It’s likely less than the stress of being dumped on from high.
Adroitly put. 
I’m not sure I want to presume to confer comparative values on physical vs emotional stress, but thank you for reminding me of it. Do fish feel fear? I’d stake my life on it. Of course they do. What else drives self-preservation? And will they suffer internal damage from being dropped from a plane, and feel pain? I think I’d stake my life on that, too. Trout aren’t as physically hardy as humans, and even with our added advantage of possibly being able to right ourselves for minimum injury, we’d still come out of it pretty sore ourselves, if not black and blue, or worse. Human injury upon hitting the water from very high places is well-documented; my money says a trout stands even less chance. Do fish feel fear and pain the same way we do? I have no way of knowing. But I don’t see differences, if they exist, as an important qualifier. Striking fear in beings and causing them pain is unfortunate enough when we have to eat, and sometimes we have no choice in that. But dumping fish from high up in a plane to hit a lake below - simply because it’s convenient - strikes me as beastly when more humane ways are to hand.
I’ll get off my high horse now.
I’d want to know much more about it and what its effects might be before I signed up to that. The images show the fish being dumped along with a load of water. What ameliorating effect might that have? Would the fish being contained within water, which then hits water, sufficiently cushion the impact? Do fish end up diving, kind of head first, into the water before impact? They might. Fish - especially fish like trout and other of the salmon family - are used to jumping. they must have a method of re-entry, as it were.
I dunno. It says thay are small fish. Small animals can survive long falls. Their terminal velocity is a lot less than a human. And fish are a better shape for entering water than humans.
And I’ll admit these things occurred to me later. I also realize that you can’t expect a curiosity-news blurb to offer real stats, but one would think anything calling itself “BBC” might go a step further than “they say”. But maybe I’m being naive there.
I do hope your points make all the difference. But we’re guessing, and that’s why I wanted better stats. 95%? What was the attrition rate before, and if it was worse, then why? Trout may be small, but in a fall like that, they can’t control how they land. This isn’t a natural leap, and landing sidewise from such a height isn’t the same as headfirst. I would still expect them to be stunned, and that can’t be good. The effects are worth considering.
All I know is that responsible sport fisherfolk make a point of slipping their returned catches into the water rather than tossing them, to land however they may. One would think that the fish’s welfare has maybe just a little something to do with it…
And no, I’m not with PETA. Those folks go way too hysterically far for me, calling milk “white blood” and fish “sea kittens”. The very idea… I have better ways to be crazy, thank you.
Here is an article about stocking fish from airplanes. Some are killed but the survival rate is higher than backpacking the fish all day into the remote mountain lakes were aerial stocking is usually conducted.
Ah! Well, thank you, then. Good article. I hadn’t even considered remote mountain lakes, nor that backpacking might be the only way to get there by land. I also wasn’t considering fingerlings, which makes more sense and shows you what a rugged outdoorsman I’m not. I do think, however, that the added reference to the book An Entirely Synthetic Fish: How Rainbow Trout Beguiled America and Overran the World suggests a hint of rationalization for the rough treatment. Just a thought. I was also surprised to read elsewhere that the Rainbow Trout is one of the top 100 invasive species in the world. Who knew? Mighty tasty, though.
Still, maybe they could use a sea plane?
Why not a helicopter - makes more sense - you could almost ‘land’ on the water. 
A helicopter close to the water would be like a huge fan over the water.. imagine what would happen to the small fish..
A helicopter could work in some areas, but range and payload are restricted when compared to fixed-wing aircraft.
I sometimes use helicopters for seed dispersal in areas that are otherwise difficult to access. We suspend a hopper below the helicopter and the pilot can open and close the orifice to control the release of the contents.
Wow! I never use a helicopter for seed dispersal! 
Oh wait …
But that’s what a sea plane does, you see. The only practical restriction I can think of is that the lake would have to be big enough for landing and takeoff.
Since the spatial requirements for safe sea plane operation make it impossible to serve all, I went on to search for sea 'copters, but found nothing beyond a YouTube vid of a small recreational-looking one with an open-air cockpit and only room for two passengers. I wonder why the general concept hasn’t found greater application, unless, as Crawforde indicated, it’s still a matter of not enough range and payload.
Always wear protection.
Protection is almost always a good idea when attempting to sow oats while airborne.
Sometimes what I think is normal, isn’t.
There are helicopters with pontoons that can land on the water, but the pontoons are heavy enough that they take up a lot of the payload.
PS. I’ve never sown oats from a helicopter
Couls that be a tune title?
An just so you don’t think I am completely off, I do wetland restoration and biological storm water treatment for a living.
Tell me about it…
How about carbon fiber?
Good for you. And I expect we have much need of your services too, of late.
This next one is quite possibly not wrong … but I have no idea what it could mean:
I’ll have a look at the article …
OK. Kind of makes sense now I’ve read what it’s about. But it’s about as clumsy a way of writing a headline as I have seen. I think the substitution of one word, “actress” (or “actor” if we’re being PC) for “lead” would have made things a lot clearer.
Abnormal is the new normal.
Please, Ben. Just have the good grace to accept that you’re no longer au courant. It’ll be less embarrassing all around. 
But seriously, I agree with you; as it stands, the headline is horrendous writing. Even a simple “The” at the beginning (which is part of the movie’s proper title, after all) would at least have gone a long way toward easier reading, even if it still would have left the reader none the wiser.
Sometimes I think these writers make headlines abstruse on purpose, as a lowly tactic to get people to read the article. But it’s really hard for me to give them as much credit as that; in the end, I think it’s just a case of chops is as chops does.
You know, it just occurred to me that people haven’t told me I’m weird in quite some time, now. Maybe it has nothing particularly to do with me.
BBC Again
“Hundreds of buffaloes drown ‘fleeing lions’ in Botswana”
https://www.bbc.com/news/world-africa-46136338
Now, I expected to read a story about a bunch of buffaloes ganging up on some scaredy-cat lions and pushing/herding them into the water, where the lions drowned.
Nope. Apparently, instead, hundreds of buffaloes were drowned. They were fleeing from lions. I don’t understand why ‘fleeing lions’ is in single (or air) quotes.
Just to point out the obvious for those of you who want to improve your English writing:
“Hundreds of buffaloes drown while fleeing lions in Botswana”. No superfluous air quotes, no confusion. Simple and easy, the way language should be.
It seems the single quotes were used to make up for having forgotten that the word “while” exists, and has a purpose.
On a personal note: I would have preferred “buffalo” as the plural, but then I’m old school.