A trial involving 21 aquaculture farmers demanding compensation from ConocoPhillips China and the China National Offshore Oil Corp. (CNOOC) over an oil spill in 2011 began Tuesday.

The sea cucumber farmers from Leting County of Hebei Province demanded a compensation of 148 million yuan (23.9 million U.S. dollars) to cover their economic losses and litigation costs, according to the Tianjin Maritime Court.

They claimed that the oil spill, which was not made public at first, led to the death of a large amount of their sea cucumbers, with losses amounting to 140 million yuan.

Ma Yong of the All-China Environment Federation said most such pollution cases face the challenge in determining economic losses from the pollution damages.

The Penglai 19-3 oilfield, jointly owned by CNOOC and the Chinese subsidiary of U.S. oil giant ConocoPhillips, experienced a severe oil leak in mid 2011.

Spills in the oilfield polluted more than 6,200 square km of sea water, with 870 square km heavily polluted.

In April 2012, the State Oceanic Administration ordered the two companies to pay a total of 1.68 billion yuan in compensation for the damages to the marine ecosystem, as well as to fund environmental protection efforts in the Bohai Sea.