The Genoese held the colony of Galata on the Golden Horn across from the city of Constantinople since 1261 as part of the Treaty of Nymphaeum, a trade agreement between the Byzantines and Genoese. However, the dilapidated state of the Byzantine Empire following the civil war of 1341-1347 was easily shown in the control of custom duties through the strategic straights of the Bosphorus. Even though Constantinople was the Imperial seat of power with its cultural and military center on the shores of the Bosphorus, only thirteen percent of custom dues passing through the strait were going to the Empire. The remaining 87 percent was collected by the Genoese from their colony of Galata. Genoa collected 200,000 hyperpyra from annual custom revenues from Galata, while Constantinople collected a mere 30,000. The Byzantine navy, a notable force in the Aegean during the reign of Andronikos III Palaiologos, was completely destroyed during the civil war. Thrace, the main imperial possession besides the Despotate of the Morea, was still recovering following the destruction of marauding Turkish mercenaries during the civil war. Byzantine trade was ruined and there were few other financial reserves for the Empire other than the duties and tariffs from the Bosphorus.
Question: What was the greatest income source of the Empire?
Answer: tariffs from the Bosphorus