Fish, which inhabit almost all types of water bodies, are characterized by the greatest variety of reproduction methods among animals. Over a huge period - more than 400 million years, fish have mastered reservoirs with fresh and salt water, they live at various depths, in high-mountain and underground reservoirs, in rivers and streams with fast current and in stagnant silted floodplains and swamps. The environment and living conditions require adaptation for the survival of not only adult individuals, but also for maximum preservation of offspring. From this point of view, it is very interesting to get acquainted with the facts of how fish reproduce and what this gave to each species.
Classification of fish according to breeding conditions
Most fish species produce reproductive cells during external environment, and fertilization, as well as their further development, occurs outside the female’s body. The majority of species do not take care of their offspring, and some even eat larvae and fry. But they choose places for spawning, which are often vast distances from their usual habitat.
There are species of fish in which the parental instinct is highly developed. They build special nests for fertilized eggs, take care of and protect the territory. Some fish bear their offspring inside the body or on its surface, in the mouth. Moreover, both females and males take care of the offspring.
According to Kryzhanovsky S.G.
According to the classification of Professor S.G. Kryzhanovsky, adaptive features in fish are observed in early period development. The habitat is different not only for various types fish, but also at different stages of life. Fish breed in certain seasons and choose the most suitable areas of water bodies for spawning. Changes in the environment cause:
- fish fertility;
- type of fertilization and type of development of offspring;
- peculiarity of sexual cycles;
- distribution and migration;
- the possibility of relocating to reservoirs with a different hydrological regime during spawning.
According to the localization of the masonry, S.G. Kryzhanovsky divided all types of spawning fish into:
- pelagophilic - spawning in water;
- lithophilous - laying eggs on stones;
- phytophilous - on plants;
- psammophilous - on sand;
- ostracophilous - into the gills of bivalve mollusks;
- protecting their eggs.
This classification has been used in ichthyology for more than 170 years. 50 years after Kryzhanovsky’s classification, the famous ichthyologist Balon compiled his own classification, more detailed, which indicates not only the location of the laying, but also the types of nests being built, the type of laying eggs on a substrate and without a substrate, and a detailed analysis of viviparous fish species.
According to Balon
The eggs of some fish are able to swim both in the thickness and on the surface of the water, being in a state of lack of oxygen, and in some fish the eggs are able to be in diapause and remain viable without water. The laid eggs can be located above the surface of the water - on coastal stones or plants, and the male periodically pours water over them so that the shell does not dry out. 
The types of nests that fish build are also very interesting:
- from foam;
- from a mixed substrate and material;
- glued sockets;
- from plants and their remains;
- from sand;
- on the surface of soil and stones, as well as in crevices and cracks;
- under the protection of sea anemones.
The adaptive features of fish that externally bear their offspring are striking. Depending on the location, there are the following types fish:
- moving their brood. They carry eggs in body cavities or on any part of it;
- bearing on accessory organs. They glue sticky eggs between the fins, on the skin. The female curls herself into a ring around her eggs;
- hatching eggs in oral cavity. Growing larvae can feed from the yolk sacs or immediately after hatching they can emerge and feed outside;
- gestating in the brood chamber or in the gill slits.
Viviparous fish also carry fertilized eggs. Internal gestation can be with complete or incomplete transformation. When incomplete, the fish lay zygotes - eggs, at a certain stage of development. When fully metamorphosed, the female gives birth to fully formed fry.
Reproduction
The predominant method of fish reproduction is sexual, in which different-sex individuals of the same species participate in fertilization. Along with sexual ones there are:
- Parthenogenesis– in which the development of the egg occurs without the fusion of male and female gametes. In some species, the development of germ cells goes through only the cleavage stage without the formation of a larva. Only in in some cases metamorphosis occurs before the larval stage. In most cases, such offspring die. In salmon, fertilized and unfertilized eggs can coexist in one clutch.
