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Salmon migration in the Great Test reach of the lower River Test, Hampshire – Determination of a salmon migration relevant river discharge threshold
G. A Fewings
Summary
In order to evaluate the potential risk of alternate abstraction regimes on the Lower River Test a simple metric was required for comparison of observed and proposed scenarios. Such comparisons were necessary to determine if the key objectives outlined in the Stage 2 Scoping report of the Testwood NEP1 will be met. This note describes the derivation of such a simple metric that is straightforward to apply and indicates a relative risk to salmon migration due to changes in river discharge.
The description of salmon migration is limited to two key months of the summer2 considered to be relevant to salmon migration since it is a period likely to experience low river discharge, and high abstraction demand. In order to estimate the probability of fish migration on each day both the number of fish migrating and the number of fish available to migrate were required. These parameters were estimated using commercial fishery data in conjunction with local fish count data.
The simple description of salmon migration has allowed some characterisation of the migratory response to changes in river discharge. It has also provided two significant metrics that are relevant to river management in the Lower River Test.
Introduction
Atlantic salmon (Salmo salar L.) are a particular feature of the River Test as one of the few chalk stream salmon populations in the UK. The population supported rod catches of up to 1100-1200 salmon3 in the late 1950’s declining to under 200 salmon4 by the early 1990’s with a lowest catch of only 49 salmon in 19975. Since that time voluntary catch and release of salmon has increased to at least 99% by 2001. Since that time there has been an increase in rod catches to a five year average of 280-300 salmon in recent years. The River Test is considered “at risk” by national stock status criteria and normally fails to meet its conservation limit for egg deposition. The population is currently
1 Natural Environment Programme 2 August and September, after the peak of arrival and before autumn behaviour 3 Five year moving average 4 Five year moving average 5 Salmon catches in the other chalk stream rivers were also poor in 1997
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not expected to rise out of this risk category by 2018. The conservation limit is set at a level below which the population is considered to be below the minimum biologically acceptable level.
Atkins (2013) describes the salmon life-cycle and outlines the pattern of arrival of adult salmon from the ocean. The pattern of arrival is complicated by the arrival of different age classes of fish at different times of the year (Figure 1). The plot below shows the average catches for the most seaward commercial catches available locally. This net was exercised at the entrance of Christchurch Harbour approximately 14km to the west of the western approaches to the Solent. Since the first arrival of one- sea winter (1SW) fish to the lowest fisheries on the Rivers Avon, Test and Itchen appears to be coincident it is assumed that the pattern of arrival at Christchurch is a reasonable representation of salmon arrival to the River Test.
0.25
0.20
0.15
0.10
0.05 Mean proportionMean of theyear's catch 0.00 0 100 200 300 Day of Year
Figure 1 Pattern of salmon catches in the Christchurch Seine net (1990-1994)
Historically, the River Test had a salmon population that comprised almost 80% multi sea-winter (MSW) fish. In common with many other rivers this proportion has declined so that 25% MSW is now the norm. As of the order of 75% of salmon returning are now 1SW salmon and these fish return in mid-summer this shift in population run timing would be expected to increase the risk of low river discharge influencing salmon migration rate. This potential impact has been recognised for a considerable time as indicated by Banks (1969) and still considered important (Thorstad et al (2008), Milner et al (2012)).
For the purposes of classification, the migration season can be split into summer and autumn seasons. From May to September inclusive the migration pattern could be described as “slow and steady” with normally only a few fish passing the fish counter each day. Some changes in count rate are apparent during this period and these are often associated with increases in river discharge. The autumn season, October to December, is considerably more volatile with marked changes in fish
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counts per day being clearly associated with changes in river discharge. At times, these migration peaks may constitute more than 100 fish per day.
It is also noteworthy that before October the time of passage at the fish counter site is predominantly nocturnal. After October the majority of fish passage is diurnal. This may indicate a changing internal “state” perhaps related to sexual maturation or readiness to migrate. Youngson and Web (1993) analysed the thyroid hormone levels in adult migrating Atlantic salmon and found that a complex set of changes in thyroid hormones occurred as salmon arrived from the sea, migrated in river and approached spawning tributaries. These changes included a decline in Tri-iodothyronine (T3) as salmon progressed from the sea and an initial decline in Thyroxine (T4) on entering freshwater and then increase when approaching spawning. They concluded that their data supported the hypothesis that motor activity level in migrant fish is a determinant of thyroid status.
