adjustment for non-catch fishing mortality due to losses from nets and non-reporting of catches was also applied.
Complete upstream and downstream trapping facilities of the SRAI, situated on the Burrishoole river system in Co. Mayo, ensured an accurate count of the numbers of
tagged adult salmon returning to the hatchery location. The number of fish entering the river was derived from total trap data and angling for the Burrishoole system. In addition
to the Burrishoole system, the trap and angling catches from 13 other rivers systems were monitored for the presence of tagged salmon to indicate the prevalence of straying.
For fresh water, the percentage return was calculated using the actual number of tags recovered divided by the number of fish migrating.
Ž .
The total number of one sea-winter 1 SW salmon returning to the Irish coast is the sum of all fish taken by the coastal commercial nets, the non-catch fishing mortality, the
Ž .
number surviving to enter fresh water angling and trap data , and finally, the number of tagged fish which may have strayed into other rivers. It was assumed that the tags were
randomly distributed throughout the fishery and that non-recognition or non-detection of tags was minimal. The percentage return of tagged fish to the coast is then expressed in
relation to the number of tagged smolts that migrated.
2.7. Data collection and analysis Ž
. Ž
. Length cm , weight kg , and sex were recorded for a subsample of the returned fish.
Gonad weight was recorded for all triploid and a proportion of diploid returns to the Ž .
Burrishoole traps from July to 24 September 1997. The gonadosomatic index was Ž
. calculated as: gonad weightrbody weight = 100. Condition factor was calculated as
Ž .
3
W = 100r L , where W is body weight converted to grams and L is the fork length in centimetres. Data on all the tags recovered for each microtag code, e.g., the recovery
location, date of capture and size, were reported through the national coded wire tag recovery programme.
Observed vs. expected return rates were compared using the x
2
statistic. A one-way ANOVA was used to detect the differences between ploidy levels and also between
stock types with respect to weight, length, and condition factor. Hartley’s F test was
max
used to test for homogeneity of variance. In cases where the assumption was violated a non-parametric Mann–Whitney U-test was used. A significance level of P 0.05 was
adopted.
3. Results
3.1. Tag recoÕery The number of 1 SW tagged fish recovered in coastal commercial fisheries for each
Ž .
experimental group is shown as actual and ‘raised’ data Table 2 . The national coded wire tag recovery programme generated over 3000 tag recoveries
in freshwater fisheries in 1997. Thirteen rivers were monitored on the west coast from Co. Donegal to Co. Cork, largely through traps and angling. These data were collected
Table 2 Ž
. The actual and raised numbers of one sea-winter Atlantic salmon 1 SW coded wire tag recoveries from Irish
coastal commercial nets and fresh water The tag recoveries in fresh water were all from the Burrishoole trap with two exceptions. See Table 1 for
abbreviations.
Release group No. migrating
Coastal commercial Fresh water
Total 1 SW tag recovery
tag recovery recovery
a
Actual Raised
Rod and trap Raisedqrod and trap
Ranched release MS2N
4897 109
224 110
334 MS3N
4867 25
51 28
79 AF2N
4860 140
281 45
326 AF3N
4801 35
71 10
81 Cage release I
MS2N 1061
28 49
– 49
b
MS3N 1084
5 11
1 12
AF2N 1065
48 88
– 88
c
AF3N 1086
6 14
1 15
Cage release II AF2N
1632 67
127 –
127 AF3N
2290 10
31 –
31
a
Ž .
Estimated number of tagged salmon taken in fisheries see text .
b
Ž .
River Eany, Co. Donegal rod caught July 1997 .
c
Ž .
River Erne Trap, Co. Donegal July 1997 .
mainly in the summer season from May to September. In the vicinity of the release sites at Lough Furnace, Co. Mayo and Inver Bay, Co. Donegal, traps were monitored for the
entire run on the Burrishoole system, and on the Erne river, which flows into Donegal Ž
. Bay, respectively Table 2 .