- Gynogenesis– reproduction, in which the germ cells of closely related fish species penetrate the eggs, but do not fertilize, but stimulate their development. As a result of gynogenesis, only female eggs emerge from the eggs. In Mexico, there are populations of mollies consisting only of females.
Most fish have male and female individuals (different sexes), but there are also hermaphrodites that combine the sexual characteristics of both sexes. In some species, sex changes occur with age. For example, in the red pagella, the ovaries begin to function as testes with age. In hermaphrodite guppies, ichthyologists have discovered the ability to self-fertilize, but the eggs are released unfertilized. 
There are also differences in the method of fertilization. Highlight:
- external– in which fertilization of eggs by sperm occurs in the external environment;
- internal- when the male, using a pseudophallus, introduces seed into reproductive system females, where fertilization occurs.
In the latter case, the development of eggs occurs inside the female’s body, and fry are born, which immediately begin to feed on their own. Such fish are classified as ovoviviparous. In addition to ovoviviparous fish, there are also viviparous fish. In them, the lower parts of the oviduct form an analogue of the mammalian placenta. These include some species of sharks. In them, the nutrition of the larvae and fry is provided by the female’s body, and not by the yolk sac of the egg.
Depending on the nature of reproduction, all fish are classified into:
- monocyclic– representatives who spawn only once in their life, after which they die;
- polycyclic– laying eggs or giving birth to offspring throughout the entire period of life. These include most types of fish.
Great importance in the life activity and reproduction of fish has the speed of their sexual maturation and productivity.
Timing of puberty
In fish puberty occurs at different periods of life. How lives less fish, the earlier it begins to reproduce. The variability of puberty ranges from 1-2 months to 15-30 years. Males mature earlier than females. The ability to procreate depends on the size of the individual. The better the fish eats and grows faster, the earlier it is able to spawn.
The metabolism of fish, and therefore life expectancy, is affected by water temperature. The higher it is, the faster the fish ages and the earlier it begins to spawn. Sexual dimorphism - visual differences between representatives of different sexes - plays a great role in creating a pair and successful fertilization. In some species it is difficult to distinguish females from males. In others, males have:
- larger size;
- more variegated, contrasting color;
- external genitalia - gonopodium or pterygopodia.
Sex differences may be present throughout life or appear only during the spawning period. This “nuptial outfit”, which occurs under the influence of activation of sex hormones, is typical for carp, whitefish, stickleback, salmon, and pink salmon.
Sex ratio and spawning characteristics
As a rule, in a population the ratio of females to males is 1:1. But under the influence of changing conditions it can shift in one direction or another. For example, infection of guuppies with the saprolegnia fungus leads to the death of the fish. In this case, the surviving females turn into males. Heat environment leads to a predominance of males in the green swordtail population. Stimulating the mark with steroids can also shift the sex ratio.
Among the fish there are:
- monogamous species– in which fertilization of the female’s eggs occurs only by one male;
- polygamous species– when there are 3-4 individuals of the opposite sex per individual.
The period of the mark is influenced by external and internal factors. Most often this is the temperature of the water in the reservoir. Each type of fish has its own comfort temperature regime, so spawning can occur in spring, summer or autumn. Caviar can ripen either simultaneously or in portions. 
The time of spawning also varies - from a few seconds (stickleback) to several months (cod, plume). The same view in different conditions can lay eggs simultaneously or in portions. This adaptive mechanism helps to make maximum use of the food supply, increase the survival rate of young animals and the fertility of females.
Natural selection ensured the best survival and prevalence of populations, depending on living conditions. Maximum adaptation to these changes is provided by various ways reproduction. They allow you to survive the largest number juveniles, expand their range and compete with other species of inhabitants of the Earth’s water basins.
