The influence of tidal streams on the pre-spawning movements of Atlantic herring, Clupea harengus L., in the St Lawrence estuary

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1 ICES Journal of Marine Science, 8: doi:.6/jmsc , available online at on The influence of tidal streams on the pre-spawning movements of Atlantic herring, Clupea harengus L., in the St Lawrence estuary Karine N. Lacoste, Jean Munro, Martin Castonguay, François J. Saucier, and Jacques A. Gagné Lacoste, K. N., Munro, J., Castonguay, M., Saucier, F. J., and Gagné, J. A. 1. The influence of tidal streams on the pre-spawning movements of Atlantic herring, Clupea harengus L., in the St Lawrence estuary. ICES Journal of Marine Science, 8: Eight Atlantic herring (Clupea harengus L.) implanted with ultrasonic transmitters were tracked to determine their movements during the pre-spawning period in the tidally energetic, upper St Lawrence estuary. Herring released near the south shore travelled upriver near the shore while herring released near an island where significant spawning occurs remained in the vicinity of that island. Herring near the shore moved upriver regardless of the tidal phase while herring close to the island swam actively against tidal currents during both phases of the tide. Herring near the coast may have been migrating to an alternate spawning site further upriver while herring near the island remained in close proximity to the island s spawning site as a result of their behaviour. Herring travelling near the coast seemed to take advantage of tidal currents by swimming with tidal streams when the tide flowed in the migratory direction (flood tide) and by swimming against them when the tide flowed counter to it (ebb tide). These fish gained more ground during the flood tide than they lost during the ebb. A circular statistical analysis of movements-through-water showed that fish were oriented against the flow during the ebb phase. Selective tidal stream transport was not observed since there was no evidence of vertical migrations associated with tides. Keywords: herring, tracking, movements over ground, movements through water, tidal streams, circulation model. Received 8 August ; accepted 3 April 1; published electronically 1 August 1. K. N. Lacoste, J. Munro, M. Castonguay, F. J. Saucier, and J. A. Gagné: Fisheries and Oceans Canada, Institut Maurice-Lamontagne, C.P., Mont-Joli, QC, Canada GH 3Z4. Current address for K. N. Lacoste: ISMER, Université duquébec à Rimouski, 3 des Ursulines, Rimouski, QC, Canada GL 3A1. Correspondence to J. Munro: munroj@dfo-mpo.gc.ca Introduction Two populations of Atlantic herring, Clupea harengus L., migrate annually up the St. Lawrence estuary to spawn, one in the spring and one in the fall (Greendale and Powles, 198; Rivière et al., 198; Lambert, 199; Munro et al., 1998). Migrating spring-spawners are first caught in March and April along the lower estuary while the largest catches are landed in May in the upper estuary, in the area of Rivière du Loup (Greendale and Powles, 198; Bérubé and Lambert, 1997). A major spawning site of the spring-spawning population has been located in the middle of the upper estuary, at the southwest tip of I le aux Lièvres by Munro et al. (1998) (Figure 1). As suggested by larval concentrations (Able, 1978; Auger and Powles, 198), spring-spawners may also use other sites around the islands distributed along the southern shore of the estuary, especially in the vicinity of the I les Les Pèlerins. The fall-spawning population migrates to the estuary at the end of the summer towards spawning grounds in the same general area as is indicated by the capture of newly hatched larvae in September (Fortier and Gagné, 199). An important mechanism used by fish during migration is selective tidal stream transport. A fish exhibiting selective tidal stream transport ascends the water column to drift or swim during the favourable tide and descends to the bottom where currents are weaker to hold position during the opposing tide (Arnold, 1974; Greer Walker et al., 1978; Arnold and Holford, 199). Selective tidal stream transport is a migratory mechanism used by plaice (Pleuronectes platessa) (Greer Walker et al., 1978; Harden Jones et al., 1979; Arnold and Metcalfe, 199), sole (Solea solea) (Greer Walker et al., 198; Greer Walker and Emerson, 199), Atlantic cod (Gadus morhua) (Arnold et al., 1994), American eel /1/ $3./

