The Soviet Union shortly will be forced to pull out a fishing depot it has leased since 1973 on the island of Bioko off West Africa.

The depot was used as a staging post for Cuban troops during the Angolan civil war and has served as a Soviet communications base, according to officials here.

The pullout will follow a turn to the West by the ruling military junta of Equatorial Guinea, a former Spanish colony. Dicator Francisco Macias Nguema was deposed and executed by the military last summer. Macias, an avowed Marxist, had signed a fishing deal with Moscow that handed over Luba under a six-year agreement expiring Dec. 31.

Reliable sources close to the junta said the new rulers had no intention of renewing the deal and conversations are well advanced to allow the Spaniards to build a fuel depot and a refrigeration plant on the site where the Soviets are.

The Soviet installation consists of several rundown buildings in a dilapidate township approached by a bad coastal road through the jungle. It lies some 20 miles from Malabo, the chief town on the island that was known as Fernando Poo before independence from Spain in 1968.

Alongside a Soviet-built jetty, guarded by Soviet seamen who bar outsiders, there is a floating dry dock and a tug. In the Bay of Luba are three large trawlers and a small coastal tanker. The trawlers are equipped with impressive antennae. Inhabitants say two to four Soviet ships normally anchor in the bay, suggesting that the depot serves a fairly large fleet.

During the post-independence civil war in the former Portuguese colony of Angola, down the coast from Equatorial Guinea, Luba served as a staging post for the estimated 9,000 Cuban troops and 500 Soviet advisers who ensured the consolidation in 1976 pro-Moscow rule of the later president Agostinho Neto in Angola.

Soviet withdrawal appears to have begun here already. Sources at Malabo airport said that the usual three weekly Aeroflot flights from the Soviet Union have been reduced to one. It reportedly arrives empty and departs full.

Sources close to the ruling junta said Equatorial Guinea had formally told the Soviets it was not prepared to renew the fishing agreement signed by the dicator Macias. They said Equatorial Guinea could grant facilities to Soviet vessels on the same terms that it would offer to other countries.

The terms of the 1973 contract were never made public but the sources said the Soviets gained a fishing monopoly off Equatorial Guinea and handed back to the Guinenans 25 percent of the catch. Rent for Luba reportedly was $600 per year.

The sources said fishing off the Guinean coast was too scarce to merit such an agreement or to necessitate a large fleet and the catch returned to the Guineans was a bad quality -- popularly believed to have been fished in areas far removed from the Guinean coast.

Col. Teodoro Obiang Nguema, 38, who heads the current junta, was trained at the Spanish military academy. He has adopted a pro-Western stand and asked for massive aid from Spain to reconstruct the country. The request came during a state visit by King Juan Carlos last week.

The junta led an Army rebellion against Macias on Aug. 3, tried him on charges of genocide and executed him by firing squad Sept. 29. During 11 years of rule by Macias, thousands of citizens were murdered, tortured and imprisoned. Up to two-thirds of the 250,000 population are estimated to have fled during the period, principally to the neighboring countries of Gabon and Cameroon.

Macias brought a once booming export industry of cocoa, coffee and timber to a standstill. All schools have been closed for at least two years.