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

Abstract

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.