2 Influence of tidal streams on pre-spawning Atlantic herring Spawning site ^ North Channel Ile Les Pelerins ^Ile aux Lievres Pot-a-l'eau-de-vie Channel South Channel 6 Quebec Study Site 48 Notre-Damedu-Portage USA km Riviere-du- Loup Figure 1. Area where herring were tracked in the upper St Lawrence estuary during spring and fall 1986 and spring Depth contours are in metres. (Anguilla rostrata) (McCleave and Kleckner, 198; McCleave and Wippelhauser, 1987; Parker and McCleave, 1997), and European eel (A. anguilla) (Creutzberg, 199; McCleave and Arnold, 1999) in various tidal systems. Among fast-swimming pelagic fish species this behaviour has only been shown in sockeye salmon (Oncorhynchus nerka) (Levy and Cadenhead, 199). Castonguay and Gilbert (199) reported that Atlantic mackerel (Scomber scombrus) avoid tidal streams in a direction opposed to migration but could not find any evidence of associated vertical migrations. Selective tidal stream transport is energetically advantageous in areas of strong currents (Weihs, 1978; Metcalfe et al., 199). The upper St Lawrence estuary is such an area (e.g. Canadian Hydrographic Service, 1997; Saucier and Chassé, ) and thus provides an opportunity to examine the hypothesis that herring uses selective tidal stream transport during their migration to spawning sites upriver. In this study, individual herring were equipped with ultrasonic transmitters and tracked during the pre-spawning period with three objectives in mind: (1) to examine the relationships between herring movements and tidal currents, () to define the movements of herring in the vicinity of spawning grounds, and (3) to locate the spawning grounds. The first two objectives are addressed in the present paper while the third one was dealt with in Munro et al. (1998). Materials and methods Study area This study was conducted in the southern portion of the upper St Lawrence estuary (Figure 1) in the spring and fall pre-spawning periods of May and September 1986 and May The upper St Lawrence estuary is funnel shaped, from 14 to 4 km wide, with an uneven topography composed of several disconnected channels and troughs separated by ridges and islands. At its downstream end the upper estuary is split by I le aux Lièvres into two distinct portions. The northern portion has a deep channel, the North Channel, while the southern portion is a broad shallow region with two channels, the Pot-à-l eau-de-vie and South channels. Herring were tracked in the area between I le aux Lièvres and the south shore of the upper estuary. The circulation in this area is characterised by mixed semidiurnal tides with a period of 1.4 h and a range of m (Godin, 1979; El-Sabh and Murty, 199; Canadian Hydrographic Service, 1997). Tidal currents are highly bi-directional along the I le aux Lièvres axis (3 T, i.e. 3 true north) and can range up to m s 1 (Canadian Hydrographic Service, 1997). The water column is well mixed and contains no internal tides (Muir, 198). Tracking The four fish tracked in 1986 were captured near the Rivière du Loup wharf (Figure 1). In 1987, three fish were caught near I le aux Lièvres, in close proximity to the Pot-à-l eau-de-vie Channel, while a fourth was caught near the I les Les Pèlerins, in the South Channel. Captures were made using monofilament and braided nylon gillnets operated from a Boston Whaler. Nets. m in height by 4. m long, with 6-mm stretched mesh size, were set in water 3 m deep, about 1 km from the shore for periods shorter than minutes. Individuals were kept on board in a 1-l holding tank

3 188 K. N. Lacoste et al. with a constant flow-through supplied with water pumped directly from the estuary. Herring were selected for their large size and the absence of visible bruise marks. Only those exhibiting normal swimming behaviour were chosen. Fish selected for tracking were mature, 8 34 cm long. A sample from the other fish caught at the same time confirmed that they were all in pre-spawning (stages or 6) or in spawning (stage 7) condition (according to Courtois et al., 1983). This strongly suggests that the individuals used for tracking were of similar maturity stages. Given that only herring mature enough to be spawners were selected and that spawning of these fish was observed a few weeks after the tracking period (Munro et al., 1998), it is assumed that the tracked individuals were exhibiting typical pre-spawning behaviour. Ultrasonic transmitters (Vemco model VB-L in 1986 and Vemco model V3P-4 in 1987) used for the experiments were set to selected frequencies between 6 and 77 khz to allow the identification of individual fish. Transmitters