Smolt migration through the River Dee and harbour. January 2018

Transcription

1 Smolt migration through the River Dee and harbour January 218 1

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3 Executive Summary To investigate the extent and cause of in-river and estuarine mortality of salmon smolts in the River Dee, 11 smolts from the upper and lower Dee catchment were tagged and tracked in 217. Mortality was high - 7% - for smolts from the upper catchment, and lower, but still significant - 13% - for smolts from the lower catchment. This equated to an overall mortality rate of.45% per km migrated. It is thought that mortality was due to predation. Smolt losses occurred in the middle and the lower river, where predator densities are greatest. The timing and location of smolt losses showed that tagged fish were surviving for, on average, at least 12 days after they were tagged, suggesting that tagging/handling was not the direct cause of mortality. However, it is considered that smolts may be made more vulnerable to predators by being tagged, and therefore it is possible that these levels of mortality may be higher than that occurring in the untagged smolt population. Although there were no confirmed losses in the harbour, as all tags were detected exiting the harbour, the behaviour of six tagged smolts (11%) was unusual and could be due to the tagged fish being eaten by a predator, and hence it was the movements of the predator that was detected. Total in-river and estuarine mortality is therefore estimated as 48%. Smolts from the upper catchment typically spent 2 days migrating through the river to the harbour, whilst it took smolts from the lower catchment less than one day. Because smolts from the upper river spent longer in the main stem Dee, they were therefore vulnerable to in-river predation for substantially longer than smolts produced in the lower catchment. The Spring of 217 was exceptionally dry, and it is unclear whether smolt migration was influenced by the unusual conditions (e.g. by delaying migration), which may have a bearing on susceptibility to predation. Therefore in 218 this study will be repeated to determine whether 217 mortality levels are standard or not. From a management perspective, this is the second study (following a pilot study in 216) that highlights a potentially high predation pressure on salmon smolts in the Dee. Although not conclusive, this work has been acknowledged by agencies and discussions are underway on what measures can be introduced. This tracking study has encouraged further investment on the Dee, and in 218, alongside the third year of this tracking study, there will be a larger study to investigate marine migration pathways of smolts. 3

4 Introduction Around the Atlantic, salmon mortality can be high when smolts first enter the marine environment (Kocik et al 29, Thorstad et al 212). This may be due to predation, and is influenced by the availability of food for post-smolts, as this sets the rate at which they can outgrow predation risk. With the recent decline in salmon stocks on the Dee ( ), the Dee s Fisheries Management Plan ( ) took a new focus to investigate the estuarine and coastal environment, where risk of mortality is thought to be greatest. The plan set out to: 1) Quantify predation impacts on smolts, 2) Identify timings of smolt migration and their presence in the lower river and harbour area, 3) Establish near-shore habitat use of smolts and migration patterns through the estuary. In 216, acoustic tagging and tracking of salmon smolts on the Dee was used to investigate migration and survival of salmon smolts in the lower river and harbour. Surprisingly, it highlighted smolt losses occurred within the river, but no losses occurred within the inner harbour. In total, 26% of the tracked smolts failed to reach the harbour in 216, and this was thought to be due to either the impact of tagging or in-river predation (Smolt migration through the lower Dee and inner harbour, River Dee Trust 216). To investigate the cause of in-river losses, in 217, additional fish were tagged in the upper catchment (at the Baddoch smolt trap, which is operated by Marine Scotland Science), as well as at the Beltie and Sheeoch smolt traps in the lower catchment. This was to investigate smolts losses throughout the length of main stem river and determine the likely cause of mortality: losses due to the impact of tagging would be expected to be high initially after tagging, but decline over time and as the fish migrated downstream. On the other hand, mortality due to predation would increase later on during the migration as smolts moved further downstream, where densities of predators are greater. The tracking of smolts within Aberdeen Harbour was also extended in 217, following reliable performance of the acoustic receivers in the harbour in 216. Aberdeen Harbour is the busiest port in the UK, and shipping traffic and harbour works could potentially interrupt smolt migrations. In addition, predators (seals, birds, estuarine fish, dolphins) are abundant in the harbour. Methods Acoustic telemetry Acoustic tags and receivers manufactured by Vemco were used for the study. The V5 tags transmit a sound every 3 seconds (randomly generated at second intervals). The sound produced by each tag is a combination of 8-1 distinct pulses that give the tag a unique code, so that individual fish can be identified. The V5 tags are 12.7 x 4.3 x 5.6 mm in size with a weight (in air) of.65g. Telemetry guidelines suggest that tags should be no greater than 5-6.5% of the fish s weight to avoid adverse effects of tagging (Prentice et al 199, Adams et al 1998, Anglea et al 24). The smolts tagged in this study were g (average 2 g) and tag weight represented % (average 3.2%) of smolt body weight. V5 tags have a 95% battery life of 77 days and power output of 143 db, presenting a maximum detection range of approximately 3 m (albeit very dependent on background noise levels). The V5 4

