[sznio]

You seem to be conflating evolution with learning. Seagulls didn't evolve to steal from trashcans, they learned to.

As for the main point of your question: the clams are replaced every so often, so they won't end up getting used to the city water and stop serving as a marker.

[n0rdy]

Got it, learning is something that can be acquired relatively quickly, while evolving needs generations. Thanks for the explanation.

[grvbck]

Valid question though, and the answer to what I think you wanted to know is >3 months:

> the clams are “paid back after three months of work by releasing them to a place from which they will never be caught again”. [---] this is done because they eventually become resistant to contamination in the water.

https://www.zmescience.com/ecology/poznan-mussel-water-plant...