Hydrobiologia 392: 217–223, 1999.© 1999Kluwer Academic Publishers. Printed in the Netherlands.

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Some characteristics of a pikeEsox luciusL. populationin an Irish reservoir

W. Roche1, M. O’Grady1 & J. J. Bracken2,∗1Central Fisheries Board, Mobhi Boreen, Glasnevin, Dublin 9, IrelandE-mail: willie.roche@cfb.ie2Zoology Dept., University College Dublin, Belfield, Dublin 4, IrelandE-mail: john.bracken@ucd.ie (∗author for correspondence)

Received 14 September 1998; in revised form 15 January 1999; accepted 24 January 1999

Key words:reservoir,Esox lucius, gill-netting, population structure, age, CPUE

Abstract

A study of a stocked upland reservoir trout fishery, using gill nets, revealed the presence of a large stock of pike andpoor trout numbers. The overall growth rate of pike was slow. Two year-classes of pike dominated, but the broadage and length frequency ranges, and the high average weight of individuals were indicative of an unexploitedstock. Higher than normal water levels during the spawning period and into the summer period in both 1985 and1986, coincided with the dominant pike year-classes recorded several years later.

Introduction

Pollaphuca, the largest reservoir in Ireland, was com-pleted in 1941. Since then, the Electricity SupplyBoard (ESB) has operated this upland reservoir as atrout fishery. Two species, brown troutSalmo truttaL.and minnowPhoxinus phoxinusL., constituted the ori-ginal fish fauna (Moriarty, 1963). PerchPerca fluviat-ilis L., pikeEsox luciusL., roachRutilus rutilusL andrudd Scardinius erythropthalmusL. were introducedto the reservoir. By the late 1950s, perch outnumberedbrown trout and were found to be significant compet-itors for available food (Moriarty,loc. cit.). Eventu-ally, the reservoir became overrun with small, stuntedperch (Kennedy & Fitzmaurice, 1971) and pike wereintroduced in an attempt to reduce perch numbers.

Between 1969 and 1974, some 1600 pike, weigh-ing in excess of 2.5 tonnes, were culled using gillnets as part of a rehabilitation programme for thetrout fishery. Despite perch culling and stocking ofthe catchment streams with juvenile trout, the reser-voir trout population continued to decline to such anextent that trout angling virtually ceased. Stockingof takeable brown trout directly into the reservoir by

ESB commenced in 1980, and rod catches improvedin comparison to previous decades. In recent yearsPollaphuca has come to be regarded as a mixed fishery,comprising of a stocked brown trout fishery, and a pikeand roach fishery.

An intensive gill netting programme was under-taken in April 1990 to assess the status of fish stocks inthe reservoir. Pike, perch, roach, trout and rudd werecaptured. Financial constraints precluded a long-termstudy of this extensive water body. Some character-istics of the pike population are reported in this paper.Trout population estimates in the catchment tributarieswere presented in an earlier paper (Kelly-Quinn et al.,1996).

The Study Area

Pollaphuca has a surface area 1970 ha. at 186.3 mO.D. Two major river systems, the River Liffey andthe Kings River, discharge into opposite ends of thereservoir, together with numerous minor tributaries(Figure 1). The reservoir is relatively shallow, witha mean depth of 6.07 m. A summary of the reser-

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Figure 1. Location of Pollaphuca reservoir, Co. Wicklow.

voir’s characteristics is presented in Table 1. Graniteand Ordovician rock dominate the catchment geo-logy. Pollaphuca reservoir is classified as being in the

oligotrophic to mesotrophic category with pH in therange 6.78–8.05 and low conductivity (range 55–150µS cm−1). High colour values, rather than excess-

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Figure 2. Length frequency distribution of Pollaphuca reservoir pike.

Table 1. Physical characteristics of Pollaphuca Reservoir,Co. Wicklow

Grid Reference N 300 100

Altitude (m) 186.3

Rainfall (mm per annum) 900–2000

Surface area (km2) 19.7

Max. length (km) 8

Max. width (km) 3.35

Minimum volume (M m3) 135

Maximum depth (m) 30

Mean depth (m) 6.07

Depth profile (at 186.0 m) 0–3 m 12.79%

3–6 m 23.84%

6–9 m 32.85%

> 9 m 30.52%

Retention time (days) 173.6

Catchment area ( km2) 305.3

Trophic status Oligotrophic -

Mesotrophic

ive algal presence, account for the low transparencyvalues often recorded.

