I'd suggest it would be relatively easy.
We have deer stations when people go out on the much larger landscape and have to have their animals weighed or measured and aged. Here you're talking about, within an estuary in a lower portion of a river, having a couple of census stations where, if you're going to be bringing.... Or someone could go along the bank and collect stomachs when the fish are opened up, and look inside them. There are relatively easy ways to do this.