Thanks everyone for the responses. Let me respond to a few of the points individually.
I’ll try to make my question more clear, since there are three interpretations I’ve seen here:
Legal: Is catch & release fishing legal where I am?
Ecological: Am I damaging fish populations too much?
Ethical: Is it OK for the fish to suffer for the sake of us observing it?
I was mostly interested in the 2. point there.
(1) is pretty straightforward. I’ve been living in Switzerland for the past four years, and never did any fishing there (I have no use for the fish, beyond observing them.) As @jonathan142 points out, the rules are pretty strict (and even more complicated) there. On the flip side, in Thailand (where we are currently), I’ve seen, on several occasions, guides going into national parks and feeding the wild animals… People are quite relaxed when it comes to regulation.
(3) is hard to answer objectively. I think there will always be those who think it is natural for fish to get eaten therefore catching and releasing them is fine. And there will always be folks who think fishing is animal cruelty. I’m more interested in what happens to the population as a whole as a result of fishing.
Re @diegoalmendras, thanks for the link to the study! I’m planning on catching rays and sharks, but still interesting.
Re @clay_s, I actually did not know barbless hooks are a thing. I certainly don’t mind using those so I’ll make sure to acquire some for next time.
Re @blue_celery, thanks for the tip. In fact, most of my fish observations are from snorkelling: it’s a lot easier to take a photo when swimming around a school of fish, plus photos of captured fish are never going to be that great. However, where we are now, visibility is 50cm at most, which means ~20cm for my hobby camera, even with the flash. Snorkelling is not an option here.
What I did try is taking some photos of fish caught by other folks. I got permission to post some of them on iNat so I did post those.
And since @tiwane mentioned microfishing: I did not know about that, so thanks for the link! That said, when I say “fishing”, what I mean is using the smallest hook you can find, with some basic equipment and catching small fish (5-15cm) with the kids, maybe 5 of them per day or so. Certainly not the kind when you pull up 5kg yellowfin tunas one after another from a boat :) (I’ve seen people do that as a tourist attraction in some places.)
Re @reuvenm, thanks for the many tips, esp. for the nets. One note on lead sinkers: I do realise the harm there, and I can see the reason against lead sinkers. That said, I’m yet to meet a fisherman who uses anything other than lead sinkers. I also often seem to find abandoned lead sinkers with leftover line at fishing places, both seaside and lakeside. In Switzerland, they are even abandoning the use of lead bullets in outdoor shooting ranges as the impact on the environment was apparently measurable. I’ll replace my sinkers, but this is the first time I heard someone voice any concerns about this. Maybe I need to read up on the impact of lead sinkers on the environment.
Thanks again everyone!