Fish are cold blooded so their metabolism slows way down in cold water. They spawn in the spring when the water warms and the days get longer. This often happens in a matter of a few days or weeks, going from too cold to spawn to good spawning conditions.
Fish egg development must take place in the late fall and winter so the fish will be ready to spawn quickly. Since cold water slows down their metabolism the eggs must start developing in the fall and be near full development before the water gets too cold for them to continue to develop.
By developing their eggs in the late fall, the eggs are ready to ripen in the few days from the time water warms up to the time they need to be laid.
The next time you catch a fish with well developed eggs in the late fall and winter, know that it is an adaption to help the the fish spawn as soon as the water temperature and amount of daylight is right.