Using spawn throwing, but different types it happens in different ways. For example, a species such as silverside grunion swims as close to the shore as possible and during high tide they find themselves on almost bare sand, it is in this wet sand that they lay their eggs, after which they become males. In silverside grunions, the process of laying and fertilizing eggs occurs very quickly, because they must be in time before the next wave overtakes them. The fry hatch from the eggs quite quickly, and when the tide comes in, the babies go to.
Red caviar salmon fish valued as tasty and healthy. It is interesting that fish migrate from salty waters to reservoirs to spawn. These fish work together - in pairs. The male and female first dig a kind of nest at the bottom of a freshwater reservoir, at the same time they protect the favored territory from competing fish. Very important point for salmon, it is necessary that the sex cells of both parents appear in the water at the same time, otherwise fertilization will not occur. Many species of salmon fish are capable of reproducing only once in their life, mostly simply after spawning. For example, this happens with Pacific salmon.
In nature, of course, there are also species of fish. They do not spawn, but produce fully formed offspring - fry, which can immediately live and develop independently. One of the strange features of viviparous fish is that after throwing fry, the mother can easily eat her own young, she simply cannot distinguish them from other food.
Reproductive organs. Fish are usually dioecious animals. The reproductive organs of females are the ovaries (see Fig. 118, p. 15), eggs - eggs - mature in them. The reproductive organs of males are the testes (milt). Sperm mature in them.
Reproduction. Females spawn eggs into the water. Males pour seminal fluid containing sperm onto the eggs. After fertilization - the fusion of a sperm with an egg - fish fry develop from the fertilized egg.
However, among fish there are also those who take care of their offspring. They lay eggs in depressions on the bottom (salmon), build nests (stickleback - see Fig. 121), carry eggs in their mouths (tilapia) or on their bodies (pipefish, seahorse). Some fish are characterized by viviparity. Among cartilaginous fish Most sharks are viviparous and give birth to young sharks. When caring for offspring, the number of eggs laid by one female and cubs born is reduced to tens and units. In caring for offspring, complex forms of fish behavior are manifested. For example, a male stickleback jealously guards his nest from predators, and a tropical catfish, carrying eggs in its mouth, subsequently allows the fry to hide there if they are in danger.
Rice. 121. Stickleback at the nest
Migrations. As you know, fish constantly move in the water alone and in groups. Many fish move long distances. Such movements of animals are called migration.
There are feeding migrations. Thus, herrings move in the ocean following the plankton that serves them as food, and sharks follow the herrings and hunt them. Migration associated with reproduction is interesting. The fact is that places with abundant food are often not suitable for fish to reproduce, since eggs must be laid in clean water, rich in oxygen, well heated sun rays. So the fish move, covering hundreds and even thousands of kilometers, from feeding grounds to spawning (reproduction) places. This is what salmon do Far East and in northern Europe, sturgeon of the Caspian Sea. Fish that move from seas to rivers or from rivers to seas are called migratory.
Adult individuals of Far Eastern pink salmon (Fig. 122) after spawning are doomed to death. Their instinct to leave behind offspring is so strong that, going up the rivers, they do not eat anything and come to the spawning grounds exhausted by hunger, lay eggs, fertilize them and die.
Rice. 122. Pink salmon (above) and male pink salmon before spawning
Fish are dioecious animals. They reproduce in water: females lay eggs, males secrete seminal fluid onto the eggs. Most fish lay many eggs and leave them unattended after fertilization. Many eggs die. Fish that care for their offspring have few eggs; most of them successfully develop into fry.
Exercises based on the material covered
- What is spawning?
- What type of fertilization is typical for most fish? Describe the characteristics of fish reproduction.
- Explain the concept of migration. What is the role of migration in the life of fish?
- Why in the life of fish, along with food, are there migrations associated with reproduction? Give examples.
- What is it connected with different number eggs from fish of different species?
Equipment: tables “Peculiarities of fish reproduction”, “ Sea fish", "Freshwater fish", "Migratory fish", "Commercial fish", "Internal structure of river perch"; geographic map; books by V. Astafiev “The Tsar Fish” and F. Abramov “Once upon a time there was a salmon”.
