From my observations, particularly with reptiles,I agree with cricketman, I think its barometric influence.
Stella, how long and how old is your giant? I spent many hours saving whitebait this season from people who were just going to eat them! so I could get some giants, have 300 or so ex-whitebait living in a soup of Daphnia in hopes I have some kokopu but all look pretty much the same to me, there are a few that are smaller and darker in colour than rest of shoal but of course those are the ones I released to main pond so retrieval is a big ask.