Fisheries Management and Ecology 1998, 5, 189–199
Changes in the density of introduced bullhead, Cottus gobio L., and its impact on juvenile Atlantic salmon, Salmo salar L., densities in a sub-Arctic salmon river in northern Finland O . P I H L A J A , M . J U L K U N E N , E . N I E M E L A¨ & J . E R K I N A R O Finnish Game and Fisheries Research Institute, River Tenojoki Fisheries Research Station, Utsjoki, Finland
Abstract A newly introduced species, the bullhead, Cottus gobio L., was observed for the first time in the River Utsjoki, a tributary of the River Teno, in 1979. The River Teno is one of the most important Atlantic salmon, Salmo salar L., rivers in Northern Europe and its tributary the River Utsjoki is an important salmon spawning and nursery area. The densities of bullhead were lowest in the areas to which the species has spread most recently and highest in the areas downstream of its introduction point. Densities were markedly lower upstream. Statistical analysis showed that the presence of the bullhead had not affected juvenile salmon densities in the River Utsjoki. bullhead, Cottus gobio, density, interaction, introduced species, northern Scandinavia, Salmo salar.
KEYWORDS:
Introduction The bullhead, Cottus gobio L., was observed for the first time in the River Utsjoki, a tributary of the subarctic River Teno, in the far north of Finland, in 1979, during electric fishing surveys carried out by the Finnish Game and Fisheries Research Institute. This species does not occur naturally in Finnish or Norwegian river systems draining into the Arctic Ocean and it is thought to have been accidentally introduced (Pihlaja, Niemela¨ & Erkinaro 1997). Electric fishing surveys carried out between 1979 and 1995 indicated that the population had spread 14 km upstream and 22 km downstream in the main river and the lower reaches of its tributaries, but its range had not yet extended into the main stem of the River Teno (Pihlaja et al. 1997). The River Teno is one of the most important salmon rivers in Northern Europe, and its tributary the River Utsjoki is a major spawning and nursery area for the Atlantic salmon, Salmo salar L. Given the low production capacity of this subarctic area and that many local fish species are at the extreme limit of their distribution, it may be presumed that a new species could have some impact on the natural fish fauna. It has been suggested elsewhere in Scandinavia (Karlstro¨m 1977; Hesthagen, Hegge, Dervo & Skurdal 1989; Gabler 1994) that the presence of the bullhead and alpine bullhead, Cottus poecilopus Heckel, may have had Correspondence: Outi Pihlaja MSc, Finnish Game and Fisheries Research Institute, River Tenojoki Fisheries Research Station, FIN–99980 Utsjoki, Finland (e-mail:
[email protected]). © 1998 Blackwell Science Ltd
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some effect on populations of salmon and trout, Salmo trutta L., by using the same food resources and favouring the same habitats. This paper attempts to find answers to two questions. How has the population density of the new invading species varied with time? Has the bullhead caused any changes in the densities of juvenile salmon in the River Utsjoki? Material and methods
The River Utsjoki area The River Utsjoki (70° N, 27° E) (Fig. 1), is the largest Finnish tributary (length 66 km, catchment area 1652 km2), of the River Teno and it consists of several lakes connected by stretches of river. Lakes larger than 1 ha make up 93% of the water area of the River Utsjoki system, the largest lakes being long, narrow and over 60 m deep in places. In addition to salmon and bullhead, the fish fauna of the River Utsjoki consists of brown trout, burbot, Lota lota (L.), grayling, Thymallus thymallus (L.), sticklebacks, Gasterosteus aculeatus L., and Pungitius pungitius (L.), whitefish, Coregonus spp., and