South Africa’s navy has detained three Chinese ships with around 100 crew on board on suspicion of illegal squid fishing, officials said on Monday.

The ships were spotted on Friday having entered South Africa’s 200 nautical mile economic exclusion zone without permits. When South African officials asked the ships to sail to port they attempted to flee but were eventually captured.

“We cannot tolerate the plundering of our marine resources, which are a source of food security,” Agriculture and Fisheries Minister Senzeni Zokwana said in a statement.

“We are also looking into the sudden influx of these vessels in our waters.”

The three vessels – Fu Yuan Yu 7880, Fu Yang Yu 7881 and Run Da 617 – had a combined total of almost 600 tonnes of squid when the navy escorted them to shore. Inspectors found all three ships had no permits to fish locally.

Crew members have been detained on the ships. Captains face fines and possible jail sentences if found guilty.

Globally, illegal fishing costs the industry around $23 billion each year, with one in four fish thought to be caught illegally in African waters.

In October, a Sao Tome and Principe court won a key victory against illegal fishing by organised syndicates when it convicted the captain of a vessel and two crew members on a number of charges, Interpol official said.