Dynamics of biannual spawning behavior in rainbow trout Oncorhynchus mykiss (Walbaum, 1792) from southern Chile

Although the rainbow trout Oncorhynchus mykiss (Walbaum, 1792) is considered a single-time spawner with a restricted annual reproduction cycle (Mylonas and Zohar, 2007), some female broodstocks of this species may display an unusual reproductive periodicity, resulting in a phenomenon characterized by two consecutive annual spawnings as a consequence of biannual spawning behavior (Hume, 1955; Aida et al., 1984; Gall and Crandell, 1992). In these broodstocks (called ‘twice-spawners’), the first and second spawning occurs during a normal reproductive cycle (NRC) and an additional reproductive cycle (ARC, respectively). Moreover, the spawning rate in the ARC is usually lower than in the NRC and is highly variable across strains and spawning seasons (Tazaki et al., 1993; Takano et al., 1995; Shrable and Orr, 1998). In addition, the gonadal maturation period previous to the second spawning event, i.e. in the ARC, is a short-term process in these broodstocks, in comparison with the single-time spawner in which gonadal maturation begins a year before ovulation (Elliott et al., 1984; Sumpter et al., 1984). Reproductive performance (Aida et al., 1984; Kincaid, 1985; Tazaki et al., 1993; Takano et al., 1995; Shrable and Orr, 1998), sex hormone profiles (Lou et al., 1984), and gonadosomatic index and gonadal histology (Estay et al., 2012) studies have found that no major reproductive disruptions occur in female twice-spawner rainbow trout. In addition, the examination of the spawning period dynamics indicates that this is longer in the ARC than in the NRC (Tazaki et al., 1993; Takano et al., 1995); furthermore, the inter cycle length (ICL), a parameter that expresses the time required until the occurrence of the additional spawning event for the same female, generally covers a period either from 126 to 180 days (Takano et al., 1995) or from 112 to 217 days (Tazaki et al., 1993). In spite of this progress, additional studies are needed to compare twice-spawner strains of different origin, to assess whether their particular spawning dynamics follow a general pattern. Therefore, in order to gain an insight into this issue, we examined twice-spawner female broodstocks from a fish hatchery in southern Chile to ascertain the pattern of the NRC, ARC and the ICL following the spawning behavior of individually-tagged breeders. Furthermore, to form a better view of the dynamics of this phenomenon, the relationships between NRC and ARC and between ICL and ARC were also assessed. This study presents the results of the spawning dynamics of biannual spawning behavior in rainbow trout, obtained by studying some characteristic reproductive parameters such as the NRC, ARC and the ICL, derived from the analysis of twice-spawner female broodstock from southern Chile.