Downriver migration of Songjiang perch

Songjiang perch is a fish that migrates short distances to the sea to spawn. Songjiang perch is distributed along the coasts of the Yellow Sea and the East China Sea. From late April to mid-June every year, their juveniles move upriver from the coast into rivers leading to the sea, where they grow and fatten in fresh water. From November to February of the following year, they gradually descend into the river. Enter the sea for reproductive activities. Songjiang perch grows and develops in the lower reaches of rivers and estuaries. The spawning grounds are generally not far from the coast, usually only a few miles from the mainland coast.

Fish that migrate down rivers for reproductive migration must have their gonads reach sexual maturity under certain salinity conditions. When Songjiang perch and other fish live in fresh water, the prolactin secretion area in the rostral part of the anterior lobe of the pituitary gland is relatively developed, while the main part of the anterior lobe The gonadotrophic zone is relatively small. Prolactin is related to osmoregulation, and increased prolactin secretion inhibits gonadotropin secretion, thus causing sterility in these fish in freshwater. When they entered seawater, the gonadotropin-secreting area of the pituitary gland expanded significantly, while the prolactin-secreting area shrank. Marine fish require a large amount of thyroid hormone when their gonads grow and reproduce. However, since fresh water does not contain as much iodine as seawater, it cannot meet the need for thyroid hormone synthesis when the thyroid function is strong. Therefore, when artificially breeding fishes that have fallen into the sea, they must gradually transition to live in seawater in order to achieve satisfactory results.

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