were 38 mm long by 8. mm in diameter in 1986 and 6 mm long by 16 mm in diameter in Only those used in 1987 were depth-sensitive. The ultrasonic signal was detected using a Vemco (model V-) directional receiving hydrophone which was rotated to locate the direction of the maximum signal. The hydrophone was connected to a receiver-decoder-amplifier (Vemco model VR-6) which allowed signals to be heard with a pair of headphones. The effective detection range was found to be approximately m. In 1986 the transmitters were inserted through the oesophageal duct and into the stomach, an operation which took approximately one minute. In 1987 the larger transmitters were inserted in the abdominal cavity using a surgical procedure similar to that used by Bidgood (198). Each herring was submerged in a -l anaesthetic tank containing a Quinaldine solution. Once the fish started to tilt on its side, it was placed on a board with its head immersed in water to allow operculum movement. A.-cm incision was made along the abdominal wall using a scalpel for the initial opening and surgical scissors to complete the cut. The transmitter was then inserted in the front and bottom of the abdominal cavity. The incision was closed by five stitches with nylon thread and the surgical suture was disinfected with an antiseptic solution. Each operation took roughly three minutes. After surgery the herring was transferred into a recovery tank to resume normal posture and activity: this took about ten minutes. It was kept for another ten minutes before release. The two protocols were tested in 1986 and 1987 prior to the tracking period by verifying that fish were exhibiting normal swimming behaviour after being kept for 4 48 h after insertion. Only fish that responded well to manipulations were released and tracked. All releases occurred near capture locations. Following release the transmitter signal was usually relocated within a few minutes. During tracking the signal from the transmitter was monitored continuously by the hydrophone operator. The ship s position was determined with Loran-C while the heading was subsequently calculated from the successive positions. As is usually the case with this type of study these data were used as a proxy of herring position. Accuracy of Loran-C positions was deemed correct and relative error for the along-estuary position was measured to be m given a clear Loran-C channel signal propagating in this direction. Each geographical fish position was converted to latitude and longitude and plotted onto a map using a Mercator projection. The herring positions were recorded whenever a change in speed or direction was detected. Over-theground speeds were estimated by dividing the distance travelled by the boat by the time between successive fish positions. The fish s direction was transformed and reported with reference to 3 T along the axis of the estuary. The depth was measured by the boat s echosounder. The net distance travelled (the straight-line distance from release point to end of tracking point) and the gross distance travelled over the ground (the actual distance travelled by fish) were calculated on the Mercator plane. Gross speed was obtained by dividing gross distance travelled over the ground by the duration of the tracking. For each individual in a given semidiurnal tidal cycle, the net displacement along the axis of the estuary was obtained by adding the incremental displacements. The tidal cycle was defined to begin with the low water as predicted from the nearby ports of reference from the Tide and Current Tables, Volume 3 (Canadian Hydrographic Service, 1986, 1987). Given the phase progression of the tide in this area this time represents the state of the tide near the individuals to within minutes. Estimation of water currents To estimate the time-dependent tidal current velocity at the positions of the individuals we made use of a three-dimensional estuarine circulation model of the St Lawrence estuary by Saucier et al. (1999) and Saucier and Chassé (). This model was used to produce tidal current charts for navigation in this area (Canadian Hydrographic Service, 1997). The model has 4-m resolution in the horizontal and -m in the vertical using a time-step of 6 seconds. It computes water levels, 3D currents, and density changes driven by the main tidal constituents that propagate from the Atlantic Ocean and freshwater from the St Lawrence River and its main tributaries. Saucier et al. (1999) and Saucier and Chassé () have shown that the model can reproduce historical current meter measurements to about % relative error for the semi-diurnal tidal phase and

