Possible effects of Rainbow trout escape from cages on Salmo caspius in the Caspian Sea

Abstract This study was conducted to investigate the possibility of population forming of Oncorhynchus mykiss in competition with Salmo caspius on the southern shores of the Caspian Sea in Mazandaran province, Iran.  One hundred fish specimens were collected seasonally from commercial purse seine from October 2017 to April 2018. Brown trout diet was analyzed and a total of 90 individuals belonging to 12 taxa (groups) were identified. Except for algae which were general prey, the other food items in the diet were rare for rainbow trout. Atherina caspia and Clupeonella cultriventris were the other prey items, accordingly.  Analysis of the brown trout diet, a total number of 2 taxa were identified summing a total of 10 individuals. The Atherina caspia for brown trout was a specialty food and the most important A% and FO% found in the diet of its had. Although the feeding habits were both carnivorous and used more than fish as food prey, but rainbow trout fed a wide range of food items. Therefore, in the competition between two species, Oncorhynchus mykiss is more successful, and under adverse habitat conditions it is possible to remove Salmo caspius. In some rainbow trout caught, all stages of sexually transmitted infections (II to V) were observed, of which stages 4 and 5 had the lowest number. Escaped farmed salmon are able to spawn successfully in rivers.