I want to build circuit which avoid birds to eat fish in dam, i need help to select a beam or electromagnetic wave which have a frequency or wavelength that interfere the view of bird so that a bird can`t see fish clearly around that zone but the beams to have no efffect on fish

 

I'm not sure how to put this nicely, [Moderator's note: If you cannot say it nicely, please do not say it]. Nothing like that exists. Sorry.

 

How big is the dam?

I'd go for a Bird Net.

 

My local carp fishing lake had trouble with birds taking the very young fish so they play a loud recording of a bird of prey and also the distress call of the birds. Seems to work ok. Otherwise get yourself a Falcon!

 

A Falcon kite should scare them away.

https://www.falconcropprotection.com/