ICES Journal of Marine Science, 56: 422–432. 1999
Article No. jmsc.1999.0453, available online at http://www.idealibrary.com on
Evaluation of an alternative strategy to enhance salmon
populations: Cage rearing wild smolts from Conne River,
Newfoundland
J. B. Dempson, V. A. Pepper, G. Furey, M. Bloom,
T. Nicholls, and G. Hoskins
Dempson, J. B., Pepper, V. A., Furey, G., Bloom, M., Nicholls, T., and Hoskins,
G. 1999. Evaluation of an alternative strategy to enhance salmon populations: Cage
rearing wild smolts from Conne River, Newfoundland. – ICES Journal of Marine
Science, 56: 422–432.
Five-thousand wild Atlantic salmon (Salmo salar L.) smolts from Conne River,
Newfoundland, were captured during their downstream migration in May 1995, and
transferred to an estuarine aquaculture rearing site at Roti Bay, 23 km away. Survival
was monitored throughout the experiment. The greatest mortality occurred in July,
approximately 6–8 weeks following transfer. Survival of smolts to one-sea-winter
salmon was 18.5%, over four times higher than the average survival of wild salmon to
Conne River during the past 6 years. Growth was monitored at monthly intervals until
November 1995, with additional sampling in the spring and early summer of 1996.
Survivors were split into two groups and released directly into the Bay d’Espoir fjord;
one group was released 27–28 June 1996, at a site approximately 7 km from the mouth
of Conne River. The second group was retained at Roti Bay and released 23 July 1996.
Lotek radio transmitter tags were used in evaluating the success of the experiment by
tracking migration timing and subsequent distribution of cage-reared salmon throughout the Conne River system. Approximately 80% returned to Conne River and 20%
strayed to other streams. Less than 50% of the surviving fish were later accounted for
in local Bay d’Espoir rivers. Results are discussed in relation to the utility of this
technique to enhance salmon populations.
1999 International Council for the Exploration of the Sea
Key words: Atlantic salmon, enhancement, growth, survival, migration, wild smolts.
Received 27 March 1998; accepted 25 September 1998.
J. B. Dempson, V. A. Pepper, G. Furey, M. Bloom, T. Nicholls: Science Branch,
Department of Fisheries and Oceans, PO Box 5667, St John’s, Newfoundland, Canada
A1C 5X1; G. Hoskins: Fisheries Division, Miawpukek Mi’kamawey Mawi’omi, Conne
River, Newfoundland, Canada, A0H 1J0. Correspondence to J. B. Dempson: tel: +1 709
772 4475; fax: +1 709 772 3578; e-mail: [email protected]
Introduction
Enhancement of Atlantic salmon (Salmo salar L.) populations is a common practice in many areas where the
species occurs naturally. The idea to enhance salmon
stocks has arisen as a consequence of a combination of
effects including poor marine survival, over exploitation
of the resource, loss and destruction of habitat, and
the introduction of foreign organisms (Jonsson and
Fleming, 1993). For example, production of Atlantic
salmon has been severely reduced in many rivers in
Nova Scotia, Canada, as a result of acidification
(Lacroix, 1992; Korman et al., 1994), while salmon have
now been eradicated from a number of rivers in Norway
1054–3139/99/040422+11 $30.00/0
(Jonsson and Fleming, 1993) due to the unintentional
introduction of the parasite Gyrodactylus salaris. Stocking of various salmon life stages (fry, parr, smolt) has
been the most common means to try to offset declines
in natural production (Jonsson and Fleming, 1993).
Hatchery propagation, however, has not been without
its problem and critics (Hilborn, 1992; Meffe, 1992;
Brannon, 1993; Hilborn and Winton, 1993; Riddell,
1993; Washington and Koziol, 1993).
Many of the concerns regarding hatchery propagation
of salmonids for release to natural habitats relate to the
loss of genetic diversity in a population and reduced
fitness of individuals (Jonsson and Fleming, 1993;
Riddell, 1993; Verspoor, 1997). Additional concerns
1999 International Council for the Exploration of the Sea
Evaluation of an alternative strategy to enhance salmon populations
423
N
illtown)
0
15 30
km
N.W. Brook
(Tailrace)
S.E. Bro
ok (M
48°00' N
47°55'
g
in
nt
n
Co
)
ill
s
on
ok
Roti Bay
Bay d' Espo
ir
47°50'
ro
i
eR
ttl
Li
55°50'
n
lli
Co
55°45'
k
oo
r
sB
2
Bois Island
55°55'
r
ve
S.
