Collecting macroalgae in Hawaii and the Caribbean

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Next month I will be on Oahu and then in the Caribbean.

I have started a macroalgae tank in preparation for my seahorse tank.

I am thinking of collecting some macroalgae.

Does anyone know the legalities of importing macroalgae.

I am assuming since these are tropical macroalgae they pose no threat to our temperate ocean environment so there shouldn't be an issue that way.

I am also wondering where to look for macroalgae.

I thought maybe near areas where a river enters the sea for more nutrients.

If anyone knows any of the answers to the above please let me know.