During the classes
Guys, listen to a Scottish folk song called "Trout".
I fished for trout for seven weeks,
I couldn't catch her
I'm all wet and all chilled,
And I tore my dress,
I caught in the forests, I caught in the gardens,
I even caught it in the oven.
So what? Trout all seven weeks
She was hiding, brothers,...
In the river! - the students finish.
Tell me, where else do fish live? ( Seas, lakes, oceans, ponds.)
The topic of our lesson is “Features of fish reproduction.” We will talk about the reproductive organs of fish, find out where fish prefer to lay eggs, how the fertilization process occurs and the further development of eggs.
Write down the topic of the lesson and the date in your notebook.
On the board you see terms that will help us in the lesson, and a table showing the internal structure of a fish. What is the name of this fish? ( River perch.)
Fish are dioecious animals. How do you understand the meaning of the word “dioecious”? ( There are males and females.)
Probably all of you have eaten herring. Can you tell what gender this herring was: male or female? ( If there was caviar in the fish’s body, then it was a female, and if not, then it was male.)
Female individuals are females. What are males called? ( Males.)
In males, the gonads are called testes. They form a liquid called milk. Does anyone know why exactly this is? ( It looks like milk, white in color.)
Milk contains millions of sperm. Sperm are male reproductive cells.
So let's do it again.
What are the gonads of males called? ( Testes.)
What liquid do they contain? ( Milk.)
What are male reproductive cells called? ( Sperm.)
Females also have paired gonads. They are called ovaries; female reproductive cells - eggs - mature in them. Does anyone know what another name is for fish eggs? ( Eggs.)
So let's do it again.
What are the gonads of females called? ( Ovaries.)
What cells mature in them? ( Female reproductive cells are eggs.)
The fish has a genital opening on the underside of its abdomen. Through this hole, females lay eggs, and males pour seminal fluid onto the eggs. The sperm that are in the liquid merge with the eggs, and the process of fertilization occurs.
So: fertilization is the process of fusion of sperm and eggs.
What can we call the egg now? ( Fertilized egg, fertilized egg.)
The fertilized egg divides many times, and an embryo is formed under its membrane. The embryo has a yolk sac. This is a supply of nutrients. Due to the yolk, the embryo grows and develops, then sheds the eggs and turns into a larva. The larva already feeds on its own and soon becomes a small fish. What is it called? ( Malek.) What does the fry look like? ( For adult fish.)
As the story progresses on the board, the teacher draws up a diagram:
fertilized egg ––> embryo ––> larva ––> fry ––> adult.
What can you call this scheme? ( Fish development diagram.)
Write the title of the diagram in your notebook.
Take a close look at the diagram again. Now I will erase two words in it (I erase the words “embryo” and “fry”). Now write down the diagram in your notebook, trying to fill in these missing words.
(After completing the task, one of the students goes to the board and restores the missing words in the diagram.)
So, you and I can conclude: each egg can grow adult fish! But most eggs, larvae, and fry die much earlier. After all, fish have many enemies - predators. Among them is a person.
Life is very difficult for fish these days. And not because people catch and eat them, but because people disturb the fish’s habitat. Therefore, it is people who are the most dangerous enemies fish
Do you know how people disturb the habitat of fish?
Student answers
– Pollution of water bodies with industrial waste: oil and oil products (oil forms a film on the surface of the water that does not allow air to pass through, and the fish become unable to breathe);
– pollution from fertilizers that flow from fields;
– pollution household waste and garbage.
Our Government cares about nature conservation. Exists the federal law about environmental protection, and it contains articles about the protection of water resources and fish.
What time do you think you can fish, and at what time is fishing limited? ( Fishing is limited during spawning season.) Why? ( Because the fish must lay eggs.)