5 tag is the second smallest tag currently available, with the smallest tag considered to have an insufficient power output to ensure tag detection against the background noise in Aberdeen Harbour. The VR2W acoustic receivers used in this study detect the sounds from the acoustic tags on the 18 khz frequency. The receivers are placed underwater and make an automatic record each time a tag is detected, recording the tag identification number, date and time, which can then be downloaded from the receiver via Bluetooth once the receiver is retrieved from the water. The receivers were placed underwater in the river and harbour, weighted onto the river bed with anchor weights. In the river, receivers were attached to a metal rod and a 4-kg anchor weight, then roped off to the bank to aid retrieval (Fig. 1). In the harbour, each receiver was attached to rope and 8 2 kg of anchor weight. The rope was held vertical by a sub-trawl float so that the receiver would face upwards in the water column. A second rope held a surface float so that the position of the receiver was known to boat traffic. The anchor weight was roped back to the shore/quayside to ensure it was not lost in heavy storms and to aid retrieval (Fig. 2). Figure 1. Receiver set up for in-river monitoring, ready for underwater deployment. 5

6 Figure 2. Deployment of receiver and mooring in Aberdeen Harbour. Study area Smolts were captured in the upper (Baddoch burn) and lower (Beltie and Sheeoch burns) Dee catchment. Rotary screw traps were used on the Beltie and Sheeoch burns, whilst a fixed trap was used on the Baddoch burn (Fig. 3). The latter trap is operated by Marine Scotland Science (MSS), and MSS personnel tagged the smolts at the Baddoch trap site 1. The Baddoch, Beltie, and Sheeoch traps were 122, 37 and 26.5 km (73, 22 and 16 miles) from the final receiver gate in the harbour, respectively. The Baddoch burn is a significant tributary of the River Clunie, with the trap site being 11 km from the main stem Dee, whilst the traps on the Beltie and Sheeoch burns were located just 28 and 96 m, respectively, above the confluence with the Dee. A total of 19 receivers were used to detect tagged fish. Nine of these were in the river and ten within the harbour (Figs 3 and 4). Because of the channel width and high background noise levels in the harbour, the receivers were paired up to form gates, to increase the likelihood of detecting tagged smolts (Fig. 4). The harbour was monitored as far seaward as the Old South Breakwater, so that the length of the harbour over which smolts were monitored was 1.5 km (i.e. gate 1 to gate 5). This was an extension to the harbour monitoring of.5 km since the 216 pilot study

7 Figure 3. Map of River Dee, showing locations of smolt traps and acoustic receivers. The harbour area highlighted by the box is shown in Fig. 4. 7

8 Figure 4. Map of Dee estuary and Aberdeen Harbour, with locations of acoustic receivers ( ). 8

9 Smolts In total, 11 smolts were tagged: 4 smolts from the Baddoch burn, 3 from the Beltie burn and 31 from the Sheeoch burn. Fish were chosen for tagging randomly, if they were in the length range of mm (average 123 mm). This size range was the average smolt length across all three trapping sites in 216. There was no significant difference in smolt lengths between the three tagging sites in 217 (ANOVA statistical test, P =.164). The body weight of smolts ranged from 16 to 23.5 g (average 2 g): the weight of Baddoch smolts was significantly greater (2.6 g) than lower catchment smolts (19.5 g; ANOVA, P =.2). All tagged fish showed the physical attributes of smolt development (e.g. Fig. 5). The condition of these smolts (Fulton Condition Factor; a measure of an individual fish s health based on weight) was (1.65 ±.8; mean ± SD) and was significantly better for Baddoch smolts (average 1.1) than lower catchment smolts (1.4; ANOVA, P =.4). Smolts were tagged between 4 and 27 April. Due to the dry spring in 217, tagging was restricted to a few days when flows were higher and most smolts entered the traps (smolts tended not to move in the tributaries during low-flow conditions). Therefore 8 out of the 11 smolts were tagged in one high-flow event between 25 and 27 April. Figure 5. Salmon smolt showing silver colouration with loss of parr markings, streamlined body and black edges to fins. 9