4 Influence of tidal streams on pre-spawning Atlantic herring 189 Table 1. Details on Atlantic herring tracked in the upper St Lawrence estuary. Maturity stage is based on Courtois et al. (1983). Net distance and direction is the shortest distance and direction between the starting and finishing point of each track. Gross distance is the actual distance covered by the fish. The direction is shown as relative to 3 T because the Estuary is aligned 3 N. Initiation of track Movement over ground Fish no. Length (cm) Sex Maturity stage Date Time (EDT) Tidal stage Direction (3 T) Area of release Track duration (hh:mm) Gross distance (km) Net direction (km) Net direction (3 T) F 6 6 May 86 1:3 F 4.4 S. shore 1: F 6 19 May 86 1:4 E S. shore : F 7 6 May 86 : F 8.6 S. shore 7: M Sept. 86 1:3 F S. shore 17: M May 87 1:9 F I. Lièvres : M 16 May 87 13: F 7.6 I. Lièvres 6: M 18 May 87 13:4 F 8.6 I. Lièvres 9: M May 87 16:3 F 14 S. Channel 14: Tidal stage: F, flood; E, ebb. amplitude for instruments that were deployed in the area south of I le aux Lièvres (see stations 19 and 41 in Saucier et al., 1999). For each fish location and time after low water, the horizontal water current vectors were extracted from the model grid using a bilinear interpolation. For tracks or portions of tracks with missing depth, the current at mid-depth was used. However, further analyses of the model results showed that since the water column is well mixed in this area, the current is quite uniform over the water column (i.e. highly barotropic) except in the bottom boundary layer (Saucier and Chassé, ). Data analysis As previously mentioned, the positions of the tracked fish were recorded whenever a change in speed or direction occurred. These raw tracking data were plotted against tidal phase to examine general patterns. The speed and direction of fish were thus taken to be constant between each pair of recorded positions. Fish positions were subsequently interpolated at -minute intervals, providing a standardised data set for intertidal and inter-herring comparisons. These standardised data were analysed using circular statistics methods (Batschelet, 1981). Over-the-ground velocity vectors were calculated for each -minute interval. The mean vector and the standard ellipse were calculated over combined flood tides as well as over combined ebb tides. The standard ellipse shows the degree of vector dispersion (equivalent to s in univariate statistics) as well as the directional trend, with the summit of the mean vector located at the centre of the ellipse. The standard ellipse is not dependent on the interval between velocity vectors so that using a different sampling interval would have yielded the same results (Batschelet, 1981). Because the sequential -minute vectors are not independent of one another it was not possible to infer statistically about the significance of the mean vector using the confidence ellipse. However, if the standard ellipse does not include the origin then it is very likely that the vectors are not randomly distributed about the origin, i.e. movements are oriented (McCleave and Arnold, 1999). Fish velocity through water was determined as the difference between the fish velocity over the ground and the current velocity for each -minute interval. The difference between these two vectors indicates whether the fish drifted with the tidal flow or swam either with or against it. The mean vectors and standard ellipses of velocities through the water were also calculated for the combined ebb and combined flood portions of the tidal cycle. Results Geographical information on movements During May and September 1986 and May 1987 a total of eight herring were tracked in the southeastern area of the upper St Lawrence estuary (Figure 1; Table 1). The fish were released in the afternoon during flood tide, except for one released at night (herring 4) and one released during ebb tide (herring ). Track duration ranged from one hour to over 7 hours and net direction ranged between 1 to 338 relative to the direction of the estuary (positive is clockwise with the origin at 3 T). The net distance travelled ranged from to 7.1 km. Net distances travelled over the ground represented less than 4% of the gross distances, with the exception of 6.% and 8.7% for herring 1 and, respectively. Differences between net and gross distances reflect changes in direction and speed of herring.

