Pike, Esox lucius, are present in Loch Callater at their highest altitude and most extreme habitat in the British Isles, with subarctic winter conditions and extended winter ice-cover. The response of pike in this environment is slower growth, due to a shorter growing season and the low availability of forage fish, giving the poorest reported length-at-age for pike in the British Isles. All pike were mature or had spawned in the same year, with gravid ovaries in April and normal recovering ovaries in June-July. As in other lochs with few prey fishes, the larger pike ate small items such as invertebrates

Treasurer, Jim

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PIKE IN A HIGH-ALTITUDE LOCH IN SCOTLAND 59 
LIFE-HISTORY STRATEGIES OF PIKE 
IN A HIGH-ALTITUDE LOCH IN SCOTLAND 
JIM TREASURER 
(Dr J. W. Treasurer, Fish Health, Lochailort, 
Inverness-shire PH38 4LZ, Scotland.) 
[Tel: 01687 470255; Fax: 01687 470279; 
E-mail: jim. treasurer® marine-harvest, co. uk] 
Introduction 
The pike, Esox lucius, a top piscivore of fish communities in north temperate 
fresh waters (Raat 1988; Craig 1996), is common in many lowland waters in 
the British Isles and is normally found in diverse fish assemblages which may 
include cyprinids, percids and salmonids. However, in Scotland where fish 
species diversity is restricted following the last glaciation, ca. 10,000 years ago 
(Maitland 1977), the pike is mainly found in simple fish assemblages and may 
become the dominant species by suppressing salmonid stocks. The pike is also 
a major predator of duck chicks and can restrict the distribution of certain 
species of waterfowl in highland lochs (R. N. Campbell, personal 
communication; Treasurer 1980; Maitland & Campbell 1992). In northern and 
eastern Scotland, pike are frequently present in simple predator-prey 
associations with perch (Treasurer et al. 1992). In these lochs, pike numbers 
(5.4 per ha), biomass (range 3.9-5.9 kg per ha) and growth rate are typical of 
pike in other waters in the British Isles (cf. Raat 1988). 
Although pike inhabit a wide climatic range and their distribution in Eurasia 
extends to the Arctic, being found at temperatures as low as 0.1 °C, they are 
best adapted to shallow, moderately productive mesotrophic-eutrophic fresh 
waters (Casselman 1996). The present study examines the life history strategies 
of pike in a high-altitude loch, Loch Callater, and compares growth and 
reproduction with pike in other locations in the British Isles. The potential 
impact of pike in pristine salmonid-dominated communities, following the end 
of the last glaciation, will be discussed. Although not an extreme environment 
in terms of the temperate zone, Loch Callater, at 495 m, is the highest lake in 
the British Isles containing a population of pike and has a subarctic 
environment with low water temperatures, falling to below 2°C, for up to six 
months of the year. The loch is 85 km from the sea and is situated in the most 
continental part of Scotland. 
Study area 
Loch Callater is located in the Cairngorm mountains in the River Dee 
catchment, 6 km south of Braemar. The loch (Fig. 1), 1.35 km long and 320 m 
60 JIM TREASURER 
PIKE IN A HIGH-ALTITUDE LOCH IN SCOTLAND 61 
wide, was surveyed in 1906 by Murray & Pullar (1908) who recorded a mean 
depth of 3.7 m, and a maximum of 9.1 m. The loch is fed by the Allt na Loch 
Burn which has a catchment area of 19.8 km2. Glacial action produced the 
steep and rocky hillsides surrounding the loch and deposited a gravel ridge 
across the glen, creating Loch Callater. It is a typical nutrient-poor highland 
loch (Table 1) with an annual mean nitrate level of 0.22 mg per litre and 0.01 
mg per litre of phosphate. The pH is higher than expected and increased from 
6.6 in 1983 to 7.0 in 1996 (R. Owen, personal communication). Other fish 
caught in the loch in this survey were salmon Salmo salar, brown trout Salmo 
trutta and eel Anguilla anguilla. Pike were present in the River Dee catchment 
in 1790 (Maitland 1977), but there is no record of the introduction of pike to 
Callater and it is probable that pike were introduced by humans. 
