Anything that changes the way water circulates or its saltiness can kill the affected mangroves. Frequently a causeway for a road, or some other construction project, may keep the sea-water from coming into the mangroves. Mangroves are also sensitive to pollution, particularly oil pollution. If an oil spill goes into a mangrove area, the oil covers the aerial roots, and the tree roots can no longer get the air they need to live. The roots will die, and with them the whole forest.
Mangroves are also very sensitive to herbicides.
With all these different threats, and the fact that few people appreciate how important the mangroves really are, it is no wonder that the area of mangroves is getting smaller. The steady reduction in mangroves means the loss of an important resource. As each little bit is taken, the remaining natural area becomes that much more important for such things as fish breeding and nursery areas. On some islands, only tiny areas of mangroves remain. Their loss could be a tragedy for coastal fisheries.