I don't think many, if any, cross it as it is. Not as a necessity in their life cycle anyway.

The strait is really narrow, with really strong currents and a huge change in salinity.

Not fun.

I didn't think about this. A quick search and I found one population of tuna that live in the Atlantic and breeds in the Mediterranean http://www.treehugger.com/clean-technology/unraveling-the-se...

Probably they're not the only sea creatures that cross Gibraltar so this is a concern. Non a minor objection, but one among the many that would be raised. Think also about the political and military implications of a bridge from Africa to Europe.

Fish catapults?

That seems like a minor objection.