Macrophyte distribution is limited in Pollaphucaand zonation is distinctly evident.Phalaris arundin-aceaL., Eleocharis palustrisR. andJuncussp. dom-inate the marsh areas in the large north-eastern andsmaller north-western basins. The tolerantLobeliadortmannaL., Mentha aquaticaL., Equisetum fluvi-atile L. andE. palustrisoccur where there is a gradual

slope to the water. Much of the shoreline shelvessteeply and, therefore, cannot support extensive mac-rophyte cover. Submerged and emergent aquatic florahave a limited distribution and species recorded in-clude: Polygonum amphibiumL., Potamogetonsp.,E. fluviatile, E. palustris, Glyceria fluitans, ElodeacanadensisMichx. andL. dortmanna.

Materials and methods

Pike were captured using gangs of gill nets. A com-plete gang incorporates seven braided nylon gill nets,each 27.5 m in length and 2.0 m in depth, ranging from50.8 mm up to 127 mm inclusive (stretched knot toknot mesh size), increasing at 12.7 mm intervals. Toreduce the possibility of permanent loss of gill netsdue to the presence of underwater obstructions andthe likelihood of entanglement/snagging (e.g. trees,buildings, etc.), floating gill nets only were used. Netsof each mesh size were arranged randomly in a gangand joined at the head rope only, to ensure a randomcatch and eliminate the problem of leading into a par-ticular mesh size. To eliminate the possibility of biasthe colour, construction, material and twine size werestandardised for each mesh size. Each gang was fishedovernight in randomly chosen sites and was servicedwithin 24 hours of being set. This gill netting pro-cedure was developed by O’Grady (1981) to samplebrown trout populations in Irish lakes. Selectivity testshave shown that this procedure can capture a rep-

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resentative cross-section of a trout population for alltrout≥ 19.8 cm and≤ 47.7 cm forklength (FL). Fiftysites were sampled over a 5-day period in April 1990.The total length of gill nets employed was 9625 m,and a total of 150 pike were captured and measured.Forklength (FL) was measured from the tip of thesnout to the fork of the caudal fin to the nearest mm be-low. One hundred and twenty eight fish were weighed(to the nearest 0.025kg) and sexed. Live pike werereturned unharmed to the water after weighing andmeasuring. Scales retained for age analysis (121 sets)were subsequently cleaned in a 5% sodium perox-ide solution, mounted on glass slides with gelatinand read under a Bausch and Lombe Tri- simplexmicroprojector (20×magnification).

Results

Apart from pike, 1460 roach, 716 perch, 10 rudd and30 brown trout (10 resident and 20 stocked) were cap-tured. Eels (Anguilla anguilla L.) were recorded insome pike stomachs.

Distribution of pike in the catchment

Pike were recorded at 35 sites. Most were recordedin bays where extensive, shallow, littoral areas occurand few were captured in the exposed areas of the ma-jor basins. The greatest numbers were captured in thebays where the major tributaries, the Liffey and theKings River enter the reservoir. CPUE was 3.0 (i.e.total number of pike captured / total number of gangsfished).

Length frequency and age distribution

Pike ranged from 34 to 103 cm FL (Figure 2). Malesranged from 34 to 74.0 cm FL and all large fish werefemales (females 38–103 cm FL). Above the min-imum selectivity limit of the gill nets the populationhad a conventional structure, with numbers tailingoff as length increased. Although females dominatedmales by 2:1 (X2, p<0.005), approximately equalratios were recorded in areas of the reservoir whererelatively large numbers of pike were captured.

Pike at the end of their growing season and thoseexhibiting plus growth were plotted together as bothwere representative of the same year class. Nine agegroups were recorded which included pike from twoto ten years old (Figure 3). Four and five year old fishdominated the sample, accounting for over 60% of the

total aged. Younger pike were poorly represented. Six,seven and eight year old pike were recorded in equalnumbers and together comprised over 22% of pikeaged. All of the nine and ten year old fish were malesand the oldest recorded female was eight years old.The reverse was found in Windermere and in the RiverFrome, where the oldest pike were females (Frost &Kipling, 1967; Mann, 1976).

