How Do Fish Reproduce? Eggs, Nests and Who Guards Them

Red sockeye salmon paired over a gravel redd in a clear stream — Photo: Roger Hoffman for the National Park Service, Olympic National Park (public domain)

Key takeaways

Most fish reproduce in the water: the female sheds eggs, the male releases milt over them in the water. Sharks and rays fertilize inside; guppies and many sharks bear live young.
Egg counts run by the pound: a largemouth 2,000–7,000 per lb, a walleye 23,000–50,000 per lb. A carp sheds over 500,000; the biggest carry up to 3 million.
Bass, bluegill and channel catfish build a nest and the male guards it. Bluegill males hold their 8–18 in disks about 10 days; a bass father fans his eggs with his tail.
Walleye and yellow perch scatter and leave: eggs over rubble, or a jelly ribbon up to 7 ft long with about 23,000 eggs draped over weeds. Nobody guards them.
A bass on a bed strikes to defend, not to eat. Every minute he is off the nest, other fish can eat his eggs, so a bed fish goes back fast, on the bed.

Most fish reproduce by laying eggs in the water, and the male fertilizes them 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.

A female walleye carries 23,000 to 50,000 eggs for every pound she weighs. A big female carp can carry up to 3 million.

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. This is called external fertilization, and it is 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. Both groups are called 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.
A walleye carries 23,000 to 50,000 per pound (50,000–110,000 per kg). A 5 lb (2.3 kg) female holds roughly 115,000 to 250,000 eggs.
A yellow perch lays everything in one long, jelly-like ribbon. The ribbon runs up to about 7 ft (2.1 m) long, with about 23,000 eggs in it on average.
A common carp can shed more than 500,000 eggs over several days, a few thousand at each spot (Iowa DNR). The biggest females carry up to 3 million (University of Minnesota Extension).
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.

Eggs scale with weight, so a 5 lb walleye carries five times the eggs of a 1 lb fish. That is part of the thinking behind slot limits that send the big females back.

Which fish build a nest, and who guards it?

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

A male largemouth bass spends several days choosing a spot, usually in 1–4 ft (0.3–1.2 m) of water and deeper where the water is clear, and sweeps a round bed in the gravel with his tail. The female lays and leaves. He stays, fanning the eggs with his tail to keep silt and debris off 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 1.3–4.5 ft (0.4–1.4 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.

A channel catfish father wants a roof. He finds a cavity in a rocky shoreline, a snag or a stump, or the pair digs a hollow in the bottom, and the female lays her eggs inside. He guards them until they hatch, about seven days depending on the water temperature. Guard duty is total: a smallmouth on a nest defends it day and night and skips its usual rest (do fish sleep?).

Species	Eggs	Nest or scattered	Who guards
Largemouth bass	2,000–7,000 per lb (4,400–15,400 per kg)	Round bed swept in 1–4 ft (0.3–1.2 m)	The male, fanning and chasing, until the fry leave
Bluegill	10–30 per dip, many dips	Disk nests 8–18 in (20–45 cm) wide, in colonies at 1.3–4.5 ft (0.4–1.4 m)	The male, about 10 days
Channel catfish	One clutch in the cavity	A cavity: rocky bank, snag, stump	The male, about 7 days to hatching
Walleye	23,000–50,000 per lb (50,000–110,000 per kg)	Scattered over rubble in 1–5 ft (0.3–1.5 m)	Nobody
Yellow perch	About 23,000 in one ribbon up to 7 ft (2.1 m)	Ribbon draped over weeds and brush	Nobody
Common carp	500,000+ over several days; big females up to 3 million	Scattered over shallow weeds	Nobody
Guppy, many sharks	Live young, no eggs laid	No nest; born swimming	Nobody after birth

Which fish scatter their eggs 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. 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.

Can a fish change sex?

A few fish switch. Gag grouper start life as females, and some turn male later. Florida State University researchers found the switch happens only when the fish are together in groups, mostly just after the spawning gatherings. Bluehead wrasses do it fast: when the dominant male is removed, the largest female acts like a male within hours and makes mature sperm within 8 to 10 days (Todd et al., 2019). Nothing in a bass pond does this; a bass is male or female for life.

How do you fish the spawn without wrecking it?

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, other fish can 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. Six steps keep a spawn working while you fish it:

Check the rules first. Some northern states protect bass through the spawn: in most of New York, bass are catch-and-release only from December 1 to June 14. Where it is open, bed fishing is legal; the choice is yours.
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.
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.
Keep the fish in the water and skip the long photo. Unhook it over the side, one picture at most, then back it goes.
Release a bed fish on its bed. Put a male bass or bluegill back where you hooked him, not fifty yards down the bank, so he can get straight back to the eggs.
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.

More fish questions, answered with numbers, are in the fish facts collection.

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