Influence of fluctuating lake levels and water clarity on trout populations in littoral zones of New Zealand alpine lakes

Brown trout (Salmo trutta Linnaeus 1758) and rainbow trout (Oncorhynchus mykiss Richardson 1836) populations in littoral zones of eight South Island, New Zealand alpine lakes were compared using gill and seine net sampling during summer. Lakes were selected to provide a matrix of lake level and water clarity variations and to assess how these variables influenced trout abundance (as reflected by catch rate), depth distribution, and size. Brown trout were small and in poor condition in three turbid lakes with shallow littoral zones and were scarce in one of these with a 14 m fluctuation. Although brown trout condition was generally higher in clear lakes, Lake Wanaka fish were an exception. Trout depth distribution was positively related to depth of the littoral zone. Brown trout were caught at most depths whereas rainbow trout showed variable depth preferences in different lakes. Rainbow trout catch rate, weight, and condition factor were unrelated to differences in depth of the littoral zone or lake level fluctuations, and it was concluded that spawning success and limnetic food supplies may be more important for this species. Spawning interactions may account for the different species composition between the stable clear Lake Wanaka, and the adjacent fluctuating clear Lake Hawea. Moderate and slow lake level fluctuations appear to have limited effects on trout in clear lakes, probably because trout are able to use deep littoral habitat and food supplies.