extinction of the populations in Morocco in 1992 (Sabatié and Baglinière, 2001). However, allis shad have not disappeared or have reappeared in small French rivers, and recent studies have revealed perennial populations and functional spawning grounds (Véron et al., 2001; Baglinière et al., 2003). In France, research on its biology and conservation status has focused mainly on populations in large river systems, such as the Loire (Mennesson-Boisneau and Boisneau, 1990), the Garonne (Cassou-Leins et al., 2000), the Gironde (Taverny et al., 2000), and the Adour (Prouzet et al., 1994). Studies on migratory behaviour have focused only on the initial phases of upstream migration, and less work has been carried out on the last phase, namely the arrival of spawners on the spawning grounds (Mennesson-Boisneau et al., 2000a). A detailed study of this last phase using acoustic tracking was conducted at the same time (Acolas et al., 2004) as the present ecological study of the population. It highlighted the existence of a separate resting, prespawning area, and the restricted visiting of the spawning ground that was partly dependent on physical (temperature and water flow) and biotic factors (sex), as well as individual variability in spawning behaviour.
The purposes of this multi-year survey were to provide a detailed description of this last phase of the migration as well as the reproduction activity on a forced spawning ground of the population of allis shad in the River Aulne, a small anthropogenically impacted stream in Brittany, and to compare the characteristics of this population with those of the populations in larger rivers.
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Study site
The catchment of the River Aulne (1875 km2 surface area) is the third largest coastal river in Brittany. The source is on granite, and the river then penetrates the schist of Châteaulin before arriving at the roadstead of Brest after meandering for 145 km, of which 70 km has been channelled since 1836 (Figure 1). There are 28 dams to maintain a depth of 2 m for navigation. Average water flow of the river is 9 m3 s–1, with a maximum value of 80 m3 s–1. Water quality is lowered by fish-farm effluents and nitrates from agriculture. Poor water quality is associated with low summer flow and the multitude of dams that lead to eutrophication in the channelled areas. The first dam met by upstream-migrating fish (Gilly Glas) is about 30 km from the river mouth and can be crossed by a fish pass at high tides (Figure 1). The second dam is located in Châteaulin, at the limit of tidal influence. This sill has a fish pass with vertical slits that allow the passage of migrating fish: allis shad, eels (Anguilla anguilla), sea lamprey (Petromyzon marinus), Atlantic salmon (Salmo salar), and sea trout (Salmo trutta). The pass is equipped with a trap and a video system that has allowed the number of migrating fish to be counted since 2000.
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The study area included 2.3 km of channelled river located between the dams of Châteaulin and Koatigrac'h, the latter being insurmountable by allis shad because of an unsuitable fish pass (Figure 1). The channel has a sandy-silty bottom and the average depth varies between 1.80 m and 2.15 m, according to the volumes of water discharge. The spawning grounds of allis shad are just downstream of Koatigrac'h dam and cover an area of 3000 m2. The physical habitat is mobile coarse gravel on a schist bed, and depths range between 0.1 m and 1.9 m. These two characteristics are similar to those described for natural spawning areas in larger rivers (Boisneau et al., 1990; Cassou-Leins et al., 2000). However, the width of the spawning area (25–50 m) and water-current speeds observed (0.1–1.3 m s–1 in 2001, 0.3–0.9 m s–1 in 2002) were on average lower than typical values recorded for natural spawning grounds (50–200 m wide, 0.9–2.0 m s–1; Cassou-Leins et al., 2000). Moreover, the "forced" character of the spawning ground was reinforced by the presence of a pool with a relatively slow current (0.2–0.4 m s–1 in 2002), while typical spawning grounds are usually followed downstream by a shallow, fast-running zone (Cassou-Leins et al., 2000). The characteristics of the spawning site contrasted with those of the channel, in terms of bottom type and water current, because of the rocky bottom habitat and a narrow jet of water flow created by an island that allowed the current to accelerate.
