Fish reproduce in many ways, depending on the species. Watching fish reproduce is one of the most fascinating things about aquaristics because there are so many forms of reproduction in fish.
There are two main forms of reproduction in fish: egg-laying fish and viviparous fish.
Concerning egg-laying fish, some people wonder why they lay so many eggs, and rightly so! Indeed, depending on their size, female fish lay an average of 3000 to 6000 eggs!
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Why do fish lay so many eggs?
It’s actually quite logical, when small fish are born, many end up being eaten by other larger species, even earlier, it can happen that fish eggs are also eaten by other species. Their survival rate is therefore extremely low.
Faced with such hazards, a species is favored if it relies on massive and rapid reproduction, as the toad does: this is what ecologists call the “r strategy”.
On the other hand, species typical of more stable environments (forests, oceans…), where resources are predictable and risks are lower, rely on the “K strategy”. They produce few young, at an older age, and provide them with parental care during a long part of their life: this is the case of large mammals such as bears, whales or humans.
How many eggs do fish lay?
The female lays a variable number of eggs on a substrate specific to each species. Salmonids such as the common trout or Atlantic salmon lay their eggs in a small depression in the gravel by the female and when reproduction has occurred, the female covers her eggs. Other species (e.g. pike) require vegetation to lay their eggs.
The size and number of eggs vary according to the species and the size of the females. The number of eggs laid can vary from a few hundred for sculpin to several million for pikeperch.
Where do fish lay their eggs?
For some species, such as the pike-perch or the spotted ictalure, the eggs are laid on a single site on which the eggs adhere in a block, or in a sort of nest. Others, like the Hypostomus, look for holes or cracks in the clay mainly, but also in the rock to lay their eggs.
The place remains very variable according to the various species. The most classical way to avoid that eggs are eaten is to use a separate breeding tank. The parents can lay or give birth to their young, and then be removed. Open water layers can be placed over a net, grate, or a bed of glass beads to protect the eggs while the fish are laying.
Bubble nest builders and mouth incubators can be left until they are no longer caring for their offspring. Ovoviviparous can lay eggs near a clump of bushy plants (natural or plastic), so that the young can hide until the mother is done giving birth, then she is removed.
A specific tank is quite easy to keep clean. The eggs and fry need very clean water to hatch and grow in good conditions. Also, there are no adults competing with the fry for food. Many breeders use a bare tank, equipped with a simple sponge/exhauster filtration. Debris and food remains are easily visible and can be siphoned off daily. Frequent water changes can be made as there are no other fish to be stressed.
Another solution is to allow the fish to lay eggs on a ball of cotton thread, a plant or a piece of slate or glass in the community tank. The eggs can then be easily moved into the breeding tank. This works well for scalars. Killies eggs can be collected in peat or mop and placed in a separate container or dried for incubation. Ovoviviparous can be bred in commercially available nesting boxes or cages (nets) placed inside the community tank. The brood box separates the babies from their mother and provides a safe place for them to grow.
Aggression of fish towards eggs
Parental care varies greatly in fish. Parents can range in behavior from wild gluttony toward their eggs and fry to jealous and persistent guarding by both parents.
Most fish consider fish eggs and young fry as appetizing snacks. Therefore, most of them will not hesitate for a second to swallow them if they find them, even if they are their own offspring. This means that open water and many substrate layers cannot breed in community tanks, as the eggs are quickly eaten by the parents and other fish. Invertebrates also eat the eggs. Ovoviviparous fish are also known to eat their young.
Some species ignore their eggs and fry, so they can be reproduced in a specific tank. Many killies will ignore their eggs; but small killies are too tempting a prey. Guppies also often ignore their young.
Other fish have a parent that guards the eggs and fry. Most bubble nest builders and mouth incubators are among them, as are some substrate layers. The parent (male or female) in charge stays with the eggs, and then the fry, until they reach free-swimming. In the case of bubble nest builders, the male maintains the nest, spits out new bubbles to replace those that burst, and prevents eggs or fry from falling out, replacing them if necessary.
He will also defend the nest from other fish. Oral incubators simply hide their offspring in their mouths, while some substrate catfish will hide them underneath.
A more common behavior among cichlids is joint guarding by both parents. This behavior is definitely the most interesting to observe. The parents take turns fanning or spitting fresh water on the eggs, and removing any unfertilized eggs attacked by rot.
They also proudly defend the breeding site, sometimes leading to severe injury or even death of some roommates. Once the eggs have hatched, the parents also guard the fry. Some even move the fry daily. Once the fry have reached the free-swimming stage, some continue to guard them, while others cease their parental duties there. Many African cichlids defend their young until the next breeding season. Discus feed their young themselves with a secretion from their mucus.
An even more extreme version of parental care is practiced by some cichlids in Lake Tanganyika. Older pups stay near the nest and help the parents defend their new clutches. The young are allowed to stay until they reach reproductive age themselves, at which time they are hunted.
