Maturation And Spawning In Fish | The Fish Site

Ovulation and fertilisation

The egg gradually enlarges and projects into the lumen of the ovary. At ovulation the follicle ruptures and the egg escapes from the aperture through amoeboid movement. The forces causing the rupture are not known and the muscle fibres present in the ovarian wall may be responsible for it. In teleosts having cystoarian ovary, the ova are discharged into the oviduct, while in those fishes which have a gymnoarian ovary, the eggs are discharged into the body cavity. The muscles of the oviduct and those of the body wall contract causing movement of the ova through the oviduct.

Fertilisation is external in water in majority of the teleosts and internal fertilisation also takes place in some species. Spermatozoa and eggs are discharged into water in close proximity. The sperms become very active in water, and survive only a few minutes, during which the fertilisation takes place which may be controlled by several chemicals released by both sperms and eggs.

In some species of teleosts, fertilisation is internal and the urogenital papilla or anal fin is enlarged or modified for the transfer of sperms. In cyprinodont fishes like Gambusia, the anal fin rays of male are modified to form an elongated copulatory organ which is tubular or grooved and is called the gonopodium. In Scorpaenidae and Embiotocidae, which include viviparous teleosts, there is no gonopodium and the sperms are transferred by genital papillae of the male coming into contact with the cloaca of the female.

In Cottidae, the genital papillae are enlarged to from the penis in the male. Wiesel (1949) has described erection tissue in the genital papillae of both males and females. In Apogon imberbis, the female has elongated genital papillae, which is introduced in the male for receiving the sperms (Garnaud, 1950). Female Rhodeus has a much elongated genital papilla, to transfer the eggs into the shell of bivalve Anodonta where they are fertilised by the sperm shed in water and develop inside the shell.

In the viviparous teleosts, fertilisation generally takes place while egg is within the follicle (follicular gestation) or development takes place within the ovarian cavity (ovarian gestation).

Eggs

The number of eggs produced by a single female differs considerably and depends upon several factors like her age, size, condition and species. The egg is generally surrounded by a shell but when it leaves the ovary, it is enclosed in a vitelline membrane. Generally, the egg is spherical or oval in shape and has some amount of yolk in it.

Eggs of bony fishes are of two main types. Pelagic eggs are buoyant and provided with a thin, non-adhesive membrane, while demersal eggs are heavy and sink to the bottom, and are covered by a hard adhesive membrane. Sticky, demersal eggs become attached to the debris of the bottom and are prevented from being swept away along the current of water at the time of deposition.

Marine fishes produce either pelagic or demersal eggs but the eggs of fresh water fishes are generally demersal. Pelagic eggs are of small size and a single large oil globule may be present on the surface of its yolk.

The eggs of some species (Scomberisocidae, Belonidae and Exocoetidae) have sticky threads for attachment with some object or with each other.

Fecundity

Fecundity can be defined as "the number of ova that are likely to be laid by a fish during the spawning season". The number of eggs produced by a fish differs in different species, and depends on the size and age of the fish. It may also differ in different races of the same species. Thus, fecundity is a measure of the reproductive capacity of a female fish, and is an adaptation to various conditions of the environment. It can be estimated by any of the following methods:

1. In the volumetric method, the total volume of the ovary is measured. Small pieces of the ovary are taken in random samples from the anterior middle and posterior parts of the ovary, and the number of ova in each sample is counted along with the volume of the sample. The total number of ova in the total volume of the ovary is then calculated.
2. In the gravimetric method, the formalin preserved ovaries are used. After determining the weight of the ovary, three small samples of 100 mg each are taken at random from the anterior middle and posterior part. The number of ova in each sample is counted under a binocular microscope. Total number of ova are then calculated as: F = S x OW / 100.

Here, F = FecundityS = Average number of ova obtained from three samples of 100 mg eachOW = Total weight of the ovary

The fecundity of a number of species has been studied by several investigators. Fecundity of Cirrhina mrigala varies from 75,900 to 11, 23,200 when the length ranges from 349 – 810mm in length (Hanumantha Rao, 1974). In Labeo gonius ranging between 270 – 490mm, the fecundity varies from 47,168 to 3,80,714 (Joshi and Khanna, 1981), but according to Chondar (1970) the values are 2,73,955 to 5,39,168 in the fish ranging from 270 – 490mm in length. The differences in the values are possibly due to the collection from different habitats.

Fecundity of the fish is closely related to the fish length and weight. A straight line relationship has been observed between the fecundity and the fish length, and the fecundity and fish weight in a number of species.

Knowledge of the fecundity of fish is useful in fishery management and the information regarding the possible number of eggs and fry likely to be produced can be used for selecting the fish of high productivity for fish culture.