10 Tagging Smolts were tagged close to the river to reduce handling and transport. The surgical procedure was carried out on a table using sterile equipment that was re-sterilised between each fish. Only staff that had been trained and demonstrated post-training competence carried out the procedure. Smolts were anaesthetised using MS-222 until they were heavily sedated. Each smolt was measured, weighed and photographed prior to the tag being inserted. The tag was inserted into the body cavity via a cut made into the belly of the fish and then the cut was closed using two sutures. The smolt was then placed into a recovery unit for a minimum of two hours, until it appeared to be fully recovered and was showing startle responses. Smolts were then released into the burn, along with other untagged smolts captured in the trap, approximately 1 m downstream of the trap. The Standard Operating Protocol worked to was based on guidelines from the Atlantic Salmon Federation. To minimise handling and stress of these fish, scale samples to age fish were not taken. Based on scale sampling of smolts caught in the traps in 216, it would be expected that these smolts, of mm fork length, would be two years old. Data and analysis Much of the subsequent information on the tagged smolts is summarised using the median value, instead of an average or mean. This simply reflects that the factor being reported on (e.g. time taken for migration) was heavily skewed and therefore the median (the middle value) better reflected the typical smolt than the average value. Various statistical analyses were done to interpret the data and the type of test used is always reported on: T-tests were used to compare differences between two groups of fish - such as between surviving and non-surviving smolts to identify what causes differences in behaviour or survival. Similarly, ANOVA was used to compare differences between three groups of fish (e.g. Baddoch, Beltie and Sheeoch groups). To determine what factors influence migration speed and timing of migration, stepwise regression models were used. For these models, each factor that potentially effects migration speed/timing e.g. smolt size, date of tagging - is added to the model, until a best fit model is produced. This process selects the factors that have the greatest influence on migration speed/timing. The model uses data collected for individual fish, so that the migration speed of any smolt is related to its characteristics (body length, date of tagging) and environmental conditions during its migration (photoperiod and river flow it experienced during its migration). River flows were obtained from SEPA s gauging station at Park (Fig. 3). This records discharge (cubic metres per second, m 3 sec -1 or cumecs ) every 15 minutes. Photoperiod was the number of minutes of daylight each day at Aberdeen, based on daylight starting 3 minutes before sunrise and continuing until 3 minutes after sunset. The strength of these statistical tests are reflected with the P-value. P-values weigh up the strength of the evidence from the data: A P-value will be between and 1 and the most crucial point is the.5 level - a P-value less than.5 indicates that the test has found strong evidence of a real effect or relationship in the data, with only a 5% chance that this could have occurred randomly. 1

11 Total mortality (%) Findings Fish detection 9 of the tagged smolts (89%) were detected subsequently by receivers. It is assumed that the 11 smolts (11%) that were never detected died because of delayed tagging effects. Delayed mortality may occur for hours after tagging (C. Adams, pers. comm.). The 11 fish that died were of similar size (average mm length, 2.1 g weight) to the surviving fish (123.4 mm, 19.8 g; t-test, P >.5), they were from all three tagging sites, tagged during the same average water temperature and tagged by different personnel. There was no difference in condition factor of these 11 fish (1.77) compared to survivors (1.56; t-test, P =.43). The remainder of the analysis is based on the 9 detected smolts. Loss rates Total mortality of upper catchment (Baddoch) smolts during their main stem migration was 7%, which was much greater than lower catchment smolts: Beltie smolt mortality was 18%, and Sheeoch smolt mortality was 8% (Fig. 6). These mortality rates were based on main stem migration distances of 17 km for Baddoch smolts, 29 km for Beltie smolts and 14 km for Sheeoch smolts. The overall loss or mortality rate in the river, for all 9 smolts tracked, equated to.45 % km -1 (per km), i.e. there was a.45 % chance of a smolt dying for each 1 km of river it travelled through. Smolts from the Baddoch had a mortality rate of.57 % km -1, whilst mortality of smolts from the Beltie was.48 % km -1 and from the Sheeoch it was.3 % km -1. Losses of Baddoch smolts was highest at 6-9 km downstream from their release site (Fig. 6), which corresponds to between Craigendinnie (above Aboyne) and Lower Crathes (below Banchory). The loss rate in this area was 1.6% km -1 travelled. Further losses of smolts from all three tagging sites occurred in the lower river between Culter and Waterside, at a rate of.76% km Baddoch 8 Beltie 6 Sheeoch Distance downstream (km) Figure 6. Total cumulative mortality (%) of smolts from detection on the first main stem receiver until exiting gate 5. 11

