Abstract
Applications of computer programs in the assessment of how a change in discharge, temperature and other physical conditions affects the available fish habitat are increasing around the world. The programs are typically used as tools for setting minimum flow regimes in regulated rivers and in habitat remedial actions. So far most of these applications have been concerned with warm water and summer conditions, and few applications incorporate winter temperatures and ice conditions. This is a flaw in the current procedure since winter may be the limiting period for fish production in a river and the introduction of ice may completely alter the available fish habitat compared to summer conditions. It is particularly important to be aware of this when carrying out habitat remedial actions, since short periods with adverse winter conditions may be the limiting factor for the fish population. A winter habitat assessment procedure will include a study of fish behaviour in winter conditions, the effects of various ice types on habitat selection and the dynamics of ice breakup on the microscale. This paper outlines how fish respond to winter conditions and how hydraulic, hydrologic and biological modelling can be applied to describe winter conditions and their impact on physical fish habitat. Some of these processes are currently being incorporated into the Norwegian habitat modelling system. This paper also points out several areas where more research is needed to describe the physical processes and the biological responses to a changing environment. Copyright Β© 2002 John Wiley & Sons, Ltd.