3 Finnmark College, N-9500 Alta, Norway. Key words: Arctic charr, brown trout, low acidity lakes, habitat use, life history. Abstract. Habitat utilization and the life ...
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Hydrobiologia 348: 113–126, 1997.
I. P. Muniz (ed.), The Høylandet Reference Area. c 1997 Kluwer Academic Publishers. Printed in Belgium.
Habitat use and life history of brown trout (Salmo trutta) and Arctic charr (Salvelinus alpinus) in some low acidity lakes in central Norway Trygve Hesthagen1 , Bror Jonsson2, Ola Ugedal3 & Torbjørn Forseth1 1
Norwegian Institute for Nature Research, Tungasletta 2, N-7005 Trondheim, Norway Norwegian Institute for Nature Research, P.O. Box 736, Sentrum, 0105 Oslo, Norway 3 Finnmark College, N-9500 Alta, Norway 2
Key words: Arctic charr, brown trout, low acidity lakes, habitat use, life history
Abstract Habitat utilization and the life history of brown trout Salmo trutta and Arctic charr Salvelinus alpinus were investigated in five sympatric populations and five allopatric brown trout populations in Høylandet catchment, a atmosphaeric low deposition area in Mid Norway. There was a significant inverse correlation in abundance of epibenthic Arctic charr and brown trout in these lakes, indicating that the latter species is dominant. The largest numbers of sympatric brown trout and Arctic charr were caught in epibenthic habitat. In two lakes, brown trout to some extent also occurred pelagically, while pelagic individuals of Arctic charr were found in all five lakes. The main food items for both epibenthic and pelagic brown trout were terrestrial surface insects and chironomid pupae. Zooplankton was the primary food item for Arctic charr in both habitats. Although the age distribution was very different in the populations, neither species seem to suffer from recruitment failure. There was no significant difference in survival rates between sympatric populations of brown trout and Arctic charr. We found a significant inverse correlation between epibenthic catches of brown trout and the mean weight of 4+ fish, the most abundant age group. However, if using weight data for three-year-old fish, no such relationship was found for Arctic charr. Brown trout and Arctic charr reached asymptotic lengths of 197–364 mm and 259–321 mm, respectively. Both species typically reached sexual maturity at age 2–3, and no maturation-induced mortality was evident. We conclude that fish populations in Høylandet lakes are regulated throughout their lifes by inter- and intraspecific competition. Introduction Acidification has caused considerable damages to lenthic fish populations in southern Norway during recent decades (Sevaldrud & Muniz, 1980; Hesthagen et al., 1994). Brown trout, Salmo trutta L. and Arctic charr, Salvelinus alpinus (L.) are widely distributed throughout this region and are among the fish species that are most sensitive to acid aluminium-rich soft waters (Rosseland et al., 1980; Andersen et al., 1984; Hesthagen & Sandlund, 1995). Sympatric populations of Arctic charr and brown trout are common in Norwegian lakes (Hesthagen & Sandlund, 1995). However, they differ in several ecological aspects, such as higher aggressiveness among brown trout (Kalleberg, 1958; Nilsson, 1963; Noakes,
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1980; Jobling & Reinsnes, 1986). The habitat selection of young fish in running water seems to confirm such differences in social behaviour (Heggberget, 1984). Langeland et al. (1991) concluded that differences in habitat and resource utilization between Arctic charr and brown trout are explained by selective differences and asymmetric competition. When allopatric, brown trout and Arctic charr have similar food preferences, while in sympatry, Arctic charr feed more on different zooplankton species (Nilsson, 1965; Sv¨ardson, 1976; Langeland et al., 1991). Therefore, Arctic charr more commonly exploit pelagic habitats than brown trout. In both monomorphic and polymorphic stocks of Arctic charr, individuals of different sizes may have partly segregated habitat utilization (Klemetsen & Grotnes, 1975; Hindar & Jonsson, 1982; Sandlund et al., 1992;
Article: hydr3978IPM GSB: Pips nr 138814 BIO2KAP hydr3978.tex; 10/09/1997; 17:04; v.7; p.1
114 Hesthagen et al., 1995a). Habitat utilization in brown trout is usually restricted to shallow littoral areas (Nilsson, 1963; Thorpe, 1974; Sv¨ardson, 1976). However, in large deep lakes with limited littoral zones, brown trout segregate according to size, and the largest specimens occupy the pelagic regions (Haraldstad & Jonsson, 1983; Jonsson & Gravem, 1985; Schei & Jonsson, 1989; Jonsson, 1989; Hegge et al., 1989; Hesthagen et al., 1995b). In this paper, we present test-fishing data on brown trout and Arctic charr from 10 slightly acidic lakes at Høylandet, Mid Norway. Five lakes supported sympatric populations and five allopatric populations of brown trout. The objectives of this study were to study their habitat utilization and life history, which we then relate to those in acid-stressed lakes. Brown trout and Arctic charr populations subjected to acidification typically exhibit ageing through failure of recruitment or juvenilization due to increased mortality of adult fish (Almer et al., 1974; Rosseland et al., 1980; Harvey, 1982; Andersen et al., 1984, Hesthagen et al., 1995). Fish populations in acidic lakes may also exhibit a pronounced increase in mortality rates at sexual maturity (Frenette & Dodson, 1984; Trippel & Harvey, 1987). Furthermore, fish in acidic lakes more frequently mature at greater age and larger size, and may have shorter reproductive life spans than individuals in circumneutral lakes (Trippel & Harvey, 1987).
