The Commission on the Merchant Marine and Defense plans four public hearings this month and in July to seek further information and ideas to deal with the sagging fortunes of the merchant fleet.

The seven-member body, appointed by President Reagan, made its initial reports last year and in January, urging extensive new steps to stimulate U.S.-flag ocean carriers.Under the law by which it was established, it will make a further series of findings and recommendations this year before disbanding.

Commission chairman Jeremiah Denton, a former republican senator from Alabama, scheduled the first hearing of the new year for May 23.

That will focus on Improving the Competitiveness of the U.S.-flag Merchant Marine and hopes to obtain views on operating subsidies, ship costs and efficiency, foreign competition and overseas subsidies, unfair trade practices abroad and related matters such as research and development.

The second meeting, to be held May 24, will dwell on Obtaining, Sustaining and Modernizing Ships for Strategic Sea-lift, the commission said. It will cover various plans for government constructing and chartering out vessels, building abroad, financing and needs for and uses of a reserve.

The second set of hearings, planned for the week of July 18, will turn to Maintaining, Modernizing and Strengthening the Shipbuilding, Ship Repair and Ship Supplier Industrial Base.

The second public session in mid-July will go into Providing Cargo and Access to Cargo for the United States Merchant Marine and take in such matters as cargo reservation programs, government sea-lift procurement policies, trade expansion efforts, coping with foreign shipping discrimination, the growing number of ship registry nation and the cargo-sharing arrangements for liners under the code promulgated by the United Nations Conference on Trade and Development.

Those wishing to testify at this month's hearings were asked to notify the

commission by May 13. Anyone wanting to submit written statements was invited to do so.

The commission indicated that its members were especially interested in specific reactions to the many recommendations made by it in January.