Marine Scotland Science Report 06/11
SAMPLING NORTH ESK JUVENILE SALMON
POPULATIONS BY MEANS OF ROTARY SCREW TRAPS
(RSTS): 12. WATER OF MARK, SPRING 2010
F MacKay and G Smith
© Crown copyright 2011
Marine Scotland Science Report 06/11
Sampling North Esk Juvenile Salmon Populations by Means of
Rotary Screw Traps (RSTs): 12. Water of Mark, Spring 2010
F MacKay and G Smith
Published by Marine Scotland – Science
Marine Scotland is the directorate of the Scottish Government responsible
for the integrated management of Scotland’s seas. Marine Scotland
Science (formerly Fisheries Research Services) provides expert scientific
and technical advice on marine and fisheries issues. Marine Scotland
Science Report is a series of reports that publish results of research and
monitoring carried out by Marine Scotland Science. These reports are not
subject to formal external peer-review.
Marine Scotland – Science
Freshwater Laboratory
Montrose Field Station
Inchbraoch House
South Quay
Ferryden
Montrose
DD10 9SL
Copies of this report are available from the Marine Scotland website at
www.scotland.gov.uk/marinescotland
Sampling North Esk Juvenile Salmon Populations by Means of Rotary
Screw Traps (RSTs): 12. Water of Mark, Spring 2010
F MacKay and G Smith
Marine Scotland Science, Freshwater Laboratory, Montrose Field Station,
Inchbraoch House, South Quay, Ferryden, Montrose DD10 9SL
Summary
The aim of this work was to estimate smolt production from the Water of Mark tributary of
the River North Esk during the Spring of 2010, to record the biological characteristics of
these smolts and to record the presence of other species.
The smolts were caught by means of two rotary screw traps (RSTs) situated in series in
the lower reaches of the tributary, 1.7km apart.
A total of 420 salmon smolts were caught at the upper of the two traps, and 932
(including recaptures) were caught at the lower trap. The age of the smolts caught
ranged from two to four years, with the majority of being three year olds. The size of fish
caught ranged from 90mm to 147mm fork length, with older fish generally being longer
than the younger fish.
A total of 368 tagged fish were released from the upper of the two traps of which 44%
were recaptured at the lower trap. 91% of those recaptured smolts reached the lower
trap within 3 days of release.
Smolt production from the Water of Mark in spring 2010, estimated by mark and
recapture, was 2814 (+/- 419). This represents two thirds of the spring smolt production
estimated for the Water of Mark in 2009.
In addition to salmon smolts, adult salmon, salmon parr, brown trout, sea trout smolts
and amphibians were also captured.
Introduction
The monitoring programme on the River North Esk is central to Marine Scotland’s stock
assessment strategy for Atlantic salmon (Salmo salar, L.). Indices of abundance are
routinely estimated for North Esk salmon at a range of life history stages and the data
1
used to infer stock status throughout the country where, for example, only catch or ad
hoc juvenile survey data may be available. Data from the North Esk also play an
important role in international salmon management. The North Esk is one of 15
monitored sites in countries bordering the North East Atlantic where stock and
recruitment data are available to assess the status of salmon populations.
An important aspect of the North Esk monitoring programme is the assessment of
salmon populations at the sub-catchment scale. Since 2002, rotary screw traps (RSTs)
have been used to sample the juvenile populations within the North Esk catchment. As
part of this long term programme, two RSTs were deployed in series in the Water of
Mark, an upper tributary of the North Esk, in the Spring of 2010 with the aim of
estimating the spring smolt production from this tributary as well as recording the
biological characteristics of these smolts. The results are summarised here.
This report is one of a series describing the juvenile sampling work on the North Esk.
The format is common throughout the series, providing a brief summary of the key
results. More detailed analyses drawing on the results from a number of exercises will
be published from time to time.
Objectives
1. To estimate the production of salmon smolts from the Water of Mark in spring 2010.
2. Record biological characteristics, behaviour and run timing of salmon smolts.
emigrating from the Water of Mark in 2010.
3. Record presence of other species.
Sites
Tagging site (NEIM2) – the RST was sited approximately 2.3km upstream of the Mark’s
confluence with the North Esk (Fig. 1).
Recapture site (NEIM1) - the RST was sited approximately 600m upstream of the Mark’s
confluence with the North Esk (Fig. 1).
Kinnaber Mill Trap (KMT) – fixed trapping facility at the mouth of the North Esk (Fig. 1)
used to produce a whole river smolt production estimate.
2
Figure 1: The Location of: A) The North Esk within Scotland, B) The Water of Mark and
KMT within the North Esk and C) The trap sites within the Water of Mark.
Equipment
Traps – Two, four foot diameter, aluminium rotary screw traps (Fig. 2a & b). Further
information on RSTs is available as MS Topic Sheet number 46 “Rotary screw traps in
Scotland” (available from
http://www.scotland.gov.uk/Topics/marine/science/Publications).
