(1) Institution d aménagement de la Vilaine, Bd de Bretagne, La Roche Bernard, France.

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1 Title : Experimental use of a lock as a migrating device for glass eel (Anguilla anguilla) in the Vilaine estuary. Authors Briand Cédric (1), Brice Sauvaget(1), Denis Fatin(1) (1) Institution d aménagement de la Vilaine, Bd de Bretagne, La Roche Bernard, France. Corresponding author : Cedric.briand@lavilaine.com. Abstract. We tested the use of a lock as a crossing device for glass eels. The experiment was carried out at the Arzal dam, a barrier between estuary and freshwater in the Vilaine. A freshwater flow of 1100 m 3.h -1 was pumped into the lock from low tide to high tide to attract the glass eels. An experimental fishery with hand nets was performed during 16 nights in March and April The density of glass eel varied from 0 to 80 glass eel.m -3. The abundance of glass eel was not homogeneous, with densities increasing near the pump, near the walls of the lock, and from mid-tide to high tide. The accumulation upstream near the attraction flow was only observed after mid-march, 10 days after the fishery was stopped, and tended to increase from March to May. The total number of glass eel present in the lock at high tide was estimated at corresponding to a weight of kg. This migration was of comparable size to the number of glass eel having used the two trapping ladders of the dam (350000, 97.2 kg). However, this comparison does not account for the number of glass eel returning to the estuary, by way of the dam gates, after having crossed the lock at high tide. Introduction. Migratory species have a particularly sensitivity to habitat destruction (MCDOWALL, 2006), and the reduction in habitat is listed in the possible causes (ICES, 2001 ) of the collapse in eel stock. While glass eel arrivals at the coasts have dwindled to a very tiny part of their historical level ((ICES, 2008 in

2 prep.)), major obstacles located in the estuaries can still play a major role in the reduction of upstream populations (BRIAND et al., 2006). Estuarine barriers are of particular importance as they stop the migration of glass eels using tidal transport, and are located at the limit of estuaries where glass eel densities are the highest. The crossing of estuarine barriers by glass eels can be done passively. Holes located in the gates of the dam, or temporary opening when the water level is higher downstream are useful to restore fish passage (LEGAULT, 1987, 1990). However, they require the transfer of large amount of saltwater upstream, which is not always possible. A siphon can be used to attract glass eel near the pipe when the sea level is below freshwater level, and subsequently suck them into the pipe towards freshwater, when the tide level rises (BULT et DEKKER 2006). The most widespread means of crossing is the use of passes (LEGAULT, 1988). The latter requires an adaptation in behaviour, for which the temperature seems to be one of the main drivers (BRIAND, 2009). The attraction of glass eels by freshwater leads them to accumulate in the vicinity of any freshwater output in the estuary (DEELDER, 1958; BULT et DEKKER 2006), and the very high sensitivity of eels olfaction(tesch, 2003) promotes this behaviour. So, very often, an attracting flow is used to guide glass eels to the bottom of ramps in trapping ladders (LARINIER et al., 1999). In the Vilaine (France) an experiment was carried out in 2007 using a lock with an attraction flow. The aim of this paper is to report the method used to estimate glass eel abundance in the lock, put forward a transition in behaviour occurring in few weeks after the estuarine commercial glass eel fishery stops, and finally compare the efficiency of lock manoeuvre and trapping ladder. Historical lock operations have been performed before the construction of the trapping ladder from 1980 to 1991, so their results are compared with those from Material and methods Since the construction of the Arzal dam, the Vilaine estuary has been reduced from an initial length of 50 kilometers to 12 kilometers. Close to the dam, the tide ranges 6 m, and an intensive