0
47°45'
(C
B
E.
Conne River
Village
em
n
fe
u
Co
Ri
ite
e
nc
Swanger' s Cove
Brook
r
ve
ne
55°40'
4 6
km
8 10
55°35' W
Figure 1. Location of Conne River, Bay d’Espoir, Newfoundland, and the Roti Bay aquaculture grow-out site, along with other
place names mentioned in the text (Scale 1: 250 000). Insert illustrates the general location of Conne River in Newfoundland.
Release sites of tagged salmon: , cage release site; , river release site.
include increased competition with local wild populations, impacts of accidental straying, and the introduction of diseases (Saunders, 1991; Hilborn, 1992;
Jonsson, 1997; McVicar, 1997). In contrast, others
believe and provide evidence that hatcheries are useful
and have been successful in various places (Bielak and
Davidson, 1993; Incerpi, 1996). The difference in opinions regarding the merits of enhancement of wild fish
populations likely is due to a historic failure to recognize
the inherent dichotomy between fish culture techniques
applied to wild stock enhancement and those appropriate to aquaculture initiatives (Pepper and Crim, 1996).
Nevertheless, a variety of enhancement techniques are
now commonly applied to Atlantic salmon (Bielak and
Davidson, 1993; Davidson and Bielak, 1993).
One strategy not discussed by Davidson and Bielak
(1993) but advocated by Thomas (1996) consists of
capturing out-migrating smolts from a river, transporting them to a salt-water rearing facility, rearing them to
maturity, and subsequently returning them to their
native stream. A modification of this was applied to
Atlantic salmon from the Big Salmon River, New
Brunswick (Bruce, 1995). Here, hatchery-reared smolts
were transported to a sea cage and reared for about 16
months to maturity. Mature fish were trucked back to
the river and released.
Such strategies lend themselves to situations in which
local stocks are perceived to require rebuilding assistance, where logistics and infrastructure are readily available, and where future reliance on natural spawning
requires minimizing the potential for domestication
selection. This situation exists in Newfoundland on the
Conne River.
Conne River flows into Bay d’Espoir on the south
coast of Newfoundland (Figure 1). It is a one-sea-winter
(1SW) salmon river with a modal smolt age of 3 years
(Dempson and Stansbury, 1991). Total returns of adult
Atlantic salmon, obtained from enumerating salmon at
a fish counting fence, have ranged from a high of 10 000
fish in 1987, to a low of 1600 fish in 1994 (Beacham and
Dempson, 1998; Dempson et al., 1998). Over this interval, marine survival from smolts to 1SW salmon
declined from 7–10% to 2.6%, and the river was closed
to recreational fishing from 1994 to 1996. The decrease
424
J. B. Dempson et al.
Table 1. Summary of the numbers of smolts transferred from
the fish counting fence trap at Conne River to the sea cage at
Roti Bay (n), along with the mortalities (M) and the numbers of
fish surviving (S) in the sea cage at various dates.
Date
1995
13 May
16 May
17 May
18 May
18 May
20 May
21 May
23 May
25 May
26–31 May
1–30 Jun
1–31 Jul
1–31 Aug
1 Sep–31 Dec
1996
1 Jan–15 May
Total
n
237
325
472
594
752
771
782
633
434
5000
M
S
1
77
3
1
9
107
50
524
2543
230
168
237
562
1034
1627
2302
3070
3851
4475
4802
4752
4228
1685
1455
1287
361
4074
926
(18.5%)
in adult salmon returns continued even though smolt
production was relatively stable.
In an effort to compensate for precipitous declines in
marine survival of Conne River Atlantic salmon smolts,
an experiment similar to that advocated by Thomas
(1996) was carried out. Wild Atlantic salmon smolts
were captured as they migrated out of Conne River,
transported to a sea cage rearing facility, and reared for
13 months. Surviving salmon were released directly into
the Bay d’Espoir fjord which extends inland from the
coast about 45 km. This paper evaluates the utility of
this technqiue as an alternative strategy to enhance
salmon populations by comparing survival and growth
of cage-reared fish with wild salmon returns. Information on homing response and return rates of cage-reared
salmon to Conne River is provided along with the
results of radio-telemetry tracking investigations in the
context of discussing modifications that may be required
before the method can be widely applied.