minnow, Phoxinus phoxinus (L.). Pike, Esox lucius L., perch, Perca fluviatilis L. and Arctic charr, Salvelinus alpinus (L.), also occur occasionally in the lakes. To evaluate changes in the annual mean densities of bullheads, the river was divided into five regions bounded by the largest lakes (Fig. 1). These regions were situated within the present known range of bullhead (Pihlaja et al. 1997). The sampling sites were situated in flowing water. Fish densities in the lakes are not known. The first region, 3.7 km downstream from Lake Ma´ttaja´vri (Fig. 1), is situated in the lowest part of the bullhead’s range. This area, with rapids and long stretches of river, is important salmon spawning and nursery habitat. Bullheads were observed for the first time in the lowest part of the area in 1993 and in the upper part in 1992. The second region (9.2 km) is situated between Lake Ma´ttaja´vri and Lake Geavoja´vri, and consists mainly of small lakes separated by rapids. This region also contains salmon spawning and nursery habitat. Bullheads have been found below Lake Geavoja´vri since 1985 and above Lake Matta´ja´vri since 1988. The third region is situated between Lakes Geavoja´vri and Ganesˇja´vri. The distance between these two lakes is 3.8 km and it includes Lake Puksalja´vri and a 1.9 km long section of river. Bullheads were found in the rapids below Lake Ganesˇja´vri as early as 1979, and above Lake Geavoja´vri in 1981. The fourth region extends 6.7 km upstream from Lake Ganesˇja´vri to below Lake Vuolib Cuoggaja´vri, and comprises small lakes connected by stretches of river which provide suitable habitats for juvenile salmon. Bullheads were observed for the first time in the rapids above Lake Ganesˇja´vri in 1979, and 6 years later below Lake Vuolib Cuoggaja´vri. The fifth region consists of the 2.7 km long Lake Pajib Cuoggaja´vri and the 3.8 km stretch of river between it and Lake Mierasˇja´vri. The upper part of the region, below Lake Mierasˇja´vri, is a particularly important juvenile salmon nursery area, and it is the upper limit of salmon spawning in the River Utsjoki. Bullheads were caught for the first time in the narrow channel between Lake Vuolib Cuoggaja´vri and Lake Pajib Cuoggaja´vri in 1988 and at the uppermost sampling site below Lake Mierasˇja´vri in 1994. The physical characteristics of these five regions are presented in Table 1.
Sampling The data on bullhead densities in the River Utsjoki (Fig. 2) were based on electric fishing surveys carried out by the River Tenojoki Fisheries Research Station of the Finnish Game and © 1998 Blackwell Science Ltd, Fisheries Management and Ecology 1998, 5, 189–199
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Figure 1. Location of the River Utsjoki and the regions 1–5. The impact study sites are marked with symbols.
Fisheries Research Institute. The first 6 years (1979–1986) data were gathered during a longterm salmon density monitoring programme (11 permanent sampling sites from 1979 onwards and 12 from 1986) (Niemela¨, McComas & Niemela¨ 1985). A special bullhead monitoring © 1998 Blackwell Science Ltd, Fisheries Management and Ecology 1998, 5, 189–199
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Table 1. Habitat characteristics (%) of the sampling regions in the fluvial stretches where the sampling sites are located. Region Area (ha)
1 2 3 4 5
29.5 20.05 39.2 42.4 57.2
Proportion Depth (m) of flowing ø 0.7 . 0.7 water (%)
35.3 2.7 13.5 28.3 12.0
35.0 44.1 51.5 55.0 53.5
65.0 55.9 48.5 45.0 46.5
Water velocity (m s–1)
Substrate composition*
, 0.5
Sand 0.2– 6.0 mm
Gravel . 6 mm
– 12.9 – 2.5 –
– 35.4 4.7 3.4 –
42.3 73.0 28.5 32.3 13.5
0.5–1.0 . 1.0
26.7 20.7 23.8 27.8 31.5
31.0 6.3 47.7 39.9 55.0
Stony bottom , 20 cm 20–50 cm . 50 cm
43.0 40.5 29.1 40.0 43.4
32.4 5.7 39.4 34.7 36.6
21.6 – 26.8 19.4 20.0
*Organic material: region 1, 3.0%; region 2, 5.5%.
