Exploring the World of Fish Spawning: How do Fish Breed

Aquatic life has allured human beings since our existence. Fishing has been a primary source of food and income since prehistoric times.

Over centuries, this relationship has only strengthened. Our ability to explore, appreciate and replicate nature has enhanced with evolution.

Fish breeding was evident in historical times.

• Sumerians bred ornamental fishes in ponds for pleasure and eating 4500 years ago.
• Ancient Egypt and Assyria (northern Mesopotamia) culture has evidence of fish keeping.
• The Chinese bred carp in 1000 BCE.
• Japan adopted selective Goldfish breeding from China.
• Ancient Romans built ponds sourced from seawater directly.
• Goldfish were kept in glass vessels in England in the middle of the sixteenth century.

We salute these pioneer aquarists for their foresight and intelligence in breeding fishes.

Initially, aquariums were vessels used to grow aquatic plants.

The first fish tank (glass aquarium) was invented in 1832 by Jeanne Villepreux-Power (a french naturalist).

However, Philip Gosse (British naturalist) modernized them as glass tanks with fish and plants.

In this article, we will learn more about how and when fishes reproduce (spawn), the impact of overfishing, and various fish spawning methods.

Did you know? Human beings evolved from fish called Tiktaalik. Fishes are our ancestors.

What is Fish Spawning

Spawning refers to the reproduction process of aquatic animals, whereby they release sperm and eggs into the water. These eggs then fertilize and metamorphose into frys.

Most fishes reproduce in this manner.

When do Fish Start Spawning

Fishes start spawning

1. Once they grow into adult fish/mature
2. Different fishes have different spawning periods (known as spawning season), which get triggered by certain factors.

A) Identifying an Adult Fish

Every fish has its life span and maturity age when they become sexually active. Even within the same species, there could be slight variations.

It is similar to human beings. We are supposed to be sexually active by a certain age. However, there are slight variations.

Few physical attributes indicate if they are ready to spawn.

Research in female fishes indicates that the size of their ovaries changes as they mature.

• Immature – Ovaries are tiny, transparent, not noticeable, and form a minuscule portion of their bodies. Egg-carrying membranes with immature ova in phases 1 and 2 are visible under a microscope.
• Early mature stage – The ovaries are slightly bigger, thicker, light yellow, and cover 50 percent of their bodies. Immature ova in phases 3 and 4 are visible under a microscope.
• Advanced maturity stage – The ovaries become larger and occupy 66 to 75 percent of their bodies.
• They are dark yellow with clearly visible nerve vessels supplying blood to them. Developed ova in phases 5 and 6 are visible.

• Pre-spawning stage – Their abdomen is egg-laden with good blood supply.
  They look fuller due to numerous developed eggs in phase 7.
  These eggs are visible through their stretched abdomen skins. The ovaries are dark yellow and awaiting the right trigger to release these eggs.

• Spawning stage – The developed, transparent eggs are present in the oviducts due to the abdominal pressure.
  The eggs get released in lots whenever the abdomen experiences the slightest pressure.

• Spent stage – The abdomen shrinks in size after releasing all egg,
  The blood supply to the ovaries normalizes, and empty ovaries occupy less space in their bodies.
  All the ova do not reach a maturity stage. Some undeveloped ova (atretic oocytes) remain in their ovaries in phases 1 and 2.
  These are visible under a microscope after spawning.

Mating Behaviors

Male fishes display specific behavioral trends when they are ready to spawn.

• They are more active.
• They may become bickerish, territorial, and aggressive while pairing or protecting their eggs/frys.
• They constantly chase the female(s). The female may swim away or reciprocate by chasing them back.
• While chasing, they keep rubbing the female fish’s abdomen from various sides, which causes the abdominal pressure needed to push her eggs out. Thus, the eggs get released into the water.
• In some species, the male fish swim in groups on the surface or sides of the ponds in search of female fish.
• Many reef fishes (both males and females) come to the surface water swiftly, release their eggs/sperm, and return to the reef quickly.
• In a few species, the male fish jumps from the surface water, signaling to the female fish that he is ready to spawn.

A female fish, which is ready to spawn, jumps from the surface water in consent. Post this they spawn in the water.

Did you know? In some fishes, it is hard to differentiate between male and female fish until they spawn.

B) Spawning Triggers

The majority of the triggers which hail the spawning season are ecological/natural.

• Changes in water temperature and salinity – Chocolate Chip Sea Star
• With tides – Beach spawning
• Increase in water level – Catfish spawn in the rainy season
• Season change – When the water temperature rises (like Discus)
• Before floods – Tire Track Eel
• Full moon day – Chocolate Chip Sea Star
• Availability of plant food (when there is thick plant growth) for the eggs and frys to stick/hide and eat when growing – Mostly in the rainy season and spring.
• Simply on attaining a certain age – Kuhli Loaches, Mickey Mouse Platy Fish
• Some fishes spawn once a year, few spawn more than once, while some spawn throughout the year (like Guppies).