Materials and methods
Smolt capture, transfer, and rearing
Five-thousand Atlantic salmon smolts, or 8% of the
total estimated run in 1995, were captured at a fish
counting fence (Dempson and Stansbury, 1991) located
in the lower section of Conne River about 0.75 km
above the head of tide (Figure 1). Smolts were caught
over a 13 day period from 13 to 25 May 1995 (Table 1),
during which time approximately 67% of the total smolt
run emigrated from Conne River. Smolts were dipped
from the fish trap into a transfer box with recirculated
water. There was no selection for smolts of any particular size. Smolts were then transported by boat 23 km to
Roti Bay. Generally, the trip was completed in 4–5 h. At
the sea cage site, smolts were dipped from the transfer
box into a sea cage that had a circumference of 70 m and
depth of 6 m. All equipment used in the transfer
process was disinfected before and after use with an
iodine-based solution (Wescodine).
During spring and summer, Roti Bay is characterized
by a large gradient in both temperature and salinity
(Sutterlin and Stevens, 1992), and the smolts could select
the salinity they desired. The feeding regime varied
among seasons and is summarized in Table 2.
Fish sampling
Dead smolts and postsmolts were removed each day and
counted. At approximately 1 month intervals, samples
of at least 25 specimens were measured for length and
weight. Monthly sampling was limited to this smallsample size to minimize handling stress. Fish were
anaesthetized with 2-phenoxy-ethanol, weighed on an
electronic balance (0.1 g), and fork length was measured
to the nearest millimetre. Specimens were allowed to
regain their equilibrium in a recovery bath before being
returned to their respective cages. Sampling continued
until mid-November, 1995, when a complete inventory
of remaining fish was carried out. Subsampling for
length and weight resumed in May 1996, when another
inventory of surviving fish was done.
Release of surviving adults and tagging
operations
Prior to releasing the salmon, a fish health evaluation
was undertaken on 15 May 1996, by collecting 60 wild
smolt specimens from the Conne River fish counting
fence, concurrent with sampling of 55 of the cage-reared
wild adult salmon from Roti Bay that were part of this
experiment. All samples were taken to Halifax, Nova
Scotia, and processed as per procedures of the Canadian
Fish Health Protection Regulations by personnel of the
Department of Fisheries and Oceans fish health laboratory. All specimens were negative for known bacterial
and viral fish pathogens.
Remaining salmon were separated into two different
cages. Group 1 fish (n=434) were tagged with Floy
‘‘T-bar’’ anchor tags inserted immediately anterior to
the dorsal fin. On 7 June 1996, fish in this cage were
towed approximately 16 km to an estuarine location
adjacent to the village of Conne River (Figure 1),
about 7 km from the mouth of the Conne River. On
19 June, radio tags were inserted into the stomachs of
37 fish that ranged in fork length from 430 to 500 mm
(Table 3). Salmon were anesthetized with Benzocaine in
Evaluation of an alternative strategy to enhance salmon populations
Table 2. Feeding regime used for the cage-reared wild Atlantic
salmon project, Roti Bay, 1995–1996 (Eff: Ewos fry feed;
SSED: Suregain smolt Excel diet).
Date
1995
13 May–25 May
26 May–30 Jun
1 Jul–20 Sep
21 Sep–28 Oct
29 Oct–12 Dec
1996
13 Dec–7 Apr
8 Apr–14 May
nfeeding d 1
Feed type
Pellet size
6
8
5
4
3
Eff
Eff
SSED
SSED
SSED
1.5–3.5
1.5–3.5
3.5–5.0
3.5–5.0
3.5–5.0
1/2
3
SSED
SSED
3.5–5.0
5.0–8.0
preparation for implant. Group 1 fish were released
27–28 June 1996.
The second group of salmon (n=433; 4 fish lost in
transfer) was held in a separate sea cage at Roti Bay.
These fish were also tagged with Floy ‘‘T-bar’’ anchor
tags but with the tag insertion immediately posterior to
the dorsal fin. This was done to assist in the identification of fish from each release group as they passed
through and were recorded on the video camera system
used at the Conne River fish counting fence. Group 2
fish, held for another month as part of a comparative
study on growth, were released 23 July. This group was
sampled for length and weight prior to release. At the
time of release 16 fish had lost their tags leaving 417
tagged individuals.
In addition, 22 wild Conne River salmon caught at the
adult salmon fish counting fence trap were also provided
with radio transmitter tags on 19 June (n=19) and 27
June (n=3). These fish, which ranged in fork length from
475 to 555 mm (Table 3), were immediately released
above the fish counting fence.
Radio-telemetry monitoring
Radio tags used were Lotek’s (114 Cabot Street,
St John’s, Newfoundland, Canada A1C 1Z8) digitally
encoded transmitter model MCFRT-3BM. This tag,
which has a rated battery life of 238 days, is 1143 mm
in size and weighs 3.7 g in water. The digitally encoded
tag provides a unique numerical identification for each
fish (channel; code). Tags transmitted at a frequency
ranging from 149.320 to 149.800 MHz.
