Stocking of brown trout (Salmo trutta L.) cohorts after liming- effects on survival and growth during five years of reacidification

The chronically acid Hovvatn and the adjoining pond Pollen were limed in March 1981. During the first 4 yr after the liming, a total of 11 437 brown trout were stocked in the two locations. The fish population was monitored by annual testfishing in a five yr period following the liming. The stocking material constituted 6 cohorts and included fish stocked at age 0+, 1+ and 2+. The stocking program assured a 7.9 times higher density of fish ha^?1 in Pollen than in Hovvatn. Consequently, the fish from Hovvatn grew significantly better and had a significantly higher condition factor than fish in Pollen. Monitoring of water chemistry showed that the cohorts were exposed to a gradual reacidification process, abrupted by episodic events of severe acidification. Within 2–3 yr after the liming, the locations had reacidified to conditions which were considered critical for fish. However, the acidinduced increase in mortality appeared 1–2 yr later than what was expected from water chemistry data. This discrepancy could most likely be ascribed to the existence of water chemical refugia which enhanced the survival of the fish. Recaptures were significantly correlated to pH, Ca, labile-Al and ANC. Growth were significantly reduced throughout the reacidification period. Estimated yield showed that fish stocked at age 2+, as opposed to fish stocked at a younger age, managed to turn the high food availability into high growthrates before the reacidification retarded growth and survival.     

Improved growth in stunted brown trout (Salmo trutta L.) after reliming of Lake Hovvatn,Southern Norway

The chronically acidic Lake Store Hovvatn and the adjoining pond Pollen in southernmost Norway were limed in March 1981. The two locations were stocked with brown trout (Salmo trutta L.) at low and high densities in Hovvatn and Pollen, respectively. After 6 yr of reacidification, the locations were relimed in July 1987. Growth depression during the reacidification process in spite of low fish densities and superabundance of food was observed in Lake Store Hovvatn. Three months after reliming, a substantial growth response was found in trout from Lake Store Hovvatn; Mean annual length increment was 68% higher than that of the preceding year. In Pollen, reliming had no apparent effect on growth. In both populations reliming caused increased swimming activity measured as an increase in CPUE-values. These results show that the growth response to liming depends on population density and food availability. Moreover, the results indicate that the food conversion rate of the trout is negatively affected in acid waters.     

Liming of lakes,rivers and catchments in Norway

Despite a slight reduction in the level of acidic deposition in Norway, acidification of lakes and rivers continues. The Norwegian Liming Project (1979–84) demonstrated that lime treatment can be an effective measure against acidification of watercourses given appropriate adaptation to local conditions. Liming in Norway is difficult because of (1) large amounts of precipitation, (2) short retention time of lakes, and (3) episodic changes in water chemistry. In 1988 NOK 14 mill. has been allocated to operational liming and research. We report here on chemical and biological responses from lime treatment of a lake, a river and a catchment. Lake Store Hovvatn was limed in 1981 and successfully stocked with brown trout. Before reliming in 1987, fish growth had ceased, but increased post liming. The River Audna has been continuously limed since 1985. Sea trout fisheries have improved, and the stocking of Atlantic salmon smolts at the mouth of the river in 1986 has already resulted in the return of spawners. Liming of the entire terrestrial catchment to the pond Tjønnstrond in 1983 by helicopter was also successful; stocked brown trout have survived to the present.     

Effects of liming and acid surface water on the mayfly Leptophlebia vespertina in Lake Hovvatn

The acidified Lake Hovvatn have been limed in 1981, 1987, 1989 and 1991. After the first liming the lake reacidified close to the prelimed condition. The reliming, which started in 1987, was planned to maintain the pH at a relatively high level for the lake. A detailed monitoring of pH and temperature was performed at depth 0.5, 1, 1,5 and 5 m since spring 1993. Quantitative samples of benthic invertebrates were taken in spring and fall in 1977 and regularly at the same seasons from 1981 at depth 0.5, 2, 5 and 10 m. A reference lake, Lille Hovvatn have been sampled with the same procedure since 1988. The acid tolerant mayfly Leptophlebia vespertina responded quickly to the first liming with a 20 times increase in density after a few months. However, the densities rapidly decreased during the first years of reacidification. The lime treatments in 1987 and 1989, resulted in a second peak in density in 1990. After this, the densities have been reduced in spite of generally good water quality in Hovvatn. During fall the density increase was significant at 0.5 m depth in 1990, at 2 m depth in 1989, 1990 and 1992 and at 5 m depth in 1988 to 1990. No significant increase was observed in the limed localities during spring. It is concluded that acid surface water, prior to ice break, affect the food resources to L. vespenina and reduce the population at all depths during spring and in the littoral zone in fall.     

