E. Kleiven

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 200μeq/L, ANC increased from −30 to +70μeq/L, and reactive-Al decreased from about 10 to 3μmol/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.

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.

Incomplete mixing of limed water and acidic runoff restricts recruitment of lake spawning brown trout in Hovvatn, southern Norway

Repeated liming of Hovvatn during the 1981–1995 period assured successful reintroduction of lake spawning brown trout, Salmo trutta. Poor natural recruitment to the population was associated with low survival during early life stages (before hatching) as shown by the 0.5, 3.5, 0.9 and 1.0% of live embryos found in natural redds during the 1992–1995 period, respectively. The low survival was most likely caused by the combination of shallow spawning areas (<2.0 m) and acidic runoff (pH 4.0–4.8) which overlayed the limed part of the water body during the ice covered period. It is therefore concluded that this type of episodic acidification poses a major threat to lake spawning salmonids, and that it can retard or inhibit biotic recovery towards preacidified conditions expected as a result of liming. Addition of limestone gravel (8–32 mm) onto spawning grounds was an efficient alternative liming strategy as 33–36% live embryos were found in this substrate. Conversely, the trout actively avoided additions of shellsand, a behaviour most likely caused by the small particle size of shellsand (3–7 mm) relative to natural spawning gravel.

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.