First of all, they are not by Venini. Venini did not do this type of elaborate glassware, not did they use these colors.
Secondly, they were most likely made in Murano (there was a suggestion on another board that they were made in Venice rather than on Murano). There was very little glass blown in Venice itself, as the dangers of fire was very real and the factories were religated to Murano until very recently, when air conditioning and modern technology has made glass blowing safer.
Third, they are most likely early 20th Century. Although people like to think that anything old is 19th Century, the truth is that very little glass was made in Murano until the later 1800s, and very little of that has survived. There are too many examples of these particular glasses out there in the world for them all to be that old.
Forth, Salviati is the top candidate, although other companies did this kind of work as well.
Fifth, some of this "traditional" style work was made throught the 20th century, and is still being made today. Certainly the familiar stems decorated with golden fish can still be purchased in Murano, although every seller on ebay likes to claim that theirs is antique. However, if I had to pin a date on these I'd say somewhere around the 1920s.
Sixth, the stems are most likely vaseline glass, which not only makes them twice as collectible, but also makes it an almost certain guarantee that they were produced prior to the 1960s.