It would depend on how you have your form set up, but my initial thought is using another field with a calculate to determine if the value is zero.
Is it set up in repeats? One station per form? One fish type per form? Is there one question to pick the type of fish, and another to say how many were seen?
Would you be looking for a specific type of fish at that time, or is this reviewing data after the fact, as that might not be a survey question but more of the analytics after.
Sorry for so many questions! Knowing the structure and goals helps to figure these things out a little clearer!