I don't think fishing violation is a misdemeanor in Iowa, it's likely an infraction punishable by fine. During the interview you will be asked to confirm yes/no on each of the questions in part 3.C of the I-485, you can choose to tell the officer that you just remember about the fishing citation, and that you wish to change one of the answers.
While the fishing citation itself is not a problem at all, failure to disclose the citation can be treated as lying under oath, which is a big deal.
The charge code is 481A.67 (SEASONS AND LIMITS):It is unlawful for a person, except as otherwise expressly provided, to take, capture, or kill fish, frogs, or turtles except during the open season established by the commission. It is unlawful during open season to take in any one day an amount in excess of the daily catch limit designated for each variety or each locality, or have in possession any variety of fish, frog, or turtle in excess of the possession limit, or have in possession any frog, fish, or turtle at any time under the minimum length or weight.
There no "misdemeanor" mentioned.
I do see some code, such as 481A.9 is
misdemeanor: SPAWNING GROUNDS.
To effect sound wildlife management and maintain biological
balance as provided in section 481A.39, the commission may set aside
certain portions of any state waters for spawning grounds where the
same are suitable for this purpose for such length of time as it may
deem advisable by the posting of notices in conspicuous places around
such area, and it shall be unlawful for any person to fish or to in
any manner interfere with the spawning of fish in this area. Any
person violating any of the provisions of this section shall be
guilty of a simple
misdemeanor