Figure 2. Annual mean (6 SE) bullhead densities (first catch 100 m–2) in regions 1–5. © 1998 Blackwell Science Ltd, Fisheries Management and Ecology 1998, 5, 189–199
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programme at 36 new sites was initiated in 1987, and a further 51 sites were added in 1993– 1995 to map the distribution of this species in the lower part of the river. All permanent sites were not fished every year, and in some years additional sites were sampled during other studies. The number of sampling sites within the present known range of bullhead and the number of occasions when the species was present are indicated in Table 2. Table 2. Number of sites sampled in each year and number of sampling sites, where species present in first fishing, in parentheses. Year
Area 1
Area 2
Area 3
Area 4
Area 5
Total
1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995
1 1 1 1 1 1 1 2 2 3 3 7 7 7 50 8 32
1 (0) 1 (0) 1 (0) 1 (0) 1 (0) 1 (0) 1 (0) 1 (0) 7 (1) 7 (6) 8 (7) 9 (7) 16 (15) 4 (4) 8 (7) 11 (11) 8 (6)
2 2 2 2 2 2 2 2 4 2 3 3 16 3 3 7 3
4 4 4 4 4 4 4 4 23 20 20 20 30 12 21 22 22
1 (0) 1 (0) 1 (0) 1 (0) 1 (0) 1 (0) 1 (0) 1 (0) 10 (0) 10 (1) 10 (1) 11 (2) 11 (2) 5 (1) 10 (1) 10 (3) 10 (2)
9 9 9 9 9 9 9 10 46 42 44 49 80 31 92 58 75
(0) (0) (0) (0) (0) (0) (0) (0) (0) (0) (0) (0) (0) (2) (3) (3) (8)
(1) (0) (1) (0) (1) (1) (2) (2) (3) (1) (2) (2) (14) (2) (2) (6) (2)
(1) (0) (1) (0) (1) (1) (3) (3) (7) (7) (9) (13) (19) (3) (10) (18) (7)
(2) (0) (2) (0) (2) (2) (5) (5) (11) (15) (19) (24) (50) (12) (23) (41) (25)
Electric fishing was performed each year when the water level was lowest, i.e. from July to September, and the areas fished varied from 10 to 235 m2 (mean 86 m2). Each site was fished one to five times. In most cases, the bullhead densities were very low, and the estimation of population densities by methods based on two or more fishings was not possible. The catchability of bullheads was low (mean 5 0.446, SD 5 0.235), as also found by Karlstro¨m (1977) in Swedish rivers. To render the abundance indices as comparable as possible, the first catch (bullhead 100 m–2) was used as a density index when comparing abundances.
Statistical methods The hypothesis that the bullhead affects salmon densities was tested in a design resembling that of an impact study (Green 1979). As it was impossible to perform a randomized monitoring experiment in one particular river, one treatment data set and two control data sets were used. The direct effect of the bullheads could not be used as treatment data relative to the salmon densities. The annual bullhead monitoring data sets for the years 1987–1994 were modified for the analysis. All the experimental units were situated in a known part of the bullhead’s range, and sites where the bullhead appeared for the first time in 1990 or 1992 were selected as treatment units, the data before impact consisting of the early density observations (years 1987–1989) and the data after impact the later ones (1990–1994). The first control data set © 1998 Blackwell Science Ltd, Fisheries Management and Ecology 1998, 5, 189–199
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comprised the annual densities at sites where bullheads were never observed and the second observations at sites where bullheads were caught throughout the monitoring period. The lowermost part of the river was left out of the analysis because bullheads were rarely caught there, exept in the last few years (Fig. 1). Two-way analysis of variance was used to test the degree to which the observed changes in salmon densities were independent of the occurrence of the bullhead. The response variable was the density of the age group ù 11. The densities of both the salmon age group 01 and those of the bullhead increased from 1987 to 1989 to 1990–1994 (Fig. 3). One time factor was used, which had two levels (1. early and 2. later data, see above), and one bullhead history factor, which had three levels (1. bullheads appeared in the middle of the monitoring period, 2. bullheads were present throughout the monitoring period, and 3. bullheads were never present). The ANOVA null hypothesis was that neither time, nor bullhead occurrence had any effect, nor was there any interaction effect between them. The interaction effect was important for this impact hypothesis that the densities of salmon parr were influenced by the existence of bullheads. The impact null hypothesis was that the densities of salmon in the early and later data were similar at the experimental sites and control sites and there was no interaction between time and bullhead occurrence factors, i.e. no effects of bullhead on salmon densities. The statistical analyses were performed using the SYSTAT software (SYSTAT 1992). A simple correlation analysis (Spearman) was used to check the association between the densities of bullhead and salmon parr, using the data for the same sites as those in the impact study design, except sites where bullheads were never found. The time period used in the correlation analysis was the later one, 1990–1994, as the densities of bullheads were higher. The total number of bullhead and salmon aged 01 and ù 11 year densities observed was 29. The similarity in physical habitat composition between regions 3 and 4 was checked by cluster analysis of the percentages of each habitat type at the sites, using an agglomerative, hierarchical Q-analysis, with simple euclidean distance as the distance measure in the resemblance matrix. The grouping method was Ward’s minimum variance method, which placed almost