The combined carrying capacity of oil tankers leaving Iranian ports last month dropped 22 percent from September, vessel -tracking data compiled by Bloomberg show.

The implied capacity of departing ships declined to the equivalent of 1.02 million barrels a day from 1.30 million barrels, according to signals gathered by IHS Maritime, a Coulsdon, England-based research company. The data may be incomplete because not all ship transmissions are captured.

U.S. and European sanctions are curbing Iran’s oil exports in an effort to pressure the Middle East country’s leaders to stop their nuclear program. Shipments dropped from more than 2 million barrels a day in 2011, before the latest rules took effect, according to figures from the website of the Joint Organizations Data Initiative.

The table below shows estimated vessel capacities in millions of barrels a day, based on signals from tankers that stopped at Iranian ports as well as movements near the nation’s terminals by its own tankers and Asian ships that previously loaded Iranian cargoes since the European Union’s embargo began in July 2012.

The tallies assume standard cargo sizes of 2 million barrels each for very large crude carriers, 1 million for Suezmax tankers and 650,000 for Aframaxes. Japanese VLCCs that also stopped at other terminals in the Persian Gulf were assumed to load 1 million barrels in Iran.

The table also shows estimates from the International Energy Agency, the Paris-based adviser to 28 industrialized countries. The IEA counts cargoes once they are received in importing countries, whereas the ship-tracking data compiled by Bloomberg tally exports when they leave Iran.

Month Ship-Tracking Estimate IEA Estimate

October 1.02 Not available

September 1.30 1.17

August 1.07 0.985

July 1.01 0.9

June 1.13 0.960

May 1.02 1.25

April 1.09 0.835

March 1.17 1.1

February 1.022 1.26

January 1.137 1.13

December 1.052 1.56

November 1.008 1.45