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Smolt migration through the River Dee and harbour
January 2018
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Executive Summary To investigate the extent and cause of in-river and estuarine mortality of salmon smolts in the River
Dee, 101 smolts from the upper and lower Dee catchment were tagged and tracked in 2017. Mortality
was high - 70% - 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 0.45% 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 20 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 2017 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 2018 this study will be repeated to determine whether 2017 mortality levels
are standard or not.
From a management perspective, this is the second study (following a pilot study in 2016) 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 2018, alongside the third
year of this tracking study, there will be a larger study to investigate marine migration pathways of
smolts.
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Introduction Around the Atlantic, salmon mortality can be high when smolts first enter the marine environment
(Kocik et al 2009, Thorstad et al 2012). 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 (2012-2016), the Dee’s Fisheries Management
Plan (2015 – 2018) 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 2016, 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 2016, 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 2016).
To investigate the cause of in-river losses, in 2017, 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 2017, following reliable
performance of the acoustic receivers in the harbour in 2016. 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 30 seconds (randomly generated at 15 - 45 second intervals). The sound produced by
each tag is a combination of 8 - 10 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 0.65g.
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 1990, Adams et al 1998, Anglea et al 2004). The smolts tagged
in this study were 16 – 23.5 g (average 20 g) and tag weight represented 2.8 – 4.1% (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 300 m (albeit very dependent on background noise levels). The V5
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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 180
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 40-kg anchor weight, then
roped off to the bank to aid retrieval (Fig. 1). In the harbour, each receiver was attached to rope and
80 – 200 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.
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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 site1. 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 280
and 960 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 0.5 km since the 2016 pilot study.
1 http://www.gov.scot/Topics/marine/Salmon-Trout-Coarse/Freshwater/Monitoring/Traps
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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.
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Figure 4. Map of Dee estuary and Aberdeen Harbour, with locations of acoustic receivers (●).
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Smolts In total, 101 smolts were tagged: 40 smolts from the Baddoch burn, 30 from the Beltie burn and 31
from the Sheeoch burn. Fish were chosen for tagging randomly, if they were in the length range of
120-125 mm (average 123 mm). This size range was the average smolt length across all three trapping
sites in 2016. There was no significant difference in smolt lengths between the three tagging sites in
2017 (ANOVA statistical test, P = 0.164). The body weight of smolts ranged from 16 to 23.5 g (average
20 g): the weight of Baddoch smolts was significantly greater (20.6 g) than lower catchment smolts
(19.5 g; ANOVA, P = 0.002). 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 0.86 – 1.29 (1.065 ± 0.08; mean ± SD) and was significantly better for Baddoch
smolts (average 1.10) than lower catchment smolts (1.04; ANOVA, P = 0.0004).
Smolts were tagged between 4 and 27 April. Due to the dry spring in 2017, 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 80 out of the 101 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.
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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 100 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 2016, it would be expected that these smolts, of 120-125
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, m3sec-1 or ‘cumecs’) every 15 minutes.
Photoperiod was the number of minutes of daylight each day at Aberdeen, based on daylight starting
30 minutes before sunrise and continuing until 30 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 0 and 1 and the most crucial point is the 0.05
level - a P-value less than 0.05 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.
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Findings
Fish detection 90 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 24 – 36 hours after tagging (C. Adams, pers. comm.). The 11 fish that died were of similar
size (average 123.2 mm length, 20.1 g weight) to the surviving fish (123.4 mm, 19.8 g; t-test, P > 0.5),
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 (1.077) compared
to survivors (1.056; t-test, P = 0.43).
The remainder of the analysis is based on the 90 detected smolts.
Loss rates Total mortality of upper catchment (Baddoch) smolts during their main stem migration was 70%,
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 107 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 90 smolts tracked, equated to 0.45 % km-1 (per
km), i.e. there was a 0.45 % chance of a smolt dying for each 1 km of river it travelled through. Smolts
from the Baddoch had a mortality rate of 0.57 % km-1, whilst mortality of smolts from the Beltie was
0.48 % km-1 and from the Sheeoch it was 0.30 % km-1.
