Doem Lao chops half a dozen large fish heads on bank of Cambodia's Tonle river for the one meal that her family will eat that day.

It is the 45-year-old farmer's fourth unseasonably cold dawn in this quiet Muslim neighbourhood on the outskirts of Phnom Penh, where her extended family has set up camp with others from their village in the southern province of Takeo.

Like tens of thousands of rural Cambodians, they have joined the annual migration to the river to buy enough fish to make a year's worth of prahoc, a pungent fermented paste that is the only source of protein for many in the country's impoverished rural regions.

But the rice brought they from home has nearly run out and the fish have yet to appear in the large nets strung across the river in front of their camp.

The crude bamboo and metal mesh processing stalls on the riverbank are silent -- and February is the last month of the fishing season.

A sudden drop-off in the numbers of prahoc fish has seen their price more than triple this year, up to as high as 50 US cents a kilogramme from around 12 cents, putting this most basic of Cambodian commodities out of reach for many.

While not normally a benchmark by which to measure food security, prahoc prices have highlighted the spiralling costs of staple goods that are threatening Cambodia's poorest with hunger.