Near the end of #4 The Message, when Visser is in his Mardrut morph, racing after the Animorphs, he is attacked by whales (real ones) just as he was about to catch up. They attack him in a highly coordinated and aggressive way, forcing him to retreat. It must have been embarrassing for him.
Now, If I was the Visser, I would have immediately assumed that the whales were actually "Andalite bandits", and that the Andalites' numbers were larger than the Yeerks had earlier estimated.
Or maybe the Visser didn't know anything about whales, and thought they were simply defending their territory? He might not have wanted to ask his human controllers, as he probably didn't want to mention the affair to anyone, given the embarrassment of being defeated by whales.
Did the Visser increase his estimate of the number of "Andalite bandits" after being attacked by the whales?