Changes in the fish community of a limed lake near sudbury,Ontario: Effects of chemical neutralization or reduced atmospheric deposition of acids?

Nelson Lake, a moderately acidic (pH 5.7), metal-contaminated (Cu 22 μg L^?1; Zn 18 ug L^?1) lake, 28 km from the smelters at Sudbury, had a degraded fish community in the early 1970's, with lake trout (Salvelinus namaycush) scarce, smallmouth bass (Micropterus dolomieui) extinct, and the littoral zone dominated by the acid-tolerant yellow perch (Perca flavescens). Liming of the lake in 1975–76 increased pH to 6.4, and decreased metal concentrations. Chemical conditions have remained relatively stable in the 10 yr following base addition. Initially, it appeared that neutralization produced dramatic changes in the resident fish community. Yellow perch abundance declined rapidly after neutralization, lake trout abundance increased to the extent that 3.26 kg ha^?1 were caught in the winter of 1980, and reintroduced smallmouth bass reproduced and established a large population. However, these changes in the fish community can not be directly attributed to liming, as water quality and the sport fisheries of an unlimed nearby lake also improved. Reduced emissions from Sudbury smelters were responsible for improvements in the untreated lake. Recovery of the lake trout population in Nelson Lake appears to have begun prior to liming. Of the lake trout sampled during the 1980 winter fishery, 65.8% were present prior to the chemical treatment. Predation by lake trout was the likely cause of the perch decline. Our results suggest that chemical conditions producing population level responses in fish have abrupt thresholds and that neutralization of lakes above these thresholds may not produce distinguishable effects.     

Effects of introduced fish on macroinvertebrate communities in historically fishless headwater and kettle lakes

Widespread fish introductions have led to a worldwide decline in the number of fishless lakes and their associated communities. Studies assessing effects of fish stocking on native communities in historically fishless lakes have been limited to high-elevation headwater lakes stocked with non-native trout. Little is known about the effect of fish stocking in historically fishless and hydrologically isolated lowland kettle lakes. We compared the effects of introduced fish on macroinvertebrate communities in kettle lakes stocked with centrarchids, salmonids, and cyprinids, and headwater lakes stocked with brook trout (Salvelinus fontinalis) in Maine, USA. Fish had significant effects on macroinvertebrate community structure in both lake types, with reduced species richness and abundances of taxa characteristic of fishless lakes. The effects of introduced fish were more pronounced in headwater lakes despite a less diverse fish assemblage than in kettle lakes. We attribute this to abundant submerged vegetation providing refuge from fish predation and reduced stocking frequency in kettle lakes. We assessed effects of stocking duration on macroinvertebrates in a subset of headwater lakes with known dates of trout introduction. Species richness and abundance of most taxa declined within 3 years following trout introduction; however, richness and abundance were least in lakes with long stocking histories (?40 years). Macroinvertebrates previously identified as fishless bioindicators were absent from all stocked lakes, indicating that trout rapidly eliminate these sensitive taxa. Conservation of this historically undervalued ecosystem requires protecting remaining fishless lakes and recovering those that have been stocked.     

Changes of planktivore fauna and development of zooplankton after liming of the acidified Lake Gårdsjön

The quantitative dynamics of the zooplankton community was studied during a period of 13 years in the manipulated Lake Gårdsjön and an acidified reference lake. Lake Gårdsjön was acid during the first 3 years, limed and fishless the following 4 years and stocked with brown trout for a period of 6 years. During the final 2 years rainbow trout was added. Changes in water chemistry and phytoplankton standing stock are associated with the manipulations. Liming had a positive effect on the overall species richness and diversity among rotifers and cladocerans. Changes in the zooplankton community during the different phases of the lake restoration are discussed in relation to the development of the planktivorous fauna of the lake, phantom midge larvae, aquatic hemipterans and fish. It can be concluded that liming creates a potential for the restoration of the zooplankton diversity, but the actual structure of the zooplankton community in limed lakes is strongly influenced by predatory and competitive interactions.     

