I would throw some Cheerios in there now.
The will float high and resist water logging for a fair bit of time.
Tiny BG should be seen flashing at them trying to eat the food. GSF or larger BG will be able to eat a whole Cheerio. If something is eating them whole, put a Cheerio on a small hook and see if you can catch one for examination. I think a cane pole (or even a stick) is easier than casting to catch fish 6' away.
