What does this sign near a waterfall in Switzerland (Saut du Doubs) mean?

I don't have any definite answer or additional information regarding this one specific sign but I have the feeling some background knowledge can help resolve the question, at least partly.

1. This interpretation can be confidently ruled out. Any half-way competent sign/graphic designer tasked with representing “no fishing” would use a red circle over the pictogram representing the fisherman. This convention comes from road signs but is widely understood in Switzerland and Europe and is used in other contexts too (buildings, public transportation).

2. This interpretation is very unlikely too, there is a commonly recognised sign for “falling rocks”. If that's what you want to represent you would draw the mountain side next to the loose rocks. Again, this is a reglementary road sign in Switzerland but is also used occasionally on mountain paths there and in neighbouring countries. This also applies to interpretation one.

3. This interpretation makes the most sense to me but, not knowing the exact location, this is admittedly a judgment call. The “rocks” or “rubbish” also reminds me of a sign I have occasionally seen on motorway bridges elsewhere and which did share the same meaning (I couldn't find an example and it's not exactly the same, in France for example such a sign would also feature a hand IIRC).

4. I cannot definitely rule this one out but in the unlikely event that someone would put a sign like that, I would expect such a sign to come from some angler's group, with some additional text or logo rather than in the form of a pictorial “road” sign from the municipality, as this one seems to be.