Some breed in pairs, while many are group breeders.

Did you know? The Avon Roach seemingly spawns on the 25th of April every year.

How do Fish Spawn

Egg Laying

Marine Vs. Freshwater Fishes

There is a difference in the spawning of marine and freshwater fishes.

Freshwater Fishes	Marine Fishes
Mostly spawn in deep, steady waters	Mostly spawn on Surface waters
Eggs are sticky/adhesive	Eggs are non-sticky and buoyant
Eggs stick to plants, rocks, etc.	Water currents sway the fertilized eggs to safer lagoons
Eggs fertilize and metamorphose through various stages	Eggs metamorphose through various stages
All eggs do not get fertilized. Some fall prey to larger fishes or perish due to natural factors, infections, etc.	All eggs do not get fertilized. Some fall prey to larger fishes or perish due to natural factors, infections, etc.
While growing, they consume algae, microorganisms, plants, etc., and remain hidden to avoid predators	While growing they eat microorganisms, plants,  etc., and remain buried/hidden in sand bottoms to avoid predators
Small frys swim back to normal waters after they become capable to defend and feed themselves (like small predators)	Small frys swim back to normal waters after they become capable to defend and feed themselves (like small predators)
They grow into adult fish and spawn	They grow into adult fish and spawn
Have some parental instinct in general, as they want their eggs to be safe	Generally lack parental instincts, as they get separated instantly

Few Exceptions

1) Migratory fishes

These fishes swim to a different habitat to spawn, or in search of food, etc.

• Fishes that live and migrate in freshwaters only are called potamodromous (like Chinook. Coho, Steelhead, etc.).
  Yoyo Loaches live in lower streams, but swim upstream / midstream to spawn. After
  spawning, they return to the lower streams.

All freshwater fishes that spend their lives in freshwater only (without migrating) also fall in this category.

• Oceananodromous are marine fishes that live and migrate in seawater only. Even non-migratory marine fishes which spend their entire lives in seawater are included in this category.
• Freshwater fishes which spawn in marine water are called catadromous. For example, the American and European eels.
• Fishes like Sturgeon, Salmons. Striped Bass, Smelt, and Sea Lamprey are anadromous, as they live in saltwater but spawn in freshwater.
• Marine fishes that spend part of their lives in freshwater and another part in marine water are called diadromous.
• Bamboo shrimp spawn in freshwater, but the egg larvae are drifted to brackish water by the water current.

They can survive in brackish water only. After developing into a fry, they swim back to freshwater.

2) Few freshwater eat their own eggs/frys – Angelfish, Kuhli Loaches, Yoyo Loaches, Cichilds, etc.

3) Some marine fishes display parental instincts.

• Clownfish and damselfish lay eggs on rocks for protection/hygiene.
• Male Jawfish and Banggai Cardinal fish carry their eggs in their mouths till they hatch.

The number of eggs a female releases in every spawn depends on her size, age, and weight.

4) Female platys can store male sperm in their bodies for up to six months. Thus, a female platy can reproduce without any male counterpart, using the stored sperm.

5) Research indicates that some female Sharks and Parthenogenetic female fishes can reproduce single-handedly.

6) A female seahorse lays her eggs in the male seahorse’s pouch after mating. Thus, a male seahorse becomes pregnant and delivers babies.
7)  Pipefish and Sea Dragons also carry eggs inside an area underneath their tails.

Livebearers

Some fishes are livebearers (like Guppies, Mollies, Swordtails, Platys, etc.). They do not lay eggs but give birth to frys directly. Their eggs are fertilized internally.

Spawning Aggregations

It refers to a gathering of an unusually high number of fish of the same species for reproduction.

It might be due to fewer fish available in an area, resulting in migration and subsequent gathering.

Group breeders also display this behavior in the wild.

During the spawning season, numerous fish gather at a commonplace and release sperm/eggs in synchronization.

Some spawn in smaller lots (as if queued up). Small batches spawn and make space for the next lot.

It results in millions of sperm and eggs getting released. The water turns milky, and a foam gathers on the surface.  It might even stink typically for a while.

Fishermen (to improve catch), researchers, scientists, and governments (to preserve fish) conduct ongoing research on spawning aggregations.

The exchange of information among these groups improves their scope mutually.

Did you know? Some fishes (like Clownfish, Damselfish, Broad-Barred Goby, Black Sea Bass, Mangrove Rivulus, Asian Sheepshead Wrasse, etc.) can change their gender from male to female and vice versa. They are known as hermaphrodites.

Methods of Fish Spawning

Natural spawning

In this, adult fishes spawn and reproduce on their own.

It can occur in the wild or in captivity.

Many fishes (like Platys, Kuhli Loaches, Guppies,  Mollies, Angelfish, Swordtail, Corydoras, Clownfish, Cardinal fish, Neon Goby, etc.) spawn in community tanks/aquariums with proper care.