3 glass eel fishery operates from December to the 11 th of March in The trapping ladder is located on the left bank of the dam and has from 1996 permitted the crossing of to glass eels and 851 to yellow eels. The lock is located on the other side of the river, across the dam s gates. It is 13 m wide and 82.9 m long, and its depth varies according to the tide from 3 to 4 m. A large flow pump (1100 m 3 /h) was located in freshwater upstream from the dam, and pumped out the water at the upstream end of the lock near the gate. Strong currents were apparent up to 10 m from this outflow. The pump was activated at low tide and kept flowing until high tide. The downstream gate was opened to let the glass eel come into the lock. A hand net was used to estimate the glass eel density at various places in the lock. The fishing was carried out using two nets of 60 cm diameter, 1m depth, and 1 to 1.5 mm mesh size, weighted and attached to a buoy at the surface. Each net was equipped with a distance recorder. (photo 1). The nets were pulled by a rope, either on the banks or across the lock by operators on the side of the lock (Figure 1). One net operated at the surface while the other fished a deeper layer. According to the tide level, the depth of fishing was regulated by the size of the rope attached to the buoy. Twenty four experimental traits were done per night. The objective was to describe the abundance of glass eel according to the proximity of the pump outflow, the distance to the lock walls and the depth. Each haul was carried out twice during the monitoring. The fishing started 2.5 h before high tide and finished at high tide. For each pull of the net, the distance was recorded along with the number of glass eel caught. A statistical analysis was performed with GLM in two steps. In the first step, the presenceabsence was predicted with a glm using a binomial dispersion and a logit link. In the second, positive data were predicted with a glm, using a gamma dispersion and a log link. The final models were selected according to the AIC criteria. The effects tested in the model were the date of fishing (categorical), the time before high tide (continuous), the depth (categorical, 0 = at the surface, 1 = deep into the water), the distance to the border (categorical, 1=haul along the walls of the lock, 0

4 haul across the lock), and the fishing in the upstream section of the lock in the area where the currents are influenced by the pump (1 =fishing in that area, 0= fishing outside from this area). The hauls were classified according to the location as indicated in table 1. Interactions between the temporal factor and the categorical variables were tested. The model was then used to predict the density of glass eel per section of the lock and it was multiplied by the volume of this area, under the assumption that the border effect was detected one meter from the walls of the lock. Results The experimental fishery was carried out 16 times from 05/03/2007 to 25/04/2007, corresponding to 360 samples of abundance. 13 operations were complete with 24 hauls. The two first fishing operations couldn t be completed before high tide. The fishing operation of 05/04/2007 was only carried out by one person, and at depth. The variation of the volume filtrated according to the hauls is expressed in box plots (Figure 2). The hauls carried out at the surface and in the deeper layers have a larger difference in volumes for hauls 1 and 2. These hauls performed in the upstream part of the lock were the shortest. This shortness makes the distance from the surface to the bottom when the net sinks or arises a larger part of the total distance fished. The density per trait is not normally distributed (Kolmogorov Smirnov, p<0.001). Given that the number of samples available is too low to perform t-tests, the glass eel density for different pairs of locations is compared using Wilcoxon signed rank test for paired samples, with pairing according to the fishing operation. The density per trait in the upstream area of the lock, (haul 1 and 2 close to the gate, median density = 4.24 glass eel.m -3 ) is higher (p<0.001) than elsewhere (haul 3 to 6, median density = 1.52 glass eel.m -3 ), and the upstream factor is highly significant to explain the change in density (Table 2,

5 Figure 3). The density is similar between the hauls done close to the borders of the lock (hauls 3 and 4, 1.66 glass eel.m -3 ) and in the middle part (haul 5, 1.52 glass eel.m -3,p>0.1). However, a haul close to the border significantly increases the density in the GLM, and partly because in the upstream areas, the densities are higher (p<0.01) along the lock gate and walls (haul glass eel.m -3 ) than in the haul done across the lock (haul 2, 2.42 glass eel.m -3 ). Densities for the haul downstream across of the lock (haul 6, 1.22 glass eel.m -3 ) are slightly lower (p < 0.1) than those of hauls 3 to 5. This result might again be the consequence of the role of attraction of the pump upstream. The influence of depth is not straightforward, and it is neither significant in the GLM for densities, nor in the GLM for presence-absence. It must be noticed that artificial light from lamps was always present except in the middle section of the lock. The surface density is sometimes higher and sometimes lower, and no clear temporal trend could be detected. We suspect that the order which the nets were pulled might have had an influence, the catch of the latest being lowered by the disruption caused when the first net was pulled (Figure 4). There has been a seasonal change in the accumulation of glass eel upstream, the difference between upstream (hauls 1 and 2) and downstream (hauls 3 to 6) densities, is not significant before the 20 th of March, and highly significant after (Figure 5). In the GLM, the interaction date:upstream was highly significant to explain density and its choice led to the best AIC. There was also an accumulation effect, with densities increasing from mid tide to high tide. For the presence absence data, the best model is presence ~ date+ time before high tide. No location factor (upstream, border, depth) is kept in the analysis. In fact only 42 hauls in 360 were without glass eel, and they were mostly concentrated in the last fishing experiment, so the absence is better explained by date alone. border. For the positive data, the best model was density ~ date* upstream + time before high tide + Using the combination of the two models, the density of glass eel is predicted at high tide for

