445 Czech J. Anim. Sci., 55, 2010 (10): 445–455 Original Paper The Caspian brown trout, Salmo trutta caspius, is a critically endangered anadromous species that has been considered for a biological conservation pro- gram in the southern part of the Caspian Sea (Kiabi et al., 1999). In recent years, because of a dramatic decline in broodstock capture and subsequently of insufficient availability of sexual gametes (sperm and ova), severe restrictions were imposed on the propagation of this species by means of artificial fertilization. Thus, many studies have been carried out to eliminate this problem by using technologies such as cryopreservation of Caspian brown trout semen in order to solve the problem of permanent access to spermatozoa (Sarvi et al., 2006). For such purpose, the selection of the best quality semen is the most critical point to increase the efficiency of artificial fertilization after sperm thawing. A survey of literature shows that the main parameters which determine the fish semen quality are as follows: (1) Composition of seminal plasma: seminal plasma has a unique composition regarding the pres- ence of organic and inorganic components which support the viability of spermatozoa as well as substances reflecting the function of the reproductive system like spermatozoa (Piironen and Hyvarinen, 1983; Lahnsteiner et al., 1993 Evaluation of semen quality of endangered Caspian brown trout ( Salmo trutta caspius ) in different times of spermiation during the spawning season S. Hajirezaee 1 , B. Mojazi Amiri 1 , A. Mirvaghefi 1 , A. Sheikh Ahmadi 2 1 Department of Fisheries and Environmental Sciences, Faculty of Natural Resources, University of Tehran, Karaj, Iran 2 Department of Animal Sciences, Faculty of Agriculture and Natural Resources, University of Kurdistan, Kurdistan, Sanandaj, Iran ABSTRACT: Sperm motility and sperm production as well as organic and inorganic components present in the seminal plasma were measured at different time periods of spermiation during the spawning season in order to evaluate the semen quality in Caspian brown trout, Salmo trutta caspius. For such evaluation, males were divided into four groups based on the date of spermiation: group A (pre-mature broodstocks), group B and C (mid-mature broodstocks), group D (late-mature broodstocks). Our results showed that the Caspian brown trout produces semen of variable quality depending on the date in the period of spermiation. In this regard, the percentage of motile spermatozoa, duration of motility, sperm density, osmolality and also the concentrations of Na + , Cl – , K + , Ca 2+ , Mg 2+ and total protein content were significantly higher in mid-mature broodstocks (which matured in the middle of spawning season, i.e. groups B and C) compared to pre-mature (group A) and late-mature (group D) samples. Statistically significant positive correlations were also found between these semen parameters: sperm production vs Na + , Cl – , K + , Ca 2+ , Mg 2+ , total protein, osmolality, pH; percentage of motile spermatozoa vs Na + , Cl – , K + , Ca 2+ , Mg 2+ , total protein, osmolality; duration of motility vs Na + , Cl – , K + , Ca 2+ , Mg 2+ , total protein, pH. The significance of such correlations is interpreted in terms of physiological implications and is discussed in relation to semen quality. Keywords: sperm motility; spermiation; spawning season; Caspian brown trout