Conclusion
Today it is not enough not to kill an animal. It is necessary to preserve the environment where the animal lives, where it can produce offspring. We must learn to exist with nature and be its protectors.
So what is your task? ( Preserve and protect, and if necessary, protect nature.)
Many writers have dedicated their works to Pisces. For example, Viktor Astafiev wrote the story “The Tsar Fish” (show book). In this story the author describes a very large and very strong fish. This is a Russian sturgeon that almost drowned a fisherman who wanted to catch it.
Another wonderful writer, Fyodor Abramov, wrote the northern fairy tale “Once Upon a Time There Was a Salmon.” The author describes the life history of a fish - salmon. The author's position is already evident in the title of the work itself. He affectionately calls the fish salmon. And the fish had a name - Belladonna. This is how the author describes the life of this fish: “In the morning, she caught bugs and spiders, which were brought by the current, and then, if it was sunny, she played: either she pushed sparkling pebbles at the bottom with her nose, or she jumped after emerald dragonflies scurrying just above the water, and sometimes, for fun, she even rushed at some unwary fry.”
What emotional words does the author use in this passage?
Salmon, about which we're talking about in the story of F. Abramov, she was born in a river, and then, together with other salmon, went to sea. Salmon spends time at sea most own life. But when fish mature, they develop a reproductive instinct. This is a complex innate feeling that makes fish swim to spawn, swim to lay eggs. Salmon lays eggs in rivers: it enters them from the sea and rises up against the current. It’s very difficult for fish; in rapids the current can be very strong, and the path is blocked by various obstacles. And yet the fish swim bravely and boldly.
“...A crazy current, ice jams, some kind of barrier made of logs all over the river” prevents the fish from swimming. The salmon makes “a furious swing of its tail - and its body flies up above the water in splashes and foam...”
“Gradually the water in the river began to fall. One after another, the salmon began to stand on the reaches - places
where they grew up." And Krasavka “swimmed up the river for a long time. At times she was overcome with despair. From threshold to threshold, the river became narrower and shallower. She often had to swim through boiling breakers and fall headlong onto sharp rocks...” But finally she reached her homeland!
This is how salmon come to spawn. They lay eggs in a hole dug in small pebbles, fill the hole with pebbles and wood debris and, completely exhausted, turn back to the sea.
“...Wounded, emaciated, with disheveled, frayed fins, it seemed that she was completely exhausted. In the stormy rapids she was spun around like a splinter and hit against the stones. But she swam and swam...
...Bending its tail, the salmon began to unfold. It was painful to look at her. Her body lost weight, dried out and became flat. There were bleeding abrasions on the belly.”
Very few spawning fish swim to the sea.
Fish that are born in rivers, then live most of their lives in the sea, and return back to rivers to spawn, are called anadromous.
Who understands why they are called that? ( Because they go from rivers to sea, and from sea to rivers.)
(Demonstration of the table “Features of fish reproduction.”)
But most fish live and spawn only in the sea or only in fresh water bodies. Freshwater fish are crucian carp, perch, pike perch, ruffe, pike, catfish... Please show on the table which fish you know.
Now let's turn to the geographical map.
I want to trace the path of migratory fish using the example of sturgeon and salmon. It was not by chance that I chose them. Sturgeon fish they give you black caviar (and we will take a black fish); and salmon have red caviar (let’s take a red fish).
Sturgeon live in the Atlantic, Pacific, Arctic oceans, the Black, Caspian and Aral seas. These are beluga, sturgeon, stellate sturgeon, kaluga, sterlet.
Let's trace the path of sturgeons, such as beluga.
Who can show the Caspian Sea? Which famous Russian river flows into the Caspian Sea? ( Volga.) Who will show it on the map from its mouth to its sources?