the same number of sites in each cluster. The data were arranged in a n 3 p matrix, in which the rows (observations) were the 12 sampling sites in regions 3 and 4 and the columns (variables), the percentages of the habitat types at the sites, one column for every habitat type. The cluster took the form of a Q-analysis for rows (sites). Results In the early years, 1979–1986, when there were only 11 sampling sites in the whole river, bullheads were only present in regions 3 and 4 which are situated near the introduction point (Fig. 1). The mean densities increased during this period from fewer than one bullhead 100 m–2 to 16 individuals 100 m–2 in region 3 and from one fish 100 m–2 to six fish 100 m–2 in region 4. From 1987 onwards, more sampling sites were fished in each region (Table 2). In region 3 the densities mostly varied around 20–30 fish 100 m–2 but in region 4, remained lower (5–10 fish 100 m–2). In region 2, which is situated downstream of region 3, the species was observed for the first time in 1987 at a density of 5 fish 100 m–2, after which it increased rapidly to 10–30 fish 100 m–2. By 1994, the figure was 58 fish 100 m–2. Mean densities were low in the lowermost region (region 1) where the bullhead was present only during the last © 1998 Blackwell Science Ltd, Fisheries Management and Ecology 1998, 5, 189–199
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Figure 3. Densities (6 SE) (first catch 100 m–2) of salmon 01, salmon ù 11 and bullheads at the impact study sites in 1987–1989 and 1990–1994.
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4 years, and in region 5, where the species had been present since 1988, the highest mean density value (4 fish 100 m–2) was observed in 1994. The cluster analysis of 12 sites based on habitat types identified three groups. The first cluster included one site in region 3 and one in region 4, the second, two in region 3 and five in region 4 and the third, just two sites in region 4. It also showed that the habitat compositions of regions 3 and 4 were to some extent similar. Four out of the five sites in region 3 were situated either in narrow channels between two lakes or in rapids very near to lakes. Alhough the habitat variables (depth, water velocity and substrate composition) differed between these locations, they contained the highest bullhead densities. The differences in density between the bullhead and the 01 and ù 11 salmon for selected impact sites are presented in Fig. 3. The two-way ANOVA for the densities of salmon in the age group ù 11 showed no statistical significant effect of time (F1, 85 5 1.07 and P 5 0.304), nor of the appearance of bullheads (F2, 85 5 0.24 and P 5 0.784), nor was there any interaction effect (F2, 85 5 0.35 and P 5 0.708). The result of the impact study was that the presence of bullheads had no influence on the densities of ù 11 salmon parr as there was no interaction between the appearance of bullheads and time. There was no statistically significant correlation between bullhead and salmon densities in 1990–1994 (Spearman correlation coefficient rs 5 0.15 [n 5 29, P 5 0.436] for age group 01 and rs 5 0.10 [n 5 29, P 5 0.602] for age group ù 11; Fig. 4). Discussion The bullhead population in the River Utsjoki spread relatively slowly (Pihlaja et al. 1997). Morgan & Ringler (1994) indicated that the territoriality and refuge requirements of the slimy sculpin, Cottus cognatus, kept it within defined areas and allowed the population density to increase (2– 33) as long as the refuge sites in the environment were not saturated with sculpins. In the River Utsjoki, the lowest bullhead densities were found at the outer limits of the species range, where it is still dispersing, while the highest values in the most recent years were found downstream of the first observation site and the probable introduction point. The difference in density between the two regions situated on either side of the introduction point where the species had been present for the longest time, is considerable. In region 3, which is downstream, the densities are much higher than those found in region 4, which is upstream. The probable reason for this difference is that the sampling sites in the regions downstream of the introduction point are situated mostly in places which are favourable habitats for bullheads and therefore have higher bullhead densities. The bullhead densities presented are underestimates, as they are based on the catch from a single electric fishing sweep and are only valid as indices of abundance. Even so, they illustrate the differences both between years and areas. Bullhead density was high in some years, up to 58 individuals 100 m–2, especially in the areas where the species had occurred for the longest period of time (Fig. 2). In 1979–1995 the estimated mean population densities of salmon fry and parr in the River Utsjoki, based on three successive removals, varied between 10 and 60 fish 100 m–2 and were highest in the lowermost part of the river (mean 01: 100 fish 100 m–2 and ù 11: 66 fish 100 m–2) (Finnish Game and Fisheries Research Institute, unpublished data). The density of bullheads (Cottus spp.) in the salmon rivers of northern Sweden varied in the range 4–19 fish 100 m–2 and in the River Rickleån were as high as 50–60 fish 100 m–2 (Karlstro¨m 1977). The density © 1998 Blackwell Science Ltd, Fisheries Management and Ecology 1998, 5, 189–199
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Figure 4. Relationships between the densities (fish 100 m–2) of bullhead and salmon parr in 1990–1994.