A radio-telemetry receiving station was set up at the
Conne River fish counting fence (Figure 1) on 27 June
1996. The station consisted of Lotek’s SRX–400 telemetry receiver/datalogger connected to two directionally
positioned 4-element Yagi antennae. The telemetry data
were downloaded from the receiver to a portable computer for processing. Downloaded information consisted
of the first and last dates a tag was detected, times of
425
detection, and the number of occurrences (events) over a
pre-set interval that a signal was obtained. The telemetry
receiver was operated until 31 August 1996. The Conne
River fish counting fence was in operation until 23
September 1996.
On 4 September 1996, the lower portions of Southeast
Brook (Milltown), Southeast Brook (Conne Mill), and
Northwest Brook (Tailrace) were surveyed on foot with
the Lotek receiver and a hand-held antenna. A section of
Little River, where a fish counting fence was also in
operation, was surveyed below the main falls. Ground
surveys were followed by three helicopter overflights: 2
October 1996; 7 November 1996; and 10 January 1997.
Watersheds covered during all overflights included:
Little River, Southeast Brook (Conne Mill), Southeast Brook (Milltown), Northwest Brook (Tailrace),
and Conne River. In addition, Collins Brook and
Swanger’s Cove Brook were added to the second and
third overflights, respectively.
During the overflights, the telemetry receiver was set
to scan sequentially through the six frequencies at a rate
of one frequency every 6 s. When a radio-tag signal was
detected, scanning was stopped and the receiver fixed on
the particular frequency in question. The approximate
position of the radio-tag in the river was determined by
adjusting the altitude and air speed of the helicopter to
obtain the maximum power signal being emitted from
the tag. At that point, latitude and longitude coordinates
were recorded from a Global Positioning System (GPS)
aboard the helicopter. Scanning then resume and the
overflight continued.
Statistical analyses
Size comparisons among groups of fish were addressed
by analysis of covariance of log transformed weight–
length relationships, unless sample sizes were too small.
In this case, analysis of variance was used to compare
lengths and weights of 1SW salmon. Comparisons of the
recovery rates of released salmon among Bay d’Espoir
rivers, and of the various tagging experiments were
conducted using a test of independence (log-likelihood
ratio testG-test).
Results
Survival and growth in cages
There were few mortalities resulting from the transfer of
smolts to the Roti Bay cage rearing site, with 95% of the
fish alive at the end of May 1995 (Table 1). Mortalities
increased during the month of June and peaked in
July leaving 1685 survivors. Subsequently, mortalities
levelled off and at the inventory carried out in midNovember, there were 1297 postsmolts (25.9%) still
surviving. Mortality remained low over the winter.
426
J. B. Dempson et al.
Table 3. Summary of length and weight data for various groups of both cage-reared and wild Conne River Atlantic salmon (C,
cage; W, wild; M, mortality; L, live; R, radio tag).
Lifestage
Smolt
Post-smolt
1SW
Category
Status
C
C
W
C
C
C
C
C
C
C
C
C
C
W
C
C
W
M
L
L
L
M
L
L
L
L
L
L
L
R
R
L
L
L
Date
May 1995
May 1995
May 1995
Jun 1995
Jul 1995
Jul 1995
Aug 1995
Sep 1995
Oct 1995
Nov 1995
May 1996
Jun 1996
Jun 1996
Jun 1996
Jul 1996
Oct 1996*
Summer 1996
n
114
25
249
25
53
25
25
25
25
31
36
35
37
22
35
22
72
Fork length (mm)
Mean
s.d.
Range
138
145
143
173
153
238
358
342
374
374
413
454
463
518
452
465
518
17
14
15
20
26
26
18
39
42
59
55
44
22
19
36
24
22
105–192
121–173
103–179
126–204
119–254
184–273
309–388
228–382
240–436
218–438
265–484
317–574
430–500
475–555
324–518
415–500
475–573
Whole weight (g)
Mean
s.d.
Range
24
28
29
49
26
168
565
577
780
762
840
1182
1281
1542
1209
8
9
8
24
36
56
73
179
236
300
328
332
223
201
278
13–49
14–51
10–51
14–91
9–205
51–260
401–715
150–878
166–1196
136–1194
190–1450
399–2275
980–1860
1160–1920
441–1946
1523**
219
1160–1920
*Caught at the fish counting fence and held in river until October.
