International Research Journal of Applied and Basic Sciences. Vol., 3 (5), 930-935, 2012 Available online at http:// www. irjabs.com ISSN 2251-838X ©2012 Comparative study of male brood stock populations of rainbow trout (Oncorhynchus mykiss) in Iran M.A. Nematollahi 1* , N. Mazloumi 1 1 Dept. of Fisheries, Faculty of Natural Resources, University of Tehran, Chamran St., PO. Box: 31585-4314, Karaj, Iran. *Corresponding Author Email: malahi@ut.ac.ir Abstract Rainbow trout, Oncorhynchus mykiss, is one of the cultured fishes in the cold and fresh waters of the world. In this project, the main objective is the evaluation of samples of all male brood stock populations of rainbow trout in Iran. Phenotype of male brood stocks was identified in three categories, namely unclear, clear and clear with orange bands. The results indicated that there were significant differences amongst the three different phenotypes, in terms of gonad weight, GSI, sperm volume and concentration. Seemingly, phenotype 1 was inferior (P<0.05). Sperm motility of phenotypes 3 and 2 were placed in the same group, while phenotype 1 and 2 were positioned in another category. Further- more, there was a positive significant correlation between gonad weight and body weight, testis weight and GSI, total sperm volume and GSI, along with total sperm con- centration and volume. This proved that there was a significant relation between the bio- logical characteristics of rainbow trout sperms and its phenotype in Iran. It can be con- cluded that phenotypes 1, 3 and 2, respectively, were the most suitable phenotypes for mating and also promoting its percentage. Keywords: Oncorhynchus mykiss, sperm biological characteristics, spermatozoa, phenotype, mating percentage. Introduction One of the most famous trout species is rainbow trout (Oncorhynchus mykiss). The reproduction season for this species is dependent on temperature, light and dissolved oxygen. Based on climate situation in Iran, the reproduction cycle of this species starts in the middle of autumn and lasts to the end of winter or even sometimes early spring. However, its time also depends on age and genetic of the species. Rainbow trout has been living for years as a migrant species in Iran. It has been cultured and produced for nearly fifty years in this country (Nematollahi and Azari Takami, 2002). The males and females of trout’s brood stocks are different in the time of reproduction due to their phenotype. Thus, their sperms and oocyte are selected appropriately based on their phenotype. Trout’s spermatozoids are immotile in seminal fluid at the end of the spermiation period, due to the presence of po- tassium ions in seminal fluid. Under normal circumstances, seminal fluid of rainbow trout contains 5 to 10 billion spermatozoids per milliliter. These fishes can dispel up to 8 milliliters of seminal fluid in each excretion time (Billard, 1992). Number of spermatozoids, their percentage and also sperm motility rate, are factors which are considered in estimating sperm quality. Sperm concentration depends to some extent on its vo- lume. Sperm volume is different among fishes from 0.6 to 200 milliliters in each excretion time. Sperm vo- lume has direct relation with age, length and weight. Moreover it has indirect relationship with sperm concen- tration (Vladi et al., 2002). In one of the rainbow trout, sperm volume in each excretion varied from 1 to 117 milliliters, which is dependent on age, body weight and gonad weight, along with length and sperm concentration (Gage et al., 1995). Several studies have been done on the biological characteristics of rainbow trout sperm (Baynes et al.,1981). The spermatozoids are immotile in testis and seminal fluid, but there are some other elements that immobilise the sperms. In 2000, Casson stated potassium subdues the sperm motility in semen liquid (Cos- son et al., 2000).