The objective of this study was to evaluate the potential use of triploid salmon in minimising the genetic and ecological impact of farmed salmon on wild populations.
The return migration to Irish coastal waters and to fresh water of triploid salmon was monitored through a controlled release programme. Triploid cultured salmon and diploid
controls, which had been microtagged, were released from freshwater and marine release sites. The retrieval of tag data was made possible by the Irish national coded wire tag
recovery programme, which was initiated in 1980 to estimate the marine survival of Irish salmon stocks and exploitation rates by high seas fisheries and home water
Ž .
commercial and recreational fisheries Browne, 1982 .
2. Materials and methods
2.1. Stock origins Experimental groups were produced in December 1994 at the Salmon Research
Ž .
Agency of Ireland SRAI hatchery facilities located on the Burrishoole river system in Ž
. Ž
. Newport, Co. Mayo, Ireland. Mixed-sex MS and all-female AF salmon stocks were
produced using ova from 32 females of ranched Burrishoole grilse origin. This stock has been line-bred in an ongoing experimental ocean ranching programme since the early
Ž .
1970s Piggins and Mills, 1985 . The pooled ova were divided into two groups. The MS stock was produced by fertilising half of the ova with milt from 22 ranched Burrishoole
grilse males. The AF stock was produced by fertilising the remaining ova with milt from three sex-inverted ranched Shannon grilse males from the Parteen hatchery located on
the river Shannon, Co. Limerick.
2.2. Triploidisation and ploidy assessment A proportion of the MS and the AF fertilised ova were exposed to hydrostatic
Ž .
pressure to induce triploidy, after Benfey and Sutterlin 1984 . Ova were subjected to 9500 psi for a period of 5 min, 3008 min post-fertilisation. Subsequently all four groups
Ž Ž
. Ž
. .
MS and AF diploid 2N and triploid 3N groups were subject to the same husbandry practices.
Ploidy levels were determined in pre-smolts to assess the success rate of the triploidisation procedure. Blood smears were allowed to air dry on slides and were fixed
Ž .
in methanol for 5 min. The blood smears were stained in 5 Giemsa pH 6.8 for 45 min at room temperature, rinsed in running water and air dried. The slides were then
Ž .
mounted in DPX Gurr . Major cell axis measurements of 10 erythrocytes per sample were made under oil immersion. The validity of using major cell axis measurements to
Ž .
assess triploidy was confirmed by Benfey et al. 1984 . In the case of returns to fresh Ž
water, where it was impossible to infer ploidy level from gonad status i.e., in the case .
of males , erythrocyte measurements were made.
2.3. Tagging Diploid and triploid pre-smolts were microtagged and cold branded ‘X’ and ‘O’,
Ž .
respectively in February 1996. Precocious males 2.7 were removed from the MS2N Ž
. stock. Pre-smolts were microtagged according to the methods of Browne 1982 . Each
magnetised microtag had a specific code, which identified the release group, stock and ploidy of the fish. The adipose fin was removed to facilitate identification of these fish
in the recovery programme. A quality control check was made on tagged fish to ensure that tags were correctly magnetised. Prior to release or smolt transfer, tag loss was
estimated and subsequent analysis was based on numbers of tagged fish migrating.
2.4. Release sites Ž
. Two release sites on the western coast of Ireland were used in this study Fig. 1 :
Ž
X X
. Ž
X X
. Lough Furnace 9855 W, 53835 N , Co. Mayo and Inver Bay 8818 W, 54838 N , Co.
Donegal. Lough Furnace is a tidal lake adjacent to the SRAI’s hatchery installations, at the lower end of the Burrishoole river system. Inver Bay, Co. Donegal, is the site of a
Ž .
Fig. 1. Map of Ireland showing: the ranched release site at Lough Furnace, Co. Mayo on the Burrishoole Ž
. river system; H the cage release site at Inver Bay, Co Donegal. The SRAI’s hatchery facility is located
adjacent to Lough Furnace. The home water fishery areas are also demarcated.
commercial aquaculture operation located approximately 150 km northeast of the Burrishoole system. Cage structures are situated 1 km from the shore in Inver Bay
within Donegal Bay.
2.5. Release groups Diploid and triploid fish were released as three groups in 1996. These were
categorised as ranched release, cage release I and cage release II groups. The release programme is outlined in Table 1. Ranched release groups were released as 1 q year-old
smolts into Lough Furnace. Cage release groups were transferred as smolts to Inver Bay in April 1996 and released as post-smolts in May and June.
2.6. Coded wire tag recoÕery Information on capture location and return data of the experimental groups was
gathered as part of an ongoing Irish national coded wire tag recovery programme. Ž
. Catches from coastal commercial fisheries drift nets, draft nets, etc. were monitored at
15 major salmon landing ports in Ireland. These fisheries operate between May and July inclusive and catches were scanned consistently during this period. Marine fishery areas
from which coded wire tags were recovered are shown in Fig. 1. Declared landings in each of the corresponding areas were collected by seven regional fisheries boards and
compiled into a national data set by the Marine Institute. Landings were also monitored by the Foyle Fisheries Commission and tag recovery information was supplied by the
Ž .
Department of Agriculture for Northern Ireland DANI . During the course of this study, no microtags were reported at Faroes or Greenland where limited sampling programmes
were implemented. Over 50 of the catch landed in Ireland is sampled for tags each Ž
. year. The number of tagged salmon taken in these fisheries ‘‘raised’’ data
was estimated by multiplying the actual number of tagged salmon in each area by the ratio of
the total declared salmon landings in these areas to the sample size examined. An
Table 1 The release date, release location, stock–ploidy combination and the number of fish migrating for each release
group AF sall-female, MSs mixed-sex.
Ž . Release group
Date Location
Stock Ploidy
n Ranched release
25 April 1996 Lough Furnace, Co. Mayo
MS 2N
4897 MS
3N 4867
AF 2N
4860 AF
3N 4801
Cage release I 30 May 1996
Inver Bay, Co. Donegal MS
2N 1061
MS 3N
1084 AF
2N 1065
AF 3N
1086 Cage release II
30 June 1996 Inver Bay, Co. Donegal
AF 2N
1632 AF
3N 2290
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
