Climate change poses a looming threat to Hawaiian fishponds, which have undergone a renaissance in recent years, University of Hawaii researchers have concluded.

The study was conducted from 2004 to 2016 at Heeia Fishpond by scientists with the UH School of Ocean and Earth Science and Technology and the fishpond stewards Paepae o He‘eia.

They measured winds, tides, temperature, water height, water movement and suspended material and phytoplankton in the water; and fish number, density and length. During this time, Hawaii experienced periodic effects of El Nino such as slackening tradewinds and warmer surface waters.

The researchers correlated two periods of extremely high fish mortality at Heeia Fishpond with weakened tradewinds in the week preceding each mortality event, as well as surface water temperatures 2 to 3 degrees Celsius warmer than normal. They suggest that the lack of tradewind-driven surface water mixing enhanced surface heating of the fishpond, leading to stagnant conditions and stress on fish populations.

“Our results provide empirical evidence regarding El Nino effects on the coastal ocean, which can inform resource management efforts about the potential impact of climate variation on aquaculture production,” said Rosie Alegado, corresponding author of the study and assistant professor of oceanography and with the UH Sea Grant College Program, in a statement. The findings were published in the journal PlosONE.

As climate change is predicted to alter the intensity, frequency and geographic patterns of El Nino, the researchers came up with three recommendations to reduce the impact of warming events on fishponds and limit the mortality of moi.

They recommended moving net pens closer to the sluice gates with the highest flow rates of ocean water entering the fishpond, which will decrease water temperatures and increase aeration in the pens. Additional steps include installing artificial aeration systems in the pens to limit stagnation and low oxygen, and implementing flexible harvest strategies at the onset of a warming event.

Historical records estimate that, in the early 1900s, an extensive network of more than 450 fishponds across the Hawaiian Islands produced upward of 2 million pounds of fish annually and supported large thriving communities. Currently, worldwide aquaculture accounts for almost half of fish consumption.

“Today, a majority of Hawaiian fishponds have been lost to coastal development and urbanization, but a growing community-based movement is working to restore and revitalize Hawaiian fishponds as a cornerstone to sustainable food fish production and resilient communities in an era of declining reef fish stocks,” said Hi‘ilei Kawelo, Paepae o He‘eia executive director.

However, these efforts are threatened by environmental changes, specifically storms and fluctuations in extreme seawater temperature that bring new sources of stress, the scientists said.