Losses of Baddoch smolts was highest at 60 - 90 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 0.76% km-1.
Figure 6. Total cumulative mortality (%) of smolts from detection on the first main stem receiver
until exiting gate 5.
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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 2009 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).
Figure 7. Smolt mortality (Baddoch, Sheeoch and Beltie combined) at each receiver (●) and average
number of goosanders counted in April and May 2017 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 (20.1 g; t-test,
P=0.43) or length (both 123 mm; t-test, P = 0.47).
There was no significant difference in the Fulton condition factor of survivors (1.056) and non-
survivors (1.076; t-test, P = 0.22).
Tagging date was similar for both survivors (21 April) and non-survivors (22 April; t-test, P = 0.45).
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 =
0.96) or for smolts from all sites together (7 days 13 hrs and 5 d, 12 hours; P=0.24) (note there were
too few non-survivors from the Beltie or Sheeoch to test these sites individually).
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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 < 0.0001). 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 20 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 1.5 - 6 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 28-29 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 < 0.001). No other factors (body weight,
condition factor, migration speed, tagging date) were helpful in explaining harbour arrival date.
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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 2015), 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 0.22 km hr-1 and 0.39 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 (0.8 – 1.6 km hr-1), whilst the Beltie smolts slowed down
(0.9 – 1.6 km hr-1). Overall, speeds through the tidal river and harbour were 0.8 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 < 0.0001). 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).
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Figure 9. Speed of migrating smolts (km per hour).
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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 2017 was an exceptionally dry spring, with average daily flows in April - May being approximately 40%
of long-term average flow levels (SEPA, http://apps.sepa.org.uk/waterlevels/). Average daily flows at
Park in April and May 2017 were 18.0 cumecs, compared to 53.4 cumecs in 2016 (Fig. 10).
During April and May 2017 there was only a single rise in water, from 27-29 April. The three gauging
stations (Mar Lodge, Woodend, Park) corresponding best to the three tagging locations all showed a
similar flow response (Fig. 10) and therefore to simplify, the flow rates at Park were used as an
indicator of flow for the movements of all tagged fish.
Figure 10. River flows (in cubic metres per second; cumecs) during the 2017 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 2017 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 < 0.001).
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.
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Figure 11. Smolt movements (initial detections at each receiver; bars) compared to river flows (line).
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f fi
sh a
rriv
ing
at r
ecei
ver Sheeoch
Culter Waterside Boating Club Gate 1 Mean daily flow at Park
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Diurnal patterns In-river migration was predominantly nocturnal, and upstream of the tidal zone, between 81 and 100%
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 < 0.001), a
pattern that was also seen in 2016.
Once smolts moved into the tidal zone and harbour, diurnal migration increased (also seen in 2016),
accounting for approximately 50% 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 < 0.0001). This differed to that observed in 2016, when diurnal
migration increased during the spring, and may be due to the overriding influence of the single high-
flow event in 2017.
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 2016, 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.
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 - 100% 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
0
2
4
6
8
10
12
14
16
18
20
0-2 2-4 4-6 6-8 8-10 10-12
Nu
mb
er o
f sm
olt
s
Hours before high tide
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 160 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 2016, gate efficiency in the harbour
was 100%.
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 0.45% 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 (70%) 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 2017 experienced much longer exposure to any dangers that are within the river,
typically spending 20 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
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
2016 (2 – 10 May). This is surprising, given the very low flows in April 2017 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 2017 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 2000). Furthermore, as river flow was found to be the crucial factor influencing the
timing of migration in both 2017 and 2016, 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 2016 and 2017, 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 2017, which was
not seen in 2016. 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.
2018 programme of work Smolt tracking will continue for a third year in 2018. The unusual flow conditions in 2017 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 2018. As in 2017, 100
smolts will be tagged, including 40 smolts from the upper catchment (Baddoch), and 30 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
21
pulled together in 2018 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 2018, 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. 100 salmon smolts will be tagged on the Dee in 2018, and will be
detected in semi-circular arrays of acoustic receivers installed at distances of 4 km and 10 km from
the mouth of the Dee. Further smolts will be tagged in 2019 and 2020, 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.
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