Premature sperm activation can reduce fertilization. Sperm extenders are a potential remedy. In Test 1, rainbow trout (Oncorhynchus mykiss) sperm motility and motility duration were compared among three diluent types, three milt:diluent dilutions and between extended and un-extended milt. Dilutions ? 1:1 were sufficient for complete activation of un-extended rainbow trout sperm with all three diluents. For extended milt, complete activation was observed in 4 of 5 replicates at 1:2 and all replicates of 1:3, but not at 1:1. Sperm motility lasted from 21 to 52 s and was unaffected by extender, diluent type, or dilution. In another test using extended and un-extended sperm to fertilize eggs at high female to male ratios (4:1), no significant difference in percent fertilization was observed between 4:1 and 1:1 ratios or between extended and control sperm treatments. For cutthroat trout (O. clarkii pleuriticus) eggs fertilized with extended sperm, there was no significant difference in survival to eye-up. The data indicated extender requires three-fold dilution, but did not negatively affect fertilization or duration of motility when common activating solutions were used
