well here is an easy solution for you
find who the property owners are after the border of the park where the river still runs and ask them can I take some stuff pretty sure they be casual about it.
I would have to side with those who say do not disturb imagine if every tourist there took just a bit of this a bit of that, in a year that is a lot of people, in ten years that is insane, in 25 years etc....
In reality though I highly doubt a lot of fish keepers are going to do that.
Just make sure your conscience is clear to follow your actions.