ICES CM 2004/ S:20 Not to be cited without prior reference to the authors

Assessment of upstream migration and reproductive behaviour of allis shad (Alosa alosa (L.)) from a small river of the Atlantic coastal zone (L’Aulne, Brittany, ).

M.L. Acolas, V. Véron, H. Jourdan, M.L. Bégout Anras, M.R. Sabatié, J.L. Baglinière.

Adult allis shad (Alosa alosa (L.)) behaviour was analysed during the last part of upstream migration in River Aulne and during reproduction on a unique spawning ground downstream of an insurmountable dam. Population-level migration and reproduction were studied by counting the number of migrating fish and the number of night acts of spawning since 2000. The influence of environmental conditions (water discharge and temperature) was highlighted. We discuss yearly characterisation of the migration and reproduction activities during the all spawning season based on the number and the sex ratio of migrating shad and on the direct counting of spawning acts.

Keywords: Allis shad, behavioural ecology, anadromous migration, reproduction. M.L. Acolas, J.L. Baglinière, H. Jourdan, M.R. Sabatié : UMR INRA-Agrocampus Ecobiologie et Qualité des Hydrosystèmes Continentaux, 65 rue de Saint Brieuc, CS 84215, F-35042 Rennes Cedex, France [tel : 02 23 48 54 41, fax : 02 23 48 54 40, e-mail : [email protected], [email protected]]. V. Véron : Institut d’aménagement de la Vilaine, Boulevard de Bretagne, BP 11 56130 La Roche- Bernard, France, [[email protected]]. M.L. Bégout Anras : Centre de Recherche sur les Ecosystèmes Marins et Aquacoles – UMR CNRS/Ifremer - B.P. 5, F-17137 L’Houmeau, France [tel : 05 46 50 06 95, fax : 05 46 50 06 00, [email protected]]. Introduction

Allis shad (Alosa alosa (L)) has a pelagic sea life mainly inshore along the coast and migrates to the higher middle watercourse of rivers to spawn. Originally, allis shad distribution area extended to the Atlantic coast from Norway to Morocco (Baglinière, 2000). Currently, this species is classified as vulnerable in Europe because of the reduction in its distribution and the threats to its freshwater habitat due to dams, pollution and deterioration of the spawning grounds (Baglinière et al., 2003). The actual septentrional limit would be the river in France (Baglinière, 2000) and the meridional limit went up to Portugal (river Mondego and Vouga) (Baglinière et al., 2003) after the extinction of the species in Morocco in 1992 (Sabatié and Baglinière, 2001). However, allis shad has not disappeared or has reappeared in small french rivers and some prospections have revealed perennial populations and functional spawning grounds (Véron et al., 2001; Baglinière et al., 2003). In France, research on the biology and conservation status of allis shad has mainly focused on populations in large river systems, such as the Loire (Mennesson-Boisneau and Boisneau, 1990), Garonne (Cassou-Leins et al., 2000), Gironde (Taverny et al., 2000) and Adour (Prouzet et al., 1994). Studies on the migratory behaviour have focused only on the first phases of upstream migration and very few work has been carried out on the last phase, namely the arrival of spawners on the spawning grounds (Mennesson-Boisneau et al., 2000a). We provide a detailed description of this last phase of migration and the reproduction activity on a forced spawning ground of the allis shad population in the River Aulne, a small human impacted stream in Brittany, using a counting trap and direct observations.

