Effects of fertilizing eggs from a summer-spawning line with cryopreserved milt from a winter-spawning line on spawning date and egg production traits in rainbow trout

Rainbow trout lines have been selected to produce gametes at different times of the year to insure a year-round supply of seedstock. Protocols for cryopreservation of milt have also been established and cryopreservation services are currently commercially available, further extending the availability of gametes. The present study was conducted to determine the impact of crossing rainbow trout with different spawning seasons on the spawning date and gamete characteristics of the offspring. Egg lots from females from an August-spawning population were divided in half and fertilized with milt from males from the same population ('purebred') or cryopreserved milt from two males from a February-spawning population ('hybrid'). The purebred and hybrid groups both exhibited spawning windows that lasted around three months with each family taking about nine weeks for the middle 10-90% of the individual females to ovulate. However, the hybrid progeny spawned on average about 10 weeks after the purebred families, peaking in November and thus exhibiting a classic phenotypic distribution pattern of heterozygous F1 hybrids. The hybrid progeny did not spawn the next spring which was the alternative intermediate to February and August. The F1 hybrids were crossed to produce an F2 generation for future genetic analysis to map quantitative trait loci (QTL) for spawning date. Consistent with the February line historically producing larger eggs than the August line, the hybrid offspring produced slightly larger eggs than the purebred August line families, as well as yielding a greater volume of eggs per unit body weight. These results are valuable for strategies to maximize hatchery production of year-round seedstock and for QTL mapping of spawning date in rainbow trout.