of salmon in these rivers varied from fewer than 1 to 9 fish 100 m–2, and was highest in the River Rickleån. In rivers where bullheads were not present, the density of salmon varied between 0 and 7 fish 100 m–2. In the River Reisaelva, northern Norway, where the density of salmon is lower than that in other rivers in northern Norway, averaging 4 fish 100 m–2, the density of bullheads was 15 fish 100 m–2 (Halvorsen, Gravem & Kristoffersen 1994). Thus the densities of both bullheads and salmon in the River Utsjoki are relatively high in comparison with other rivers in northern Scandinavia, especially in the lower part of the river. By contrast, the density of Alpine bullhead in the Atna watershed in southern Scandinavia varied between 18 and 59 fish 100 m–2 over 3 years and the mean densities of trout aged 01 and ù 11 were both about 7 fish 100 m–2 © 1998 Blackwell Science Ltd, Fisheries Management and Ecology 1998, 5, 189–199
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(Hesthagen et al. 1989). The density of bullheads in some rivers in southern Sweden was much higher, varying between 100 and 2500 fish 100 m–2 (Andreasson 1969). On many occasions it has been suggested that sculpins compete for food with juvenile salmonids (Andreasson 1971; Mason & Machidori 1976) and indirectly reduce the food resource by decreasing the numbers of drifting organisms (Broksen, Davis & Warren 1968; Andersson, Bro¨nmark, Herrman, Malmqvist, Otto & Sjo¨rstro¨m 1986). Sculpins are also thought to influence the use of habitat by juvenile salmonids, causing them to avoid areas with high sculpin densities (Gaudin & Caillere 1990; Bardonnet & Heland 1994). In the River Reisaelva, northern Norway, the Alpine bullhead was found to use the same food resources as juvenile salmon (Gabler 1994) and to prefer habitats where the water velocity and substrate composition were similar to those occurring at sites commonly inhabitated by salmon parr (Halvorsen et al. 1994). This work, especially the impact study, was based on a set of carefully selected sites from an extensive monitoring programme (Pihlaja et al. 1997). No effect of the distribution history of the bullhead on salmon densities, or any other association between the abundances of the two species could be detected. It may be possible that there are enough resources for both species and that the factors which limit the size of the salmon populations are only effective at higher densities. Nevertheless, sculpins have had some effect on salmonid populations in other Scandinavian rivers despite the lower salmonid densities. The density of salmon in the River Reisaelva was lower in the areas where Alpine bullhead were present (Halvorsen et al. 1994), and a similar relation was found between sculpins and juvenile salmon and trout in the River Kalix (Karlstro¨m 1977), and between the Alpine bullhead and juvenile brown trout in the Atna system (Hesthagen et al. 1989). The design of the present study entailed some methodological restrictions. Sites could not be replicated (Hurlbert 1984; Stewart-Oaten, Murdoch & Parker 1986), because the River Utsjoki is the only river in the River Teno system where bullheads are present. The sampling methods and the time scale are adequate for defining changes in density, but the spatial scale would have to be different to study the interactions between the bullhead and salmon. Such a study would require extensive random site sampling in the regions. The interaction between salmon and bullheads could be studied by means of field experiments, but the present fishery management practices in the River Teno hinder the transplantation and removal of fish. As the bullhead is a relatively new species in the River Utsjoki and its population is still spreading to the lowest part of the river (Pihlaja et al. 1997), where salmon densities are highest, it