5 19 K. N. Lacoste et al. Herring 1 (/6/86) Herring (/19/86) :3 1 1:4 :44 13: Riviere-du- Loup Riviere-du- Loup 1 km Herring 3 (/6 7/86) ^ Ile Les Pelerins ^Ile aux Lievres : Notre-Damedu-Portage : km Herring 4 (9//86) ^ Ile Les Pelerins ^Ile aux Lievres :31 1:3 Notre-Damedu-Portage Riviere-du- Loup km Figure. Movements of herring 1,, 3, and 4 released along the south shore of the upper St Lawrence estuary. The position of the fish is plotted on the hour as well as at hourly intervals with circles corresponding to tidal phase ( Flood, Ebb). Times of start and end of tracking are given (dates as month/day/year). The direction of movement is indicated by arrows. Depth contours are in metres. Herring released near-shore in 1986 made progress upriver by following the coastline in shallow waters < m deep (herring 1,, 3, and 4: Figure ). Herring tagged near I le aux Lièvres in 1987 remained in the proximity of the island (herring, 6, and 7: Figure 3). Release site and movements of herring 8 were different from the other herring (Figure 3). Upon release in the South Channel, farther away from shore than the others, it travelled towards the south shore and then along the shore during the last hours of tracking. Interestingly, after the termination of continuous tracking of herring 4 due to bad weather, its signal was picked up again the next day based on a prediction of its position from its average upriver progress rate. The fish was relocated at the western end of the I les Les Pèlerins (Figure 1) in shallow waters approximately km upriver from its last recorded position. Movements over the ground Examination of herring tracks according to tidal phase (Figures and 3) suggests a general fit between the direction of movement over the ground and the prevailing tidal currents, with the exceptions of the erratic movements of herring 6 and the shoreward movement of herring 8. Stick diagrams of over-the-ground velocities of herring (raw data) also indicate that tidal phase strongly determined the direction in which herring travelled at release and afterwards (Figure 4). The direction in which the fish initially travelled was generally related to tidal phase (Table 1); most fish released at flood tide travelling upriver while the only fish released at ebb tide (herring ) travelled down river. However herring 3 and 7 did not travel in the tidal direction immediately following their release. This was probably due to the timing of their release, which occurred within one hour before or after current reversal (Figure 4). All other herring were released more than 1. h after current reversal. Throughout the tracking period the agreement between fish direction and tidal current direction can also be assessed from the stick diagrams (Figure 4). However, there appears to be a lag between fish direction and tidal direction for herring 3, 4, and. This lag

6 Influence of tidal streams on pre-spawning Atlantic herring 191 Herring (//87) ^Ile aux Lievres ^Ile du Pot l'eau-de-vie a 1: Herring 6 (/16/87) ^Ile aux Lievres :6 13: ^Ile du Pot l'eau-de-vie a 18: km km Herring 7 (/18/87) 47 1 ^Ile aux Lievres :6 13:4 ^Ile du Pot l'eau-de-vie a Herring 8 (/ 1/87) :3 South Channel ^Ile les Pelerins 6: km km Figure 3. Movements of herring, 6, 7, and 8 released along the south shore of I le aux Lièvres and near the South Channel. Details as in Figure. could be a result of the fish continuing to swim upriver after the tidal streams change direction until the tidal streams exceed its mean swimming speed. Circular statistics analysis of over-the-ground movements during a flood tide indicates that herring 1, 3, 4, and were travelling upriver along with tidal currents (Figure ). Movements of herring 6, 7, and 8 were more dispersed during the flood tide since their ellipses include the origin. The results from herring 8 are somewhat misleading since it swam towards the coast during the first flood tide while travelling upriver along the south shore during the second flood tide (Figure 3). During an ebb tide analysis of over-the-ground movements shows that herring tended to travel downstream (Figure 6) as expected from Figures and 3. However, movements were defined by a large ellipse indicating that they were dispersed, especially in the case of herring 3, 4, and 7. The net progress-rate calculation revealed a net upriver movement of the fish during a flood tide while the opposite prevailed during an ebb tide (Table ). For herring 3, 4, and, which travelled upriver during flood tides, the progress rate during a flood tide was greater than the progress rate in