Effects of intra- and interspecific competition and hydropeaking on growth of juvenile Atlantic salmon (Salmo salar)

The effects of hydropeaking and intra- and interspecific competition on the growth performance (growth in length, mass and lipid content) of juvenile Atlantic salmon Salmo salar and brown trout Salmo trutta were studied in six experimental channels (three experiencing hydropeaking and three controls with a stable discharge of water). Changes in the water-covered area in the hydropeaking channels were small to avoid fish stranding. Each channel was divided into three similar-sized sections and stocked with either low or high density of Atlantic salmon, or a mix of Atlantic salmon and brown trout, with the density of the latter equalling the high-density treatment of Atlantic salmon. A marked effect of competition was visible as salmon in the low-density treatment were significantly larger (27-33%) and had a higher mass (30-38%) than salmon in both the high-density salmon treatment and the high-density salmon and trout treatment. Hydropeaking had only minor and insignificant effects on the growth performance: overall final length, mass and body lipid content in the salmon experiencing hydropeaking differed by -9%, -7% and +2% compared with controls. Furthermore, there was no indication that the competitive regime influenced hydropeaking effects. The increase in both intra-and interspecific competition among the juvenile salmon had a pronounced and significant effect on growth. Our study adds to the growing evidence that energetic consequences of hydropeaking are likely to be small for Atlantic salmon and that stress and mortality associated with stranding represent the main source of population impact.