Let's look at the original sentence as a whole. The original sentence is not ungrammatical, but it is unidiomatic except in unusual circumstances.
Fish are jumping in summertime.
The complement in summertime (or in the summertime) refers to the recurrent season, and thus it is used to expresses a universal or general fact, not a particular fact, analogous to:
Trees lose their leaves in (the) autumn.
Notice the simple present, lose. The simple tense is used when we want to express a universal or general fact.
So, if we want to say that the season of the year when fish jump is the summer, we would say:
Fish jump in the summer.
If we use the particular-fact form to express a general fact, to say what always happens, the speaker is making a sort of hybrid statement like "What is happening now is an instance of what always happens":
Trees are losing their leaves in the autumn.
Fish are jumping in the summer.
An example of a situation that might call for such a statement: let's say that you are a climate-change skeptic. You could say that everything is still honky-dory on planet Earth: Trees are losing their leaves in the autumn and fish are jumping in the summer.
The meaning would be: trees continue to lose their leaves in the autumn as always and fish continue to jump in the summer as always.