**Weight data based on 19 fish.
Another inventory was carried out 16 May 1996, after
the sample of 55 fish had been removed for disease
analysis. At this time 871 fish remained. Thus, overall
survival, with the inclusion of the 55 salmon sacrificed
for disease analysis was 18.5%.
A comparison of the transferred smolts that died
showed that they were smaller than the average size of
wild smolts sampled as they emigrated from Conne
River in 1995 (Table 3; Figure 2). Analysis of covariance
of weight–length relationships indicated significant differences in slopes (r2 =0.913, Fslopes =5.09, p=0.025)
with the length-specific increase in weight less in those
fish that died.
Of the 53 dead smolts sampled in July, 49 had a whole
weight that was less than the mean of the group transferred in May (Figure 2). Apparently, these fish had not
taken to feeding and exhibited characteristics of failed
smolt syndrome (McCarthy et al., 1996). This is
undoubtedly the cause of the increased mortality during
this period.
Rapid increase in size occurred from July to August,
and by October 1995 caged-reared wild salmon averaged
374 mm in length and 780 g in weight (Table 3; Figure
3), 82 and 66% of their subsequent size achieved by June
1996, respectively. Little growth occurred over the
period October 1995 to May 1996.
Two samples of surviving adults were obtained from
June 1996. One sample (n=37) was measured 19 June
1996 (Table 3). These had been taken off feed in
mid-May, then transferred to the Conne River estuary
and were subsequently implanted with radio transmitter
tags. An additional sample of 1SW salmon was retained
at Roti Bay and maintained on feed. An analysis of
covariance indicated that neither slopes (Fslopes =0.15,
p=0.699) nor intercepts (Fintercept =2.09, p=0.153) were
statistically different between these two groups. In contrast, wild 1SW Atlantic salmon returning to Conne
River in 1996 were significantly larger in length
(F=190.27, p<0.001) and weight (F=19.30, p<0.001)
than 1SW caged-reared salmon.
Distribution of tag returns among rivers
Cage-reared wild salmon from each of the two groups
released in Bay d’Espoir subsequently returned to Conne
River and Little River (Table 4), both monitored by
fish counting fences, as well as to Northwest Brook
(Tailrace) and Southeast Brook (Milltown). In total,
38.5 and 45.8% of the salmon from release groups 1 and
2, respectively, were accounted for. Based upon known
recoveries, there was a strong affinity for fish from each
release group to return to Conne River (80%).
The distribution of salmon among rivers, after pooling information from Northwest Brook and Southeast
Brook (Milltown) due to low expected cell frequencies,
was independent of release group in the likelihood ratio
test (G=2.264, p=0.322). That is, the distribution and
straying of fish among these Bay d’Espoir Rivers was the
same for each of the two release groups. Similarly, the
recovery rates of salmon in Conne River or in Little
River were independent of the early vs. late release
groups (Conne River, G=2.964, p=0.085; Little River,
G=2.508, p=0.113).
Evaluation of an alternative strategy to enhance salmon populations
60
8
(a)
427
(c)
7
50
Frequency
6
40
5
4
30
3
20
2
10
0
35
1
0
5
10 15 20 25 30 35 40 45 50
0
0
50
(b)
30
25 50 75 100 125 150 175 200 225 250 275
(d)
40
Frequency
25
20
30
15
20
10
10
5
0
0
5
10 15 20 25 30 35 40 45 50
Whole weight interval (g)
0
0
25 50 75 100 125 150 175 200 225 250 275
Whole weight interval (g)
Figure 2. Weight–frequency distributions of: (a) wild Conne River salmon smolts, May 1995 (n=249, mean=28.6 g); (b) smolt
mortalities in the sea cage, May 1995 (n=114, mean=24.2 g); (c) surviving smolts in the sea cage, July 1995 (n=25, mean=167.7 g);
and (d) smolt mortalities in the sea cage, July 1995 (n=53, mean=25.5 g).
