M.F. O'Grady
Central Fisheries Board
Glasnevin, Dublin, Ireland
ABSTRACT
The relative success of a series of brown trout (Salmo trutta L.) stocking experiments in a range of Irish lakes are outlined. Data indicate that in the case of yearling plantings, the direct offspring of wild trout survived in greater numbers than an inbred strain of trout. Size on stocking can be more important than genotype in relation to plantings of two-year old fish. The larger individuals tend to survive in greater numbers and contribute more to angling catches. Repeated stockings of trout in spring and autumn indicate that the former are more successful both in terms of survival and subsequent angling catches.
RESUME
En Irlande, on a procédé, avec des résultats variables, à toute une série d'expériences de repeuplement des lacs en truites (Salmo trutta L.). On a constaté, en cas de repeuplement avec des sujets d'un an, que les descendants directs de truites sauvages avaient un taux de survie supérieure à celui d'une race consanguine de truites. Si l'on utilise des poissons de deux ans, leur taille au moment du repeuplement peut jouer un rôle plus important que le génotype. Les truites les plus grosses ont tendance à avoir un taux de survie supérieur et représentant une part plus importante des captures. On obtient de meilleurs résultats (taux de survie, captures) en repeuplant les eaux au printemps plutôt qu'en automne et donne des exemples de leurs effets sur les stocks de truites. Il passe en revue les problèmes qui subsistent et indique brièvement le travail qui reste à accomplir.
The Central Fisheries Board (formerly the Inland Fisheries Trust) was founded in 1951 to develop and promote brown trout (Salmo trutta L.) and coarse fish (principally pike (Esox lucius L.), bream (Abramis brama L.), rudd (Scardinius erythrophthalmus L.) and tench (Tinca tinca L.)) sports fishing in the Republic of Ireland.
There are many lake brown trout fisheries in Ireland. The Central Fisheries Board developed these lakes by improving the trout spawning and nursery areas in inflowing streams and by controlling the trout's predators (pike) and competitors (perch, Perca fluviatilis (L.)) in the lakes.
In the late sixties a combination of arterial drainage schemes and increased instances of organic pollution in streams resulted in serious reductions in wild trout recruitment rates to lake fisheries. A mjaor stocking programme was undertaken at that time in many lakes to maintain optimum trout stocks. In 1975 a long-term research programme commenced to evaluate the overall development programme on trout lakes (O'Grady, 1981). The current manuscript outlines some of the results of this study - the relative importance of genotype, size on stocking and the actual stocking date to the survival of planted brown trout in a range of Irish lakes.
Experimental stockings were carried out in nine lakes. They ranged in size from 1 to 1 883 ha. Details of the size, depths, water chemistry, geology and resident fish stocks in these waters are presented (Table 1).
Five discrete brown trout strains were used in these experiments. The Central Fisheries Board have developed a fast-growing variety (Roscrea trout) over the past 25 years. A brood pool of 1 000 breeding pairs of Roscrea trout are retained annually. Seven different strains of wild trout have been crossbred with the brood pool at intervals to prevent excessive inbreeding. Details of the evolution of this stock are presented in O'Grady (1981). The four additional strains of trout used in these experiments were F' wild trout - the direct offspring of wild Irish lake trout which were reared in fish-farm conditions from the egg stage.
In four of the lakes, trout population estimates were carried out at intervals to assess the survival of fish stocked experimentally. A Petersen tag recapture type exercise was used. Tagged trout were stocked and the water was subsequently gillnetted for a three to five day period to obtain a sample of both the introduced tagged fish and the resident stock for estimate purposes. Data indicate that the standing crop estimates obtained in this way were accurate (O'Grady, 1981a, in press).
In the remaining waters, samples of trout from experimental stockings were recaptured in gillnet gangs which were capable of catching a random cross section of the population present (O'Grady, 1981a).