3.2. Percentage returns In each of the three release groups, the percentage return of triploid MS and AF
Ž .
stocks was significantly lower than diploid MS and AF stocks Table 3 . Returns of Ž
2
ranched release triploid groups were significantly lower x s 21.97, df s 1, P - 0.001
2
. for AF salmon and x s 46.55, df s 1, P - 0.001 for MS salmon than ranched release
diploid groups. The relative proportions of triploid to diploid ranched release returns Ž
. were of similar magnitude to the coast and to the river Table 3 . The percentage return
Ž
2
of ranched release AF2N stock to fresh water was significantly lower x s 27.22,
. df s 1, P - 0.001 than the percentage of ranched release MS2N returning. Ranched AF
groups were more heavily exploited in coastal waters by commercial nets than ranched Ž
. MS groups Table 4 . In diploid groups, 86.2 and 67.1 of AF and MS stocks,
respectively, were recovered from commercial nets, the remainder being taken in fresh water.
Table 3 The return of one sea-winter Atlantic salmon to Irish coastal waters and the Burrishoole river expressed as a
percentage for each release group Ž .
Release group Return to coast
Return to Ž .
Burrishoole river Ranched release
MS2N 6.83
2.25 MS3N
1.62 0.58
AF2N 6.71
0.93 AF3N
1.69 0.21
Cage release I MS2N
4.60 MS3N
1.02 AF2N
8.28 AF3N
1.27 Cage release II
AF2N 7.78
AF3N 1.35
Cage release I and cage release II groups show a similar pattern in percentage returns to coastal waters where triploid returns were four- to six-fold lower than those of their
Ž
2 2
diploid counterparts x s 29.71, df s 1, P - 0.0001 and x s 74.19, df s 1, P -
. Ž .
0.0001 Table 3 . None of these groups returned to the hatchery location on the
Burrishoole river system and only two triploid salmon were recovered in other rivers. Both of these were in the Donegal region: one MS3N in the River Erne and one AF3N
Ž .
in the River Eany Table 2 . 3.3. Growth performance
Mean length, weight and condition factor of coastal and freshwater returns are shown in Table 5. Although triploid fish tended to be lighter than diploid fish and the condition
factor was lower, there were no significant differences in weight, length or condition factor between triploid and diploid coastal returns, within stocks. However, there was a
Table 4 The percentage of the total one sea-winter ranched release recovery, which was exploited in Irish coastal
waters by commercial nets and in the Burrishoole river by freshwater angling and traps See raw data in Table 2.
Ž . Release group
Total 1 SW Coastal commercial
Fresh water Ž .
Ž . recovery n
nets Ranched release
MS2N 334
67.1 32.9
MS3N 79
64.4 35.6
AF2N 326
86.2 13.8
AF3N 81
87.7 12.3
Table 5 Ž
. Ž
. Ž
. Mean length cm , weight kg and condition factor SE of one sea-winter Atlantic salmon recovered for
each release group, in coastal waters and fresh water from the commercial drift net catch and the SRAI upstream traps on the Burrishoole system, respectively
See Table 1 for abbreviations. Ž
. Ž
. Different letters superscript denote significant differences ANOVA at P - 0.05 .
Ž .
Ž .
Release group n
Length cm Weight kg
Condition factor MeanSE
MeanSE MeanSE
Coastal net catch 1 June to 31 July 1997 Ranched release
a a
MS2N 63
62.9 0.43 2.9 0.06
1.160.015 MS3N
21 62.20.83
2.70.11 1.110.028
b b
AF2N 69
64.0 0.33 3.1 0.04
1.170.013 AF3N
20 63.20.44
2.90.08 1.140.023
Cage release I MS2N
14 62.50.90
2.80.10 1.150.024
MS3N 3
62.72.19 2.60.17
1.080.086 AF2N
14 61.90.92
2.70.13 1.150.034
AF3N 3
60.01.16 2.40.07
1.130.040 Cage release II
AF2N 29
61.70.57 2.70.07
1.160.025 AF3N
5 61.70.99
2.70.20 1.120.046
Freshwater SRAI upstream traps June to September 1997 Ranched release
c c
MS2N 66
64.50.65 2.8 0.08
1.00 0.011
d d
MS3N 17
62.51.18 2.4 0.13
0.95 0.021 AF2N
23 63.50.70
2.60.10 0.990.018
AF3N 6
63.62.04 2.40.21
0.930.068
significant difference between ranched diploid MS and AF stocks returning to the coast; Ž
AF2N salmon were significantly heavier and longer than MS2N salmon ANOVA, df 1, .