The Chironomid (Diptera) Subfossil Record of Lille Hovvatn (Norway), with a Comparison with a Similar Study in Adjacent Store Hovvatn

The subfossil chironomid community of Lille Hovvatn during the last 150 years was analysed. A short sediment core was taken in 1999, sliced at 0.5 cm intervals and dated using the ²¹⁰Pb method. A total of 1588 specimens were identified, most of them to species level, and 37 chironomid taxa were found. Effects of acidification were recorded on several species up-core, especially in the distribution of Micropsectra insignilobus and Stictochironomus rosenshoeldi. Ordination analysis showed that the chironomid community of Lille Hovvatn is different from the one in Store Hovvatn based on the abundance of the various species, but that the species composition is almost identical in the two lakes. The reappearance of S. rosenshoeldi in the upper 1 cm of the sediment in Lille Hovvatn probably is the first response among the chironomids to the increased pH values measured in the lake during the last decade.     

Liming of acidified lakes: Induced long-term changes

This study presents data concerning long-term trends after neutralization of four acidified lakes in two regions on the Swedish west coast. Neutralization was achieved by a di-Ca-silicate with 52% CaO and about 11.5% MgO. Between 61 and 74% of the spread lime product dissolved during a 5 to 7 yr period. The liming increased pH, from a range of 4.5 to 5.2 to near neutral and restored alkalinity in the range of 0.2 to 0.3 meq l^?1 and the Ca-content became 3 to 4 times higher than before liming. In two lakes transparency decreased significantly presumably due to changed phytoplankton composition. These changes successively declined due to dilution and continuous acid loading. The changes in water chemistry and development of stocked brown trout (Salmo trutta) populations initiated biotic changes. Phyto- and zooplankton communities reacted both instantly and later with successions in species composition. Changes of benthic macroinvertebrate species occured over several years, but some pelagic species, e.g. corixids were rapidly reduced due to predation of fish. Observed changes were predominantly due to expanding populations of species present at very low abundances even during acid state of the lakes. Some organisms found during preacid state of the lakes did not establish new populations and this process may need a prolonged time with favorable conditions. Reacidification towards the end of the study period significantly stressed the brown trout population and also favored expansion of the filamentous algaMougeotia sp. andSphagnum sp. that almost vanished during the first year after liming. Decreasing concentration of total P was not influenced by neutralization and may be mostly dependent on negative changes in the soils surrounding the lakes. If generally valid, this process may be an important factor for the oligotrophication of lakes in areas where acid deposition is high.     

Population dynamics of brook trout (Salvelinus fontinalis) during maintenance liming of an acidic lake

Maintenance liming of an acidic lake in the Adirondack Mountains of New York state (Woods Lake) was conducted three times over a 5 yr period in an attempt to establish a self maintaining brook trout population. Various strains and age classes of marked brook trout were stocked annually and the population was inventoried semi annually to evaluate survival, growth, and reproductive success. The Woods Lake brook trout population was dominated by young, stocked fish throughout the maintenance liming period of 1985-89. Based on spring emergent fry trap catches and fall trap net catches of unmarked fish, only one naturally produced year class (1986) was successfully recruited to the Woods Lake brook trout population. Low annual survival rates (G 20%) of juvenile trout were observed throughout the study period. Although initial growth rates and condition of young trout were satisfactory, increased intraspecific competition for food resulted in declining growth rates and condition of older age classes. Fall standing crops of brook trout remained at relatively low levels of 6 to 10 kg ha^?1 and both production per unit biomass and growth efficiency decreased over the 5-yr. Repeated whole lake liming and limited spawning habitat improvement were not sufficient to sustain brook trout natural reproduction in Woods Lake. Low productivity, marginal spawning habitat quality, and low survival rates of stocked trout in Woods Lake resulted in the failure to establish a self maintaining, productive brook trout population in Woods Lake.     