All of the trout stocked during the course of these experiments were either fin-clipped or tagged for subsequent identification purposes. The tags utilized were Dennison floy anchor tags. Fin-clipped and tagged trout were never used in the same experiment in order to avoid the problem of possible differential mortalities from each marking technique. In experiments where fin clips were used for identification purposes and two genotypes were stocked, the same fin type was clipped in each group (i.e., a left and right pelvic or pectoral fin). Data indicate no differential mortality (P>0.05) in trout of identical genotype which has one or other pectoral or pelvic fin clipped (O'Grady, 1981).
In 1977 and 1978, seven relatively small lakes (0.4–130 ha) were stocked with autumn fingerlings or spring yearling trout. Each water was planted with fish of two strains, Roscrea trout and one F' wild group. Each group were marked by clipping a single fin. In October 1979, these waters were test-netted to establish the relative survival of the experimental stockings. Reasonable numbers of these fish were captured in four of the seven lakes. In all four waters there was a significantly higher (P < 0.05 or P < 0.001) survival of the F' wild strain (Table 2). In the remaining three lakes poor survival of both strains was probably caused by a combination of factors - predation by the resident pike stocks and both inter-and intra-species competition with resident trout and perch populations.
The Roscrea and F' wild trout used in these experiments were not homogeneous in terms of mean length values on stocking. The former group were larger in each case and maintained this differential (P<0.001) over the experimental period in at least three of the experiments (Table 3).
When the samples of fish recaptured in each lake were being removed from the gillnets, it was noted that some of the immature fish were soft-fleshed. Subsequently, when individual fish were being weighed and measured, the flesh texture of each immature trout was recorded. A distinct pattern emerged. All of the immature Roscrea trout were soft-fleshed. Their F' wild counterparts had much firmer musculature. Trout which would have matured later in the year were not utilized in this comparative test because maturing females tend to become soft-fleshed and ripening males usually develop a toughened skin.
The relevance of size on stocking to the subsequent survival of planted trout was illustrated in a number of experimental programmes. In each case, the larger group of fish survived and/or were cropped by anglers in greater numbers. In each experiment, the larger group of fish were significantly (P<0.001) bigger than their counterparts.
A. Roscrea two-year old and yearling trout
Both Roscrea two-year old and yearling fish were planted in Lough O'Flyn in April 1976. A large percentage (56.4 percent) of the former group were cropped by anglers in 1976 (April to September) and a significant number of these fish (33.1 percent) were still present in the fishery in October 1976. An additional 12 and 6 percent of this two-year old group were caught by anglers during the 1977 and 1978 angling seasons respectively. In contrast, none of the yearling Roscrea fish were recorded in angling catches in 1977 or in subsequent years. Five trout population estimates carried out in Lough O'Flyn at intervals from October 1976 to October 1979 indicate that few of these fish has survived (<3.0 percent) 17 months after planting.
A similar stocking programme in Lough Inchiquin yielded identical results with a greater number of the two-year old Roscrea fish surviving for a longer period (Tables 4 and 5).
B. Roscrea and F'Owel two-year old trout
Roscrea and F'Owel two-year old trout were stocked concurrently on three occasions. In one instance (O'Flyn, 1977) F'Owel trout were larger than their Roscrea counterparts. In the other two experiments (O'Flyn, 1976 and Inchiquin, 1976) the Roscrea trout were the larger fish. In each of the three experiments the larger batch of trout, irrespective of genotype, survived and/or were caught in greater numbers by anglers (Tables 4 and 5).
Repeated planting of autumn yearling (19 months old) and two-year old (22–24 months old) Roscrea trout in Loughs O'Flyn and Sheelin illustrate the importance of the stocking date to the subsequent survival of planted trout. In each case, the two-year old fish survived in greater numbers for a longer period and thus more of these fish were cropped by anglers (Tables 6 and 7).
