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.

High mortality in introduced brown trout depressed potential gene flow to a wild population

“…No differences in daily food rations and qualitative food intake were found between the introduced and the wild parr in the Øyreselv Stream, a year after the release of the hatchery parr (Berge, 1993), which is in accordance with results obtained in different stocking experiments with one summer-old hatchery brown trout (Johnsen, 1995;L'Abée-Lund & Langeland, 1995). Parr of brown trout and Atlantic salmon in Øyreselv were predated by larger, resident male brown trout (Vik et al, 2001). Furthermore, heron and American mink were observed frequently in the stream, and bird and mammal predation is probably an additional cause of mortality for parr.…”

“…Thus, the experiment was carried out in a stream with high initial abundance of salmonid parr, which theoretically could have affected survival due to density-dependent mortality (Elliott, 1993). The number of 0+ year wild parr, however, was not lower in August 1991 than in some of the previous years, before the introduction of hatchery brown trout (Vik et al, 2001), indicating that the introduced fish had no substantial effect on the survival of wild 0+ year parr during the first months in the stream. Since the size of wild 0+ year parr in the study site was negatively correlated with the number of brown trout <100 mm L T (Vik et al, 2001), a high initial number of introduced fish may indirectly influence the survival rate of native fish due to resource competition.…”

“…The number of 0+ year wild parr, however, was not lower in August 1991 than in some of the previous years, before the introduction of hatchery brown trout (Vik et al, 2001), indicating that the introduced fish had no substantial effect on the survival of wild 0+ year parr during the first months in the stream. Since the size of wild 0+ year parr in the study site was negatively correlated with the number of brown trout <100 mm L T (Vik et al, 2001), a high initial number of introduced fish may indirectly influence the survival rate of native fish due to resource competition. Thus, the potential production of wild smolts may be reduced, as also indicated in the River Imsa, where the production of smolts of Atlantic salmon was depressed after introduction of spawners of farmed Atlantic salmon (Fleming et al, 2000).…”

Interactive effects of cover and hydraulics on brown trout habitat selection patterns

“…Greenberg et al, 1996;Mäki-Petäys et al, 1997;Roussel and Bardonnet, 1997;Teixeira et al, 2006). Despite physical habitat is the major determinant of spatial variations in the abundance and distribution of most salmonid populations , the distribution of younger individuals at a reach scale is affected by older fish through intercohort competition or predation (Bult et al, 1999;Vehanen et al, 1999;Nordwall et al, 2001;Vik et al, 2001). This results in deeper areas being occupied by larger trout and younger individuals living in shallow habitats.…”

Perspectives on the Utilization of Aquaculture Coproduct in Europe and Asia: Prospects for Value Addition and Improved Resource Efficiency

“…However, many aquaculture species exhibit very different life strategies to cattle and sheep for which the regulations were targeted. Brown trout (Salmo trutta), pike (Esox lucius), other 72 carnivorous fish species (Vik et al 2001, Hawkins et al 2005, Klemetsena et al 2002 and…”