Run timing of cage-reared wild salmon
2400
2200
2000
1800
1600
1400
1200
1000
800
600
400
200
0
Radio-telemetry monitoring
M
ay
1
Ju 995
n
19
Ju 95
l1
A 995
ug
1
Se 995
p
19
9
O
ct 5
19
N
9
ov 5
19
M
ay 95
1
Ju 996
n
19
Ju 96
l1
99
6
Whole weight (g)
Cage-reared wild salmon from group 1 first entered
Conne River 2 days following their release on 27–28
June, but most of the fish delayed their return for several
weeks. The median date of return was 13 July (Group 1)
with the last fish recorded at the fish counting fence on
29 July. Run timing of wild salmon returning to Conne
River in 1996 was the earliest since 1989, with a median
return date of 27 June and 90% of the run completed
by 12 July. The last wild fish was recorded on 23
September. In contrast, cage-reared salmon from the
second group released at Roti Bay 23 July, had a median
return date of 31 July with the first record of these fish at
Conne River on 26 July. The last salmon from this
release group was recorded at Conne River on 21
August. Return of these salmon was more rapid than
those from the first release even though they had to
migrate a greater distance to return to Conne River.
Month and year
Figure 3. Summary of the change in weight (g) of Conne River
Atlantic salmon smolts reared in a sea cage to the one-seawinter (1SW) adult stage. Vertical line represents the minimum
and maximum values, the rectangle the 95% confidence intervals of the mean, and the marker within the rectangle is the
mean.
Of the original 37 cage-reared radio tagged fish released
in Bay d’Espoir, 15 were identified as having returned to
Conne River. The first of these was detected at the
fixed receiving station on 27 June, the last on 20 July.
There was no difference in the recovery rate of
radio tagged fish vs. the Floy tagged fish at Conne River
(G=0.889, p=0.346). The ground survey conducted 4
September, found a single radio tag in Little River. No
signals were detected in the lower sections of the other
rivers. However, a radio tagged salmon was angled in
428
J. B. Dempson et al.
Table 4. Distribution of Floy tagged (1 and 2: cage-reared) and radio tagged (WC, cage reared; W,
caught and released directly in Conne River) Atlantic salmon among Bay d’Espoir rivers by release
group (CR, Conne River).
Group
Number released
Release site
Recovered
Conne River
Little River
Northwest Brook (Tailrace)
Southeast Brook (Milltown)
Collins Brook
Total
Percentage recovered
1
434
CR Village
n (%)
2
417
Roti Bay
n (%)
134
21
6
6
80.2
12.6
3.6
3.6
152
31
6
2
79.6
16.2
3.1
1.0
167
100.0
38.5
191
100.0
45.8
the lower section of Southeast Brook (Milltown) on
7 July 1996.
The 2 October aerial survey of Conne River detected
14 of the cage-reared radio tagged salmon; seven in the
main stem, five in the Bernard Brook tributary and two
in Twillick Brook (Figure 4). There was also one radio
tag detected at the fixed receiving station that was not
found on the first overflight. Thus, 15 of the 37 cagereared radio tagged salmon were accounted for in Conne
River. In addition, aerial overflights detected one other
radio tag in Little River. No signals were detected at any
of the other rivers surveyed at this time.
Sixteen of the radio tagged wild fish released above
the fish counting fence were found during the October
overflight; eight in the main stem, two in the Bernard
Brook tributary, and six in Twillick Brook (Figure 4).
Two tags had been recovered earlier above the fence
where fish had regurgitated them. Thus, 18 of the 22
river-released fish were accounted for. The distribution
of radio tagged salmon within Conne River, after pooling information from the two tributaries because of low
expected cell frequencies, was independent of release
group (G=0.016, p=0.898).
Subsequent aerial surveys carried out on 7 November
1996, and 10 January 1997, provided similar results at
Conne River and Little River, with three additional
radio tagged river-released salmon located. Some of the
cage-released and wild salmon detected in Conne River
had moved downstream or into ponds. At Collins
Brook, surveyed for the first time during the November
overflight, two radio tags were detected approximately
10 km upstream from the river mouth.
Based on three overflights, 21 of 22 (95.5%) radio
tagged wild salmon released in Conne River were
accounted for (including two regurgitated tags found).
With respect to the radio tagged cage-reared
salmon (Table 4), 20 of 37 (54%) were accounted for
[2, Little River; 2, Collins Brook; 1, Southeast Brook
(Milltown); 15, Conne River]; 75% were found in
Conne River.
WC
37
CR Village
n (%)
15
2
15.0
10.0
1
2
20
5.0
10.0
100.0
54.1
W
22
CR
n (%)
21
100.0
21
100.0
95.5
Discussion
Wild Atlantic salmon smolts were successfully cagereared at sea to 1SW adults with a resulting survival rate
four times greater than the average survival of wild
salmon returning to Conne River during the period
1991–1996 (4.2%; Dempson et al., 1998). Results of the
radio-telemetry investigations confirmed that cagereared wild salmon that returned to Conne River were
able to navigate and utilize virtually the entire watershed, and had a distribution in the river similar to their
natural counterparts. Thus, our results demonstrate that
this technique could be used as a strategy in stock
rebuilding. The technique was similar in some respects
to the delayed release approaches carried out in the
Baltic (Eriksson, 1991; Eriksson and Eriksson, 1991)
where survival was similarly improved. To our knowledge, this was the first time that a project of this nature
had been attempted using wild smolts and releasing
adults into an ocean–estuary environment rather than
directly into the river. The 18.5% survival to 1SW
salmon realized is comparable to 20% obtained in the
Big Salmon River experiment (Bruce, 1995).
