Physiological deceiving of Rutilus kutum fry with a saline diet in freshwater to stimulate the ionosmotic regulation system and direct transfer to the Caspian Sea

Abstract

Caspian Rutilus kutum with the scientific name of Rutilus Kutum is one of the most important economic fish in the southern region of the Caspian Sea. Millions of juvenile fish are released into the rivers each year to resource reconstruct sea fish. At this time the rivers are not suitable due to water pollution for the release of juveniles. One of the most common methods in the world is the readiness of juveniles to transfer directly from freshwater to seawater by stimulating its osmotic regulation system in freshwater. This study was carried out on the feasibility of increasing the physiological capacity of juvenile fish with a salt diet in freshwater. Fish fry were fed and reared for 35 days in two earthen ponds with commercial feed up to a weight of 1±0.1 grams. In an earthen pond and the last 15 days of the rearing period, the fry was fed with a 2% salt diet. Then the fish in 2 experimental groups and 3 repetitions were directly introduced to the Caspian Sea water. Fish gill and kidney tissues were sampled before and after the experiment under salt stress. The results of the biometric examination of the fry showed that there was no disturbance in their growth process by feeding them with a salty diet. The relationship between the length and weight of fry fed with a salty diet (F1) and without feeding with a salty diet (F2) had a high correlation coefficient (0.99 and 0.98, respectively). The condition factor was no significant difference between groups F1 and F2 (P>0.05). As well as the survival rate of juvenile fish was 100% during the assay period in brackish water (15 days) and there was no adverse change in the tissues in the two groups. Also, investigation of gill tissue sections showed that after introduction to brackish water, the chloride cells were observed in all two groups. However, fish fed with a salty diet in fresh water and after exposure to brackish water had a larger size and more chloride cells than the control group. Therefore, there is the possibility of directly releasing of juveniles about one gram into the Caspian Sea based on their physiological ability.