Tags – Glass encapsulated passive integrated transponder (PIT) tags (Fig 3). For more
information regarding PIT tags and RFID (radio frequency identification), please refer to
www.trovan.com.
3
Figure 2a: The rotary screw trap at the lower Mark site (NEIM1).
Figure 2b: The rotary screw trap at the upper Mark site (NEIM2).
4
Figure 3: A passive integrated transponder (PIT tag).
When possible, the upper RST (NEIM2) was operational for five days during each week
of the study, or until approximately 100 fish were sampled for that week, between
24 March and 8 June 2010 (Fig. 4). A total of 420 smolts were caught at this site, of
which 368 were weighed, fork lengths recorded and a small sample of scales were taken
for age determination. From these scale samples, the age of 362 fish was determined.
These fish were then marked using PIT tags and adipose fin clips before being released.
The smolts were released a minimum of 15m downstream of the RST, to reduce the
incidence of recapture.
The lower RST (NEIM1) was operational between 24 March and 11 June 2010 (Fig. 5).
A total of 932 smolts were caught at this trap and examined for fin clips and the 163
marked fish were scanned for PIT tags. Where practical, these fish were also weighed
and measured. A total of 209 of the unmarked fish were also weighed, measured and
had a sample of scales taken. From these scale samples the age of 203 fish could be
determined. The smolts were released a minimum of 15m downstream of the RST,
below a crump weir, to reduce the incidence of recapture.
5
Wetted Area Estimates
O.S. digital mapping was used to estimate the fluvial wetted areas associated those
parts of the Mark and North Esk previously designated as comprising salmon habitat
(Maclean et al in prep).
Results and Commentary
RST Operation and Daily Catches
The daily catch of salmon smolts at NEIM2 varied from 0 to 75 (Fig. 4) and at NEIM1 the
catch varied from 0 to 130 (Fig. 5).
80
70
No. smolts
60
50
40
30
20
10
09/06/10
02/06/10
26/05/10
19/05/10
12/05/10
05/05/10
28/04/10
21/04/10
14/04/10
07/04/10
31/03/10
24/03/10
0
Date
Smolts not sampled
Smolts sampled & tagged
Figure 4: The daily catch of salmon smolts at NEIM2 in 2010, showing the number of fish
sampled and tagged and the number of fish released unsampled.
6
140
120
No. smolts
100
80
60
40
20
09/06/10
02/06/10
26/05/10
19/05/10
12/05/10
05/05/10
28/04/10
21/04/10
14/04/10
07/04/10
31/03/10
24/03/10
0
Date
Unmarked smolts (not sampled)
Unmarked smolts (sampled)
NEIM2 marked smolts
Figure 5: The daily catch of salmon smolts at NEIM1 in 2010, showing the number of
smolts sampled, those released unsampled and the number of smolts which were
tagged at NEIM2 and subsequently recaught at this site.
In addition to salmon smolts, salmon parr, brown trout, sea trout smolts, amphibians and
adult salmon were also captured. An interesting point to note is that the number of
amphibians caught in 2010 (8 common frogs and 32 common toads), greatly exceeded
anything seen previously.
There was some damage observed on the fish sampled, with 39 (6.8%) showing minor
damage. The incidence of damage observed in 2010 was greater than in previous
studies on this tributary when the prevalence has always been less than 5%.
The overall increase in the incidence of damage in 2010 was driven primarily by an
increase in the proportion of fish carrying damage typical of that made by predators. The
proportion of fish seen in 2010 with this type of damage has increased from a previous
high value of 2% to 4.2%. It is interesting to note that the predator marked fish were not
evenly distributed through the sampling period, with a quarter being caught on the 27th
April and more than a fifth being caught on the 6th of May. In an attempt to identify a
possible source of this damage, additional information will be recorded during future
sampling exercises on individuals of other species caught.
The next most common type of damage observed (2.3%) was damage to the tail, the
majority of which was typical of the tail clipping carried out during a previous study
7
collecting genetic material from the Water of Mark. The remaining damaged fish had
suffered damage to the dorsal fin (1 fish), or the loss of a pectoral fin (1 fish).
In addition to the damaged fish, three salmon smolts were found dead in the trap, one
was killed as it could not maintain balance, one had a deformed tail and, on the basis of
appearance, one was suspected to be a salmon/trout hybrid.
Age Composition
As in previous years at this site, 3 age groups of salmon smolts were caught at NEIM1
(Fig. 6). The majority of these were 3 year olds (82.8%), followed by 2 year olds (16.7%)
and 4 year olds (0.5%). No one year old smolts were caught.