The numbers of autumn yearling trout which survived this first overwintering period, post-stocking, were checked in relation to four specific plantings (Sheelin, October, 1977 and 1978; O'Flyn, October 1976 and 1977). In both the Lough Sheelin stockings and in the O'Flyn October 1976 planting, only a small (8.9–25.6 percent) proportion of each stocked group survived overwinter - a 5–6 month period.
The 95 percent confidence limits obtained for the estimated number of autumn yearlings (October 1977) still present in Lough O'Flyn in February 1978 were too broad to indicate the precise overwintering mortality for this group. However, very few of these trout were caught by anglers in 1978 and 1979 (9.9 and 10.3 percent) and none of these fish were captured two years after planting during a population estimate (Tables 6 and 7). These data indicate a poor survival rate for this stocking.
The results of these experiments indicate that genotype can influence the survival of stocked trout. Significantly greater numbers of F' yearling trout survived than their Roscrea counterparts in four small lakes. A similar pattern was noted for domesticated and F' wild rainbow trout (Borgeson, 1966; Webster and Flick, 1974 and Reisenbichler and McIntyre, 1977) and brook trout (Flick and Webster, 1962 and Mason et al., 1967). The same trend has also been observed between different domesticated trout strains. (Cordone and Nicola, 1970: Brown, 1970; Rawstron, 1973 and Ayles, 1974) and also between transplanted wild and domesticated brown trout (Miller, 1953 and Strange and Kennedy, 1979).
The higher survival of F' genotypes in this study might be related to the fact that they were introduced into waters, similar ecologically to those occupied by their parental stocks. Ayles (1974) and Webster and Flick (1974) have stressed the importance of matching planted strains to specific habitats. The F' genotypes may also have higher stamina than their Roscrea counterparts. Green (1964) noted that F' brook trout have higher stamina than domesticated varieties. Fishfarm experiments indicated that Roscrea fish were tamer than the F' strains (O'Grady, 1981), a factor which might render the former group more prone to predation. Greene (1951), Vincent (1960) and Butler (1974) quoting McClaren also record that domesticated trout display a lack of wariness even after release from fish farms. Fishfarm experiments also indicated that F' genotypes will adopt a territorial stance given the opportunity (O'Grady, 1981).
Roscrea fish have difficulty in recognizing invertebrate food items for at least ten days after release and do not feed at the same rate as resident fish during this period (O'Grady, 1981). F' wild trout may not experience this difficulty. A combination, of some, or all, of these factors probably resulted in the higher survival of the F' genotypes. The unusually soft musculature of the Roscrea fish which did survive may also reflect the social stress which these fish were subjected to in an alien and competitive environment.
Size on stocking is of critical importance to survival and also influences the number of fish caught by anglers. The importance of this factor in relation to yearling and 2+ fish has been recognized previously for both brown trout (Brown, 1970; Cooper, 1974 and Fleming-Jones, 1978) and rainbow trout (Borgeson, 1966; Brown, 1970; Cooper, 1974 and Fleming-Jones, 1978). In Loughs O'Flyn and Inchiquin size on release was more important than genotype in determining subsequent angling returns. The cropping of larger trout in two-year old plantings has been noted previously (Shetter, 1944; Needham, 1947; Larson and Ward, 1955; Mullan, 1956; Hallock et al., 1961; Wagner and Wallace, 1963; Hansen and Stauffer, 1971; Templeton, 1971 and Cooper, 1974). The higher returns to anglers of larger trout might be related to an increased ability of these individuals to survive in competitive situations.
The date of planting can, of itself, seriously influence survival. Repeated autumn yearling plantings in Loughs O'Flyn and Sheelin failed to overwinter successfully. Comparable two-year old spring stockings survived in much greater numbers. Many authors report a similar differential survival pattern for autumn and spring stockings of non-migratory salmonids (Shetter and Hazzard, 1940; Needham and Slater, 1944; Shetter, 1944; Reimers, 1957; Buettner, 1961; Anderson, 1962; Brown, 1970; Templeton, 1971 and Cooper, 1974). A paucity of food in the autumn has been suggested as the reason for the poor survival (Brown, 1970). This does not hold in Irish lakes because autumnal standing crops for many invertebrate species are relatively high (Whelan and O'Connor, pers.comm.). Data indicate that, initially in Irish waters stocked fish tend to be surface feeders, ignoring the benthic invertebrates (O'Grady, 1981). Thus, an absence of surface food items in the autumn may result in the poor survival of fish stocked at this time of year.
