Impacts of water level regulation on trophic niche and growth of Arctic charr (Salvelinus alpinus) and brown trout (Salmo trutta) in Norwegian hydropower reservoirs

Abstract

Hydropower is among the largest renewable energy sources globally. However, it can have drastic environmental and socio-economic impacts on dammed lakes (i.e., reservoirs) and rivers where water levels are regulated due to hydropower operations. Water level regulation in hydropower reservoirs is known to be a large environmental problem, leading to changes in the abiotic conditions, which subsequently affect biological productivity and diversity and thereby may also change the trophic niche and life-history traits of top predators. Arctic charr and brown trout often coexist in subarctic lakes and hydropower reservoirs, and as top predators, they are suitable indicators for changes in the lake ecosystems. In the present study, the effects of water level regulation on trophic niche and life-history traits of Arctic charr and brown trout were studied in 14 subarctic lakes in northern Norway. Age, length, maturity, diet and stable isotope data were compared from seven regulated and seven unregulated lakes. Both charr and trout were expected to show different diet, lower littoral reliance, lower trophic position, slower growth and later maturation in regulated lakes compared to unregulated lakes. This study indicated less effect on fish predators than were expected based on previous studies of reservoir fish populations, but some expected results were also found. Charr did not show any differences in diet, trophic position or growth between the lake types. Moreover, they were found to rely more on littoral resources and to mature at a smaller size in regulated lakes than in unregulated lakes. As hypothesised, trout were found to have a different diet, lower trophic position, slower growth and higher age at maturation in regulated lakes than in unregulated lakes. However, trout also relied more on littoral resources in regulated lakes compared to unregulated lakes. The results were supported by previous studies indicating that charr with a more plastic niche can utilize alternative food and habitat resources, whereas trout are generally specialized on littoral resources and therefore suffer more from water level regulation impacts. The unaffected growth of charr in regulated lakes may result from decreased abundance in regulated lakes due to unsuccessful reproduction and increased predation on juveniles, leading to reduced intraspecific resource competition. Unexpected higher littoral reliance in regulated lakes may be linked to interspecific competitive interactions and to the effect of altitude on lake productivity. All in all, trophic niche and life-history traits of salmonids were different in regulated lakes compared to unregulated lakes. Species showed different responses to water level regulation, while possible confounding effects of the environmental factors on trophic niche and growth remain unknown.