We used warm summer temperatures as an analogue of climate change to estimate the potential impacts on a population of smelt (Osmerus eperlanus) in a boreal lake. A combination of hydroacoustics, seine and gillnet catches with temperature and dissolved oxygen profiles was used to study the changes in the abundance, vertical distribution and age structure of the smelt population. Smelt were squeezed between their temperature and dissolved oxygen tolerance limits during most summers. Conditions in 2002 were, however, extreme and mean surface-water temperatures exceeded 21°C while at the same time waters below 6 m had low (< 0.5 mg l.sup.-l) dissolved oxygen concentration. Pelagic fish density during summer declined from 5000 to 200 fish ha.sup.-1. However, some young-of-the-year smelt survived in 2002, leading to a recovery in abundance to 4500 fish ha.sup.-1 by 2004. A similar recovery is not likely if the high temperature and low oxygen conditions persist during critical periods over a generation time.