Evidence of Lower Productivity in Long Term Limed Lakes as Compared to Unlimed Lakes of Similar pH

Ecosystem development in lime-treated waters in Sweden has been followed since 1989 in a programme for integrated studies of the effects of liming acidified waters (ISELAW). Observations after prolonged liming (>10 y) indicate a phosphorus depletion in the limed lakes which contrasts to the increased phosphorus supply often following within the initial years after lime treatment. After prolonged liming, the levels of total phosphorus are lower as compared to neutral reference lakes at identical TOC, and the phosphorus/TOC -ratio is consequently lower in limed lakes. Depletion of dissolved inorganic nitrogen during the summer is also lower in limed as compared to neutral reference lakes. Phytoplankton biomass and species number also lower in the limed lakes as compared to unlimed neutral references. Furthermore the bacterial number per unit TOC is lower in the long term limed lakes, possibly as a result of phosphorus limitation. As to the higher trophic levels, the benthic soft-bottom fauna of limed lakes (specifically the sublittoral fauna) is poorer in terms of species diversity and abundance. Also fish community composition indicates lower productivity in the limed lakes. Taken together there is thus evidence that the long term limed lakes have a lower trophic level than reference lakes.     

An update on the lake acidification mitigation project (LAMP)

Limestone addition is a management tactic to mitigate acidic conditions in lakes. The Lake Acidification Mitigation Project is a long-term integrated project to study the ecological effects of liming and develop the technical information necessary for an effective liming program. Three Adirondack lakes have been limed: Cranberry Pond, a small, 60-day hydraulic residence time fishless lake; Woods Lake, a larger fishless lake with longer residence time (214 days); and Little Simon Pond, a large lake (160 ha) containing populations of brook trout, lake trout and other fishes, with a residence time of 450 days. Woods Lake has been limed twice and the stocked brook trout fishery has been maintained; Cranberry has been limed once and then allowed to reacidify with loss of the stocked fishery. Little Simon Pond showed no ill effects of liming on the extant populations. In general, short term evaluation of ecological effects has shown no major deleterious effects of liming. Methods for predicting reacidification are extremely accurate and use of different limestone particle-size fractions is recommended to achieve longer term in-lake neutralization.     

Biotic Effects in Planktonic Crustacean Communities in Acifified Swedish Forest Lakes after Liming

In Sweden, approximately 16 000 of a total of about 85 000 lakes have been acidified due to acidic deposition. Of these about 8000 have been treated with limestone powder in order to detoxify the acidified waters and protect sensitive fauna. The present study was performed in ten lakes in the southern part of the country. The lakes belong to four different catchments and were in different stages of acidification at the time of lime treatment. The composition of the zooplankton and fish communities also differed and three lakes were empty of fish at the beginning of the studies. Quantitative sampling of planktonic crustaceans was performed during the ice free season between 1976–87 in five of the lakes and between 1977–87 in the other five. After treatment the pH increased significantly in all lakes except one. The average number of crustacean taxa found per sampling occasion increased in all lakes. Increases were statistically significant in four of the lakes. In the lakes empty of fish, increased abundances of chaoborids inhibited, by predation, the increase of species richness. Species richness increased after the introduction of fish and the subsequent reduction of the chaoborids. At the end of the study, more taxa were found in the limed study lakes than in non-treated west coast lakes with an alkalinity of 0.04–0.10 meq L^-1. Most species normally occurring in oligotrophic forest lakes were found. It was shown that the water quality after liming made the occurrence of sensitive species possible and that predation from fish and interactions within the zooplankton assemblage were of great importance to the species composition and structure of the zooplankton community.     