Materials and Methods

Study site The catchment of river Aulne (1 875 km2 in surface) 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 after vast meanders for 145 km, of which 70 km are channelled since 1836 and have 28 dams to maintain a depth of 2 m for navigation (Figure 1). Average water flow of the river is 9 m3s-1 with a maximum value of 80 m3s-1. Water quality is deteriorated by fish farm effluents and nitrates from agriculture activity. This poor water quality associated to a low summer flow and the multitude of dams that slow down the flow lead to eutrophication of the channelled part. 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 allows the passage of migrating fish species present on the watershed: allis shad, eels (Anguilla anguilla (L.)), sea lamprey (Petromyzon marinus (L.)), Atlantic salmon (Salmo salar (L.)), and sea trout (Salmo trutta (L.)). The pass is also equipped with a trap and a video system that allows the number of migrating allis shad to be counted since 2000. 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. The study area was divided into four subsections: Sector B was the deepest (>3 m deep) while sectors A, C and D have average depths that varied between 1.80 m and 2.15 according to water discharge levels (Figure 1). All the sectors have a sandy-silty bottom. Allis shad spawning area was located just above sector D, downstream Koatigrac'h dam and covered an area of 3000 m2 . Its physical habitat was characterized by mobile coarse gravel on a schist bed and depths ranged between 0.1 and 1.9 m (Figure 2). Those two characteristics were similar to what is described for natural spawning area in larger rivers (Cassou-Leins et al., 2000; Boisneau et al., 1990). However, the spawning area width (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 slow current (0.2-0.4 m s-1 in 2002) while typical spawning ground are usually followed downstream by a shallow fast running zone (Cassou-Leins et al., 2000). The spawning site contrasted with the homogeneity of the channel (habitat and water current) thanks to the narrow part of water flow created by the island which allowed a current acceleration and a rocky habitat (Figure 2). Brest Roadstead Châteaulin

Area A Fish Pass 200 m

Studied area Area B Spawning Tributary area Doufine Area C Koatigrac ’h

Area D

Gilly Glas dam

Châteaulin dam Fish video 2 km France counting and trapping Koatigrac’h dam Forced spawning area River Aulne

Figure 1: study area

Direct observation post on the jetty Lock Koatigrac'h Dam

50 m Fish pass

Spawning area

Depth (m) Island 0 - 0.5

0.5 - 1

1 - 1.5

1.5 - 2 Figure 2: Spawning area Protocol for migration and reproduction assessment

The number of migrating allis shad was counted by the aquatic observatory of Châteaulin (SMATAH). 74 and 239 individuals were sampled respectively during the second half of the 2001 migration and during the whole 2002 migration period at the rate of 3 to 4 times per week using the trap system. Sex was determined by gentle pressure on the abdomen (males were all ripe, with running milt), overall length measured (Lt ± 5 mm) and weight (Mf ± 50 g). A scale sample was taken from the optimum zone to estimate age (Baglinière et al., 2001).Population reproduction phase was documented during one month in 2000, and during all the spawning season in 2001 and 2002 (Table 1). An observer was positioned on the jetty to count spawning events by hearing them (Figure 2). Reproduction acts were counted every night between 22:00 and 6:00 (U.T. +2) in 2000 and 2001 and 3 nights per 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 to 10 s) of minimum two spawners side by side. Its sound intensity at 1 m far was measured between 35 and 50 dB (Cassou-Leins et al., 2000). Further, surface current speed was visually estimated. When environmental conditions were favourable (low water turbidity and fish close to the observer), the number of fish participating to a spawning act was noted. Water flow was measured by DIREN Brittany. Water temperature on the spawning area was recorded hourly by a datalogger (Minilog, Vemco Ltd).

Table 1: Migration and reproduction period

Year Migration Total Number of Reproduction Number of Maximum of period number of migrants period spawning acts spawning acts migrants sampled counted counted in one night 2000 07/04-08/08 2182 0 13/05-16/06* 2917 306 2001 16/04-22/07 4325 74 12/05-11/07 16851 675 2002 06/04-15/08 2331 239 24/04-11/07 6310 531 * In 2000 reproduction was partially followed

Data analysis

Statistical analysis of migration and reproduction were carried out with EXCEL and SPADâ. Migration and reproduction trends were evaluated by calculating an order 7 mobile mean. Average daily temperature were calculated during all the study periods. To study the environmental factors that influence migration, migrating population was considered as closed (total number of migrants known) and was expressed by the percentage of migrating individuals per day (Xj) in relation to the number of migrant left. æ n j-1 ö ç Percentage of migrants per day: Xj=Nj çåNi-åNj-i÷ è i=1 j=1 ø (j=date of the day; i=day between 1 and n with n=total number of day; N=number of migrants.)