6 each of the 6 areas, the depth factor is not used in the prediction. The total number of glass eel present in the lock at high tide is estimated at corresponding to a weight of kg. This migration is of comparable size to the number of glass eel having used the two trapping ladders of the dam (350000, 97.2 kg). The biomass per night seems mainly to depend on tidal levels and the migration peak was observed after a lessening in the flow level below 100 m 3.s -1 (Figure 6). Discussion The use of the lock during 16 nights during the main period of glass eel accumulation has led to the trapping of glass eels before lock operation at high tide. Amid these trapped glass eels, the number of glass eel that has successfully crossed the dam and enter into freshwater is unknown. It will depend on the number of glass eel remaining in the lock after operation, and the number of glass eel returning in the estuary by way of the dam s gate. In contrast, the particular behaviour of glass eel having ascended the trapping ladder, and their release in a place far from the gates, ensures that probably only a few will return to the estuary. So the apparent similar efficiency of the sluice and trapping ladder has to be considered with caution. A similar experiment conducted in the Netherland led to a 6 folds greater catch of a siphon than of a the trapping ladder, but in this case, the trapping ladder was operated with a same duration than the siphon (BULT et DEKKER 2006). LEGAULT (1990) used opening of sluice gates to transport glass eels from the Loire to the Briere marshes. The density of glass eel reported 0.03 à 3.2 ind.m -3 was lower. than in the current experiment. However, the transit estimated during only six operations ( glass eel) it larger than is our study, as it corresponds to large volumes transiting into freshwater. In the same lock in the Vilaine, from 1986 to 1991, 73 nights of experiments have been carried out from February to May, using an attracting flow of 30 (31 nights) or 130 m 3.h -1 (36 nights) or without attracting flow (6 nights). The overall experiment was the same but the net was 1.1m in

7 diameter, diagonal traits such as those used to estimate the density in the centre of the lock were only operated from 1987 to No crossing trait was realised close to the pump, however one was carried out at the downstream gate. The largest densities were found in the upstream part of the lock, near the borders, and in contrast to 2007, at the surface, possibly indicating that the lock was not illuminated at that time. According to the year, the densities near the upstream gate were 0.52 to glass eel.m -3 with a mean value of 9.5 glass eel.m -3, about twice that found in this study. Given the 7 fold drop in recruitment since that time, the reduction of a factor two can be surprising, but it mostly reflects the low densities in the estuary during the fishing season, as there is no escapement in the Vilaine estuary (BRIAND et al., 2003; BEAULATON et BRIAND, 2007). Some lock operations were also done without attracting flow in 1982 by (ELIE et RIGAUD, 1984). Abundances of 20 ; 1,2 ; 0,4 glass eel.m -3 were found at 3 ; 25 and 50 meter from the upstream gate of the lock during the period of largest densities in May. Conclusion It is possible to estimate the number of glass eel attracted within the sluice before operating, and the experimental procedure used in 2007 the Vilaine could be simplified as there is no need to account for a variation in glass eel density according to the depth. Most importantly, the density varies according to the distance of the freshwater flow and this factor, along with a possible trend in attraction to freshwater, should be accounted for in future studies, in particular, in the conception of the a new lock, equipped with a device to count the glass eels. Bibliography BEAULATON L. and BRIAND C., Effect of management measures on glass eel escapement. ICES J. Mar. Sci., 64, BRIAND C., Dynamique de population et de migration des civelles en estuaire de Vilaine. Population dynamics and migration of glass eels in the Vilaine estuary AGROCAMPUS OUEST,