Studies that have examined the interaction of wild and
escaped farmed salmon occasionally report that farmed
fish enter the rivers later (Gausen and Moen, 1991), tend
to spawn more in the lower reaches of a river, and later
in the year (Webb et al., 1991) than wild salmon
although this is not consistent among all studies (Okland
et al., 1995). In our experiment, the first group was
released to coincide with the timing of the natural
salmon return to Conne River, but the cage-reared wild
fish generally delayed their return to the river for several
weeks. In contrast, the second group which was released
later returned to Conne River more rapidly. There was
no evidence from the radio-telemetry investigations that
the cage-reared wild salmon remained in the lower
sections of the Conne River.
The strategy to use cage-reared wild salmon to
enhance salmon populations is novel by comparison
Evaluation of an alternative strategy to enhance salmon populations
429
48°15' N
Conne Pond
48°10'
Be
rn
ar
48°00'
dB
Tw
ill
ick
Br
oo
k
48°05'
roo
N
em
St
Counting fence site
55°40'
em
St
k
n
ai
M
47°55'
n
ai
M
0
55°35'
2
4 6
km
55°30'
8 10
55°25' W
Figure 4. Distribution of cage-reared ( ) and wild ( ) radio tagged Atlantic salmon throughout the Conne River watershed as
determined from an aerial overflight survey conducted 2 October 1996. Scale 1: 250 000.
with conventional fry- or smolt-stocking programmes.
However, there are several aspects that would require
attention before this method could be widely applied.
These can be partitioned into three stages: capture and
transfer; cage rearing; and release.
Although initial smolt mortality following transfer to
the estuarine pen was low relative to the mortality that
occurred 6–8 weeks later, survival might be enhanced by
further optimizing catch and transfer procedures to
reduce handling. Stress associated with transporting
smolts has been found to reduce survival (Hansen and
Jonsson, 1988). Secondly, there is a strong need for
improvements to cage design and feeding strategy
to encourage a greater incidence of feeding among
smolts on introduction to the estuarine cage. This could
include use of a locally-formulated moist diet, the
addition of aromatic fish oils, and more frequent
feeding during the day. In the experiment with
enhancement-oriented aquaculture of wild Conne
River Atlantic salmon smolts, greater experimental
flexibility would be required to allow for various
treatments and replicates of experimental protocols
which was not feasible using the 70-m circumference
production cage.
Initial feeding of smolts on introduction to sea cages
has been problematical in aquaculture operations, even
with hatchery-reared smolts that are predisposed to
commercial dry diets. In light of these observations, the
performance of wild Conne River salmon smolts in
the estuarine cage is remarkable given that most of the
smolt had been feeding naturally in the river for at
least 3 years. While the observed feeding response
among some of the smolts in our experiment
confirms that commercial diets are acceptable to some
wild smolts, feeding disposition would have to be
improved greatly before protected rearing of wild smolts
in sea pens could be advocated as a viable stock
enhancement tool.
The decision to release surviving cage-reared wild
1SW salmon to the ocean–estuary environment of Bay
d’Espoir provided the opportunity to examine the local
home-water homing response in fish that originated in,
and subsequently were intercepted as they emigrated
from Conne River as wild smolt. The results obtained
have implications for the release phase where additional
improvements might be possible to ensure maximum
contribution to the spawning populations and to prevent
straying. This is because only 42% of the survivors that
430
J. B. Dempson et al.
were assumed to have retained their external tags were
subsequently accounted for, of which 80% returned to
the Conne River and 20% strayed into other Bay
d’Espoir rivers. Overall, 5.7% of the 5000 smolts transferred returned to Conne River, which is almost identical to the return rate of wild smolts that returned as 1SW
spawners in 1996 (Dempson et al., 1998). The fate of the
remaining survivors of the cage-reared wild fish is
unknown, but there was no evidence of any returning as
two-sea-winter fish to either the Conne River or Little
River fish counting facilities in 1997.