Growth 
Pike were captured with multifilament gill nets (mesh sizes ranging from 49 to 
240 mm when stretched) set overnight in April 1989 and 1990. April was 
62 JIM TREASURER 
chosen because it was planned to sample fish prior to spawning, in order to 
examine reproductive investment - for example fecundity, but the numbers of 
fish captured were low (5 pike, 2 brown trout). Therefore samples were also 
taken in May and August 1992 (4 pike, 1 brown trout) and July 1995 (7 pike, 2 
brown trout and 4 salmon). Pike were measured to fork length (mm) and wet 
weight (g). Opercular bones and the cleithra (large paired bones forming the 
pectoral girdle) were removed, immersed in cedarwood oil and examined on a 
dark background with reflected light. Annual growth rings were easily 
distinguished in cleithra as a translucent zone followed by an opaque zone of 
high intensity, but in opercula the inner annulus was obscured. Therefore 
cleithra were solely used for back-calculation of length at previous ages. The 
distance from bone origin to each annulus was measured on the dorsal carina of 
cleithra under a low power (x40) microscope with an eyepiece micrometer 
(Casselman 1978). No "plus" growth (+) was observed on the outer edge of 
cleithra from pike captured on 18 April 1989 when water temperature was 
0.5°C at 50 cm depth, indicating that somatic growth had not commenced, 
although additional growth beyond the last annulus was identified from pike 
captured on 11 April 1990 when water temperature was 6.0°C. Total lengths of 
opercula were plotted against fish length to apply a correction for allometric 
growth of back-calculations at previous ages: X = 1.0027 (SE 0.0232) + 1.0013 
(SE 0.0142), n = 16, with a highly significant correlation, r = 0.993 (df = 14). 
As the slope almost passed through the origin, the growth of cleithra relative to 
fish length was isometric, permitting back-calculations of length at previous 
age by the direct proportionality formula (Tesch 1971). 
The range in body length (fork length) of captured pike was 205 to 701 mm, 
wet weights were 75 to 2905 g, and ages ranged from 2 to 11 years. 
Unusually, from 4 to 7 years of age, males were slightly longer than females 
(f-test at each age, p>0.05), although not significantly (Table 2). Length-at-age 
was low compared with Deeside lochs at lower altitude (Treasurer & Owen 
1991): the mean length of pike in Callater was 231 to 236 mm at age 3 (Table 
2) compared with 400 mm in Loch Davan and 320 mm in oligotrophic Loch 
Kinord. Length-at-age was also lower than in other highland lochs (Fig. 2), 97 
to 106 mm at age 1, ca. half the length of pike in Loch Lomond (Shafi & 
Maitland 1971) and Windermere (Kipling 1983), and similar to pike in Loch 
Choin, a highland loch with poor pike growth (Munro 1957). In Callater, 
growth of pike was slower than in Loch Choin from age 2 years and similar to 
growth of pike in the dystrophic Dubh Lochan (Shafi & Maitland 1971). The 
slow growth of pike in Loch Callater may be due to the low availability of 
alternative prey and a short growing season, as water temperatures were as low 
as 2.0°C from November to mid-April, frequently with continuous ice-cover 
through winter. 
PIKE IN A HIGH-ALTITUDE LOCH IN SCOTLAND 63 
Reproduction 
In 1989, spawning commenced in mid-April at a water temperature of 6.0°C 
(Table 3). The gonadosomatic indexes (GSIs as [gonad wt÷ fish wt] x 100) of 
two fish were 5.5 and 5.7%, with 1802 and 3515 eggs respectively and relative 
fecundities of 4.7 and 6.7, at the lower end of the ranges reported by Raat 
(1988), i.e. 3700 to 234,000 eggs and relative fecundities of 4 to 46. The range 
in egg diameter, from 2.1 to 2.8 mm (Table 3), was typical of pike in Eurasia 
and North America. All fish sampled were mature or had spawned previously, 
males at 2 to 7 years old and females at 6 to 10 years. 
The mean maximum GSIs of pike in Callater were lower than in two other 
Deeside lochs, Davan (8%) and Kinord (9%) respectively, but were similar to 
the Loch of Skene (4 to 9%) in the lower Dee Valley (Treasurer 1990). GSIs 
were much lower than those found in southern lakes in the British Isles; for 
64 JIM TREASURER 
FIG. 2. The back-calculated fork-lengths (mm) at various ages of male pike (above) and female 
pike (below) in Loch Callater, compared with other locations in the River Dee Valley (Loch 
Kinord, Loch Davan and Loch Skene) (Treasurer & Owen 1991), with other highland lochs in 
Scotland, Loch Lomond and Dubh Lochan (Shah & Maidand 1971) and Loch Choin (Munro 
1957), and with Windermere in northwest England (Kipling 1983). 
PIKE IN A HIGH-ALTITUDE LOCH IN SCOTLAND 65 
example, mean maximum GSIs of 15 to 20% were reported from pike in 
Windermere (Kipling & Frost 1969) and 17% was reported from pike in 
Slapton Ley (Bregazzi & Kennedy 1980). Fecundity of pike is dependent on 
several factors, e.g. temperature and food availability (Billard 1996). Food is 
restricted in Callater by the limited number of prey species; there are no small 
prey such as sticklebacks, perch and minnows. The extreme winter conditions 
experienced at Callater may also influence fecundity. In support of this view, 
the lower reproductive investment of pike in Loch Kinord and Loch Davan, at 
a lower altitude in the same Dee valley, is associated with the relatively cold 
winters, as these lochs are situated in the most continental part of Scotland. 
