Fish reproduce sexually. In the process of spawning, the female gametes, which develop in the ovaries, merge with the male gametes, which are secreted in large numbers by the male sexual organs in the milk. As the fusion of sexual products takes place outside the body, this process is called external fertilization.
The maturation of sexual products in fish is decisively influenced by water temperature, then the type and amount of food, day length, light, barometric pressure and density of the parent fish per unit of water surface. This last factor must be given special attention by fish farmers in ponds. The ripening time of eggs and jelly - the first spawning - is different for different species of fish. Most of our fish species have their first spawn in the third, fourth or fifth year of life. Males usually mature one year before the females. The size and abundance of sexual products is also different in different species of fish and depends on the age of the female.
The properties of the area that attract certain species of fish for spawning are: richness of aquatic flora of higher plants, salinity, water flow rate, temperature and more. These properties affect the rapid maturation of sexual products in parent fish, as well as the subsequent development of eggs and young.
When laying eggs, the fish choose a certain substrate. Salmonids lay eggs at the bottom of the riverbed by digging a previously spawning pit in the gravel. An uncovered caviar that is exposed to light dies in a short time. Carp and pike spawn on the vegetation base of aquatic and flooded plants. Catfish and perch prepare nests sunk into the bottom and covered with parts of the plants in which they lay their eggs, and the males keep them until they hatch. The spawning itself is done in such a way that the male and the female rub against each other or rub their side parts of the body against the surface on which they want to deposit sexual products, with the females doing so first, and then the males in the same way.
Fish eggs are usually sticky, and stickiness has the task of attaching the eggs to the substrate where they are deposited. The stickiness can also be between the eggs themselves, so that the eggs are laid on the substrate in continuous rows (with perch) - this is an example of the double stickiness of the eggs, to the substrate and to each other.
The vast majority of our fish have eggs with a single adhesive property and only on the substrate, so that each caviar is independently glued to a specific substrate. This property of the stickiness of the eggs protects the developing eggs from falling to the muddy bottom, where they would perish.
The time that the eggs spend from laying and fertilization to the formation of fish, ie. larvae, we call egg incubation. This is the most important phase on which the creation of the young fish of the new generation of fish largely depends. At this stage, in our natural open water conditions, most already hatched and fertilized eggs perish. The most common cause of failure is in water level oscillations to which open waters are constantly subjected. Since the incubation of eggs in fish species lasts differently, from at least a few days to several months, and water levels change, these eggs are most often exposed to various harmful effects, even remaining dry and then immediately decaying. Therefore, in our open waters, the successful development of eggs through all its phases is very rare, up to the young capable of independent life. From experience, it can be expected that on average we have one successful spawn every five years. As most of our lowland fish spawn on vegetation in floodplains outside riverbeds, the success of hatching fish fry depends on the size of the floodplain and the duration of the flood, so if the floodplain is larger and water retention is longer, all stages of egg development the younger ones are safer.
In Croatia, there are only a few farms and hatcheries for restocking our lowland waters, so restocking our rivers depends on the success of spawning in natural hatcheries located in large floodplains. Lonjsko polje is the largest and most important in our country, and we can claim that today most of the waters of the Sava basin are stocked with mostly younger fish hatched and fed in this field. However, due to the impact of other negative factors, which reduce the fish stock in our rivers faster than it is naturally restored, it will be necessary to take extensive measures to restock our rivers with juvenile fish. Speaking of spawning and incubation of eggs, we must emphasize the third important developmental phase in the development of the new generation of fish fry, and that is the development from the moment of hatching of larvae to young capable of independent life.The fourth phase is also important, rapid development, which requires the young to stay in the floodplain for as long as possible, where there is an abundance of natural food that the young find all around, without having to make efforts and expose themselves to various enemies in search of food. In floodplains, natural food is produced in much larger quantities (twenty or more times) than in rivers. Therefore, since the young feed in the first year of their life very intensively, it is desirable to stay as long as possible in flood zones. In all fish, the eggs do not mature at once and at the same time, but gradually on several occasions, which can last up to 2-3 months. If favorable conditions prevail for each spawning, incubation and development of young larvae, we will have more age classes of young in one year. But this rarely happens in the natural conditions that prevail in nature on open waters. As an example of this natural spawning phenomenon, we will cite the behavior of the parent fish, our most important species and its main representative from the carp family: the river carp, the inhabitants of most of our lowland-type waters. River carp caviar matures gradually, in our latitudes and climatic conditions, usually at intervals of 3 to 9 days, if during this time none of the basic factors that cause spawning are absent. When this happens, the maturation of the eggs is interrupted, and therefore further spawning. Then the queen waits for a possible re-creation of conditions that affect the maturation of the eggs in her body. In the event that the mother fish with mature eggs for any reason prevented the act of spawning, mature eggs for 5 hours despise and become incapable of fertilization, and after 8 hours begin to degenerate. To maintain the normal ability of carp to spawn, they need to be completely spawned each year. The absence of complete spawning may result in poorer spawning in subsequent years. Such cases of uncluttered carp are very common and affect the overall reproduction (creation of new generations) of carp in our waters. This is evidenced by the application of modern technology in fish farming, which has eliminated many natural influences that negatively affect the reproduction of fish in open waters, and where a small number of parent fish get a large number of fish fry. Of course, in this process of reproduction of our fish there are many other, even insufficiently known factors that also affect reproduction, that they are different in different species of fish, and that we do not meet them every year in the same relationships in natural conditions. on open waters. That is why we have very different spawning results from year to year.
For the sake of understanding and practical guidance, we will list the most favorable temperatures within which only the spawning of some of our more important fish species takes place. The most favorable temperature for spawning of various species of trout is from 6 to 9 ° C, for young and grayling from 9 to 12 ° C, for pike from 10 to 13 ° C, for perch from 11 to 15 ° C, and for carp from 18 to 20 ° C. Embryonic development of eggs (hatching) lasts a shorter or longer time, depending on the average daily temperature and is measured by the so-called. degrees-days (D ° C). Thus, trout need about 390-420 D ° C, juveniles 280-320 D ° C, grayling 160-200 D ° C, pike 130-140 D ° C, and carp only about 100 D ° C. This means that the larva will hatch from the carp eggs in 5 days if the average daily water temperature was 20 ° C.
After incubation from the eggs, the fish larva first feeds from the yolk sac and this stage of development also lasts differently in different species of fish.
Icant