Salmon spawn in late December to January and fry emerge in the following April/May. Previous analysis of spawning success suggests that egg survival is a key limiting factor with poor spawning success associated with years of high suspended solids load.
Site description
The river channels of the lower River Test are complex and require description in conjunction with a map (Figure 4 Schematic map of the Lower Test)6.
Progressing downstream from Romsey, an abstraction for a disused fish farm (Broadlands Fish Farm Carrier) removes approximately 0.5 to 1.0 m3s-1 (43-86 Mld-1) of the whole River Test flow. This abstraction is returned to the Great Test but via the River Blackwater and to a point below the Testwood PWS abstraction but above the head of tide.
Continuing downstream on the main River Test, the river splits below the M27 Motorway to form two channels, the Little River Test and the Great Test. This bifurcation was the subject of dispute in the early 19th century leading to a settlement known as the Coleridge Award (1831). In simple terms, the Great Test was agreed to receive at least two thirds of the River Test Flow through the operation of the hatches at this bifurcation. Monitoring indicates that this arrangement has not been observed in recent decades and in times of low river discharge the Great Test receives less than the Coleridge Award suggests. The Little River Test flows toward Southampton Water and rejoins the Great Test in tidal waters.
Following the Great Test, after a few hundred yards the river passes through and around a historic mill at Nursling. During summer flows the discharge under the mill is minor and the vast majority of discharge flows around the mill by-pass through sluices and over a disused gauging weir. This gauging weir is also the site of the salmon counter. Some 740m downstream of the salmon counter water is abstracted by Southern Water for public water supply.
6 Other maps are included in Appendix A
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Approximately 260m below the PWS intake the River Blackwater joins the Great Test augmented by the flow from the Broadlands Fish Farm Carrier. This combined flow of the Great Test, River Blackwater and Broadlands Fish Farm Carrier continues with little loss to Testwood salmon pool (head of tide pool) and on to rejoin the Little River Test in tidal waters.
A consequence of this configuration of the channels and of abstractions is that a significant portion of the River Test flow bypasses the Southern Water abstraction either via the Broadlands Fish Farm Carrier or the Little River Test. Whilst little control is exercised over the flow through the carrier, the Little River Test has the potential to route a substantial portion of the River Test flow direct to the estuary. This effectively bypasses the Great Test and therefore has the capability to substantially deplete the flow of the Great Test.
To control abstraction in the lower River Test area, the public water supply licence has a flow condition, calculated as the flow in the Great Test downstream of the River Blackwater confluence. Flows at this point are a good approximation for the flows that salmon would experience in Testwood Salmon Pool. Therefore analysis presented in the remainder of this note uses river discharge data calculated for this location (Mininum Residual Flow location [see Figure 4)
Derivation of a salmon migration risk criterion
It is clear that because of a variable availability of salmon to potentially respond to environmental cues some account of salmon availability is required in order to understand the complex patterns displayed. This is the same principle as adopted by Greest et al (2006) and discussed by Bendall et al (2013). The fish counter on the Great Test is located at Nursling Mill, the closest suitable structure to the head of tide, some 2.3km downstream. The site is approximately 740m upstream of the SWS abstraction intake and 1000m upstream of the outflow of the River Blackwater. Thus there was ample opportunity for salmon to aggregate below the fish counter.
The distribution of river flow (estimated at the historic MRF location) observations for the included dataset is shown as a histogram and summary statistics.