The study period was reduced by taking out 2% of the total length of migration period at the beginning and the end. Then the following periods were considered: 29/04-18/06 in 2000, 27/04-30/06 in 2001, 20/04-19/06 in 2002. In 2002, the estimation of the number of spawning acts during nights without observation was made by giving to that date the average of the number of spawning acts the night before and the night after. For all the years, the estimation of the number of spawning acts per female (X) was estimated as:

X = å spawningacts å females A sex-ratio expressed by the sampled males number/females number has been calculated in 2001 and 2002. Both years, the estimation of the number of act of spawning per female was made by using the number of spawning acts counted on the spawning area and the number of migrants counted at Châteaulin pass considering the following hypotheses according to Cassou-Leins et al. (2000): 1) Spawners visit only one spawning ground, 2) a balanced sex-ratio of the population exists, 3) there is only one spawning act per night per female and 4) to one act corresponds only one female. According to these hypotheses, Cassou-Leins et al. (2000) considered a female could spawn between 5 to 7 times during a reproduction season.

Results

Upstream Migration

Demographic characteristics In 2001, a sex-ratio of 1.51 in favour of females was observed on 1.7 % of the population (N=74) during the second half of the migration period. In 2002 sex-ratio estimated over the whole migration period on 10.2% of the population (N=239), 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) whereas it was in favour of female in the middle of the migration period (sex-ratio=0.6 in May, N=150). According to those values, the population in 2002 was estimated to be composed of 1103 males and 1195 females and the global sex ratio raised at 0.92. Sampled individuals in 2001 and 2002 were between 3 and 7 years old (3-6 in males and 4-7 for females). For males, sizes (Lt) and weights (Mf) ranged respectively from 370 to 585 mm and from 510 to 2000 g. For the females, they respectively ranged from 445-670 mm and from 600-3100 g.

Migration Trends Upstream migration took place mostly between April 29 and June 18 in 2000 (96% of the migrants), between April 25 and June 16 (94% of the migrants) in 2001 and between April 19 and May 27 in 2002. From one year to the other, the total number of migrants varied by a two fold order (Table 1). Migration trends calculated for on 7 days basis were uni-modal in 2000, bi-modal in 2001 and in 2002 (Figure 3).

200 2000 2001 2002

150

strength 100

Migrants 50

0 01/04 16/04 01/05 16/05 31/05 15/06 30/06 Figure 3: Migration trends Environmental influence on upstream migration Over the 3 years, temperature observed during 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 and was recorded in 2001 only (Table 2). A positive linear relation between daily temperature (Td°C) and the percentage of daily migrants (Nmig) was highlighted in 2001 only (p<0.01; Table 2). However, there was a positive correlation between the daily temperature variation between two consecutive days (VTd°C) and the percentage of daily migrants in 2002 (p<0.01; Table 2).

Over the 3 years, water flows observed varied strongly during upstream migration (2.7-86.7 m3s-1) and at the beginning of the migration (17.5 - 82.7 m3s-1) a correlation between the daily water flow (D) and the percentage of daily migrants was observed to be positive in 2000 (p<0.01; Table 3) and negative in 2001 (p<0.01; Table 3). In 2002, no significant correlation between the two parameters have been established even if migration activity seemed to decrease during rapid water flows increase (Figure 4). Furthermore, the percentage of daily migrants in 2001 et 2002 was probably influenced both by temperature and water flow variation as an increase of daily migrants seemed to be linked to a continual temperature increase while water flow decreased (Figure 4).

Table 2: Temperature variation during migration and reproduction

Year T Migration Migration Relation between daily T during T during Relation between (Td°C) observed activity activity T (Td°C) or daily T reproduction reproduction or (VTd°C) and the during started stopped variation (VTd°C) and period Min- activity number of counted migration the percentage of daily Max Min-Max spawning acts (Ns) period migrants (Nmig) Min-Max

2000 11.8-21.9 - - - 13.3-17.2* 13.9-17.2 Ns=70.63 Td°C -923.34 r2=0.62 ; N=35

2001 10.5-23.0 11.3 10.7 Nmig=1.06Td°C-11.93 14.0-23.0 14.3-23.0 Ns=193.74 Td°C -2686.40 r2=0.44; N=60 r2=0.66; N=13

2002 11.6-19.9 11.7 - Nmig=6.96VTd°C + 5.26 13.4-19.3 13.9-19.3 Ns=242.37VTd°C +99.61 r2=0.27; N=65 r2=0.45 N=42

T=Temperature; * In 2000 reproduction was partially followed (Cf. Table 1).

