Spaw, fucking fish is purely a cultural practice of opportunity. In the Amazon there are fresh water dolphin who are orten raped by the non tribal population. While a dolphin is not a fish, you indeed could have a good ol fashioned fish fucking party in Brazil.
If there were fresh water dolphin in Texas there would be fewer sheep jokes.

I have seen the participants speak about the practice on PBS.
It seems that lots of alcohol is part of the ceremony.