12 Mortality (%) Goosander numbers Counts of goosanders are done monthly during Winter and Spring on the Dee. The river is canoed and counted in four sections: 1. Feardar burn Dinnet burn 2. Dinnet burn Banchory Bridge 3. Banchory Bridge Waterside 4. Waterside Harbour Counts since 29 show that during the Spring (April and May), the number of goosanders is greatest in the middle and lower river (sections 2 and 3), and low in the upper river and tidal waters, which corresponds to the overall locations of smolt losses (Fig. 7) Section Section Section 1 Section Distance downstream (km) Figure 7. Smolt mortality (Baddoch, Sheeoch and Beltie combined) at each receiver ( ) and average number of goosanders counted in April and May 217 in each river section ( ). What influenced survival of smolts to the harbour? Overall, 57 (63%) smolts made the journey to the harbour whilst 33 (37%) smolts failed to reach the harbour. There was no obvious factor influencing whether a smolt survived or not: There was no significant difference in weight of survivors (19.8 g) and non-survivors (2.1 g; t-test, P=.43) or length (both 123 mm; t-test, P =.47). There was no significant difference in the Fulton condition factor of survivors (1.56) and nonsurvivors (1.76; t-test, P =.22). Tagging date was similar for both survivors (21 April) and non-survivors (22 April; t-test, P =.45). There was no difference in the time taken to reach the first receiver after being tagged, for surviving and non-surviving smolts from the Baddoch (5 days, 14 hours and 5 days 11 hours, respectively; P =.96) or for smolts from all sites together (7 days 13 hrs and 5 d, 12 hours; P=.24) (note there were too few non-survivors from the Beltie or Sheeoch to test these sites individually). 12

13 The only significant difference between survivors and non-survivors out of all the factors recorded was water temperature on the day of tagging, which was warmer for survivors (5.2 C) than non-survivors (3.5 C; t-test, P <.1). This may be due to most of the non-survivors being from the Baddoch, which had lower water temperatures than the other two sites, and not for any biological reason. Journey times As smolts are expected to take time to recover from tagging before returning to normal migration, the time taken for smolts to reach the first receiver (between 9 and 15 km below the tagging sites) was discounted from the rest of their in-river journey. Journey times are therefore measured once the fish is recorded on the first receiver. The Baddoch smolts typically spent 2 days (median value) in the river, from the first receiver to the tidal waters in the lower river (Waterside receiver). This ranged between individual fish from 7 to 28 days. In contrast, the Beltie smolts typically took 8 hours (range from 4 hours - 12 days) to reach the tidal waters, whilst the Sheeoch fish took 22 hours (range from days). This variation is despite most smolts being tagged within a three-day period. Smolts from both the upper and lower catchment spent little time in the tidal part of the river (a 4.5 km stretch), typically moving through it within 3-6 hours. Migration through the harbour The time smolts spent in the harbour was brief: from arriving at gate 1 to leaving at gate 5 (a distance of 1.5 km), median journey time was 1 hour and 17 minutes, equivalent to a migration speed of 1.2 km hr -1 ; km per hour), with the quickest smolt taking just 37 minutes (2.4 km hr -1 ). However, there were a few fish that spent a lot longer in the harbour and showed unexpected movements. Five smolts spent between one and six days in the inner harbour, being detected on different receivers which suggested that they were swimming back and forth. All five fish did subsequently exit through the final gate and were not recorded again. A sixth smolt moved through the habour initially but then spent 28 hours being recorded at the final gate. Furthermore, this was during the high flow period of April, making it seem unlikely that a smolt would choose to hold station here. It is thought likely that the vastly different behaviour of these six fish (median time in harbour was 4 days, 19 hours) compared to the other tagged smolts (median time 1 hour 9 min) was because the smolts had been consumed by a predator and the tags were showing the predator s movements. Beltie smolts reached the harbour significantly earlier (median 28 April) than Sheeoch (6 May) and Baddoch (17 May) smolts. The date that a smolt arrived at the harbour was influenced by river flow (mean daily discharge during the fish s migration; Fig. 8) and the day they started their main stem migration (date of arrival at the first receiver). Together, both factors explained 62% of the variation in arrival dates at the harbour (Stepwise regression, P <.1). No other factors (body weight, condition factor, migration speed, tagging date) were helpful in explaining harbour arrival date. 13