Protocol for migration and reproduction assessment
The number of migrating allis shad was counted by the aquatic observatory of Châteaulin (SMATAH). Fish sampling was done during the second half of the 2001 migration (74 fish) and during the whole 2002 migration period (239 fish) at a rate of three to four times per week, using the trap system. Sex was determined by gentle pressure on the abdomen (males were all ripe, with running milt), and total length (Lt ± 5 mm) and weight (Mf ± 50 g) were also recorded. A scale sample was taken from the optimum zone to estimate age (Baglinière et al., 2001). Population reproduction was documented during one month in 2000, and during all spawning seasons in both 2001 and 2002 (Table 1). An observer was positioned on a jetty in the middle of the spawning area, to count spawning events by seeing and hearing them or just hearing them (Cassou-Leins and Cassou-Leins, 1981; Boisneau et al., 1990). The number of spawning acts was enumerated every night between 22:00 and 06:00 in 2000 and 2001 and for three nights a week in 2002. A spawning act was defined according to Boisneau et al. (1990) as a fast, nocturnal circular movement at the water surface (1–1.2 m in diameter during 2–10 s) of a minimum of two spawners side by side. Its sound intensity at a distance of 1 m was measured between 35 and 50 dB (Cassou-Leins et al., 2000). Surface current speed was estimated by eye. When water turbidity was low and fish were close to the observer, the number of fish participating in a spawning act was noted.
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Water flow was measured by Brittany DIREN (Direction Régionale de l'Environnement). Water temperature on the spawning area was recorded hourly by a datalogger (Minilog, Vemco Ltd.).
Data analysis
Statistical analysis of migration and spawning patterns were performed with EXCEL and SPAD®. Trends in migration and reproduction were evaluated by calculating a seven-day moving average. Average daily temperature was calculated for each study year. A sex ratio, expressed as the number of males sampled divided by the number of females sampled was calculated for both 2001 and 2002.
To study the environmental factors that influence migration, the migrating population was considered as closed (total number of migrants known), and was expressed as the percentage of migrating individuals per day (Xj) in relation to the number of migrants remaining.
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To analyse migration activity, the study period was reduced by removing 2% of the total duration of the migratory period at the beginning and end. Therefore, the following periods were considered: 29 April–18 June in 2000, 27 April–30 June in 2001, and 20 April–19 June in 2002.
In 2001 and 2002, the evolution of the daily migrants and reproductive activity were analysed with ANOVA and a global linear model (GLM). The Fisher test was used to indicate the significance of some of the results. Temperature, water flow, and tidal influences on migration were monitored using the following variables: daily average temperature and water flow, and derived variables; and daily average of the two highest tide coefficients. Spawning activity was analysed using the following variables: daily average temperature and water flow, and derived variables; and the number of migrants recorded one day earlier.
For each year, the number of spawning acts per female was estimated by using the number of spawning acts counted in the spawning area and the number of migrants counted at Châteaulin pass, following the assumptions of Cassou-Leins et al. (2000), viz. (i) spawners visit only one spawning ground; (ii) a balanced sex ratio of the population exists; (iii) there is only one spawning act per night per female; and (iv) one act corresponds to just one female.
Based on these assumptions, Cassou-Leins et al. (2000) considered that a female could spawn between five and seven times each spawning season.
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Results |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Upstream migration
In 2001, a sex ratio of 1.51 in favour of females was observed in a sample of 1.7% of the population (N = 74) during the second half of the migration period. In 2002, the sex ratio was estimated over the whole migration period through a sample of 10.2% of the population (N = 239). It tended to be in favour of males at the beginning (sex ratio 1.3 in April, N = 52) and at the end of the migration (sex ratio 1.2 in June, N = 28), but in favour of females in the middle of the migration period (sex ratio 0.6 in May, N = 150). According to these values, the population in 2002 had a total sex ratio of 0.92 and was estimated to consist of 1103 males and 1195 females.
Sampled individuals in 2001 and 2002 were between three and seven years old (three to six years for males and four to seven years for females). In males, sizes and weights ranged from 370 to 585 mm and from 510 to 2000 g, respectively. In females, sizes and weights were higher, ranging from 445 to 670 mm and from 600 to 3100 g, respectively (Figure 2). The percentage of multi-spawners was very low (2.1–2.5%).
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During the three years of the study, upstream migration took place between 6 April and 15 August, predominantly between 29 April and 18 June in 2000 (96% of migrants), between 25 April and 16 June in 2001 (94% of migrants), and between 19 April and 27 May in 2002 (91% of migrants). The total number of migrants in 2001 was twice that in 2002 (Table 1). Migration trends calculated as a seven-day moving average were unimodal in 2000, but bimodal in both 2001 and 2002 (Figure 3).
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Over the three years, the temperature observed during the migration varied between 10.5°C and 23°C. The temperature threshold under which migration would be inhibited seemed to be close to 11°C; such a temperature was recorded in 2001 only. In the same period, the observed water flows varied strongly during upstream migration (2.7–86.7 m3 s–1; Table 2). Migration activity in 2001 and 2002 increased while temperature increased and water flow decreased, and slowed when the temperature decreased with or without decrease in water flow (Figure 4).
