Besides energy saving behaviour, the metabolic rate has been depressed due to the physicochemical effects of cold, typically a reduction of 2-3x per 10 degrees.

Besides energy saving behaviour, the metabolic rate has been depressed due to the physicochemical effects of cold, typically a reduction of 2-3x per 10 degrees. In terms of having enough energy, energy stores accrued during summer and autumn go a long way, but many fish are indeed a negative energy flux state over winter. In fact, over-winter starvation in the first year or three is a common ecological bottleneck, where even if fish are capable of reproducing and adults are fine and can survive the first year, the 0+ age class may not have had sufficient time to store enough energy before winter to survive until spring.