14 27-Apr 28-Apr 29-Apr 3-Apr 1-May 2-May 3-May 4-May 5-May 6-May 7-May 8-May 9-May 1-May 11-May 12-May 13-May 14-May 15-May 16-May 17-May 18-May 19-May 2-May 21-May 22-May 23-May 24-May 25-May 26-May Number of fish arriving at harbour Mean flow at Park (cumecs) Total fish Park flows Figure 8. Arrival date at the harbour (gate 1), compared to river flows (line). Smolts exited the final gate in the inner harbour between 28 April and 26 May. The main smolt migration through the harbour, when between 25% and 75% of the smolts moved (standard for migration time described by Malcolm et al 215), was 28 April - 8 May. Migration speeds In-river migration speeds depended on where the smolts had originated from: Smolts from the Beltie burn moved very rapidly, at around 2.8 km hr -1, whereas smolts from the Baddoch and Sheeoch moved much slower, at.22 km hr -1 and.39 km hr -1, respectively (Fig. 9). In both the tidal river (downstream of Waterside) and the harbour, the swimming speeds of the Baddoch and the Sheeoch smolts increased ( km hr -1 ), whilst the Beltie smolts slowed down ( km hr -1 ). Overall, speeds through the tidal river and harbour were.8 and 1.2 km hr -1, respectively. Migration speed of the smolts in the river (from first receiver to gate 1) was most influenced by river flow (mean daily discharge during each fish s migration) and explained 41% of the variation in migration speeds (Stepwise regression, P <.1). None of the other tested factors had a significant influence on migration speed (tagging date, body weight, condition factor, arrival date at first receiver, photoperiod). 14

15 Migration speed (km per hour) Migration speed (km per hour) Migration speed (km per hour) 7 6 Baddoch River Tidal river Harbour Trap site to first receiver 1st receiver to Waterside Waterside to Gate 1 Gate 1 to Gate Beltie River Tidal river Harbour Trap site to first receiver 1st receiver to Waterside Waterside to Gate 1 Gate 1 to Gate Sheeoch River Tidal river Harbour Trap site to first receiver 1st receiver to waterside Waterside to Gate 1 Gate 1 to Gate 5 Figure 9. Speed of migrating smolts (km per hour). 15

16 1-Apr 5-Apr 9-Apr 13-Apr 17-Apr 21-Apr 25-Apr 29-Apr 3-May 7-May 11-May 15-May 19-May 23-May 27-May 31-May Mean daily flow (cumecs) However, whilst most smolts at all tagging sites were tagged prior to a rise in water, only smolts in the Beltie migrated during the subsequent high flow. In contrast, Baddoch smolts reached the first receiver during the high flow, but subsequently moved slowly, whilst Sheeoch smolts did not respond to the rise in water and suffered a delay of several days before leaving the tributary and arriving at the first receiver. River flows 217 was an exceptionally dry spring, with average daily flows in April - May being approximately 4% of long-term average flow levels (SEPA, Average daily flows at Park in April and May 217 were 18. cumecs, compared to 53.4 cumecs in 216 (Fig. 1). During April and May 217 there was only a single rise in water, from April. The three gauging stations (Mar Lodge, Woodend, Park) corresponding best to the three tagging locations all showed a similar flow response (Fig. 1) and therefore to simplify, the flow rates at Park were used as an indicator of flow for the movements of all tagged fish Mean daily flow at Park Mean daily flow at Woodend mean daily flow at Mar Lodge mean daily flow at Park 216 Figure 1. River flows (in cubic metres per second; cumecs) during the 217 smolt run. The low flows before 27 April meant that there were few fish moving out of the tributaries, and therefore available to tag. The single high flow event in the spring seemed to trigger nearly all fish to move, allowing us to tag fish. Further evidence that the unusually low river flows in spring 217 delayed early migration of smolts was seen in the Baddoch burn, where it was possible to tag a few fish before the high flow event: The nine smolts that were tagged early in April typically took 25 days (median value) to reach the first receiver at Lower Invercauld. In contrast, the 28 smolts that were tagged at the end of April took 2 days and 5 hours (median) to reach Lower Invercauld (t-test, P <.1). The relationship between flow and main stem smolt migration varied between location (Fig. 11): Baddoch smolts showed movement patterns that followed flow, whilst Beltie smolts used the high flow event to move out of the river quickly, therefore providing only a narrow window for monitoring their movements. In contrast, Sheeoch smolts did not move in the high flows after they were tagged, but delayed migration for five days and then migrated in low flow conditions. 16

