IAG Group's load factor fell 2.2 percentage points in April.
The airline group cites "a policy to improve until revenues through yield, rather than load", as well as the incidence of Easter, as a factor in the performance.
Revenue passenger-kilometres declined 5.6%, while capacity fell 2.9%.
Trends in passenger traffic differed widely between the group's two airlines, British Airways and Iberia. Iberia's RPKs fell 19.3% as capacity was cut by 13%. BA grew capacity by 0.8%, but its RPKs shrunk by 0.4%.
Cargo tonne-kilometres were down 7.1% at BA and 6.3% at Iberia.
About three-quarters of IAG's passenger traffic and 80% of its cargo traffic are accounted for by BA.