Cannibalism governing mortality of juvenile brown trout, Salmo trutta, in a regulated stream

Abstract

Although lake-dwelling trout frequently eat smaller conspecifics in the absence of other fish prey, there are fewer reports of cannibalism in stream populations. However, a coarse riverbed structure and low discharge may facilitate cannibalism by providing pools for large fish whilst limiting refuges for small ones. We estimated the autumn size structure and density of the seatrout population in a regulated stream in western Norway over a period of 12 years. Bioenergetics models were used to relate energy intake in large trout (≥ 17 cm) to the biomass of small fish present. The number of 0 + trout in autumn decreased with increasing abundance of large trout (log-log regression, nonlinear R 2 = 0.80, n = 10, p = 0.0005). The size distribution of 0 + trout in autumn had a lower mean, but more positive skewness, when many small fish were present. Twenty percent of the large trout sampled had parr of trout or salmon Salmo salar in their stomach. The total food consumption estimated for large trout exceeded the total energy content of small fish in years with few small per large trout. We conclude that cannibalism appears to be an important cause of small-fish mortality in this stream, regulating the abundance and size distribution of small trout. In streams with low discharge, weir building can improve conditions for large trout, presumably reducing the density of small fish. However, it is difficult to predict the net effect on smolt production of improved growth but increased mortality.