8 Rennes. BRIAND C., FATIN D., FEUNTEUN E. and FONTENELLE G., Estuarine and fluvial recruitment of European glass eel in a fished Atlantic estuary. Fish. Man. Ecol., 10, BRIAND C., FATIN D., FONTENELLE G. and FEUNTEUN E., Effect of re-opening of a migratory axis for eel at a watershed scale (Vilaine river, Southern Brittany). Bull Fr Pêche Piscic, 378, 67:86. BULT T. P. and DEKKER W., An experimental field study on the migratory behaviours of glass eel (Anguilla anguilla) at the interface of fresh and salt waters, with implications to the management and improvement of glass eel migration. ICES J. Mar. Sci., 64, DEELDER C. L., On the behaviour of elvers (Anguilla vulgaris Turt.) migrating from the sea into fresh water. J Cons Int Explor Mer, 24, ELIE P. and RIGAUD C., Etude de la population d'anguilles de l'estuaire et du bassin versant de la Vilaine : examen particulier de l'impact du barrage d'arzal sur la migration anadrome (civelles), proposition d'amélioration du franchissement de cet obstacle. Cemagref/Université de Rennes, Rennes. ICES, 2001 Report of the ICES/EIFAC Working Group on Eels. ICES/EIFAC. ICES, 2008 in prep. Report of the ICES/EIFAC Working Group on Eels, Leuven, Belgique. LARINIER M., PORCHER J. P., TRAVADE F. and GOSSET C., Passes à poissons, expertise, concetion des ouvrages de franchissement. LEGAULT A., Amelioration du franchissement des barrages estuariens des marais du Brivet. Ecole Nationale Supérieure Agronomique de Rennes, Rennes. LEGAULT A., Le franchissement des barrages par l'escalade de l'anguille. Etude en Sèvre Niortaise. Bulletin Français de la Pêche et de la Pisciculture, 308, LEGAULT A., Gestion des barrages estuariens et migration d'anguilles. Int. Revue ges. Hydrobiol., 75, MCDOWALL R. M., Particular problems for the conservation of diadromous fish. Aquatic Conservation: Marine and Freshwater Ecosystems, 2, TESCH F. W., The eel (ed J. E. Thorpe), pp Blackwell Publishing, London. WHITE E. and KNIGHTS B., Dynamic of upstream migration of the European eel, Anguilla anguilla (L.), in the River Severn and Avon, England, with special reference to the effect of man-made barriers. Fish. Man. Ecol., 4,

9 Table 1.- Classification of hauls Haul number depth upstream border 1 surface/ deep 0/ surface/ deep 0/ surface/ deep 0/ surface/ deep 0/ surface/ deep 0/ surface/ deep 0/ 1 0 0

10 effect Df Deviance AIC F Pr(F) A/ glm model, family=binomial, link=logit,presence = date+upstream+time+border+depth Full model date *** upstream NS time before high tide ** border * depth * B/ glm model, family=binomial, link=logit,presence = date +time Full model C/ glm model, family gamma, link=log, density=date+upstream+time+border+depth Full model date *** upstream *** time before high tide *** border *** depth NS D/ glm model, family gamma, link=log, density=date*upstream+time+border Full model Table 2.- A/ Single term deletion from the presence absence model using all variables without interaction and computation of the change in fit and associated F probabilities. B/ Best model for presence absence. C/ Single term deletion from the density model using all variables without interaction and computation of the change in fit and associated F probabilities. D/ Best model to explain density in the lock.

11 Figure 1.- Schematic view of the Lock. The arrows illustrate the net pulls. For each number, the net was pulled both at the surface and deep into the water. The white arrows indicate the possible migration pathways outside from the lock.

12 Photo 1.- The net, equipped with a distance recorder.

13 volumes filtered per haul Filtered volumes (m3) gates banks middle downstream 1-d 1-s 2-d 2-s 3-d 3-s 4-d 4-s 5-d 5-s 6-d 6-s Haul location Figure 2.- Box-plot of the volumes filtered per trait for 16 experimental fishing experiments. The number corresponds to the trait location in figure 1, the letter to the depth of fishing: d= depth, s= surface. Gates = near the pump upstream in the lock, middle and downstream, hauls across the lock far from the borders, banks hauls along the border, downstream from the attracting flow.

14 Glass eel density per haul Glass eel / m gates banks middle downstream 1-d 1-s 2-d 2-s 3-d 3-s 4-d 4-s 5-d 5-s 6-d 6-s Haul location Figure 3.- Box-plot of the glass eel density per trait for 16 fishing experiments. The number corresponds to the location of the haul in figure 1, the letter to the depth of fishing: d= depth, s= surface, horizontal bar indicates statistically different group (Wicoxon sign rank test for paired observations).

15 Densities in the sluice according to the fishing depth depth surface Glass eel density / m * date Figure 4.- Box-plot of non null glass eel density (log scale) according to the fishing depth. * The fishing of 5 th of April was only carried out deep in the water but be assured that the second fisher slept soundly.

16 Seasonal change in the upstream accumulation of glass eel Relative density of glass eel downtream upstream Fishing operation (dates) Figure 5.- Box-plot of relative density (density per location per date/ mean density of the night of fishing), the colors are according to the distance from the pump outflow.

17 Glass eel weights (kg) tide coeff/10 flow m3.s-1/10 temperature C glass eel weight tide coefficients flow temperature 05/03 12/03 19/03 26/03 02/04 09/04 16/04 23/04 night of experimental fishery Figure 6.- Temporal variation in estimated biomass of glass eel in the lock at high tide, and relation to tidal coefficients, flow and temperatures level.