Beluga from the Caspian Sea (the teacher takes a black fish and leads it upward along the map) moves against the current to the upper reaches of the Volga River. But now the fish do not swim to the upper reaches of the river. Why? ( A lot of hydroelectric dams have been built on the Volga. They block the fish's path.) Although passages are left in the dams, often the fish do not have time to realize that they need to swim through them. They have to spawn along the river, clinging to the shore.
Salmon fish, known for their red caviar, live in the seas Pacific Ocean. These are chum salmon, pink salmon, sockeye salmon, chinook salmon, coho salmon, and masu salmon.
Who can show the Pacific Ocean? Who can show the Amur River?
From the seas of the Pacific Ocean, salmon enter the Amur River and other Far Eastern rivers and swim against the current (the teacher takes a red fish and leads it along the map). Having reached the spawning grounds and laid eggs, Pacific salmon die, i.e. They reproduce once in a lifetime. But the European noble salmon that live in the seas of the Atlantic Ocean - salmon, brown trout - can spawn by entering rivers and then sliding back into the sea several times. Among them there are also forms that constantly stay in rivers or lakes. Such small salmon that constantly live in fresh water, called trout.
Guys, do you know how much caviar costs?
Red (Pacific salmon) – 130 rubles per 100 g. Black – about 4000 rubles per 100 g, and abroad $2000 per 1 kg. The most valuable caviar is from Kaluga. And in 1922, a beluga 5 m long was caught, containing 245 kg of caviar.
To summarize the lesson, please answer the following questions:
What material did you remember from the lesson?
What did you find interesting?
Homework
On the map, find geographical objects associated with the names of fish.
Task 1. Fill in the gaps in the text.
Fish are dioecious animals. They have gonads. In males they are called testes, in females they are called ovaries. Mature in the ovaries eggs-eggs, in the testes - sperm. The reproductive period in fish is called spawning. Fertilization in most fish is external. After fertilization, the fry, fry, develops in the ovaries. Substances necessary for the nutrition of fry are contained in caviar. Fish that lay small amounts of eggs have developed caring for offspring.
Task 2. Fill out the table.
Task 3. Write down the numbers of the correct statements.
Statements:
1. Fish are dioecious animals.
2. Gonads of fish - unpaired testes and ovaries.
3. Among fish there are viviparous species.
4. The fertility of fish is associated with high mortality of eggs.
5. Fertilization in fish is always internal.
6. Male and female fish do not differ in appearance from each other.
7. Sharks and rays are ovoviviparous fish.
8. All fish spawn in the spring.
9. The fish larvae emerging from the eggs initially feed on the smallest organisms.
Correct statements: 1, 2, 3, 4, 7.
Task 4. Explain why different species of fish have different timing of the onset of sexual maturity, different fecundity and duration of embryo development.
The time for the onset of sexual maturity varies greatly among different fish species. It is subject to significant changes, varies among different populations of the same species, as well as within the same population. Therefore, than slower fish grows, the later her puberty occurs and vice versa.
Thus, the time of puberty is associated with food availability, i.e., the abundance of food, and the duration of the feeding period. How smaller fish, the earlier she becomes sexually mature.
The rate of embryo development is affected by water temperature: at high temperature they develop faster, and at low temperatures they develop more slowly. In this regard, there is a direct relationship between the weather, the progress of spawning and the rate of development of fish embryos.
Task 5. Cod sweeps great amount eggs - up to 9.3 million pieces. The fertility of three-spined stickleback compared to other fish is very low - from 65 to 550 eggs. Make a guess as to what explains this difference in fish fertility.
The fact is that male sticklebacks selflessly protect their offspring. Predators avoid the male standing guard near the eggs. The survival rate of stickleback offspring is many times greater than that of cod. Cod does not protect its offspring in any way. After external fertilization, she abandons her eggs to the mercy of fate. Of course, there are always plenty of animals who are not averse to profiting from such food. Therefore, cod needs to spawn millions of eggs in order for some small part Of these, she survived through numerous feasts given by a variety of predators and other animals.
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