Natural spawning in captivity may involve replicating their natural spawning triggers in a breeding tank/pond like

• Raising the water temperature, salinity, PH
• Increasing water level
• Providing mosquito larvae, increasing plantation,
• Creating a drought/rainy season-like environment, etc.

On the other hand, many fish species spawn effortlessly in captivity once they mature.

a) Types of Egg layers

• Mouth brooders – In these species (like Red Zebras, Lake Malawi Cichlids, etc.), the fertilized eggs are held by either of the parents in their mouth.

Sometimes, the female fish holds the eggs in her mouth for the male fish to fertilize them.

Some release developed frys from the mouth, while a few carry both – the eggs and the frys

in their mouths.

In some species, either of the parents carries the developed frys (not the eggs) in their

mouth to protect them.

The frys also return to their parent’s mouth for security or to sleep in the initial stages.

• Egg scatterers – Some fishes (like Tetras, Goldfish, etc.), release their eggs into the water.  Fertilized eggs are carried away by the water current quickly.

Hence, they lack parental instinct in a captive environment.

• Egg burriers – Some fishes (like South American and African Killifish) bury their eggs in mud during the rainy season.

They remain buried during the drought season. The onset of the next rainy season hatches them naturally.

• Egg depositors – Some species (like Clownfish, damselfish, etc.) openly deposit their eggs on rocks, leaves, etc. They sometimes dig a small crater in the substrate to deposit eggs. 

While some species hide their eggs for safety (in caves, folds, substrate, etc.)

• Nest builders – Male fish in a few species (like Dwarf Gouramis, Pearl Gouramis, Siamese Fighter, etc.) build a bubble nest with saliva.

It has some plant fragments inside.

The eggs are gathered and deposited into this nest by either of the parents, or the eggs auto-float inside the bubble in some species.

Explore in depth about protecting fish eggs.

b) Livebearers

As discussed earlier, some fishes give birth to frys directly. The eggs are fertilized and developed internally in the female fish’s ovary.

The male fish has a gonopodium (instead of an anal fin), which is sharp. It ejects sperm into the female fish’s abdomen through the gonopodium.

The eggs fertilize and grow inside the female fish’s abdomen. Newborn frys are released directly, just like human beings.

The male impregnates several female fish throughout the spawning season. Likewise, a female fish spawns several times in the spawning season.

Induced Spawning

This includes semi-natural artificial spawning.

Induced spawning helps

• Reproduce hybrids (cross-breeds) to satisfy the need for newer varieties.
• Meet the ever-increasing demand for fish for ornamental/consumption around the year.
• Create polyploid fish. These have an extra set of chromosomes due to scientific intervention,  improving their performance and quality.
• All fish might not spawn at the same time in the same spawning season on their own. Induced spawning makes it possible.

Thus, it enables large-scale spawning for commercial production and sale, reducing costs.

• Improve the fertility rate of eggs and survival rates of frys in a protected environment.

Semi-natural Spawning

Stages in natural spawning and artificial methods are mixed in semi-natural spawning to improve the survival rate and quality of frys.

Mostly, the female fish are administered with chemical injections to trigger spawning. Then the male and female fish are spawned in a special breeding pond/tank.

The fertilized eggs are gathered and nurtured into frys in a better artificial/natural environment.

Artificial Spawning

The entire process is managed artificially using chemical injections in a fully controlled environment.

• Female fish are injected several times to ripen their eggs.
• When the eggs are ready for spawning, they are carefully removed from their abdomens.
• Male fish are injected, and their sperm is collected.
• The eggs and sperm are fertilized artificially and nurtured in a controlled environment.

Conclusion

The super-intelligence of nature is evident in the unending varieties of fishes, their detailed anatomies, and their unique abilities to reproduce and survive.

Several pointers are very concerning.

• Many Pacific Salmon fish die/rot in the freshwaters after spawning – caused due to long migration distance to lay eggs, high stamina consumed during spawning, reduced immunity, and infections.
• Similar instances of fish dying naturally due to spawning stress are reported in Connecticut.
• High pollution of rivers, beaches, and seawater – is caused by reckless waste dumping, deteriorating the quality of aquatic fish and plants.
• Many varieties of fish are perishing at an alarming rate due to overfishing.
• Knowledge about spawning aggregations, their timing, etc., is used by many to get a huge catch of adult fish from a single spot for commercial gain, year after year.
• In the process, some known spawning aggregation spots have disappeared now.
• The dams we build, impact the river water flows, oxygen levels, and temperature, affecting the natural triggers that hail the spawning season.
• Use of genetic science to produce new fish varieties. (like polyploid fish, Fluorescent or Glowing Angelfish).

With every progress, we are breaking the ecological chain, causing irreversible damage to nature, our planet, and aquatic life.

We need to act responsibly and use every knowledge we have, to improve ecological balance, the quality of natural resources (including water) and aquatic life.

Any human intervention should be aimed to heal them or provide them with a better environment.