Hypoxia may be the culprit of sex transposition in fish

A recent study by local marine scientists reveals that hypoxia may be the cause of sex transposition in fish. In this research, carps growing in the hypoxic water have smaller sex organs, lower sperm activity and survival rate. After growing in the hypoxic water for several generations, the sex ratio tends to become imbalance as evident by the domination of male and virtually no female in the population of the progeny.

Researchers explained that an enzyme CYP19, an important protein in sex determination, is interfered under the hypoxic environment and renders fish inability of producing female sex hormone and this leads to the domination of male in the progeny.

The problem of hypoxia in marine environment is a global problem. The water is defined as hypoxic if the dissolved oxygen is lower than 2.8mg per liter. The area of hypoxic ocean is equivalent to the size of three Japan(s). The condition in Hong Kong is not too bad. Occasional hypoxia is found in Deep Bay and the dissolved oxygen in the other nine water pollution control zones is well above 4mg per liter.

There are natural and human causes of hypoxia. The meeting of sea water with freshwater results in the formation of water layers of different densities and this may form barriers slowing down the dissolution of oxygen in the water body. Global warming leading to the rise of sea temperature further aggravates the problem. Besides, organic pollution and eutrophication promote the extensive growth of algae, which leads to the red tide and it is one of the major human causes of marine hypoxia.

Sex transposition caused by hypoxia may further drive some rare but commercially important species such as Cheilinus undulates (NAPOLEON WRASSE), already over-fished, to the verge of extinction. To save our precious renewable resources such as fish, we need not only the effort of scientists but also the strong political will and concerted efforts of leaders from governmental and non-governmental organizations all over the world.