Reproduction and development of fishes
The carp has separate sexes: the female has a large ovary in the body cavity, the male a pair of testes; during spawning the testes fill with thick milt containing millions of sperm. When sex cells ripen, the breeding instinct appears and the fish seeks a suitable place to spawn. The carp goes to calm shallow places: the female lays roe on plants and the male sprays milt over it to fertilise it (external fertilisation). Fry hatch in 4–6 days; they feed first on microorganisms, then on small invertebrates, and reach 5–10 cm by year’s end. Some migratory fishes travel far to breed: chum and pink salmon from sea to river, the eel to the Sargasso Sea. Fishes that lay many eggs (perch, cod) do not care for offspring and most die. The male stickleback builds a nest and guards the eggs, the male seahorse carries eggs in a belly pouch, and the tilapia (often the female) carries them in its mouth – these lay few eggs.
Table: perch – carp – stickleback – seahorse: egg number (many/few), care (none/yes), place. Safety: fish only with adults, mind hooks and lines; wet shores are slippery; do not take more fish than allowed and follow the rules.