The results of these experiments illustrate that several factors, genotype, size on planting and the stocking date, can seriously influence survival and therefore affect angling catches. These data will allow a rationalization of stocking procedure in Irish lake brown trout fisheries and contribute to a more efficient fisheries management programme. There are many small (1–30 ha) productive lakes in Ireland which are not fished intensively. Annual spring plantings of yearling F' genotype fish in these waters will ensure the maintenance of high stocks in circumstances where natural production is limited or absent. In larger waters, annual spring plantings of large two-year old trout will successfully supplement natural stocks. A 50 percent reduction of stock densities in Central Fisheries Board fish farms since 1978 has resulted in an increased growth rate of stocks (O'Grady, 1981). Two-year old Roscrea trout now (1981) average 30 cm in length. The Roscrea strain fish are the most useful for stocking purposes where two-year old trout are required because they grow more quickly in fishfarm conditions than F' strain fish (O'Grady, 1981). The reduction in fishfarm production because of the decrease in pond stock densities has been offset by the adoption of a cage-culture system in lakes.
I am very grateful to the Central Board for providing me with the facilities to carry out this research. Special thanks are due to T. Sullivan and P. Green for technical assistance. I wish to gratefully acknowledge all assistance received from members of both the development and fishfarm staffs of the Central Fisheries Board. Sincere thanks are due to Drs. K. Whelan and J. O'Connor for information on invertebrate standing crops in Irish lakes.
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Table 1 Details of the study areas
| Lake | Area (ha) | Max.depth (m) | Mean depth (m) | Alkalinity | Conductivity | Solid geology | Fish species |
| Sheelin | 1 883 | 14 | 5 | 2.70 | 287 | Carboniferous limestone | Bt, Pe, Pi, Ro, Ru, Br, T, St(3),St(9) |
| Mount Dalton | 8 | 4 | 3 | 5.00 | 544 | Carboniferous limestone | Bt |
| Doolin Pond | 2.2 | 2.7 | 1.3 | - | - | Carboniferous limestone | Bt, St(3), C |
| O'Flyn | 130 | 19 | 4.5 | 2.80 | 282 | Carboniferous limestone | Bt, Pe, Pi, Ru, Br, St(3), St(9) |
| Inchiquin | 113 | 27 | 6 | 3.70 | 400 | Carboniferous limestone sandstone, millstone grit, flagstone, shales | Bt, Pe, Pi, Ru, T, St(3), St(9) |
| Muckanagh | 300 | 20 | 3 | 4.00 | 405 | Carboniferous limestone | Bt, Pe, Pi, Ru, Br |
| Kilkee Reservoir | 2.4 | 3.6 | 1.8 | 0.85 | 300 | Millstone grit, flagstone | Bt |
