How Do Fish Reproduce? Eggs, Milt, Nests and the Fathers Who Guard Them

How do fish reproduce? Most of them lay eggs in the water, and the male fertilizes the eggs right there. The female sheds her eggs, the male releases a cloud of milt over them, and the two never touch. That is how bass, bluegill, walleye, perch, carp and catfish all begin. Sharks and rays fertilize inside the female, and guppies and many sharks give birth to live young. Below: how fish mate, how many eggs they lay, which fathers guard the nest, and what it means in spring at the water.

A female walleye carries 23,000 to 50,000 eggs for every pound she weighs. A big female carp can shed a million in a single spawn.

How do fish mate?

Fish mating is mostly timing and place. When the water warms in spring, a male and a female swim side by side over the spot they have chosen. She releases eggs; at the same moment he releases milt, the fish word for sperm. The eggs are fertilized in the water, not inside her body. Britannica calls this external fertilization, the rule for nearly every bony fish.

It works because fish lay so many eggs. Most are eaten, buried in silt or washed away, so a fish makes thousands and lets the numbers do the work.

Do fish lay eggs or give birth?

Most lay eggs. Sharks and rays are the big exception: the male fertilizes the eggs inside the female, the way mammals do. Some sharks then lay a leathery egg case, the “mermaid’s purse” found on beaches. Many others keep the young inside until they are born, already swimming. Aquarium guppies and mollies do the same: a female drops a brood of tiny fish, not eggs. The Merck Veterinary Manual lists both groups as live-bearers. So nearly every fish you will catch in a lake hatched from an egg, and a small club of fish were born.

How many eggs does a fish lay?

More than you would guess, and the bigger the female, the more she carries. Biologists count eggs per pound of fish.

• A largemouth bass lays 2,000 to 7,000 eggs per pound (4,400–15,400 per kg) of body weight, according to the North Carolina Wildlife Resources Commission.
• A walleye carries 23,000 to 50,000 per pound (50,000–110,000 per kg), Iowa DNR reports. A 5 lb (2.3 kg) female can hold a quarter of a million eggs.
• A yellow perch lays everything in one long, jelly-like ribbon. North Dakota Game and Fish puts the ribbon at up to about 7 ft (2.1 m) long, with about 23,000 eggs in it.
• A common carp sheds about 300,000 eggs in a typical spawn. The biggest females carry a million and more (USGS).
• A bluegill female drops only 10 to 30 eggs each time she dips into a nest. But she dips again and again, often in more than one nest.

That quarter-million walleye is the thinking behind slot limits: one big female out-lays ten small ones, so she goes back.

Who builds a nest, and who guards it

Some fish are parents for a few weeks, and the father does the work.

A male largemouth bass picks a spot in 1–4 ft (0.3–1.2 m) of water and sweeps a round bed in the gravel with his tail. The female lays and leaves. He stays, fanning the eggs with his fins so silt does not smother them, and chasing off anything that comes near, from a bluegill to your lure.

Bluegill do it as a neighborhood. Males sweep saucer-shaped nests 8–18 in (20–45 cm) across, packed side by side in 1–4.5 ft (0.3–1.4 m) of water. From a dock the colony looks like a honeycomb of pale circles. Each male guards his disk for about 10 days, until the fry swim up and scatter (North Dakota Game and Fish).

A channel catfish father wants a roof. He finds a hole in a bank, a hollow log or an old muskrat den, and the female lays a sticky egg mass inside. He guards the doorway until the eggs hatch (Iowa DNR). Guard duty is total: a smallmouth on a nest defends it day and night and skips its usual rest (do fish sleep?).

The fish that scatter and leave

Walleye and yellow perch take the other road. A walleye pair scatters its eggs over rubble in 1–5 ft (0.3–1.5 m) of water and swims away (Iowa DNR). The eggs drop between the stones, where moving water keeps them clean. Perch drape their ribbon over weeds and brush and leave it, too. Their bet is numbers, not babysitting.

Fish that change sex

A few fish switch. Gag grouper all start life female. When the big male of a social group dies, a large female turns male and takes his place (FSU Coastal and Marine Laboratory). Wrasses do it faster: a 2021 study in Scientific Reports timed a full switch at about 60 days. Nothing in a bass pond does this; a bass is male or female for life.

What the spawn means when you go fishing

Every spring, the spawn pulls fish into shallow water on a schedule, which is why anglers watch water temperature the way farmers watch the calendar. Each species has its own band; the water temperature chart lists them, and when bass spawn gives the month by region. The fish you find in spring may be a parent on duty.

A male bass on a bed hits almost anything that lands near his eggs. That is defense, not hunger, and it is why bed fishing works. But every minute he is off the bed on your line, the bluegill move in on his eggs. A bluegill colony is the best day of the year for a kid with a bobber; bluegill spawn temperature tells you when to look for the pale disks. A perch ribbon over the weed tops says the perch spawned days ago and are close. How to fish a spawn and leave it working:

1. Check the rules first. Some northern states close bass season until the spawn is over. Where it is open, bed fishing is legal; the choice is yours.
2. Look before you cast. Pale circles in 1–4 ft (0.3–1.2 m) are beds; a honeycomb of them is a bluegill colony. Cast to the edge and you still catch fish.
3. Use tackle that ends the fight fast. A bed fish played for five minutes is a bed left open for five minutes. Heavy enough line, barbs pinched flat.
4. Keep the fish in the water and skip the long photo. Unhook it over the side, one picture at most, then back it goes.
5. Release a bed fish on its bed. Put a male bass or bluegill back where you hooked him, not fifty yards down the bank. He settles back over the eggs in minutes.
6. Take from the colony, not the guards. If you keep bluegill for the pan, keep the mid-size fish. The biggest males are the ones holding the disks.

The facts worth repeating

• A walleye carries 23,000 to 50,000 eggs per pound; a big female carp sheds a million or more in one spawn.
• A yellow perch ribbon can be 7 ft (2.1 m) long and hold about 23,000 eggs.
• A bluegill father guards his disk for about 10 days; a bass father fans his eggs in 1–4 ft (0.3–1.2 m) of water.
• Gag grouper are born female and can turn male; a wrasse makes the switch in about 60 days.
• Sharks, rays and guppies give birth. Almost everything else you will catch hatched from an egg.

More of the questions kids ask, answered with numbers, live in the fish facts collection.