the opposite direction during an ebb tide, resulting in a net overall upriver displacement ranging from.4 to.71 m s 1 (Table ). Upriver movements during a flood tide also corresponded to higher gross speed (Table ) and higher over-the-ground speed than those during an ebb tide (Table 3). Movements through the water Comparisons of current velocities with velocities over the ground revealed that herring did not passively drift with the current but rather swam actively (Figures 7 and 8). In fact herring often swam and were oriented against the flow, this being especially the case during ebb tides. During flood tides, herring (1, 3, and 4) swimming near the coast swam more often with the current while herring (, 6, and 7) swimming near the island as well as herring 8 swam more often facing the current (Figure 7). Herring 1 was oriented and swam upriver while herring 3 was also oriented but toward the upriver/centre of the estuary. Herring 6 swam in an oriented fashion downriver while herring also swam in an oriented fashion toward the island, across the

7 19 K. N. Lacoste et al Herring 1 Herring Herring Herring 6.4 Over the ground velocity (m s 1 ) Herring 3 Herring Herring 4 Herring Time of day (h) Time of day (h) Figure 4. Over-the-ground velocity vectors of herring in relation to tidal cycle. Stick length indicates swimming speed while stick angle indicates the fish s heading. The vertical axis follows the longitudinal axis of I le aux Lièvres; upriver movements are represented by sticks oriented towards the bottom of the page. Solid horizontal bars on the x-axis indicate tidal cycle (above axis: ebb; below axis: flood). Tracking data of each fish are presented in chronological order. 4 8 current. Similarly, herring 4 appeared to swim upriver and herring 7 and 8 appeared to swim downriver, but changes in direction did occur such that their overall movements were not oriented as shown by their ellipses (Figure 7). Through-water mean vector speeds during flood tides indicate that herring were swimming at similar speeds (.448 m s 1, s.e.=4, n=7) regardless of whether they were swimming with or against the tide (as indicated by direction in Table 3). During ebb tides, herring 3, 4, 6, and 7 swam in an oriented fashion upriver against the tide while herring,

8 Influence of tidal streams on pre-spawning Atlantic herring 193 Over the ground speed (m s 1 ) (northwest-southeast) (1) () (6) (3) (7) Over the ground speed (m s 1 ) (northeast-southwest) (4) (8) Figure. Over-the-ground velocities of herring calculated for successive -minute intervals during flood tide. The x- and y-axes correspond to 3 T (I le aux Lièvres axis) and 3 T, respectively. The number at the top left of each graph corresponds to the herring number. The mean vector is in bold. The standard ellipse represents the degree of dispersion of vectors. Over the ground speed (m s 1 ) (northwest-southeast) () () (6) (3) (7) (4) (8) Over the ground speed (m s 1 ) (northeast-southwest) Figure 6. Over-the-ground velocities of herring during a ebb tide. Details as in Figure., and 8 swam in an oriented fashion across the current toward the shore of the estuary (Figure 8). As a result all fish remained in the same general location during this phase of the tide (Figures and 3). Through-water mean vector speeds during ebb tides indicate that the seven herring were swimming against the tide at similar speeds (.447 m s 1, s.e.=4) and in similar directions (Table 3). Vertical movements In 1987 the tracked herring were equipped with depthsensitive transmitters. These fish did not vary their depth

9 194 K. N. Lacoste et al. Table. Gross speed and upriver progress rate of herring relative to 3 T during flood and ebb tides. Area of travel Fish number Duration (hh:mm) Gross speed (m s 1 ) Upriver progress rate (m s 1 ) Flood Ebb Flood Ebb Net South shore 1 1: : : : I. Lièvres : : : South Channel 8 14: Note: Herring 1 was not tracked during ebb tide while herring was not tracked during flood tide. Table 3. Over-the-ground (O.G.) and through-water (T.W.) speeds and directions of mean vectors for herring tracked during flood and ebb tides. See also Figures 8. Fish no. Speed (m s 1 ) Direction (3 T) Flood Ebb Flood Ebb O.G. T.W. O.G. T.W. O.G. T.W. O.G. T.W Note: Herring 1 was not tracked during ebb tide while herring was not tracked during flood tide. according to tidal phase (Figure 9). Instead they generally maintained a constant distance from the bottom but this distance varied among fish. Mean fish-depth was.6 m (s.e.=.18) and ranged from to. m while mean fish distance off the bottom was 3.84 m (s.e.