17 1-Apr 5-Apr 9-Apr 13-Apr 17-Apr 21-Apr 25-Apr 29-Apr 3-May 7-May 11-May 15-May 19-May 23-May 27-May 31-May Number of fish arriving at receiver Mean daily flow at Park (cumecs) 1-Apr 5-Apr 9-Apr 13-Apr 17-Apr 21-Apr 25-Apr 29-Apr 3-May 7-May 11-May 15-May 19-May 23-May 27-May 31-May Number of fish arriving at receiver Mean daily flow at Park (cumecs) 1-Apr 5-Apr 9-Apr 13-Apr 17-Apr 21-Apr 25-Apr 29-Apr 3-May 7-May 11-May 15-May 19-May 23-May 27-May 31-May Number of fish arriving at receiver Mean daily flow at Park (cumecs) Baddoch L Invercauld Monaltrie Craigendinnie L Woodend Culter Waterside Boating Club Gate 1 Mean daily flow at Park Beltie Culter Waterside Boating Club Gate 1 Mean daily flow at Park Sheeoch Culter Waterside Boating Club Gate 1 Mean daily flow at Park Figure 11. Smolt movements (initial detections at each receiver; bars) compared to river flows (line). 17

18 Number of smolts Diurnal patterns In-river migration was predominantly nocturnal, and upstream of the tidal zone, between 81 and 1% of fish detections on each receiver occurred during night time. Diurnal movements occurred later in the spring (median date 3 May) than nocturnal movements (median date 28 April; t-test, P <.1), a pattern that was also seen in 216. Once smolts moved into the tidal zone and harbour, diurnal migration increased (also seen in 216), accounting for approximately 5% of all recordings on the receivers. In the tidal zone of the river, the preference for diurnal migration did not differ over the spring period. In the harbour, there was a preference for diurnal migration earlier in the spring (median date 28 April) and nocturnal migration increased later in the spring (4 May; P <.1). This differed to that observed in 216, when diurnal migration increased during the spring, and may be due to the overriding influence of the single highflow event in 217. Tidal influences Smolts entered the harbour (gate 1) throughout the tidal cycle, although more smolts entered during a falling tide (72%) than a rising tide (28%; Fig. 12). This was not evident in 216, when slightly more smolts arrived preceding low tide. It is also possible that tidal influences are a by-product of fish moving during the high flows on 28 April Hours before high tide Figure 12. Number of smolts arriving at the harbour (gate 1). Black vertical line represents low tide. Receiver detection performance Maximum efficiency of each receiver was estimated based on the number of smolts that the receiver failed to detect (i.e. smolts that were subsequently detected on receivers further downstream), relative to the total number of smolts that could have been detected. Based on this, most receivers (16 out of 19) had 95-1% maximum detection efficiency. However, there were two receivers in the river (at Abergeldie and Lower Crathes) that had substantially lower efficiencies (73 and 83%, respectively). It is thought that the location of these two receivers was 18

