Here you go - good question btw.
It is believed that the lipid composition in the fish muscle plays a vital role in the ability of fish to adapt from one temperature to another . Due to cellular disfunction from low lipids the fish lock up stiffen and heart and respiration stop.
For example, the Nile tilapia Oreochromis niloticus does not store excess lipids in the musculature but rather relies on visceral deposits that it is incapable of mobilizing at low temperatures, which results in high mortalities between 8°C and 6.5°C (Satoh et al. 1984).
The dynamics of lipid composition of cells occurs in order to maintain a constant fluid matrix for enzymes associated with membranes (Greene and
Selivonchick 1990). Different species of fish differ in their patterns of fat deposition and mobilization, which in turn affects the temperature range in
which the species can grow and survive. the ability of a fish to alter its lipid composition when placed in colder water is one factor that determines survival.
In these fish, the fat apparently hardens in the colder water, causing the fat-impregnated muscles to stiffen and the fish to become exhausted and lose movement.
Physiologically, fish are affected by variations in water temperature in two ways (Hochachka and Somero 1984). First, temperature
determines the rate of chemical reactions, and secondly, temperature dictates the point of equilibrium between the formation and disruption
of the macromolecular structures in biological membranes. Structural flexibility, therefore, is a requirement for integrity of biological membranes
(Hazel 1993). Cold temperatures constrain this flexibility and, as a result, stabilize less active conformations.
The rate of resistance to lower temperatures is governed in part by the rate of metabolism, which is depressed at lower environmental temperatures.
See this thread
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=111414&page=1
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