Table 3: Water flows and current speed variation during migration and reproduction Year Water flows Relation between Water flows Water flows Current speed Current speed (m3s-1) during the daily water (m3s-1) during (m3s-1) during (ms-1) during (ms-1) during migration flow (D) and the the reproduction the reproduction Period percentage of reproduction activity Min-Max reproduction activity Min-Max daily migrants period period Min-Max (Nmig) Min-Max Min-Max 2000 10.1-86.7 Nmig=0.27D-1.02 10.1-47.6 26.6-47.6 NA NA r2=0.22; N=39 2001 2.7-64.0 Nmig=-0.22D+8.56 2.7-26.9 20.5-26.3 0.10-1.25 0.10-0.80 r2=-0.25; N=61 2002 5.6-44.3 - 5.6-44.3 8.3-19.9 0.30-1.50 0.30-0.75 NA=Data not available 2001 30 70 (°C) A B A A 25 60 ) _1 s 3 50 20 (m

Temperature 40 15 flow 30

10 Water 20

5 10 Migration (%); 0 0 16/04 01/05 16/05 31/05 15/06 30/06

Temperature Percentage of daily migrants Water flow ) 1

2002 - s 50 B A 30 3 A B A B A (m 25

40 flow 20 30 Water 15

20 (°C); 10 Migration (%) 10 5

0 0 Temperature 20/04 05/05 20/05 04/06 19/06

A : Temperature increase and flow decrease B : Temperature decrease and/or flow increase

Figure 4: Percentage of migrants regarding the number of migrants left function of temperature and water flow in 2001 and 2002.

Reproduction activity

Period and trends During 2001 and 2002, reproduction periods took place between April 24th and July 11th. The estimation of the total number of spawning acts varied yearly in a proportion from 1 to 2.5 (Table 1). Reproduction activity trend, calculated on a 7 days basis, was clearly bi-modal in 2000, uni-modal to bi- modal in 2001 and tri-modal in 2002 (Figure 5). In 2001 occasional presence of allis shad downstream of Châteaulin dam and acts of reproduction were observed. 600 2002 2001 500 2000 400 acts

300 spawning

of 200

100 Number

0

15/05 15/05 15/05 22/05 22/05 22/05 29/05 29/05 29/05 05/06 05/06 05/06 12/06 12/06 12/06 19/06 19/06 19/06 26/06 26/06 26/06 03/07 03/07 03/07 10/07 10/07 10/07

24/04 24/04 24/04 01/05 01/05 01/05 08/05 08/05 08/05

Rapid and important water flow increase (+50% to +240% of increase)

Figure 5: Reproduction activity trends Environmental factors influence on reproduction activity Over the 3 years, temperatures observed during reproduction period varied between 13.3 and 23°C (Table 2). The temperature threshold beyond reproduction activity seemed to be inhibited was between 13.9 and 14°C. No maximum temperature threshold was recorded. A positive correlation between the number of counted spawning acts (Ns) and daily temperature (Td°C) was established for values not exceeding 17°C during 2000 and 2001 spawning seasons (p<0.01; table 2). In 2002, a positive correlation was found between the number of counted spawning acts and the daily temperature variation between two consecutive days (VTd°C) for values not exceeding 17°C (p<0.01; Table 2). Over the 3 years, water flows observed during reproduction period varied between 2.7 and 47.6 m3s-1. In 2001 and 2002, current surface speeds ranged between 0.1 and 1.5 ms-1 (Table 3). Generally reproduction activity was stopped for values above 0.75-0.80 ms-1. In 2000 a low correlation between water flow and the number of spawning act was established (r2=0.17, p<0.05, N=25) and no correlation was noticed in 2001 and 2002 owing to the occurrence of some spates (Figure 4). During 2001 and 2002, reproduction acts hourly distribution fluctuated during reproduction period and seemed to be linked to water temperature. Indeed water temperature reduced the length of the nocturnal spawning activity by progressively shifting the reproduction peak towards the end of the night (between 4:00 to 5:00 am T.U. +2) (Figure 6).