19 unsuitable, and in the case of the Lower Crathes receiver, may have been exposed above the water surface due to the low flow conditions. One of the receivers in the final gate within the harbour (gate 5) also had a lower efficiency, recording 91% of the smolts. However, given that the width of gate 5 was 16 metres (due to the need to keep equipment out of the shipping channel), we had not expected the receivers to fully monitor this distance. Nevertheless, between the two receivers comprising gate 5, every fish was detected - as was the case with all the other gates in the harbour. Therefore, as in 216, gate efficiency in the harbour was 1%. Conclusions This was the second year of a three year smolt tracking programme on the Dee to investigate the estuarine and coastal environment, where risk of salmon mortality was thought to be greatest. The objectives set out in the Fisheries Management Plan are discussed below. Quantifying predation impacts on smolts 37% of tagged smolts died in the river, equating to a mortality rate of.45% km -1 of river migration. Although no losses occurred in the harbour, it is thought that an additional 11% of smolts were taken by predators, as the movement patterns of these smolts were unexpected. In total, therefore, 48% of smolts did not make the journey from their natal stream and out through the harbour. Due to the efficiencies of receivers in the harbour area, it is assumed that 63% of fish did not reach the harbour and either died in the river or decided not to migrate. The latter is considered unlikely as all smolts were well advanced in the smolting process and all had migrated downstream since being tagged, as they were detected on receivers below the traps. Tag failure rate is less than 2% (for all VEMCO tags; specific failure rate for V5 tag not available) and so would not be expected to account for more than two missing fish in the study. It is therefore considered that loss of tags from the study is due to mortality. Mortality was substantially higher for smolts from the upper catchment (7%) than the lower catchment (13%). The only difference identified between tagged smolts from the upper and lower catchment was that upper smolts were significantly heavier and had higher condition factor, which if anything, should have been advantageous to survival. However, smolts travelling from the upper catchment in 217 experienced much longer exposure to any dangers that are within the river, typically spending 2 days to reach the tidal limit, whereas lower catchment fish reached the tidal limit in less than 24 hours. The 33 fish that did not reach the harbour were tracked for an average of 11.5 days (range 1 34). This demonstrates that fish survived for many days after undergoing the tagging procedure, suggesting that the tagging was not the direct cause of mortality. It is possible that the tagging procedure, or presence of the tag, affected smolt behaviour such as swim speed or manoeuvrability, such that they were more vulnerable to predation. If this was the case, then the tagging could have indirectly caused mortality and the level of mortality in these tagged smolts may not be representative of untagged smolts. However, unlike most acoustic tracking studies, a particularly small acoustic tag was used, which represented about 3% of body weight. Until further work is done on behaviour and survival of smolts in the wild after tagging, indirect tagging effects cannot be ruled out, however, such effects are expected to be smaller than in most other studies. The results suggest that predators are the cause of the smolt mortalities, although the level of mortality may be inflated because of tagging. Most mortality occurred in the middle river (very 19

20 approximately, between Aboyne and Crathes), and the main predator in this area would be goosanders. Goosander densities are also greatest in the middle and lower river, excluding the tidal waters. Mortality in the lower river (between Culter and Aberdeen) may also have been due to predation from seals and kelts. Identifying timings of smolt migration and presence in the lower river and harbour The peak time for smolt arrival at the harbour was 28 April 8 May, just a few days earlier than in 216 (2 1 May). This is surprising, given the very low flows in April 217 that appeared to delay smolts in initiating their migration in the tributaries as evidenced by the lack of fish captured in the fish traps. However, after rainfall in late April, smolts from the Beltie then moved rapidly, and Sheeoch smolts also subsequently moved. Although Baddoch smolts had a late migration time, due to their high in-river mortality they only represented 19% of survivors, and therefore did not have a significant impact on the overall picture of migration timing. The timing of migration and arrival at the harbour differed for smolts from the different tributaries. Smolts from the upper catchment were much later to reach the harbour (17 May) compared to smolts from the two tributaries in the lower catchment (28 April and 6 May). It is possible that this is a result of the unusual flows in 217 and needs to be looked at further, as the significance is that the timing of smolt arrival into the marine environment is thought to be critical in determining survival at sea (Friedland et al 2). Furthermore, as river flow was found to be the crucial factor influencing the timing of migration in both 217 and 216, the potential for climate-related changes to river flow could also have a bearing on timing of smolt arrival in the marine environment and hence survival. Establishing near-shore habitat use and migration patterns through the estuary In both 216 and 217, smolts spent very limited time in the estuarine area: approximately 3-6 hours in the tidal river and 1¼ hours in the harbour. This time probably represents continuous travelling along the total distance of 5.5 km. Despite the short length of time in estuarine water, mortality was thought to be 11%, based on unusual tracks of six tagged smolts that were most likely consumed by predators. The estuarine area would be the area of greatest predation risk, as fish-eating birds (goosanders, mergansers, cormorants), seals and predatory marine fish would be in this area, and this could perhaps explain why smolts spend so little time in this area. Tidal patterns appeared to be related to the timing of smolts entering the harbour in 217, which was not seen in 216. However, this is possibly a coincidence resulting from many fish migrating through the harbour during high flows on 28 April; indeed, as the smolts moved quickly through the estuarine area, this suggests that they did not need to wait for the tidal cycle. 218 programme of work Smolt tracking will continue for a third year in 218. The unusual flow conditions in 217 may have contributed to delayed migration and therefore greater vulnerability to in-river predation, so it is important to repeat this work to determine if mortality rates remain high in 218. As in 217, 1 smolts will be tagged, including 4 smolts from the upper catchment (Baddoch), and 3 smolts from each of the Beltie and Sheeoch tributaries in the lower catchment. Given the emerging picture of the importance of in-river mortality, five extra receivers will be deployed to provide greater insight into where losses occur in the river. Although it is not possible to rule out that tagging the smolts made them more vulnerable to predation, i.e. indirectly caused mortality, there is national and international work ongoing to look at the issue of tagging effects. Through the national Tracking and Telemetry group, this work will be 2

