Time of year will definitely affect it. I would wager that when the rivers flood and the fish disperse more, that insects would become more prominent. And it's not like there were no fish found in their bellies during the study. I did find it interesting though that they are not quite the strict piscivores that most hobbyists believe them to be though.
I can't recall how long the study was conducted over, if it was even mentioned. Am curious now though, next time I'm in the storage unit I'll see if I can find it.