Study area Høylandet catchment is located in the county of NordTrøndelag in Mid Norway (Figure 1), and drains into the River Namsen. The catchment covers an area of 551 km2 (Nøst, 1982). The bedrock in the drainage area consists mainly of slowly weathering granite and grando-diorite. In the northeastern part of the drainage area, there is Cambrosiluric bedrock, mainly glimmer. Our study lakes ranged from Bl˚arøyvatn (415 m a.s.l.), where the catchment area is mainly covered by grass and shrubs, to Ors-Grønningen (134 m a.s.l.) whose catchment is forested (Table 1). The local climate is marine, and the average annual rainfall is about 2000 mm. Chemical analysis of precipitation at Høylandet started in February 1987 with monthly samples (SFT 1994). The acidity of the precipitation is the lowest measured of any station in Norway, with weighted annual mean pH and sulphate ranging between 4.98–5.16 and 0.30–0.40 g S l 1 respectively (1987–1993). Total annual deposition of sulphate
is estimated at 220–269 mg S m 2 yr 1 and that of nitrate at 120–125 mg (N) m 2 yr 1 (SFT 1994). The study lakes have generally slightly acid water (pH 5.86–6.83) and low concentrations of calcium (0.26–1.45 mg l 1 ) except for Brynntjønna and Litltjønna, which are located near agricultural land (Table 1). The concentration of labile aluminium was low in all lakes (1–22 g l 1 ). The water colour and secchi disc transparency vary considerably, ranging between 5–55 mg Pt l-1 and 4–9 m respectively. Five of the study lakes were inhabited by only brown trout, while five lakes contained sympatric stocks of brown trout, Arctic charr and three-spined sticklebacks, Gasterosteus aculeatus L. (Table 1). Several lakes also support European eels (Anguilla anguilla (L.).
Methods In lakes inhabited by sympatric populations of brown trout and Arctic charr, sampling was carried out using fleets of bottom and pelagic nets. One benthic gill net series consisted of 8 different nets with bar mesh sizes between 10 and 45 mm (Rosseland et al., 1979). Each net was 27 m long and 1.5 m deep, and one series covered an area of 324 m2 . Benthic nets were set independently both from the shore and in chains consisting of four nets, covering areas from the shoreline to the deepest parts of each lake. In those lakes, sampling was performed on two successive nights except in Lake Grassjøen. The net panels were changed between successive nights so that all mesh sizes were equally represented at each depth interval. We used a multi-mesh survey net to sample fish in the pelagic zone at depths of 0–6 m. One net was 54 m long and 6 m deep (i.e. 324 m2 ) and consisted of the same eight mesh sizes as were in the benthic nets, i.e. panels of 6.75 m of each mesh size. In lakes inhabited by allopatric brown trout stocks, only benthic nets were used, set independently from the shoreline. Relative abundance is expressed as catch 100 m 2 net area 12 h 1 fishing (CPUE). All test-fishing was carried out between August 12–29, 1987. Data on total length (L, mm), weight (W, g) and degree of sexual maturity (Dahl, 1917) were obtained for each fish. Tests of significant differences between sexes in age at sexual maturity were based on the approximation to the binomial distribution (Siegel, 1956; Jonsson et al., 1988). Ages of brown trout were determined from scales and otoliths (Jonsson, 1976)
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Figure 1. Outline of the Høylandet catchment area, showing the geographical location of the study lakes.
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116 Table 1. Different fish species present (brown trout = BT, Arctic charr = AC, three-spined sticklebacks = SB and eel = EL), and some physical and water chemical parameters in some Høylandet lakes. AltiSize BT AC SB EL tude (m) (ha)
Max depth (m) pH
Cond
Alk
Lake
S/cm ek/l
Ca Ala Alo Ali Colour Turb. Secchi disc mg/l g/l g/l g/l mg Pt/l ftu transp. (m)
Bl˚arøyvatn L. Fisk˚avatn S. Fisk˚avatn Brynntjønna Litltjønna Teintjønna Elgsjøen Grassjøen Langvatnet Ors-Grønningen
x x x x x x x x x x
– 5 12 7 7 11 25 22 25 19
12.2 14.0 14.5 40.3 42.0 26.9 23.0 23.6 22.5 20.2
10 18 11 274 284 83 37 24 28 30
0.26 0.35 0.33 4.80 5.00 1.45 0.87 0.79 0.74 0.65
x x x x x
x x x x x x x
x x x x x x x
415 260 250 142 141 204 170 158 150 134
42.5 7.5 32.5 8.5 7.5 60.0 190.0 70.0 120.0 87.5
Figure 2. Regressions between epibenthic catches of Arctic charr and brown trout (CPUE) from sympatric populations in five lakes in Høylandet.
and of Arctic charr from otoliths only (Kristoffersen & Klemetsen, 1991). For comparison of relative abundance (catches on single benthic nets) and growth in brown trout, we related catches from single benthic nets to the mean weight of age 4+ fish, as this age
5.86 6.10 6.05 7.14 7.23 6.83 6.38 6.19 6.22 6.33
40 44 54 71 87 27 58 61 51 46
19 27 32 55 65 23 51 57 50 41
21 17 22 16 22 4 7 4 1 5
5 8 9 51 51 22 30 33 28 18
0.45 0.42 0.40 0.48 0.62 0.49 0.40 0.53 0.43 0.43
9.0