Also there is a restricted food supply, with a low availability of prey fish such 
as perch and other forage fish such as minnows and sticklebacks (Treasurer 
1990). Another example is the low absolute fecundity of pike in the Vilyuy 
Reservoir, Russia, which is only free of ice-cover from the end of June, 
compared with more southern lakes (Sukhanova 1979). 
Food 
The food of pike in Callater was analysed by the occurrence method, i.e. the 
percentage of stomachs containing a particular food item. There was a narrow 
range of benthic organisms: Ephemeroptera larvae (75% occurrence), 
Gammarus pulex (25%), chironomid pupae (8%), Odonata (8%) and also 
zooplankton (8%). The two largest pike, 672 and 701 mm respectively, had 
eaten pike. This diet was similar to that of pike in other lakes; for example, 
invertebrates are the main food of small pike and larger pike are piscivores 
(Mills 1964). Gammarus was also the main food of pike in Loch Choin (Munro 
1957) where, similarly, larger pike ate considerable numbers of invertebrates 
because suitable larger prey was absent. 
Parasites 
The exterior of the fish, gills, eyes and viscera were examined for protistan and 
metazoan parasites, with a binocular microscope at x40 magnification. No 
parasites were found on the skin or viscera, and only one in the eye humor, a 
Diplostomum species in one fish. 
The impact of pike on pristine salmonid communities in highland lochs 
There were few fishes in Scottish fresh waters following the end of the last 
glaciation ca. 10,000 years ago (Maitland 1977). By 1790, five additional 
species had colonised southern Scotland and by 1890 five more species were 
introduced by humans. Many of the lochs in the Cairngorm mountains retain 
their original simple fish communities, dominated by the salmonids - brown 
66 JIM TREASURER 
trout and salmon, and also containing Arctic charr Salvelinus alpinus and eels 
(Maitland 1972). The original fish fauna of Loch Callater comprised eels and 
brown trout, with migratory adult salmon and salmon parr largely confined to 
feeder streams. Although Arctic charr are found in nearby lochs, e.g. Loch 
Muick, they have not been recorded from Callater as the loch is probably too 
small and shallow. Campbell (1979) concluded that Arctic charr were mainly, 
although not exclusively, present in large deep lochs. Pike and perch are 
common fishes in the smaller lowland east coast lochs. Although perch 
colonised the Dee valley, they are not found in the rivers and lochs of the 
Cairngorm mountains. In contrast, pike are present in many of the lochs and 
rivers in the Dee and Spey valleys with catchments in the Cairngorms. Pike 
were probably introduced into Callater by humans, as the efferent stream, the 
Callater Burn, is fast-flowing and has several small waterfalls. 
The impact of the introduction of pike to the original indigenous salmonid-
dominated fish community of Loch Callater would have been to rapidly reduce 
the numbers of an abundant brown trout population with a low mean weight, 
typically found in many small relatively shallow Scottish lochs (Campbell 
1971). Pike would also prey on migrating salmon smolts in the shallow and 
narrow loch. For example, Mills (1964) calculated that ca. 10% of the smolt 
run on the River Bran catchment, Ross-shire, was eaten by pike. 
Conclusions 
Pike are present in Loch Callater at their highest altitude and most extreme 
habitat in the British Isles, with subarctic winter conditions and extended 
winter ice-cover. The response of pike in this environment is slower growth, 
due to a shorter growing season and the low availability of forage fish, giving 
the poorest reported length-at-age for pike in the British Isles. In some extreme 
environments, for example in high-altitude Alpine lakes, brown trout spawn 
only in each second year, as the summers are too short to permit recovery of 
the gonads following spawning and oogenesis (Cavalli & Chappaz 1996). This 
was not evident from the sample of female pike examined from Loch Callater; 
all were mature or had spawned in the same year, with gravid ovaries in April 
and normal recovering ovaries in June-July. There was an impact on 
reproductive potential, however, as absolute and relative fecundities were low 
compared with reported values. As in other lochs with few prey fishes, the 
larger pike ate small items such as invertebrates. 
Acknowledgements 
I am grateful to Invercauld Estates for permission to sample pike in Loch 
Callater and to the Scottish Environmental Protection Agency (East Region) 
for access to water chemistry data. 
PIKE IN A HIGH-ALTITUDE LOCH IN SCOTLAND 67 
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Life-history strategies of pike in a high-altitude loch in Scotland

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Life-history strategies of pike in a high-altitude loch in Scotland

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