14 - 16°C N=1922 16 - 18°C N=2525 20h-23h 20 20 10 10 04h-05h 0 00h-01h 0

2 02h-03h 0 18 - 20°C 20 - 23°C N=6260 N=2598 0 1 30 30 20 20 10 10 0 0

14 - 18°C N=1297 18 - 20°C N=1298 2 30 20 0 20 0 10 10 2 0 0

Figure 6: Hourly spawning acts distribution in relation to water temperature fluctuations (N=Number of spawning acts counted). Yearly characterisation of the spawning activity In 2000, the reproduction activity was followed at the population scale only during the first month (mid-May/mid-April) and the number of spawning acts per female was estimated at 2.3 for an hypothetical balanced sex-ratio. In 2001, the average number of spawning acts per female during the spawning season observed was estimated at 7.7 if we considered a balanced sex-ratio of 1 and at 9.9 if we referred to the sex ratio estimated (1.5). In 2002, the average number of spawning acts per female calculated for the whole spawning season considering the estimated sex-ratio (0.9) raised at 5.2. During the 2001 reproduction period, the number of spawners was higher than 2 for 52.3% of the spawning acts (197 observations). In 2002, the important water turbidity limited the number of observations to 20 resulting in a possible overestimation of the spawning acts involving 2 spawners. However, distribution observed in 2002 was similar to that observed in 2001 (Figure 7).

1% 2% 2001 N = 197 4%

5% Number of spawners 18% 2 3 20% 4 5 2002 47% N = 20 6 8 60% 15%

28%

Figure 7: Number of spawners observed participating to a spawning act in 2001 (external annular) and 2002 (internal annular). . Discussion

Upstream migration

In the river Aulne, allis shad upstream migration took place between mid-April and mid-August. The migration beginning is similar to what was 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 annual fluctuation of migrating fish recorded during the three years in the River Aulne was much lower than that observed in larger rivers like the Dordogne and the Garonne respectively at Tuillère (factor 45) and Golfech (factor 8) dam (Bellariva, 1998). The observation of a bi-modal migration trend in 2001-2002 might be explained by lower flows during those 2 years compared to 2000. Moreover, in the Loire system, Mennesson-Boisneau et al. (1993) noticed that migration trend tend to be uni-modal years of medium flows and to become bi-modal to dissymmetrical years of low flows. A temperature decrease would also modify migration trend toward a bi-modal or tri-modal distribution (Mennesson-Boisneau et al., 2000a; Rochard, 2001). The high 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). However, the influence of water discharge upon migration activity was modest except during spates when a decrease in migration activity was observed as it was also shown by Mennesson-Boisneau and Boisneau (1990). It did not seem to exist a minimum or a maximum water flow threshold for migration which was observed between 2.7 and 86.7 m3s-1 according to the years. The positive influence of water flow in 2000 became negative in 2001 probably in relation with associated temperature fluctuation and the fact that migration observations were made at the level of an obstacle crossing. Indeed, in 2001, high flow increase (+161 and +62 %) induced negative temperature variations and caused a decrease of migration activity. In 2000, the water flow increase (+240%) induced better hydraulic condition, without modification of temperature conditions, that helped fish-pass crossing by allis shad. Temperature may supplant water flow influence although the latter could make the obstacle crossing easier. No link with tide and migration flow was established despite previous observation by Mennesson-Boisneau et al. (2000a). Tide would rather be a factor of control of shad migration flow between sea and freshwater environment. The absence of correlation in the river Aulne study may be explained by the dam position at the estuary saline influence limit.

Reproduction

General characteristics In the Aulne, the beginning of shad reproduction period (mid-April - mid-May) was close to that observed in larger rivers (Bellariva, 1998; Mennesson-Boisneau and Boisneau, 1990). Globally, reproduction activity showed trends modulated by variations of water flow and temperature as observed in larger rivers (Mennesson-Boisneau et al., 2000a). No correspondence appeared between the migration and spawning trends. That might be explained by: 1) a different influence of environmental factors on the two activities, 2) a larger influence of biological and internal factors (maturity state) as showed by Mennesson-Boisneau and Boisneau (1990) during the end of usptream migration and 3) a possible impact of the forced spawning zone resulting in a high concentration of spawners and an increase in social interactions. According to Cassou-Leins et al. (2000a), water temperature would be an important factor that initiated and then controlled reproduction activity but the threshold values would vary according to year and river. In the river Aulne, water temperature influence appeared to be prominent at the beginning of the reproduction when temperatures did not exceed 17 °C with a minimum threshold (14°C) observed in 2000 and 2001. Above this value, water temperature did not seem to influence spawning activity. As for migration activity further, the influence of water discharge upon spawning activity was difficult to underscore except a strong reduction of activity during flood period.

