Effects of hydrological change and the cessation of stocking on a stream population of Salmo trutta L

Abstract

The North Esk-St Patricks river system, northern Tasmania, was electrofished at 27 sites in 1985, 30 years after the same sites had been electrofished in a previous study on the survival of released brown trout. All sites were dominated by brown trout, Salmo trutta L. Before 1955, stocking of brown trout fry and yearlings had been heavy. Stocking ceased after 1956 and few releases were made to 1985. At all but 4 sites, the number and total biomass of brown trout were higher in 1985 than in 1955. The estimated total population of brown trout had increased by 63%, accompanied by a 55% increase in the number of fish of legal size (>22 cm). Previously described 'nursery streams' still maintained high densities of 0+ fish, despite considerable changes in the age composition at other sites. Little or no change had occurred in riparian habitat at 23 of 27 sites. Change in year-class strength was highly correlated with change in total annual river flow in the natal year. This is explained in terms of mortality in 'nursery streams' during periods of low river flow. At 19 sites out of 21, changes in age composition were related to relative changes in year-class strength due to interannual variability in river flow. Four sites where major changes in riparian habitat occurred exhibited decreased brown trout biomass but still showed changes in age structure due to variation in annual flow. Mean annual river flow had increased by three times since the 1950s and this was attributed to a doubling in the proportion of cleared land in the catchments. Effects of changes in river hydrology on the trout population are discussed. Growth of S. trutta was essentially independent of density. The number of anglers, total effort and the total harvest in 1985/86-1986/87 were significantly higher than in 1945/46-1953/54. A shift toward higher catch per season per angler was observed in 1985/86-1986/87 compared to the 1945/46- 1953/54 seasons, but catch per day had not changed. Total annual mortality remained at 70%. The effects of increased mean flow and interannual flow variability on the brown trout population of the North Esk river system are discussed.