... and Wikipedia states that the category of "fish" is paraphylatic, meaning that it is defined by convention rather than 'fact' and its boundaries can be argued, since it excludes some of the descendants of fish.
also, as pointed out by another commenter, we use the word fish to describe lots of things that are not included in this definition, like starfish and crayfish.
Having recently learned about trees, I actually don't think I know what a tree is. at least, not enough to create a reasonable, non-blurry definition of "tree."
You've defined fish as being vertebrates, and as such I cannot find an invertebrate that fits that definition. But what you can't do is just say the word "fish" and expect me to know what you mean - you have to provide a definition, and I could provide a different definition in a different context and neither of us would really be "wrong."
If you did just say "fish" without providing a definition, I would be tempted to either exclude sharks or include crabs, depending on context.