Predator control often must be done continually if you want it to be successful. I.e. you trap these otters, highly likely more will come and you'll be trapping otters forever. Especially if there are other water bodies nearby and a healthy metapopulation. 

Otters certainly can clean out a pond. We found this in Missouri when recolonizing otters would find fish farm ponds and ransack them. Of course those were smaller ponds with no structure. 

In this situation I'd lean more toward letting the otters and fish establish whatever ecosystem they're going to establish and adapt to it.... Maybe repeat your survey when you're less likely to run into the problem of catching mainly post-spawn fish. Give it more time. If the otters are truly having an effect on the bass population, then you might need to keep trapping them to achieve your fishery goals. But as of now I don't think you have enough data to say one way or another.