Hong Kong fish population under threat from oxygen-starved water, breakthrough study finds
Nutrient pollution from sewage creates conditions that can lead to reproductive problems that are passed down to offspring
Fish exposed to low or depleted oxygen levels in their marine environment – a phenomenon linked to algal blooms, or “red tides” – can develop reproductive impairments that are passed down to future generations, a team of Hong Kong scientists have found.
Defects include reduced testis size, sperm quantity, sperm motility and hormonal imbalances that cause more males to be born, disrupting the population, according to the latest breakthrough research.
The findings were relevant to Hong Kong where hypoxia, as the low-oxygen phenomenon is known, is commonly observed in bodies of water such as at Deep Bay, Shing Mun River and a major hotbed for nutrient-infused algal blooms – Tolo Harbour.
When algae cells die, they sink to the ocean floor in large concentrations, consuming all oxygen as they decompose, and causing mass deaths of fish in the process. A hypoxic condition usually occurs when oxygen concentration falls below 2.8 milligrams per one litre of water.
“Hypoxia can cause [reproductive] impairments in fish and their offspring, [even if the latter] have never been exposed to the condition,” co-author of the study, professor Rudolf Wu Shiu-sun of Education University’s science and environmental studies department, said.
“Our research suggests that hypoxia is a more serious and long-lasting threat to fish populations and the sustainability of fish resources than what was previously thought.”
The research, involving scientists from the University of Hong Kong, City University, Chinese University, Baptist University and Education University was recently published in the academic journal Nature Communications.
The findings might also shed further light on the potential transgenerational effects of hypoxia on other vertebrates – including humans. Previous studies found men suffering from sleep apnoea exhibit lower levels of male and female sex hormones as well as a reduced sex drive.
Researchers exposed adult medaka fish and two subsequent generations of offspring to hypoxic conditions, regular conditions and a mix of both. Those under normal conditions turned out predictably normal but those starved of oxygen were found to have impaired sperm motility – their ability to move properly – reduced testis weight and a lower percentage of mature sperm cells.
More shocking was the fact that two subsequent generations produced by parents who had lived under hypoxia recorded lower levels of male hormones, poorer sperm quality and lower sperm motility and fertilisation success, despite having never been exposed to hypoxia. Other defects included reduced growth.
Wu said the problem would only get worse with climate change which, together with the large amount of nutrient-rich wastewater being discharged into the ocean, caused excessive phytoplankton growth, leading to hypoxia.
Baptist University assistant professor of biology Dr Jill Chiu Man-ying said the key source of nutrient pollution in Hong Kong was sewage. She pointed to the reduction of coral in places like Hoi Ha Wan, where “primitive” wastewater treatment – namely septic tanks – caused highly eutrophic waters.
The Drainage Services Department said it conducted regular inspection and maintenance to ensure proper operation of the public sewage system, so as to safeguard against any nutrient discharge into the Shing Mun River and other inland watercourses in the Tolo Harbour catchment.
There are over 400 such hypoxic “dead zones” around the world, one of the largest of which is in the Gulf of Mexico. These areas no longer support life.