if you're the one creating the work, i fail to see how they could
"own" the copyright.
Unless theyre somehow claiming that the fish are a work of their own,
which you would be reproducing. Id like to see that one tested.
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I assume the aquarium is private property, so they can make the
rules. And it's not a matter of the fish being copyrighted. They
don't have to allow you inside. By signing the waiver, they are
making sure that if people want photos of their aquarium, people have
to come to them and buy their photos.
This type of agreement is very common at concerts and it's starting
to pop up more at sporting events. People are realizing that there is
money to be made from photos and they want to control who makes the
money.
I'm not saying I like the rules, I don't, I'm just trying to explain
how it is.