The age structure at NEIM1 in 2010 was relatively unchanged from that seen in 2009 (3
year olds – 82.2%, 2 year olds - 17.2% and 4 year olds – 0.6%).
Three age groups of salmon smolts were also caught at NEIM2 (Fig. 6). Again, the
majority of these fish were 3 year olds (79.0%), followed by 2 year olds (19.9%), then 4
year olds (1.1%) and no one year old smolts were caught.
The age structure at NEIM2 in 2010 was also broadly similar to that seen in 2009 (3 year
olds – 75.4%, 2 year olds – 23.4% and 4 year olds – 1.2%).
90
80
70
60
%
50
40
30
20
10
0
2
3
4
Age
NEIM1
NEIM2
Figure 6: The percentage of smolts in each age class caught at the two trap sites in
2010.
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Size of Smolts
The size of fish caught at the two sites ranged from 90mm to 147mm, with older fish
generally being bigger than the younger fish (Table 1 and Figs. 7 & 8).
Table 1
The size of fish caught emigrating from the Mark in 2010 showing the minimum, mean
and maximum values for length and weight in the all age classes
Smolt age
(years)
2
Length (mm)
3
4
2
Weight (g)
3
4
Site
NEIM1
NEIM2
Min
97
90
Mean
111.6
114.1
Max
139
142
Min
97
96
Mean
116.9
119.1
Max
143
147
Min
108
Mean
Max
126*
128.5
143
Min
8.8
8.5
Mean
14.8
15.83
Max
26.9
29.2
Min
8.8
9.0
Mean
16.8
17.76
Max
31.7
34.2
Min
Mean
Max
* Actual measurements for the one four year old caught at NEIM1.
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17.2
18.6*
23.4
29.3
Figure 7: Box and whisker plot of the length of fish caught emigrating from the Mark in
each age class and at each site during 2010. Box shows interquartile range, whiskers
show non-outlier maximum and minimum values, dark circles show median values and
open circles show outliers.
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Figure 8: Box and whisker plot of the weight of fish caught emigrating from the Mark in
each age class and at each site during 2010. Box shows interquartile range, whiskers
show non-outlier maximum and minimum values, dark circles show median values and
open circles show outliers.
Estimate of smolt production
Of the 368 tagged fish released from NEIM2, 44.3% (163 smolts) were recaptured at
NEIM1, down 4.4% on 2009. The majority of these fish (91%) were recaptured within 3
days of release, which again is down marginally (4%) on 2009.
The smolt production estimate was calculated using DARR (Darroch Analysis with Rank
Reduction) 2.0.1 software (Bjorkstedt, 2005). For further information on the methods
used to calculate the smolt production estimate, see MacKay and Smith (2011). In
spring 2010, a total of 2,814 (+/- 419) smolts emigrated from the Water of Mark (smolt
production estimate +/- 95% confidence intervals) – a smolt production rate of less than
3 smolts per 100m2 (based on an estimated wetted area available for smolt production
above NEIM1 of 123,890 m2). The smolt production estimate from the Water of Mark in
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the Spring of 2010 represents 63% of the 4,482 (+/- 1,343) salmon smolts that emigrated
in 2009.
While the year on year variation in the numbers of smolts emigrating from the Water of
Mark thus appears to be considerable, this may simply reflect the extent of natural
variability within the system, which may become apparent as the time series of these
data develops.
Acknowledgements
We thank Marine Scotland personnel (colleagues at FRS FL - Bryce Whyte, Ian
Simpson, Jackie Anderson and Julian MacLean) whose tireless efforts made this project
possible. Thanks are also extended to the Esk District Salmon Fishery Board, and
Dalhousie Estate for their permission and support during this project.
Software
DARR 2.0.1. U.S. Department of Commerce, National Oceanic and Atmospheric
Administration, National Marine Fisheries Services, Southwest Fisheries Science Centre.
http://santacruz.nmfs.noaa.gov/publications/software/439/
References
Anon. 2006. Rotary screw traps in Scotland. Fisheries Research Services Information
leaflet, FW 27/06/06
Bjorksted, E. P. 2005. DARR 2.0: Updated software for estimating abundance from
stratified mark-recapture data. NOAA-TM-NMFS-SWFSC-368
Darroch, J. N. 1961. The two-sample capture-recapture census when tagging and
sampling are stratified. Biometrika (48), pp 241- 260.
MacKay, F., Smith, G. W. 2011. Sampling North Esk juvenile salmon populations by
means of rotary screw traps (RSTS): 11. Water of Mark, Spring 2009. Marine Scotland
Science Report, No. 2/11.
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© Crown Copyright 2011
Marine Scotland – Science
Freshwater Laboratory
Montrose Field Station
Inchbraoch House
South Quay
Ferryden
Montrose
DD10 9SL
Copies of this report are available from the Marine Scotland website at
www.scotland.gov.uk/marinescotland