The dominance by two year-classes (four andfive year old fish) coincided with high water levels(>186 m O.D.) recorded from April to August inclus-ive in 1985 and 1986 when these fish were spawned.Available spawning habitat, which is limited in thereservoir, would have been flooded which affordedpike increased spawning opportunities. Extremely lowwater levels were recorded in the reservoir in 1982and 1984. Water levels began to recede in March anddropped steadily until autumn. The presence of olderyear classes suggests that low water had a limitedimpact on the population.

Age and size at maturity

The majority of pike had spawned; 87.5% of malesand 72.8% of females were spent. A further 7.5% ofmales and 24.7% of females were mature. The re-maining fish (5% of males and 2.5% of females) wereimmature. Pike would seem to mature at the end oftheir second year given that 75% of two year old fishwere either mature or spent. The smallest, mature malecaptured was 34 cm FL, while the smallest maturefemale was 38 cm FL. Mature one year old pike, gen-erally males, have previously been recorded in LoughGlore (Healy, 1956) and also in the larger limestonelakes (Kennedy & Fitzmaurice, loc. cit.). In these stud-ies females tended to mature as two year old fish inwaters where growth rate was rapid. Although the vastmajority of three and four year old pike of both sexesin Pollaphuca had matured, some immature fish wererecorded which suggests that breeding may not occurannually in these age groups. All five year old andolder pike had matured.

Length-weight relationship

The length-weight regression equation (logW = log a+ b log L, whereW is weight (g) andL is forklength(mm)) was calculated for male and female pike. Formalesn = 42, loga = −2.075,b = 3.011,r2 = 0.98.For femalesn = 86, log a = −2.006,b = 2.986,r2

= 0.98. The highr-values indicated a strong relation-ship between the two dimensions. In both sexes the

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Figure 3. Age frequency distribution of Pollaphuca reservoir pike.

regression coefficientb = 3 implied that growth wasisometric. Allometry is common in most fish andbtends to be greater or less than 3 in most cases (Ba-genal & Tesch, 1978). The absence of smaller fish inthe sample may account for this result.

Backcalculated growth

The greatest mean annual increments occurred in thefirst two years of life for both sexes (Table 2). Sub-sequently, these increments decreased slowly in males.In females, the increments remained relatively con-stant from age 2 onwards, ranging from 8 to 10 cm,and did not display any pronounced decrease overtime.

Length (FL) at age for males and females wassimilar at L1, L2 and L3 but female pike exhibitedgreater length at age values than males from L4 on-wards (ANOVA, p < 0.05). Pollaphuca pike growthis comparable to that of pike from Barnagrow Lakeby Healy (loc. cit.) (Table 2), where the growth rate isamongst the slowest recorded for the species in Irelandand other temperate lentic waters in Europe detailedby Raat (1988).

Discussion

Individual gill nets are highly selective (Hamley,1975) but a set of mixed gill nets of differing meshsizes at specified intervals, will capture representat-ive samples of the different fish sizes present in agiven population (Lagler, 1978). Pike gill-net selectiv-ity data from lakes in northern Minnesota (Pierce etal., 1994) suggested that the Pollaphuca pike popula-tion was adequately sampled with the gill-nets used,because the minimum length of pike captured waswithin the effective range predicted for the mesh sizesemployed.

By 1969, when the trout fishery rehabilitation pro-gramme commenced on Pollaphuca, it was knownthat a substantial stock of pike existed in the reser-voir. Approximately 1600 pike, averaging 1.6 kg inweight, were removed by gill nettings over a fiveyear period up to 1974. Using data from gill nets ontwo large productive trout lakes in the Irish midlandsin the mid-1960s, Fitzmaurice (1983) demonstratedhow intensive pike control measures reduced the meanweight of pike fromcirca 2 kg to 1 kg. The meanweight of pike taken in survey nets (45 m in length× 1.5 m deep, mesh sizes ranging from 20 mm to100 mm stretched mesh) in Lough Erne, where a se-lective commercial fishery is operated, was 0.758 kg

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Table 2. Backcalculated growth (cm FL) of pike from Pollaphuca reservoir and selected waters in Ireland and Lake Windermere