Reproduction activity The forced studied spawning zone was the most important spawning area in the River Aulne for allis shad population. However, a temporary spawning ground was observed downstream the Châteaulin dam in 2001 probably in relation to low water and migration delay due to the dam (Mennesson-Boisneau et al., 1993). Moreover, simultaneous observation at the individual scale using acoustic tracking showed a large number of tagged fish located in the deepest area of the channel and a restricted frequentation of the spawning ground in time (both day and night) suggested that this area might be considered as a rest area/pre-spawning area (Acolas et al., in press). Such fish concentration areas during migration have been observed on the river Loire, particularly downstream from sites of crossing of obstacles (Steinbach et al., 1986). The large use of this zone might be reinforced by the presence of the insurmountable dam leading to a gathering of both spawners and fish going on to migrate upstream. That was confirmed by the following observation made at the end of the period of migration in 2001 while two acoustically tagged males attended the spawning ground only during day-time, reflecting more an exploratory phase in front of the obstacle than reproduction attempts (Acolas et al., in press). Allis shad were observed on the spawning ground during both the day and night (acoustic tracking results and visual observations). Also, time spent on the spawning grounds was limited and, at most, accounted for 30% of the total time tagged shad were resident in the river between the Châteaulin and Koatigrac’h dams (Acolas et al., in press). The hourly distribution of spawning acts during all the spawning season was similar to that observed in the Rivers Loire (Mennesson-Boisneau and Boisneau, 1990) and Garonne (Cassou-Leins and Cassou-Leins, 1981). However, we noticed that this hourly distribution of the spawning acts varied during the reproduction period what was not documented previously. A reduction in duration of the reproduction activity and a shift in time of the reproduction peak were observed. This shift was lined up with the daily average water temperature increase suggesting that shad were looking for a thermal optimum during the night. In 2001 and 2002, the visual observation of the number of spawners participating to a reproduction act showed that more than two individuals were involved in 45 % of the spawning acts. The proportion of spawning acts involving 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 to a spawning act might be due to the forced character of the spawning zone which induced high concentration of spawners. Otherwise, this observation would suggest a reconsideration of the method of spawners evaluation based on the counting of spawning acts over the whole spawning season based on the hypotheses proposed by Cassou-Leins et al. (2000). Nevertheless in 2002, the one year when the sex ratio has been estimated over the whole migration period, the average number of spawning acts per female during the reproduction period was included in the values recorded in larger rivers (5 to 7) (Cassou-Leins et al., 2000). Moreover, sex-ratio seemed to vary according to river, year and during reproduction season and the observed figures may be biased owing to the difficulty to estimate it truly during upstream migrating of shad (Mennesson-Boisneau et al., 2000b).

Conclusion

Many similarities between the allis shad population of the river Aulne, a small channelled river at the northern limits of the species distribution, and larger French rivers were highlighted about upstream migration and reproduction trends, environmental influences on these activities. The observations made during that three years study confirmed the presence of one forced spawning zone for allis shad in the lower part of the river Aulne. It showed that the spawning zone was functional, resulting in the presence of a self-sustainable population of the species. Nevertheless, some negative aspects have been evoked in relation to the high concentration of spawners in this zone. Such concentration and the presence of a limiting environment for the development of eggs and larvae downstream the spawning zone, might lead to a low spawning success (eggs losses and lower survival) resulting in a very high variability of yearly population abundance. Furthermore, this impact might be reinforced in the future as the population abundance size seems to increase (7000 shad adults counted in 2004). So, it appears necessary to collect further informations and data in order to analyse the true impact of the forced spawning zone upon the evolution of the population unless upstream dams were to be equipped with a fish pass as there exist potential spawning zones in the upper parts of the River Aulne.

Acknowledgements

This study was carried out with the financing support of 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 wish to thank the SMATAH representatives who allowed us to trap and hande shad at Châteaulin, the members of the APPMA of Châteaulin for communication to the fishermen, GILAP dairy plant as well as the town of Châteaulin for the utilisation of their buildings, the oar club of Châteaulin and the assistance of field students: P.-Y. Brioche, R. Keller.

References

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