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120
100
80
60
Frequency
40
20 0 (m3s 1) 2 4 6 8 10
(Mld 1) 200 300 400 500 600 700 800 Residual discharge
Figure 2 Frequency histogram of river discharge during observations
Table 1 Summary of residual flow observations in count dataset
Summary of flow data in count dataset for days 210 to 274 m3s-1 Mld-1 Min 2.26 195 Q99 2.47 213 Q95 2.67 231 Q90 2.82 244 Q80 3.12 270 Q50 4.03 348 Q10 6.73 582 Qmean 4.49 389 Number of observations 700
To account for the availability of fish below the counter it was necessary to assume that the majority of salmon had arrived from the ocean by July. This assumption is supported by observations of commercial salmon catches in the local salmon fishery at Mudeford (Figure 1). This location represents a site below the head of tide on a chalk stream salmon river.
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Another assumption was that once salmon had entered the Lower River Test then losses of salmon were minimal. Given these constraints it was reasonable to step backwards by day from the last day of the year subtracting the count on that day from the cumulative number of salmon remaining below the fish counter. Thus for each day from August to the end of the year a number of fish counted per day (C) and the number of fish at the start of the day below the counter was available (N). An estimate of the daily transfer probability (p) could be calculated by division of C by N. This allowance for the number of salmon available to migrate was reported by Greest et al (2006).
The August to September period (days 210 to 274 of the year) was selected as from October onward a clearly different behaviour pattern was evident7 which was less likely to be relevant to abstraction issues during low discharge8.
Estimates of daily transfer probability show considerable variability due to the low numbers of counts per day with many days having no counts at all. To indicate trends, the observations were split into groups of approximately equal numbers of observations. In this case the dataset was split into 30 groups with median residual flow9 and mean daily transfer probability ( ) listed in Table 1 and plotted in Figure 3. The mean daily transfer probability was calculated as the sum of the fish migrating divided by the sum of the fish below the observation point. Within each group the distribution of flows was expected to be biased, as the occurrence of river discharge was not evenly distributed throughout the range, and therefore the median flow for each group was calculated. To indicate trends in the plot a LOESS smoothed trend line was plotted. This is a statistical method allowing a locally weighted polynomial regression to be fitted to the data and does not require prior assumptions about the nature of trends in the data. In this case, suitable data was available for eleven years giving 700 observations of daily transfer probability for the selected seasonal period.
7 A greater “eagerness” to migrate in the autumn was evident in the Greest et al (2006) model and is supported by evidence of Thyroid hormone changes reported by Youngson and Webb (1993). 8 High abstraction rates are less likely from October onwards due to reduced consumer demand. 9 Measured below the confluence of the R Blackwater and R Test (MRF location)
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Residual discharge (Mld 1)
200 300 400 500 600 700 800
0.015
0.010
0.005
Daily transfer probability 0.000 2 4 6 8 10 Residual discharge (m3s 1)
Figure 3 Mean daily transfer probability and river discharge
Notes: The vertical blue line indicates the residual discharge on the loess regression line where decreases to 75% of the average for the observations between flows of 3.5 to 6.5m3s-1 and is estimated at 3.51m3s-1 [303Mld-1]. The horizontal blue line indicates the average of in the flow range previously described. The vertical grey line indicates the estimated river discharge where the loess regression falls to =0.
The plot of these points has at least three notable features. At river discharge less than approximately 3.5m3s-1 [302Mld-1] the mean daily transfer probability ( ) decreases to effectively zero at around 2.53m3s-1 [219Mld-1]. From 3.5 to 6.5m3s-1 (302Mld-1 to 561Mld-1), remains relatively constant.
At river discharge above 6.5m3s-1 [561Mld-1], increases abruptly, although this is based on relatively few observations due to the selected season.
In order to select a flow criterion relevant to additional migration risk from low flow, a discharge was selected that represents 75% of the average that is approximately static between the discharges of 3.5 to 6.5m3s-1 (indicated by a horizontal dashed blue line). The flow coincident with a of 75% of this intermediate maxima was considered a suitable transition point between the two sectors of the
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curve to indicate the transition of salmon behaviour. In this case the discharge was estimated at 3.51m3s-1 [303Mld-1] and has been described as Qmig. It is considered that this threshold residual flow could be used a criterion on which to rank alternative flow management scenarios. In particular, the number of days that the residual flow falls below this flow criterion for each scenario.