WHOLE-CATCHMENT LIMING AT TJØNNSTROND,NORWAY: AN 11-YEAR RECORD

In June 1983 a whole-catchment liming experiment was conducted at Tjønnstrond, southernmost Norway, to test the utility of terrestrial liming as a technique to restore fish populations in remote lakes with short water-retention times. Tjønnstrond consists of 2 small ponds of 3.0 and 1.5 ha in area which drain a 25-ha catchment. The area is located at about 650–700 meters above sea-level in sparse and unproductive forests of spruce, pine and birch with abundant peatlands. A dose of 3 ton/ha of powdered limestone were spread by helicopter to the terrestrial area. No limestone was added to the ponds themselves. The ponds were subsequently stocked with brown and brook trout. Liming caused large and immediate changes in surface water chemistry; pH increased from 4.5 to 7.0, Ca increased from 40 to 200 μeq/L, ANC increased from –30 to +70 μeq/L, and reactive-Al decreased from about 10 to 3 μmol/L. During the subsequent 11 years the chemical composition of runoff has decreased gradually back towards the acidic pre-treatment situation. The major trends in concentrations of runoff Ca, ANC, pH, Al and NO₃ in runoff are all well simulated by the acidification model MAGIC. Neither the measured data nor the MAGIC simulations indicate significant changes in any other major ion as a result of liming. The soils at Tjønnstrond in 1992 contained significantly higher amounts of exchangeable Ca relative to those at the untreated reference catchment Storgama. In 1992 about 75% of the added Ca remains in the soil as exchangeable Ca, 15% has been lost in runoff, and 10% is unaccounted for. The whole-catchment liming experiment at Tjønnstrond clearly demonstrates that this liming technique produces a long-term stable and favourable water quality for fish. Brown trout in both ponds in 1994 have good condition factors, which indicate that the fish are not stressed by marginal water quality due to re-acidification. The water quality is still adequate after 11 years and >20 water renewals. Concentrations of H⁺ and inorganic Al have gradually increased and approach levels toxic to trout, but the toxicity of these are offset by the continued elevated Ca concentrations. Reduced sulphate deposition during the last 4 years (1990–94) has also helped to slow and even reverse the rate of reacidification. The experiment at Tjønnstrond demonstrates that for this type of upland, remote terrain typical of large areas of southern Norway, terrestrial liming offers a suitable mitigation technique for treating acidified surface waters with short retention times.     

Low Success Rate in Re-Establishing European Perch in Some Highly Acidified Lakes in Southernmost Norway

In order to test whether major reductions in acid inputs had improved water quality sufficiently for fish populations to recover, we stocked wild European perch (Perca fluviatilis) in three highly acidified lakes that had previously supported this species, and in one limed lake. The fish, which were introduced from a local lake (donor lake), generally ranged from 12 to 16 cm in total length, and were stocked at densities of 117–177 fish ha^?1. The untreated lakes were highly acid, with minimum pH values and maximum inorganic aluminium concentrations (Al_i) during the spring of 4.6–4.7 and 118–151 µg L^?1 respectively. In the limed lake, the corresponding values for pH and Al_i ranged between 5.8 and 6.6 and 5 and 19 µg L^?1 respectively. Gill-netting in two subsequent years after the introduction yielded only a few recruits (0+) and one adult in one of the three acidified lakes in one year only. However, stocked perch reproduced successfully in both years in the limed lake. There was a significant linear relationship between the catches (CPUE) of juvenile perch (age 0+) in the different lakes in the autumn and the water quality in May (time of hatching), both in terms of Al_i (r ²=0.934, P<0.05) and pH (r ²=0.939, P<0.05). Our data suggest unsuccessful recruitment in waters of pH <5.1 and Al_i>60 µg L^?1.     

Liming acidic waters in Norway: National policy and research and development

Since 1983, the Norwegian environmental authorities have given financial support to operational liming and research following recommendation given by the Norwegian Liming Project (1979–1984). Liming all acid waters in Norway would require an annual expenditure exceeding NOK (Norwegian Kroner) 300 × 10⁶. The funding level for 1988 is NOK 14 × 10⁶ and will probably increase to NOK 30 mill. by 1990, including NOK 1 to 3 mill. for research and development. Priority is given to lakes and rivers whose fishing is open to the public, and to save or restore valuable fish populations. Local support in the form of voluntary labor is a condition for financial aid. Two salmon rivers are currently included in the program. T h e Norwegian Institute for Water Research (NIVA) plays an important role in liming research and development in Norway. The aims of this research are twofold: to document the chemical and biological response to neutralization and liming, and to improve liming strategy to obtain cost efficient liming activities.     

The Swedish liming programme

To mitigate the acidification problem in surface waters the Swedish government is funding a liming programme. Limestone or dolomite powder has been applied to acidified waters since 1976 and on a large scale since 1982. In most projects, limestone is applied directly to the lake, but in several cases supplementary liming is carried out on wetlands and in streams using dosers or other techniques. At present 7,500 Swedish lakes and more than 11,000 kilometers of streams are limed repeatedly with a total of some 200,000 tonne of limestone every year. In 1994 about US$ 25 million was invested by the Swedish government in the liming programme. The biological objective of the liming operations is to detoxify the water so that the natural fauna and flora can survive or recolonize. The chemical aim is to raise the pH above 6.0 and the alkalinity above 0.1 meq/l, which gives an acceptable buffering capacity. In addition, dissolved metals will be deposited after liming, thus reducing their toxicity. Overdosing must be avoided, with natural softwater characteristics being the objective. The chemical and biological effects in water of the liming operations are encouraging. The Swedish liming programme has so far resulted in restoration in 80–90% of the limed surface waters. The fauna often shows an initial dominance by a few species but diversity increases with time, In general, flora and fauna in limed waters show a great resemblance to those in waters not acidified. An undesired effect of liming is significant changes in mosses and lichens after wetland liming.     

