Brenda Carpenter, age 15, of Santa Rosa, Calif., for her question:

HOW DO FISH REPRODUCE?

All fish reproduce sexually. In sexual reproduction, a sperm unites with an egg in a process called fertilization. The fertilized egg develops into a new individual.

The eggs of most fish are fertilized outside the females body. A female releases her eggs into the water at the same time that a male releases his sperm. Some sperm come in contact with some eggs and fertilization takes place. This process is called external fertilization.

The entire process during which eggs and sperms are released into the water and the eggs are fertilized is called spawning. Almost all bony fish reproduce in this way.

Most fish have a spawning season each year, during which they may spawn several times. But some tropical species breed throughout the year. The majority of fish spawn in spring or early summer, when the water is warm and the days are long.

Most fish return to particular spawning grounds year after year. Many fresh water fish have to travel only a short distance to their spawning grounds. They may simply move from the deeper parts of a river or lake to shallow waters near shore.

But other fish may migrate tremendous distances to spawn. For example, European fresh water eels cross 3,000 miles of ocean to reach their spawning grounds in the western Atlantic.

At their spawning grounds, the males and females of some species swim off in pairs to spawn. Among other species, the males and females spawn in groups.

Depending on the species, a female may lay a few eggs or many eggs    even millions    during the spawning season. Most fish eggs measure one eighth of an inch in diameter or less.

Some fish, such as cod and herring, abandon their eggs after spawning. A number of fish, including bass, salmon and trout, protect their eggs. The females of these species lay far fewer eggs than do the females of the cod and herring groups.

A female cod may lay as many as nine million eggs during a single spawning season. Cod eggs, like those of many other ocean fish, float near the surface and scatter as soon as they are laid. Predators eat many of the eggs. Others drift into waters too cold for hatching. Only a few cod eggs out of the millions develop into adult fish.

A female herring lays about 50,000 eggs in a season. But herring eggs, like those of certain other marine fish, sink to the bottom and have an adhesive covering that helps them stick there. As a result, herring eggs are less likely to be eaten by predators or to drift into waters unfavorable for hatching.

Salmon and trout cover their fertilized eggs with gravel but abandon them soon after. Male fresh water bass guard the eggs fiercely until they hatch.

The eggs of most fish species hatch in less than a month. Eggs laid in warm water hatch faster than those laid in cold water.