Backcalculated length (cm)

Water L1 L2 L3 L4 L5 L6 L7 L8 L9 L10

Pollaphuca Reservoir Combined 17.6 30.26 39.6 49.52 58.98 66.78 72.64 78.07 68.35 71

(current study) Male 18.02 29.87 38.35 47.69 55.83 61.29 65.14 67.57 68.35 71

Female 17.39 30.45 40.18 50.37 60.46 69.03 76.92 87.07

Barnagrow Lake Combined 18.2 29.4 39.6 49.8 58.4 63.5 70.6 86.8

(Healy, 1956)

Lough Glore Combined 21.7 36 49.2 60.6 73.6 86.1 95.3 105.5 111.8

(Healy, 1956)

Lough Arrowa Female 23.1 45.5 58.2 72.9 80.8

(Bracken & Champ, 1971)

Lough Derravaragha Female 22.7 36.8 45.2 52.6

(Bracken & Champ, 1971)

Lake Windermerea Female 23.9 41.9 56.9 67.1 74.9 81 85.1

(Frost & Kipling, 1967)

aFrom Fitzmaurice (1983).

(Rosell, 1994). In 1990, pike in Pollaphuca reservoirhad a broad length and age range, typical of an un-exploited stock (Kipling & Frost, 1970, 1972; Otto,1979). The mean weight of individual fish was 2.12kg. On this basis, it is reasonable to suggest that themean weight of individual pike, taken by gill nets, inan unexploited stock in lentic Irish waters is likely tobe≥ 1.5 kg.

Usually 50% of gangs used by O’Grady (loc. cit.)were bottom-set and the remainder was floating on thesurface. The results obtained during the current studywere comparable to similar Irish studies because thereservoir is relatively shallow and low water condi-tions prevailed during sampling (185 m O.D.). TheCPUE value of 3.0 is amongst the highest values re-corded for similar studies in Irish lakes. Annual CPUEvalues ranging from 0.5 to 4.07 (mean 1.5) have beenrecorded for Lough Sheelin, a managed trout fishery,between 1978 and 1996. On Loughs Ennell and Corribvalues ranging from 1.27 to 1.93 have been recorded(O’Grady et al., loc. cit.). Rosell (loc. cit.) recordedlower CPUE values (≤ 0.8) in Lough Erne. These datasuggest that a large pike population was present inPollaphuca in 1990, and that despite the intensive net-ting programme carried out between 1969 and 1974,

the pike population has become re-established in theintervening period.

Studies have shown that intensive gill netting ef-fects changes in pike population structure by eliminat-ing larger pike (Kipling & Frost, 1970 & 1972; Otto,loc. cit.; Le Cren, 1987), which is accompanied by areduction of the average weight of individual pike cap-tured. In Windermere, although the pike populationsize remained stable post-netting, it was dominated bygreater numbers of younger fast growing fish (Kipling& Frost, 1972). Literature was cited by Raat (loc. cit.)which showed that this pattern was also observed inHeming Lake, Manitoba, where following cessationof gill netting the average length and weight of pikeincreased and the population structure rapidly revertedto a pre-netting or unexploited state. Similar trendshave been noted in some Irish lakes (O’Grady et al.,loc. cit.).

Water level fluctuation is probably the most sig-nificant influence on fish populations in reservoirs(Vostradovsky, 1990; Cohen & Radomski, 1993). InIrish waters pike tend to spawn between Februaryand April, in shallows on macrophytes or marginalgrasses, when water temperatures approach 9◦ or10 ◦C (Healy, loc. cit.; Fitzmaurice, loc. cit.). InPollaphuca reservoir, where macrophyte distribution

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is limited, low lying margins and swamp areas areof paramount importance for successful pike spawn-ing. Strong year-classes of pike were recruited tothe population when hydrological conditions (risingwater levels and suitable water temperatures) wereparticularly favourable.

The findings of this study should assist decision-making in relation to the future management of thisimportant fishery.

Acknowledgements

The authors wish to acknowledge the Electricity Sup-ply Board Fisheries Conservation Section who facil-itated this work, but do not necessarily endorse ourconclusions. Thanks are also due to Dr Tony Mc Nally,Dublin Corporation for providing water chemistrydata.

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