| Gortglass | 31 | 15 | 3 | 0.,50 | 104 | Millstone grit, flagstone | Bt. Ch |
| Gloster Pond | 1 | - | - | - | - | Carboniferous limestone | Bt, Pe, Pi, Ru |
Bt = brown trout;
Pe = perch;
Pi = pike;
Ru = rudd;
Ro = roach;
Br = bream;
T = tench;
St(3) = three-spined stickleback;
St(9) = nine-spined stickleback;
C = carp;
Ch = char
Eels present in all lakes
Alkalinity as milli equivalents per litre
Conductivity as reciprocal megohms
Table 2 The comparative survival of Roscrea and F' wild trout strains, stocked in a number of small lakes, as indicated by x2 tests
| Lake | Strain | Number stocked (1977/78) | Number recaptured (1979) | X2 | Significance |
| Gortglass | Roscrea | 829 | 4 | ||
| Mask F' | 829 | 39 | 27.8 | P<0.001***Mask F'>Roscrea | |
| Mount Dalton | Roscrea | 1 000 | 23 | ||
| Mask F' | 1 000 | 38 | 3.6 | P<0.05* Mask F'>Roscrea | |
| Doolin Pond | Roscrea | 600 | 14 | ||
| Sheelin F' | 600 | 104 | 62.8 | P<0.001*** Sheelin F'>Roscrea | |
| Kilkee Reservoir | Roscrea | 500 | 23 | ||
| Sheelin F' | 500 | 38 | 3.5 | P<0.05 * Sheelin F'>Roscrea | |
| O'Flyn | Roscrea | 4 008 | 3 | ||
| O'Flyn F' | 4 008 | 5 | - | Insufficient data | |
| Muckanagh | Roscrea | 3 175 | 0 | ||
| Inchiquin F' | 3 175 | 0 | - | - | |
| Gloster Pond | Roscrea | 500 | 0 | ||
| Sheelin F' | 500 | 0 | - | - |
*** = very highly significant difference
Table 3 A statistical comparison of the mean (x) length values of Roscrea and F' wild trout on stocking and subsequently on recapture in the Mount Dalton Lake, Kilkee Reservoir and Doolin Pond experiments
| On stocking (November 1977) | On recapture (October 1979) | ||||||||||
| Lake | Trout strain | Mean(x) length cm) | SE | t value | Sig. | Sample | Mean(x) length (cm) | SE | t value | Sig. | Sample |
| Mount Dalton | Roscrea | 13.9 | 0.31 | 6.74 | P<0.001 *** | 50 fish | 36.1 | 0.41 | 6.20 | P<0.001 *** | 23 fish |
| Mask F' | 11.5 | 0.29 | 50 fish | 33.5 | 0.37 | 38 fish | |||||
| Kilkee Reservoir | Roscrea | 13.6 | 0.28 | 6.10 | P<0.001 *** | 50 fish | 41.5 | 0.40 | 4.26 | P<0.001 *** | 23 fish |
| Sheelin F' | 11.2 | 0.24 | 50 fish | 38.9 | 0.38 | 38 fish | |||||
| Doolin Pond | Roscrea | 12.5 | 0.27 | 3.20 | P<0.001 *** | 50 fish | 30.1 | 0.36 | 4.03 | P<0.001 *** | 14 fish |
| Sheelin F' | 11.6 | 0.30 | 50 fish | 28.4 | 0.31 | 50 fish | |||||
*** = Very highly significant difference
SE = Standard error of the mean
Table 4 Data illustrating the importance of size to the survival of stocked trout
| Lake | Strain | Age (months) | X Length (S.E.) | No. Fish Stocked | Date | Standing Crop Estimates ( ) | |||||
| Oct '76 | Feb '77 | Oct '77 | Feb '78 | Oct '79 | |||||||
