I love game jams, but it is really hard for me to come up with ideas, and to stick to them, for my games , so when you say FISH my mind goes blank and then at least 50 ideas start flowing in , so I need to have a filter. 

Is it possible to have an "optional" 2nd requirement that is announced at the start of the jam.   for  example  the last game jam I did there the main requirement was to make a game that fools the player , and the 2nd requirement was  "FALL"

I second this. Being challenged by the theme is a big part of jam experience. While it seems the theme is "fish", the fact that there is a requirement of prominent appearance of concrete fish makes it not really a theme, as proper jam themes are not such concrete requirements and are open to interpretations.