is difficult to predict how the coexistence of these two species will affect the future production of juvenile salmon. Acknowledgements The authors wish to thank Lisbeth Jørgensen for useful comments on the manuscript. References Andersson K.G., Bro¨nmark C., Herrman J., Malmqvist B., Otto C. & Sjo¨rstro¨m P. (1986) Presence of sculpins (Cottus gobio) reduces drift and activity of Gammarus pulex (Amphipoda). Hydrobiologia 133, 209–215. Andreasson S. (1969) Ta¨thetsbesta¨mning av stensimpa (Cottus gobio L.) i skånska vattendrag [Estimation © 1998 Blackwell Science Ltd, Fisheries Management and Ecology 1998, 5, 189–199
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of population density of Cottus gobio L.]. Information från so¨tvattenslaboratoriet, Drottningholm. No. 8, 12 pp. (In Swedish.) Andreasson S. (1971) Feeding habits of a sculpin (Cottus gobio L. Pisces) population. Report of Institute of Freshwater Research Drottningholm No. 51, 30 pp. Bardonnet A. & Heland M. (1994) The influence of potential predators on the habitat preferenda of emerging brown trout. Journal of Fish Biology 45 (Suppl. A), 131–142. Brocksen R.W., Davis G.E. & Warren C.E. (1968) Competition, food consumption and production of sculpins and trout in laboratory stream communities. Journal of Wildlife Management 32, 51–75. Erkinaro J. (1995) The age structure and distribution of Atlantic salmon parr, Salmo salar L., in small tributaries and main stems of the subarctic River Teno, northern Finland. Ecology of Freshwater Fish 4, 53–61. Gabler H.M. (1994) Na¨ringsinteraksjoner mellom laksunger (Salmo salar) og steinulke (Cottus poecilopus) i Reisaelva. Cand. Scient. thesis, University of Tromsø, 66 pp. (In Norwegian.) Gaudin P. & Caillere L. (1990) Microdistribution of Cottus gobio L. and fry of Salmo trutta L. in a first order stream. Polskie Archiwum Hydrobiologi 37, 81–93. Green R.H. (1979) Sampling Design and Statistical Methods for Environmental Biologists. New York: Wiley, 257 pp. Halvorsen M., Gravem F.R. & Kristoffersen K. (1994) Fiskerbiologiske undersøkelser i Reisaelva. Fylkesmannen i Troms, miljøvernavdelingen Rapport No. 58, 54 pp. (In Norwegian.) Hesthagen T., Hegge O., Dervo P.K. & Skurdal J. (1989) Utbredelse, fordeling og interaksjoner hos fiskebestandene i Atnsjøen og Atna. Miljøvirkninger av Vassdragsutbygging -Rapport, Trondheim B 60, 59 pp. (In Norwegian.) Hurlbert S.H. (1984) Pseudoreplication and the design of ecological field experiments. Ecological Monographs 54, 187–211. ¨ . (1977) Biotopval och besa¨ttningsta¨tthet hos lax och o¨ringungar i svenske vattendrag. Karlstro¨m O [Habitat selection population densities of salmon (Salmo salar L.) and trout (Salmo trutta L.) parr in Swedish Rivers]. Information från So¨tvattenslaboratoriet, Drottningholm No. 6, 72 pp. (In Swedish.) Mason J.C. & Machidori S. (1976) Populations of sympatric sculpins, Cottus aleuticus and Cottus asper, in four adjacent salmon-producing coastal streams on Vancouver Island, B.C. Fishery Bulletin 74, 131–141. Morgan C.R. & Ringler N.H. (1992) Experimental manipulation of sculpin (Cottus cognatus) populations in a small stream. Journal of Freshwater Ecology 7, 227–232. Niemela¨ E., McComas R.L. & Niemela¨ M. (1985) Salmon (Salmo salar L.) parr densities in the Teno river. International Council for the Exploration of the Sea C.M. 1985/M: 23, 14 pp. Pihlaja O., Niemela¨ E. & Erkinaro J. (1998) Introduction and dispersion of the bullhead, Cottus gobio L., in a sub-Arctic salmon river in Northern Finland. Fisheries Management and Ecology 5, 139–146. Stewart-Oaten A., Murdoch W.W. & Parker K.R. (1986) Environmental impact assessment: ‘pseudoreplication’ in time? Ecology 67, 929–940. SYSTAT (1992) SYSTAT for Windows: Statistics, Version 5 edn. Evanston, IL: SYSTAT Inc., 750 pp.
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