| O'Flyn | Roscrea | 24 | 28.3 | (0.13) | 3,323 | April '76 | (1,262-694) 1,100 | (1,395-586) 960 | (209-10) 201 | (332-20) 211 | 0 |
| Roscrea | 13 | 14.7 | (0.13) | 8,735 | May '76 | (280-47) 235 | (265-1) 221 | (254-1) 206 | (247-1) 204 | 0 | |
| Aug. '76 | Aug '77 | Sept '78 | |||||||||
| Inchiquin | Roscrea | 24 | 19.8 | (0.27) | 2,806 | April '76 | (1,000-233) 617 | 0 | 0 | ||
| Roscrea | 12 | 14.5 | (0.21) | 6,313 | April '76 | 0 | 0 | 0 | |||
| Oct '76 | Feb '77 | Oct '77 | Feb '78 | Oct '79 | |||||||
| O'Flyn | Roscrea | 22 | 21.4 | (0.13) | 850 | Feb '77 | - | - | (354-32) 196 | (396-87) 203 | 0 |
| F'Owel | 22 | 23.4 | (0.16) | 1,175 | Feb '77 | - | - | (566-116) 364 | (883-141) 568 | 0 | |
| O'Flyn | Roscrea | 22 | 28.2 | (0.13) | 3,323 | April '76 | (1,262-694) 1,100 | (1,395-586) 960 | (209-1) 201 | (332-20) 211 | 0 |
| F'Owel | 22 | 19.8 | (0.13) | 4,612 | April '76 | (892-580) 784 | (1,247-612) 1,199 | (640-280) 530 | (690-261) 533 | (70-1) 50 | |
| Aug. '76 | Aug. '77 | Sept '78 | |||||||||
| Inchiquin | Roscrea | 24 | 19.8 | (0.27) | 2,806 | April '76 | (1,000-233) 617 | 0 | 0 | ||
| F'Owel | 24 | 17.3 | (0.32) | 3,300 | April '76 | (666-93) 380 | (386-1) 197 | 0 | |||
(S.E.) - Standard error of the mean length on stocking;
(-) - 95% Confidence limits.
Table 5 Data illustrating the importance of size in relation to angling catches of stocked trout
| Lake | Strain | Age (months) | X Length | (S.E.) | No. Fish Stocked | Date | Angling Catch Estimates () | ||||
| 1976 | 1977 | 1978 | 1979 | 1980 | |||||||
| O'Flyn | Roscrea | 24 | 28.3 | (0.13) | 3,323 | April '76 | (2,137-1,740) 1,876 | (419-379) 399 | (209-87) 199 | 0 | 0 |
| Roscrea | 13 | 14.7 | (0.13) | 8,735 | May '76 | 0 | 0 | 0 | 0 | 0 | |
| Inchiquin | Roscrea | 24 | 19.8 | (0.27) | 2,806 | April '76 | (1,629-1,132) 1,352 | (322-1) 159 | 0 | 0 | 0 |
| Roscrea | 12 | 14.5 | (0.21) | 6,313 | April '76 | 0 | 0 | 0 | 0 | 0 | |
| O'Flyn | Roscrea | 22 | 21.4 | (0.13) | 850 | Feb '77 | - | (86-3) 480 | 0 | 0 | 0 |
| F'Owel | 22 | 23.4 | (0.16) | 1,175 | Feb '77 | - | (437-128) 294 | (440-101) 320 | 0 | 0 | |
| O'Flyn | Roscrea | 22 | 28.2 | (0.13) | 3,323 | April '76 | (2,137-1,740) 1,876 | (419-379) 399 | (209-87) 199 | 0 | 0 |
| F'Owel | 22 | 19.8 | (0.13) | 4,612 | April '76 | (1,422-1,325) 1,384 | (401-278) 387 | (331-74) 270 | 0 | 0 | |
| Inchiquin | Roscrea | 24 | 19.8 | (0.27) | 2,806 | April '76 | (1,629-1,132) 1,352 | (322-1) 159 | 0 | 0 | 0 |
| F'Owel | 24 | 17.3 | (0.32) | 3,300 | April '76 | (617-273) 462 | (321-0) 155 | 0 | 0 | 0 | |
(S.E.) - Standard error of the mean length on stocking;
(-) - 95% confidence limits.
