Accra - Ghana's navy has freed a tanker ship hijacked off the coast of Nigeria and arrested eight pirates believed to be responsible for seizing it, a military spokesperson said on Sunday.Pirate attacks have increased in West Africa in recent years, jacking up insurance costs for shipping companies. Experts say gangs based in the waters off Africa's top oil producer Nigeria are extending their reach across the region's Gulf of Guinea.Colonel Aggrey Quarshie would not say when the MT Mariam was seized by pirates. The small tanker's owners, using an on board tracking device, informed Ghanaian authorities of its position in Ghanaian waters on Saturday."The Ghana Navy responded swiftly with a patrol team to the area and they were able to overpower the pirates and free the ship. But when they got there, the cargo had already been transferred to another vessel," Quarshie told Reuters.It was not immediately clear what cargo the ship was transporting. Its crew members were unharmed, Quarshie added.The pirates, armed with weapons including AK-47 rifles, were arrested and handed over to Ghana's Bureau of National Investigations. Cash was also recovered during the operation."They are all suspected to be Nigerians," Quarshie added.He said Ghana's navy and other forces from Togo, Benin and Nigeria had launched a search for the ship carrying the stolen cargo.