This is a good point. The only issue I see is that the Christmas trees form a brushpile where fish can live and breed. However, they will not ATTRACT fish. Therefore, the trees need to be deposited in an area where fish already populate. Otherwise the brushpile will do nothing but dam up the water source.
Fish do move around in the water. Cover, like that made by depositing old trees will in fact attract fish. Small fish hide and bigger fish look for a meal. Same principle as a man made reef without the salt.