Table 6 Data illustrating the importance of the stocking date to the survival of planted brown trout
| Lake | Strain | Age (months) | X Length | (S.E.) | No.Fish Stocked | Date | Standing Crop Estimates () | ||||
| March'78 | March'79 | ||||||||||
| Sheelin | Roscrea | 23 | 22.4 | (0.22) | 33,296 | March'77 | (21,723-10,699) 16,211 | (13,937-6,163) 10,281 | |||
| Roscrea | 19 | 21.3 | (0.26) | 20,827 | Oct '77 | (4,897-1,471) 3,184 | (5,956-3,971) 4,972 | ||||
| Sheelin | Roscrea | 23 | 26.3 | (0.18) | 34,133 | March'78 | - | (8,180-5,454) 6,820 | |||
| Roscrea | 19 | 25.7 | (0.2) | 15,200 | Oct '78 | - | (4,685-3,123) 3,900 | ||||
| Oct'76 | Feb '77 | Oct '77 | Feb '78 | Oct '79 | |||||||
| O'Flyn | Roscrea | 19 | 24.3 | (0.14) | 3,338 | Oct '75 | (331-21) 163 | (460-130) 280 | (290-1) 131 | (285-1) 127 | 0 |
| Roscrea | 24 | 28.2 | (0.14) | 3,323 | April '76 | (1,262-694) 1,100 | (1,395-586) 960 | (209-1) 201 | (332-20) 211 | 0 | |
| O'Flyn | Roscrea | 19 | 21.5 | (0.15) | 3,978 | Oct '76 | - | (490-260) 354 | (560-73) 303 | (530-71) 301 | (47-1) 20 |
| Roscrea | 22 | 21.4 | (0.13) | 850 | Feb '77 | - | - | (354-32) 196 | (396-87) 203 | 0 | |
| O'Flyn | Roscrea | 19 | 21.5 | (0.15) | 1,265 | Oct '77 | - | - | - | (1,202-291) 759 | 0 |
| Roscrea | 22 | 22.3 | (0.17) | 1,081 | Feb '78 | - | - | - | - | (510-112) 216 | |
(S.E.)- Standard error of the mean length on stocking;
(-) - 95% confidence limits.
Table 7 Data illustrating the importance of the stocking date in relation to angling catches of stocked trout
| Lake | Strain | Age (months) | X Length | (S.E.) | No. Fish Stocked | Date | Angling Catch Estimate () | ||||
| 1977 | 1978 | 1979 | |||||||||
| Sheelin | Roscrea | 23 | 22.4 | (0.22) | 33,296 | March '77 | (*) | (1,841-1,287) 1,584 | (1,423-364) 814 | ||
| Roscrea | 19 | 21.3 | (0.26) | 20,827 | Oct'77 | (*) | (*) | (190-0) 28 | |||
| Sheelin | Roscrea | 23 | 26.3 | (0.18) | 34,133 | March '78 | - | (*) | (1,315-211) 701 | ||
| Roscrea | 19 | 25.7 | (0.2) | 15,200 | Oct '78 | - | - | 0 | |||
| 1976 | 1977 | 1978 | 1979 | 1980 | |||||||
| O'Flyn | Roscrea | 19 | 24.3 | (0.14) | 3,338 | Oct'75 | (390-110) 294 | (108-30) 68 | 0 | 0 | 0 |
| Roscrea | 24 | 28.2 | (0.14) | 3,323 | April'76 | (2,137-1,740) 1,876 | (419-379) 399 | (209-87) 199 | 0 | 0 | |
| O'Flyn | Roscrea | 19 | 21.5 | (0.15) | 3,978 | Oct'76 | - | (251-90) 163 | (230-43) 141 | (135-22) 93 | 0 |
| Roscrea | 22 | 21.4 | (0.13) | 850 | Feb'77 | - | (86-3) 48 | 0 | 0 | 0 | |
| O'Flyn | Roscrea | 19 | 21.5 | (0.15) | 1,265 | Oct'77 | - | - | (241-21) 125 | (165-95) 130 | 0 |
| Roscrea | 22 | 22.3 | (0.17) | 1,081 | Feb'78 | - | - | (490-141) 224 | (460-100) 227 | (170-30) 113 | |
(*) - Trout which were below the angling size limit and not cropped;
(S.E.) - Standard error of the mean length on stocking;
(-) - 95% confidence limits.