21 pulled together in 218 and will help determine whether the mortality found in the Dee smolts is influenced by the use of tags. A further smolt tracking project will start on the Dee in 218, which is being delivered jointly by the River Dee Trust and Marine Scotland Science, with funding from Aberdeen Offshore Wind Farm Ltd. The focus of this tracking is to determine the migration route of smolts after they have left the river in their early marine migration. 1 salmon smolts will be tagged on the Dee in 218, and will be detected in semi-circular arrays of acoustic receivers installed at distances of 4 km and 1 km from the mouth of the Dee. Further smolts will be tagged in 219 and 22, including salmon and sea trout smolts from the Rivers Don and Ythan, and arrays of acoustic receivers will be extended further offshore to follow migration routes. The information from these tracked smolts will be used by Marine Scotland Science to develop a model that can predict smolt migration pathways from other Scottish rivers, to help establish where the sensitive marine areas for salmon exist. Acknowledgements This work has been possible due to support from various people and groups: Marine Scotland Science, in particular Rob Main helped with tagging and deploying receivers, Aya Thorne, Stephen McLaren and Denise Stirling helped with tagging, and Iain Malcolm and John Armstrong assisted in study design. Aberdeen Harbour Board provided vessel, crew and maintenance staff to deploy and retrieve receivers in the harbour and assist with moorings for the receivers. We benefitted from advice from people with expertise in salmon acoustic telemetry to design this study. Jon Carr (Atlantic Salmon Federation, Canada) trained staff in tagging procedures and offered advice on study design, Dr Matt Newton and Professor Colin Adams (University of Glasgow) undertook range testing and provided advice on study design and equipment. SEPA provided river flow data from their gauging station at Park. 21

22 References Adams NS, Rondorf DW, Evans SD, Kelly JE & Perry RW (1998). Effects of surgically and gastrically implanted radio transmitters on swimming performance and predator avoidance of juvenile Chinook salmon (Oncorhynchus tshawytscha). Canadian Journal of Fisheries and Aquatic Sciences 55, Anglea SM, Geist DR, Brown RS, Deters KA & McDonald RD (24). Effects of acoustic transmitters on swimming performance and predator avoidance of juvenile Chinook Salmon. North American Journal of Fisheries Management 24, Friedland KD, Hansen LP, Dunkley DA & MacLean JC (2). Linkage between ocean climate, postsmolt growth, and survival of Atlantic salmon (Salmo salar L.) in the North Sea area. ICES Journal of Marine Science 57, Kocik JF, Hawkes JP, Sheehan TF, Music PA & Beland KF (29). Assessing estuarine and coastal migration and survival of wild Atlantic salmon smolts from the Narraguagus river, Maine using ultrasonic telemetry. American Fisheries Society Symposium 69, Malcolm IA, Millar CP & Millidine KJ (215). Spatio-temporal variability in Scottish smolt emigration times and sizes. Scottish Marine and Freshwater Science 6(2). Marine Scotland Science, Scottish Government, pp.15. Prentice EF, Flagg TA & McCutcheon CS (199). Feasibility of using implantable Passive Integrated Transponder (PIT) tags in salmonids. American Fisheries Society Symposium 7, River Dee Trust (216). Smolt migration through the lower Dee and inner harbour. Thorstad EB, Whoriskey F, Uglem I, Moore A, Rikardsen AH & Finstad B (212). A critical life stage of the Atlantic salmon Salmo salar: Behaviour and survival during the smolt and initial post-smolt migration. Journal of Fish Biology 81,