For Ung Bun, a 39-year-old fisherman from Cambodia’s southern Kep province, the days when he would come home with plentiful catches of flower crabs appear to be long gone.

Pulling in his net one recent morning, he found just one crustacean. Ung Bun dropped the crab – a male that was too small – back into the sea.

“I feel despair that I cannot harvest even one crab after a day, when about five years ago, I would have caught about 10-20 kg (22-44 pounds) of crabs. Yesterday morning I caught about four to five crabs,” he said.

He later took three gravid crabs – females laden with eggs – out of a bucket and released them back into the sea.

Releasing them and the small male crab were acts Ung Bun wouldn’t have done a few years ago, part of a conservation campaign he joined this year that seeks to ensure a more sustainable future for crab catching.

The provinces of Kep and Kampot are famous among locals and foreign visitors for their delicious flower crabs but fishermen there are anxious about their small catches – a development that experts attribute to a warmer sea.

According to data from the University of Maine’s Climate Change Institute, temperature spikes above normal have become increasingly common in oceans along Cambodia’s coastline since 2010.

Escalating emissions also result in higher levels of carbon dioxide being dissolved in the sea, lowering its pH level. The warmer and more acidic water reduces the concentration of carbonate, a compound necessary for shellfish to create their shells.

Overfishing as demand from customers increases also hasn’t helped.