=1.17). Discussion Catch data (Côté and Powles, 1978) and tag returns (Greendale and Powles, 198) indicate that herring travel upriver during their pre-spawning migration into the St Lawrence estuary. This study is the first to describe movements of clupeoids in relation to tidal currents and is also the first to describe pre-spawning behaviour of herring in that estuary and is therefore of some importance despite its small sample size. In 1987, Munro et al. (1998) discovered a major spawning ground along the north side of the southwestern tip of I le aux Lièvres for which they provided a comprehensive description. They located the spawning site only three weeks after herring were tracked near I le aux Lièvres in 1987 (this study). Therefore it is likely that the herring tracked near the island in 1987 formed a pre-spawning aggregation (Dragesund, 198; Haegele and Schweigert, 198; Hae, 198) which remained in the area until favourable spawning conditions occurred (see location of I le aux Lièvres spawning site on Figure 1). This site does not seem to be used every year. According to larval concentrations in the Pot-à-l eau-de-vie Channel, herring eggs were probably abundant there in 198 and 198 (Fortier and Gagné, 199). The spawning ground also seemed to have been used in 199 according to the local concentration of belugas and other recognised egg consumers (Lesage and Kingsley, 199). However, eggs could not be found at the tip of I le aux Lièvres in 1996 (Bédard et al., 1997) nor in 1998 (J.M., unpublished data). Although no spawning sites have yet been found along the south shore of the St Lawrence most islands along the southern littoral, from Rimouski to Kamouraska, are also believed to be used as spawning sites concurrently with or alternately to that of the I le aux Lièvres. Among these sites the I les aux Pèlerins area

10 Influence of tidal streams on pre-spawning Atlantic herring 19 Swimming speed (m s 1 ) (northwest-southeast) (1) (3) () (6) (7) (4) (8) Swimming speed (m s 1 ) (northeast-southwest) Figure 7. Through-water velocities of herring during a flood tide. Details as in Figure. Swimming speed (m s 1 ) (northwest-southeast) () 1. () (3) (6) (7) (4) (8) Swimming speed (m s 1 ) (northeast-southwest) Figure 8. Through-water velocities of herring during an ebb tide. Details as in Figure. is thought to be the most important one, comparable, in fact, to the I le aux Lièvres site (Munro et al., 1998). Concentrations of newly hatched larvae were reported downstream of I les Les Pèlerins in June 1973 and 1974 (Able, 1978), and in 1978 (Auger and Powles, 198). Concentrations were also found directly downstream from Pointe aux Orignaux and I les Kamouraska in June 1974 (Able, 1978), i.e. upriver from I le les Pèlerins. The presence of other spawning sites along the south shore of the Estuary, in particular the I les Les Pèlerins and I les Kamouraska sites, might explain why herring tracked near the coast (1986) swam upriver and parallel to the coastline. The tracked herring adopted different swimming behaviours depending on the area where they were tracked. Those travelling near the coast were generally travelling upriver regardless of tidal phase and swam actively with the current at flood tide and actively

11 196 K. N. Lacoste et al. Depth (m) Time of day (h) Herring Herring Seafloor Herring 6 Herring 7 Herring 8 Figure 9. Depth of herring tracked in relation to tidal cycle and seafloor. Solid horizontal bars on x-axis indicate tidal cycle (above axis: flood; below axis: ebb). Note (1) identifies maximum seabed depth (up to 3.1 m) reached in tracking of herring. against it during ebb tide to counteract the effects of the downstream current. This swimming behaviour allowed them to take advantage of flood currents and to travel considerable distances upriver (e.g. herring 3 travelled close to km in a single day while herring 4 travelled close to 3 km in 1. days). In contrast the fish near the island (1987) swam in an oriented fashion against tidal currents during both phases of the tide. The net result of such swimming behaviour was that the fish remained in the same location. In contrast, herring tracked near the southern shore of the estuary seem to have been migrating upriver towards an unknown spawning site. 8 1 Maximum current speeds in the study area are of the order of 1 m s 1 (Canadian Hydrographic Service, 1997). Currents.4 m s 1 (the mean swimming speed of tracked herring) opposing upriver movements occur during the 3 h before low tide. At that time the fish did not swim fast