NAIROBI—Despite the recent release of US and French nationals, Somali pirates continued holding at least 18 ships and close to 300 crew members ransom on Tuesday, in one of the worst spates of hijackings in the past two years.

Here are some facts about the booming operation:

• Pirates have used ransom money from previous hijackings and favorable weather conditions to launch a barrage of attacks in recent days, hijacking at least 10 ships since the start of April.

• The latest ship captured by pirates is the MV Irene, a 35,000-ton Greek-operated merchant vessel captured Tuesday and flagged in Saint-Vincent and the Grenadines and carrying a crew of 22 Filipinos.

• Pirates currently hold close to 300 seamen in total. The largest national contingent is from the Philippines, accounting for more than 120 of the hostages.

• According to Ecoterra International, an environment NGO monitoring illegal marine activities in the region, pirates attacked more than 130 ships in 2008 and hijacked close to 50 of them.

• One single hijacking can involve many nations. For example, a ship can be owned by country A, operated by country B, flagged in country C, hijacked in the waters of country D, and carry crew members from countries E, F and G.

• The longest-running hijacking is that of the Nigerian tugboat T/B Yenegoa Ocean and its 11-member crew, which have been held for eight-and-a-half months.

• Equipped with small skiffs, firearms, grapnels and ladders, pirates have proven their ability to attack all types of vessels, from tourism yachts to oil supertankers, as well as cargo box ships, fishing vessels, barges and tugboats.