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In 2001, the water temperature-derived variable "difference of temperature from the day before" explained the highest variance of the model (49%), followed by the daily average temperature (45%), water flow (39%), and tide coefficient (21%). In 2002, the water temperature-derived variables "temperature five days before" and "difference of temperature from the day before" explain the highest variance (54% and 44%, respectively), the next highest being water flow (34%; Table 3).
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Spawning activity
Reproduction took place between 24 April and 11 July. The total number of spawning acts was highest in 2002 and lowest in 2000, when spawning was only partially followed (Table 1). Temporal trends in spawning activity, calculated on a seven-day moving-mean basis, were clearly bimodal in 2000, unimodal to bimodal in 2001, and trimodal in 2002 (Figure 5). In 2001, the occasional presence of allis shad downstream of Châteaulin dam resulted in the observation of some spawning acts.
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Over the study period, the temperatures observed during spawning varied in the range 13.3–23°C (Table 2). The minimum temperature below which reproduction seemed to be inhibited was between 13.9°C and 14°C. No maximum temperature threshold was recorded. Temperature explained 72% of the variance in the model in 2001, and 33% in 2002 (Table 3).
Water flows observed during the spawning period over the three years varied between 2.7 and 47.6 m3 s–1. In 2001 and 2002, current surface speeds ranged between 0.1 and 1.5 m3 s–1 (Table 2). Generally, spawning activity ceased when water speeds were >0.75–0.80 m s–1. In 2001, the water flow-derived variable "difference of water flow from the day before" explained the highest variance of the model (81%), and water flow per se (78%) was the next most important factor. In 2002, the water flow-derived variable "difference of water flow between the day and three days before" explained the highest variance in the model (75%), and was again followed by the water flow per se (52%; Table 3).
During 2001 and 2002, the hourly distribution of spawning acts fluctuated during the spawning period, but 50% of the spawning acts were observed in a short period of time (02:00–04:00 UT +2; Figure 6). Otherwise, water temperature reduced the length of the nocturnal spawning activity by progressively shifting the spawning peak towards the end of the night (04:00–05:00 UT +2; Figure 7).
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In 2000, the reproductive activity was documented only at its start (the first month, mid-April/mid-May), and the number of spawning acts per female was estimated at 2.3 assuming a sex ratio of 1:1. In 2001, the average number of spawning acts per female was estimated at 7.7, assuming a sex ratio of 1:1, and 9.9 if we used the calculated sex ratio (1.5). In 2002, the average number of spawning acts per female estimated for the whole spawning season was 5.2 with a calculated sex ratio of 0.9.
During the 2001 spawning period, the number of spawners was >2 for 52.3% of the spawning acts (197 observations). In 2002, water turbidity limited the number of observations to 20, resulting in possible overestimation of the number of spawning acts involving two spawners. However, the trend observed in 2002 was similar to that observed in 2001 (Figure 8).
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Life history traits (first maturity age) and biological features (age range, older females growing faster than males, semelparity) in the population of allis shad on the River Aulne were similar to western Atlantic populations and may be closer to those of the Loire stock (Mennesson-Boisneau et al., 2000b).
Upstream migration
In the River Aulne, upstream migration of allis shad took place between early April and mid-August. The early migration was similar to that described in the Loire (end of March/mid-April; Mennesson-Boisneau et al., 2000a) and in the Garonne (Cassou-Leins and Cassou-Leins, 1981; Bellariva, 1998). The number of migrating fish recorded during the three years in the River Aulne was much lower than the number of migrants observed in larger rivers like the Dordogne and the Garonne at the dams at Tuillère (by a factor of 45) and Golfech (by a factor of 8; Bellariva, 1998).
The observation of a bimodal migration trend in 2001 and 2002 might be explained by lesser flows during these two years than in 2000. Moreover, in the Loire, Mennesson-Boisneau et al. (1993) noticed that migration tended to be unimodal in years of medium flow and bimodal to asymmetrical in years of low flow. A temperature decrease would also modify migration towards a bimodal or trimodal distribution (Mennesson-Boisneau et al., 2000a; Rochard, 2001).
The significant influence of temperature on upstream migration, with the presence of a minimum threshold (10–11°C) for migration activity, was confirmed (Sabatié, 1993; Mennesson-Boisneau et al., 2000a). The effects of water temperature and its variations were highlighted during 2001 and 2002. On the other hand, the influence of water discharge on migration was low except during spates, when migration rate decreased, as also shown by Mennesson-Boisneau and Boisneau (1990). An extreme flow threshold for migration did not seem to exist while water flow varied between 2.7 and 86.7 m3 s–1. In 2001, greater flow (+161% and +62%) induced negative temperature variations and caused a decrease in migration. In 2000, the increase in water flow (+240%) led to better conditions in that there was no change in temperature, thus facilitating the passage of allis shad. Temperature may supplant the influence of water flow, although high flow could make dam passage easier. Migration activity appeared slightly related to tidal height (coefficient), as observed by Mennesson-Boisneau et al. (2000b).