Although the derivation of the criterion uses data from August to September (days 210 to 274) due to the assumption of salmon arrival time from the ocean, it is considered reasonable that the description of salmon migration behaviour, and therefore the use of the criterion, could also apply to earlier months in the summer. In principle, it could be used to compare flow management regimes from June to September inclusive and is intended to be thought of a comparative risk measure.
Table 2 Transfer probability by flow group
Group Median Median Daily residual residual transfer discharge discharge probability
(m3s-1 ) (Mld-1 ) 1 2.52 218 0.00021 2 2.67 231 0.00064 3 2.77 239 0.00023 4 2.87 248 0.00029 5 3.01 260 0.00066 6 3.08 266 0.00150 7 3.14 271 0.00013 8 3.22 278 0.00111 9 3.33 288 0.00247 10 3.44 297 0.00132 11 3.54 306 0.00219 12 3.59 310 0.00165 13 3.66 316 0.00297 14 3.83 331 0.00161 15 3.99 345 0.00189 16 4.11 355 0.00155 17 4.25 367 0.00102 18 4.42 382 0.00244 19 4.58 396 0.00213 20 4.79 414 0.00094 21 4.98 430 0.00259 22 5.05 436 0.00328 23 5.31 458 0.00258 24 5.64 487 0.00261 25 5.95 514 0.00212 26 6.38 551 0.00247 27 6.67 576 0.00222 28 6.93 598 0.00685 29 7.80 674 0.01164 30 9.08 785 0.01081
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Discussion
Estimation of the probability of salmon migration past the salmon counter on the Great Test has been possible given certain assumptions on the arrival of salmon from the sea as indicated by commercial salmon catches. This description of salmon behaviour associated with residual flow appears to indicate some non-linearity and hence the suggestion of behavioural influence. The pattern of -1 10 migration indicated that above around 303Mld (Q93.6Act) remains reasonably constant at approximately 0.0021 and could be described as “facilitated”. At observed river discharge below this threshold the probability of migration appears to decline to near zero at around 2.53m3s-1 [218Mld-1]
(Q99.7Act, M0) and could be described as “constrained”. This flow was also slightly above the lowest observed flow in the fish count dataset (2.26m3s-1,195Mld-1)11 and therefore it is unclear what behaviours might be observed below M0 as there were few observations below this discharge.
Milner et al (2012) makes the point that the impact of anthropogenically altered river flow and impacts on salmon are not well understood but asserts that locally derived and adopted protection strategies are likely to be the most successful. Another point made is that deviation from natural flow regimes may not necessarily be detrimental for salmon migration. In this case the flow of the Great Test has been abstracted for several decades which has increased the proportion of the year that flows fall below 303Mld-1.
The salmon population of the River Test has exhibited a slow recovery since the advent of catch and release angling but is considered “at risk” and expected to continue in this assessment category at least until 2018. The current extent of flow depletion in the Great Test does not appear to exert sufficient impact to cause the population to decline given the extent of the other pressures on the population. It is, however, difficult to predict the consequences of additional flow depletion below the zero migration (M0) and Qmig thresholds.
In particular, an increased frequency of flows below the Qmig threshold is likely to reduce the rate of migration through the lowest reaches of the Great Test. Under recently observed (1996-2011) flows and abstraction the residual flow of the Great Test falls below the Qmig threshold approximately 6.5% of the time but under naturalised conditions only 1% of the time. Again, under recent actual conditions the residual flow fell below the M0 threshold only 0.2% of the time. Under naturalised conditions the 3 -1 -1 QN99.9 was estimated at 3.14m s [272Mld ] and so the flow in the Great Test was not anticipated to fall to this threshold. If a longer dataset is considered, from 1918-2011 the QN99.9 was estimated at 3 -1 -1 2.77m s [240Mld ], indicating that even in naturalised and under drought conditions the M0 threshold would not have been breached.
Whilst that consequence of increased migration constraint may seem benign, Solomon (1991) found that salmon that were delayed in their early freshwater movements then became more torpid and
10 Percentile excedence flow (Actual) is derived from the agreed flow dataset 1996-2011. 11 Observations of salmon migration were derived from only the months August and September.