One strategy to alleviate the straying problem and
thus ensure that cage-reared wild salmon survivors contribute to the spawning escapement would be to simply
release surviving fish directly into their natal stream as
was done in the Big Salmon River experiment in New
Brunswick. Transferring smolts from the river in an
open cage that allowed continued contact and exchange
of the river and estuarine water is another option that
could decrease the rate of straying.
Salmon have a remarkable ability to home to their
natal river (Harden Jones, 1968; Stabell, 1984; Hansen
et al., 1993). Recent studies have shown that farmed
(Heggberget et al., 1993) or hatchery reared salmon
(Jonsson et al., 1991; Hvidsten et al., 1994) stray more
than wild fish. According to the ‘‘sequential imprinting’’
hypothesis (Harden Jones, 1968; Hansen et al., 1993;
Dittman and Quinn, 1996), juvenile salmon learn
sequentially from various cues associated with their
home river which enables them to orient back to their
natal stream. The most sensitive period for learning
appears to occur during the parr–smolt transformation
stage (Hasler and Scholz, 1983; Dittman and Quinn,
1996). Quinn (1993) states that disruptions in the
sequence of odours associated with their natal freshwater streams and subsequent seaward migration could
result in straying. Also, smolts may gain essential
experience during their outward migration allowing
them to navigate with precision to their home stream
(Hansen et al., 1993; Hansen and Jonsson, 1994). In our
experiment, smolts were captured in the lower part of
the river about 0.75 km above head of tide. Given the
largely fresh-to-brackish characteristics of the Conne
River estuary, transferred smolts could have been prevented from fully ‘‘learning’’ and thus imprinting
to their natal stream and estuarine characteristics.
This may have contributed to a higher straying
rate than wild salmon typically have (Stabell, 1984;
Quinn and Tallman, 1987; Thorpe, 1994) and was more
comparable with rates observed in hatchery-reared
(Jonsson et al., 1991; Hvidsten et al., 1994), or smolt
transportation experiments (Mills, 1994). Alternatively,
maintaining smolts in a grow-out sea cage for
13 months could have also acted to alter the normal
sequence of cues that fish key on to ensure precise
navigation.
There are a number of advantages in using cagereared wild smolts as an alternative enhancement strategy to assist in rebuilding natural salmon populations.
First, the naturally present genetic strains are utilized
and maintained. In addition, nature has already contributed to part of the selection process, in our case,
throughout the first 3–4 years of freshwater life. Past
enhancement strategies that stocked salmon fry into a
river system required 4 years following stocking before
any adult production materialized. Few if any of these
projects were thoroughly evaluated. In contrast, adult
salmon were produced here in just 1 year. Given the
infrastructure at Conne River, it was rather straightforward to monitor and subsequently evaluate the success
of the project.
On the basis of results obtained from this initial
experiment, the approach appears to offer considerable
potential as a stock rebuilding tool. In situations where
the infrastructure already exists in support of aquaculture operations, or in which wild stock declines are
sufficiently critical to warrant immediate implementation of recovery strategies, rearing of wild smolts in
estuarine pens appears to have more immediate benefits
than traditional enhancement techniques. Given either
of these conditions, and good quality freshwater habitat
for both natural reproduction and early survival, broodstock enhancement is one means of minimizing domestication effects on salmon populations during the
interval it takes to increase wild salmon stocks to
sustainable levels of conservation egg deposition
requirements.
Acknowledgements
The authors thank Anne-Margaret MacKinnon of the
DFO Moncton Fish Health Laboratory for coordinating the fish health analyses. A research team from the
University of Waterloo (Dr Scott McKinley, Dr Rick
Booth, and Mr Eric Bombardier) was instrumental in
applying the radio tags and helping to set up the
monitoring equipment. Technical assistance and the
telemetry receiver were provided by Dave Scruton, DFO
St John’s. The authors also wish to acknowledge the
technical and logistical support provided by Lotek
Marine Technology staff, in particular Keith Stoodley
(St John’s, Newfoundland) and Mitch Sisak (Aurora,
Ontario), and the cooperation of the Canada Coast
Guard helicopter pilots, Casey Buckles and Gordon
Simmons, for their patience and careful attention to our
demands during the aerial overflight surveys. Drs Bror
Jonsson, John Browne, and Niels Daan, kindly reviewed
the manuscript and provided editorial advice. Funding
support for this research was provided by the CanadaNewfoundland Cooperation Agreement on Salmonid
Enhancement/Conservation and by the Aboriginal
Fisheries Strategy (Government of Canada).
Evaluation of an alternative strategy to enhance salmon populations
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