Effects of liming on crayfish and fish in Sweden

The effects of lime treatment on crayfish (Astacus astacus and Pacifastacus leniusculus) populations in 17 lakes and fish populations in 47 lakes and 7 rivers within the trial period 1976–82 have been evaluated. An increase in the catch of crayfish per unit effort was observed in 7 lakes, although significantly in one lake only. The varying results in the other lakes indicate that factors other than pH may be of greater importance for the development of crayfish populations after liming. Recruitment of fish improved in waters where liming resulted in pH >5.5. In lakes with pH <5.5 before and pH >5.5 after treatment, there was a significant increase in the number of fish caught, from 12 to 34 per unit effort. Due to improved recruitment the individual average weight was smaller and hence the catch in weight per unit effort was about the same as before liming. After lime treatment in streams which resulted in a stable pH of >5.5, the abundance of juvenile salmonids increased to numbers found in non-acidified streams. In other streams acid spates reduced the positive influence of liming on the abundance of juvenile salmonids.     

Development of zooplankton in relation to lime treatment in two acidified lakes

Zooplankton has been studied in Lakes Östra Nedsjön and Ömmern (1974–94), two acidified lakes in South-Western Sweden. The former lake was first limed in 1971–73, and secondly in 1982. The first liming increased the pH-level from ca. 5.3 to 6, and the second one from ca. 5.8 to 7–7.5. The pH-value in Lake Ömmern was about 6.1 in 1973, but decreased to 5.3 in 1981, when liming raised the value to 6.5–7. After the first liming in Lake Ö. Nedsjön, the rotatorians represented in average 1% of the total volume of zooplankton. Among the copepods, which made up ca. 22% of the volume, Cyclops was frequent, and both Eudiaptomus gracilis and Heterocope appendiculata were recorded. After the second liming on the other hand the rotatorians increased to in average 28% of the total volume, while the volume of copepods was only ca. 6%. Heterocope disappeared, and Eudiaptomus and Cyclops, usually favoured by liming, were rare. Consequently the supply of larger forms of crustaceans, suitable as fish food was severely reduced. In Lake Ömmern the effects of acidification within the zooplankton were moderate. The species richness was about the same as in unacidified lakes. After liming the development of zooplankton was similar to that in most other limed lakes, i. e. increased frequency of rotatorians, cladocerans and Cyclops spp. In Lake Ö. Nedsjön, however, the small zooplankton volumes and the elimination of copepods was in contrast to the zooplankton development in other limed lakes and in unacidified ones.     

The effects of liming on juvenile stocks of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) in a Norwegian river

The effects of liming on juvenile stocks of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) in the river Vikedalselva in southwestern Norway were assessed. From 1987 to 1989, the river was limed only during the spring snow melt, and pH varied in the range between 5.5 and 7.0. In 1990 to 1993, the river was limed to pH 6.2 from 15 February to 1 June and to pH 5.7 during the rest of the year. Since 1994, the pH during late winter and spring was maintained above 6.5. Prior to liming fish kills were evident during spring snow melt, but these have not occurred since liming. Electrofishing in the autumn between 1981 and 1994 showed no significant change in densities of juvenile salmon and brown trout after liming, mean densities ranged between 19–50 and 9–32 individuals 100 m^–2 respectively. A significant linear correlation between production and biomass of both species was found, indicating that factors directly controlling density affect juvenile production and cause production to remain below carrying capacity. In spite of a clear increase in pH and a reduction in the concentration of labile aluminium after liming, the conditions still do not seem to be optimal for juvenile salmonids. We suggest that a complexity of different factors impose limitations on fish production in the river: inadequate egg deposition, environmental factors such as water temperature and flow, osmoregulatory failure in mixing zones between limed and acidic water and gill damage through deposition of aluminium and iron. However, there are several indications of a reduction in toxic effects after the pH was raised to 6.5 during spring snow melt.