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were often reluctant to progress until much later in the season. Since water temperatures generally increase from headwaters to the head of tide salmon would be exposed to less favourable temperatures (higher) if they remain in the lower reaches for longer. Since salmon do not feed in freshwater the increase in metabolic rate associated with longer exposure to higher temperatures would necessarily decrease the energy reserves available for gamete production and reduce egg survival.
Solomon and Lightfoot (2008) report that egg survival and egg size are impacted even at temperatures as low as 18°C and that acute spikes in temperature can be as damaging as longer duration elevated temperatures. The relevant period of egg development for these impacts was reported as mid-August to mid-September in the northern hemisphere.
In addition, the highest fishing effort location on the Great Test is at Testwood Salmon Pool at the head of tide of the reach. It is a relatively deep pool fed by water from the river through undershot sluices. Although a catch and release policy is exercised at the fishery it is likely that fish that stay longer in the high fishing effort areas will have a greater risk of capture and hence repeat capture. Some mortality of these fish might be expected to be associated with both the number of times captured and the conditions during capture.
Salmon, in common with most if not all fish, exhibit rheotaxis, an ability to orient in relation to water movement. They have several mechanisms with which to sense water movement including sensory mechanisms on the surface of the body and associated with the lateral line. In addition, they have good eyesight and can use visual cues to hold station and recognise changes in water velocity as particles move past them in the water column (Montgomery et al (2000), Arnold (1974)).
No specific velocity thresholds are available for Atlantic salmon but a range of fish species have been shown to exhibit response thresholds from 0.2 to 5 cms-1. These threshold responses can be influenced by a range of internal and external factors and should be considered part of multisensory system through which fish are able to perceive and react to their immediate environment. Whilst the complete functionality of these systems are not fully understood it is clear that fish can detect changes in water velocity consistent with those anticipated in the Great Test (0.34-0.83ms-1, Atkins, 2013).
Salmon migration into freshwater is considered a critical step in success toward reproduction (Solomon and Sambrook (2004)) but the observations of fish passage through this reach cannot take account of the factors affecting river entry. If a significant proportion of salmon were not successful in river entry the method of working back from the total count of the year could not indicate the fate of unobserved fish. It would therefore be reasonable to assume that at low flows, that would not be observed under naturalised conditions, that some impact on river entry success could occur. Solomon (1991) tracked over 400 salmon from a seaward location at the mouth of the River Avon estuary, the next chalk stream to the west of the River Test. He observed that at low river discharge a substantial proportion of salmon tagged did not enter freshwater. This proportion increased as river discharge fell below 9m3s-1[777Mld-1] at the head of tide. The Hampshire Avon is larger than the River Test and therefore to allow comparison river discharge may be transformed into exceedance proportions. It is noteworthy that if the decline in river entry on the River Avon is compared to the in-river migration on
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the River Test there are striking similarities. Taking the static “facilitated” on the River Test as equivalent to unconstrained river entry on the River Avon it is possible to compare proportions of decline from these maxima. The exceedance discharge coincident with 90% of the migration maxima was Q87.9Act and Q86.4Act for the Rivers Test and Avon respectively. Similarly, for 75% of the migration maxima Q93.5Act and Q92.5Act, and for 50% Q96.9Act and Q98.4Act. This suggests that impacts on river entry to the River Test might be expected to occur in the same range of flows as described earlier as the “constrained” band of flows below around Q93 Act. Caution would therefore be advisable where man-made activities would increase the frequency of river discharge below this criterion as this represents impacts where salmon do not enter the river at all.
Analysis of salmon spawning success on the River Test appears to be related to the availability of suitable spawning habitat and the occurrence of low suspended solids concentration during incubation. Low suspended solids concentration is more likely the further upstream12 fish migrate, and therefore the extent of salmon migration upstream is likely to be a factor affecting population success.