F ratio s 5.34, P - 0.05 for weight and df 1, F ratio s 4.58, P - 0.05 for length . Mean weights for coastal returns were 2.9 and 3.1 kg for MS2N and AF2N groups,
Ž .
respectively Table 5 . There was a significant difference between ranched release triploid and diploid MS
Ž salmon returning to fresh water with respect to mean weight ANOVA, df 1, F
. ratio s 4.61, P - 0.05 but not for length. MS3N salmon returning to fresh water had a
mean weight of 2.4 0.13 kg compared to 2.8 0.08 kg for MS2N returns. Conse- Ž
quently, there was also a significant difference between condition factors ANOVA, df .
1, F ratio s 4.44, P - 0.05 for the two groups. When the male and female components of both diploid and triploid MS stock were examined, although females were smaller
than males, the difference between males and females within each ploidy level was not significant.
3.4. Sexual maturity Gonadosomatic indices were measured in returns from July until September 1997.
Ž .
Differences in gonad development were most evident in September Table 6 . Develop-
Table 6 Ž
. Mean gonadosomatic index GSI of triploid and diploid one sea-winter Atlantic salmon sampled in August
and September 1997, from SRAI upstream traps on the Burrishoole system Ploidy
Sex Gonadosomatic index
August September
Ž . Ž .
n MeanSD
n MeanSD
Triploid Male
3 0.2790.071
4 4.080.90
Diploid Male
3 0.2240.027
4 3.650.80
Triploid Female
3 0.080.025
5 0.160.08
Diploid Female
3 3.350.650
4 12.431.70
ment in males was similar for triploid and diploid salmon with mean gonadosomatic Ž
. indices GSI of 4.08 0.89 and 3.65 0.80, respectively. In triploid females, however,
gonad development was very much reduced, resulting in a low mean GSI of 0.158 0.083 compared to 12.44 1.68 for diploid females. The sex ratios of MS ranched
returns examined in fresh water were 62 F:43 M and 12 F:14 M for diploid and triploid salmon, respectively.
Ž .
Ž .
Ž .
Ž .
Fig. 2. The distribution of diploid 2N and triploid 3N mixed-sex MS and all-female AF tag returns, from the ranched release group, cage release I group, and cage release II group of Atlantic salmon. The tag
Ž .
recoveries have been raised see text .
Table 7 The number, length, and weight of two sea-winter ranched release returns to the Burrishoole system from each
release group of Atlantic salmon in 1998 See Table 1 for abbreviations.
Ž . Ž
. Ž
. Release group
No. of returns n Sex
Length cm Weight kg
a
MS2N 1
– –
– MS3N
2 F
83.0 5.5
F 74.0
3.5 AF2N
2 F
79.8 5.5
F 79.0
3.7 AF3N
2 F
80.1 4.7
F 74.7
4.2
a
Rod caught fish, no data available.
In view of the observed gonad development in triploid males, milt was stripped from a triploid male in December 1997 and was used to fertilise ova from a diploid female.
Only 1.6 of the resulting fry survived to first feeding compared to 92.7 of fry from diploid parents reared as controls. Yolk sac absorption was poor and survivors were
weak with no prospect of survival.
3.5. Distribution of coastal tag recoÕeries The distributions of tag recoveries from the three release groups are shown by area in
Fig. 2. Ranched release tag recoveries were distributed from Donegal to west Cork. The recoveries occurred with greatest abundance from Donegal to Galway with a peak in the
Mayo fishery area. Tag recoveries from cage release groups I and II extended from Donegal to Kerry and
from Donegal to the south coast, respectively. In both triploid and diploid groups, recoveries were predominantly made in the Donegal and Mayo areas. In isolated cases
the numbers of recoveries can sometimes be distorted due to the raising factor; specifically where the number of fish sampled is low compared to the total reported
commercial catch, resulting in a high raising factor. The percentage of AF3N recoveries
Ž .
made in the south coast was a manifestation of this Fig. 2 . The actual number of tags recovered in that area was one vs. five actual recoveries in the Mayo fishery area for the
AF3N group.
3.6. Two sea-winter returns In 1998, three diploid and four triploid 2 SW salmon were recovered in the
Ž .
Burrishoole river system from the ranched release group Table 7 .
4. Discussion