Spawning
In the Aulne, the beginning of the spawning period (late April/mid-May) was close to that observed for allis shad in larger rivers (Mennesson-Boisneau and Boisneau, 1990; Bellariva, 1998). Overall, spawning activity was modulated by variations in water flow and temperature, as in larger rivers (Mennesson-Boisneau et al., 2000a). There was no correspondence between peak periods of upstream migration and peak periods of spawning. This might be explained by, first, a different influence of environmental factors on the two activities, second, a greater influence of biological factors (e.g. maturity state), as shown by Mennesson-Boisneau and Boisneau (1990) at the termination of upstream migration and, finally, the possible influence of the forced spawning area resulting in a high concentration of spawners and an increase in individual fish interactions.
According to Cassou-Leins et al. (2000), water temperature would be an important factor that initiated and then controlled reproductive activity, but the threshold values would vary according to year and river. In the River Aulne, the temperature threshold of this activity was 14°C, a value confirmed by the telemetry study conducted in 2001 and 2002 (Acolas et al., 2004). Water flow influence and its variations were highlighted during the two years of complete observation.
Because further upstream migration to potential spawning grounds is rapidly obstructed in the downstream course of the river, the forced spawning area described is currently the only permanent spawning area in the River Aulne for allis shad. However, a temporary spawning ground was observed downstream of the Châteaulin dam in 2001, probably related to low water and a delay in migration attributable to the dam, as observed in other rivers (Mennesson-Boisneau et al., 1993).
The hourly distribution of spawning acts during the whole spawning season was similar to that observed in the River Loire (Mennesson-Boisneau and Boisneau, 1990) and the River Garonne (Cassou-Leins and Cassou-Leins, 1981). However, we noticed that the timing of the spawning acts varied during the reproductive period, a phenomenon not previously documented. A reduction in duration of reproductive activity and a shift in the timing of peak spawning were observed. This shift was coincident with the daily average water temperature increase, suggesting that allis shad were seeking a thermal optimum during the night for spawning.
For 2001 and 2002, analysis of the number of spawners participating in a spawning act revealed that >2 fish were involved in 45% of the spawning acts. The proportion of spawning acts involving only two spawners (55%) in the River Aulne was lower than that observed in the River Loire (78%; Boisneau et al., 1990). The increase in the number of spawners participating in a spawning act might be due to the forced character of the spawning area that induced a higher concentration of spawners. Otherwise, this observation would suggest a reconsideration of the method of estimating total spawners based on the counting of spawning acts over the whole spawning season, under the assumptions proposed by Cassou-Leins et al. (2000). Nevertheless, in 2002, the year when the sex ratio was estimated over the whole migration period, the average number of spawning acts per female during the reproductive period was included in the range of values recorded in larger rivers (five to seven; Cassou-Leins et al., 2000). This estimate differed from that obtained at the same time using acoustic tracking (one to two spawning acts per tagged female; Acolas et al., 2004), a difference possibly explained by the variability in individual fish behaviour, the limited surveyed area, and the small number of tracked fish. Moreover, the sex ratio in the spawning area seemed to vary according to river, year, and spawning season, though this could be due to the difficulty in estimating it accurately during upstream migration of allis shad (Mennesson-Boisneau et al., 2000b).
In conclusion, it would seem that there are many similarities between the population of allis shad in the River Aulne, a small channelled river at the northern limits of the species distribution, and larger Atlantic rivers in terms of biological and ecological characteristics, suggesting homogeneity in European populations of the species.
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Acknowledgements |
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The study was developed in the framework of the PhD study of Véronique Véron. It was carried out with the financial support of the Conseil Régional de Bretagne, Direction Régionale de l'Environnement de Bretagne, Agence de l'Eau de Loire-Bretagne, Ministère de l'Ecologie et du Développement Durable, and Conseil Général du Département du Finistère. We thank the SMATAH representatives who allowed us to trap shad at Châteaulin, the members of the APPMA of Châteaulin for links with the fishers, the rowing club of Châteaulin, and field students P-Y. Brioche and R. Keller. Finally, we gratefully acknowledge John E. Olney for his helpful comments on an earlier version of this manuscript.
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References |
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