Salmon migration is not constant through the season and follows a regular pattern that appears to be influenced by environmental factors. There is a clear general pattern of salmon arrival and migration in-river that appears to be modulated by environmental factors, the most significant of which is river discharge or some factor related to it. Approximately 60% of salmon migration takes place from October onward in most years although occasionally river flows do not increase substantially until late November. This suggests that abstraction may have the potential to influence salmon migration until late in the year.
Conclusions
Analysis of salmon counter data in conjunction with local commercial catch data has allowed the local characterisation of salmon migration in relation to river discharge in the Great Test reach of the lower River Test. This characterisation includes the estimation of a flow threshold that indicates a transition flow below which salmon migration appears to be constrained. This threshold, termed Qmig, is approximately equal to an excedence flow of Q93.6Act and QN99. The threshold was intended to be used as a metric with which to compare alternative abstraction scenarios. It is considered logical that the greater the frequency of flows below this threshold the greater the risk of impacting salmon migration.
A notable feature of the observed relationship was the trend towards reduced migration below the Qmig threshold. Near the lower limit of flows in the fish count dataset the mean daily transfer probability approached zero. The estimated flow for zero migration was 2.53m3s-1 [218Mld-1] and is a flow that is unlikely to occur under naturalised conditions.
12 as indicated by long term suspended solids sample observations
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Potential mechanisms for River Test salmon population impacts are discussed due to changes in migration rate in the Great Test. These mechanisms included, increased risk of capture, increased risk of exposure to higher water temperature and decreased egg incubation success. In addition, salmonids may not be able to sense river discharge per se but are known to be able to detect minute changes in water velocity within the range of changes caused by abstraction. The potential impacts of river entry success could not be assessed using the method described here.
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References
ARNOLD, G. P. (1974). Rheotropism in fishes. Biological Reviews of the Cambridge Philosophical Society, 49(4), 515-576.
ATKINS (2013), Lower River Test NEP Investigation Volume 1: Report (October 2013)
BENDALL, B., MOORE, A., MAXWELL, D., DAVISON, P., EDMONDS, N., ARCHER, D., SOLOMON, D., GREEST, V., WYATT, R. and BROAD, K. (2012), Modelling the migratory behaviour of salmonids in relation to environmental and physiological parameters using telemetry data. Fisheries Management and Ecology, 19: 475–483.
GREEST, V., WYATT R. & BROAD K. (2006) Flow Protection Criteria for Adult Salmonids. Bristol: Environment Agency Science Report No. SC010016/SR, 56 pp.
MILNER, N. J., SOLOMON, D. J. AND SMITH, G. W. (2012), The role of river flow in the migration of adult Atlantic salmon, Salmo salar, through estuaries and rivers. Fisheries Management and Ecology, 19: 537–547
MONTGOMERY J., CARTON G., VOIGT R., BAKER C. AND C. DIEBEL (2000) Sensory processing of water currents by fishes.Phil. Trans. R. Soc. Lond. B 29 September 2000 vol. 355 no. 1401 1325-1327
SOLOMON, D.J. (1991) Hampshire Avon Salmon Radio Tracking 1986-1990, Final Report September 1991, 109p.
SOLOMON, D.J. AND LIGHTFOOT, G.W. (2008). The thermal biology of brown trout and Atlantic salmon: A literature review. Environment Agency Science Report No. SCHO0808BOLVE-P
SOLOMON, D. J. AND SAMBROOK, H. T. (2004), Effects of hot dry summers on the loss of Atlantic salmon, Salmo salar, from estuaries in South West England. Fisheries Management and Ecology, 11: 353–363.
THORSTAD E.B., ØKLAND F., AARESTRUP K. & HEGGBERGET T.O, (2008) Factors affecting the within river spawning migration of Atlantic salmon, with emphasis on human impacts. Reviews in Fish Biology and Fisheries 18, 345–371.
YOUNGSON, A. F. AND WEBB, J. H. (1993), (ABSTRACT) Thyroid hormone levels in Atlantic salmon (Salmo salar) during the return migration from the ocean to spawn. Journal of Fish Biology, 42: 293–300.
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Fish counter MRF point (disused gauging weir)
Figure 4 Schematic map of the Lower Test
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