Apparently back in 1998 a lethal red algae knows as Karenia digitata wiped out 80% of the fish in local fish farms. This is the time of year (March-June) when "red tides" are most prevalent in Hong Kong, fueled by a combination of nutrients in the water, warming seas, and pollution from the Pearl River. Most species of red algae are harmless and pose no threat to the environment, however every once in a while, a "deadly" strain of algae comes along and kills off a wide array of marine life.
On the same day I paddled through this red tide, I also noticed plenty of dead fish - both in the water and on the beach. In the photo to the right, you can count 9 dead fish within one square meter. Is there a connection?