That's definitely not true that they are forced. Can't speak about any part of the world but it's easy to train a sea lion just as you can do with your dog, just with treats and some patience.Plus we have sea lions? Sea lions are forced to do shows/tricks 3 times a day all over the world.
I've seen quite a few sea lion (and also dolphin) trainings and also did an internship in a seal resue centre where they also have permanent residents which are trained just like sea lions in zoos, and they are free to join the training and don't get any disadvantage if they don't want to participate. They get their fish later if they don't want to participate. And this is the standard procedure in modern zoological institutions.
Keep in mind that seals and dolphins are very playful and curious animals also in the wild. There are several examples of dolphins and seals "surfing" on waves behind boats for example or doing jumps just for fun. You really don't have to force them to do such things, you just have to encourage them to do their natural behaviour and that's what zoos do.
Of course, there are some tricks that are not really natural and more like "circus", that's also something which I don't like to see but in most zoos I've been to the trainings and shows are really harmless. More like medical training and some jumps (which are not unnatural as stated above).
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