enough to completely counteract the effects of the tide, which resulted in net loss of ground. Since swimming speeds were virtually constant during the period of tracking, regardless of tidal phase, the herring probably swam at preferred speeds in order to minimise energy consumption thus optimising endurance during migration. They swam at speeds much lower than m s 1, which is their estimated maximum sustainable swimming speed (i.e. the maximum swimming speed the fish can maintain for minutes; He and Wardle, 1988). They actually maintained an average speed of.44 m s 1, well within the range of preferred swimming speeds (.3 6 m s 1 ) reported by He and Wardle (1988). Herring swimming near the island showed no signs of using selective stream transport as they were swimming against tidal streams during both phases of the tide, presumably to maintain position. In contrast herring tracked near the coast would have seemed more likely to use selective tidal stream transport since they did not appear to be close to their spawning site and thus presumably were still en route. Typically, selective tidal stream transport is defined by two components: the use of tidal currents to assist in horizontal movements when currents flow in the migratory direction and a vertical movement to hold position on the bottom when currents flow in the opposite direction. Herring tracked near the coast appear to have made use of tidal currents to facilitate their upriver migration but without resorting to selective tidal stream transport. During flood tides, they swam with the transporting tide while during ebb tides, they swam against tidal currents and seemed more oriented than during flood tides. Herring 8, the only herring tracked near the coast with a depth-sensitive transmitter, did not perform semidiurnal vertical migrations synchronised with the tides. It seems unlikely, in any case, that herring migrating near the coast would make regular vertical movements synchronised with tides since the area is characterised by shallow depths (< m) with vertically homogeneous currents (Canadian Hydrographic Service, 1997; Saucier and Chassé, ). Although the herring did not change depth in relation with the tides, tracks of fish near the coast showed some behaviour patterns similar to those in studies demonstrating selective tidal stream transport in other fish species (e.g. Greer Walker et al., 1978). Ground speeds were low and tracks convoluted when tides were against the direction of fish migration, for example, while ground speeds tended to be high and tracks straight when tides were in the migratory direction. Furthermore the

12 Influence of tidal streams on pre-spawning Atlantic herring 197 tracked herring did not drift passively with the current but rather swam actively, either with the current if it was in the migratory direction or against the current if counter to the migratory direction. Selective tidal stream transport is a migratory behaviour that is energetically advantageous in areas of strong currents (Weihs, 1978; Metcalfe et al., 199). When considering the lengths of our herring and current speeds in the study area it seems that selective tidal stream transport would have been energetically more efficient than continuous swimming (see Figure in Metcalfe et al., 199). It is possible that only those species ecologically associated with the bottom and that can take advantage of the weaker currents of the bottom-boundary layer by holding position on the bottom, will use selective tidal stream transport. As a pelagic species herring is not usually associated with the bottom, although it is a bottom spawner. Swimming behaviour was characterised by changes in fish heading associated with changes in the direction of tidal streams. It seems unlikely that fish were in frequent visual contact with the bottom since they were in turbid waters of the St Lawrence estuary and changed headings at night as well as during daylight hours. This suggests that herring can orient according to tidal flow by non-visual clues, as has been suggested or demonstrated before in other species (e.g. Metcalfe et al., 1993). In summary, there is no evidence of tidally synchronised movements normally associated with selective tidal stream transport (with the important caveat that there was only one depth-sensitive transmitter among the fish moving along the coast). Herring migrating up the estuary swam in the same direction on both ebb and flood tides gaining more ground on flood tides than they lost on the ebb. 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