President Carter yesteday exercised the "pocket veto" for the first time refusing to sign a bill that would have provided federal assistance to the aquaculture industry - the commercial raising of catfish, prawns crabs and other forms of aquatic life.

The bill sponsored by Rep. Robert L. Leggett (D - Calif.) would have established a new federal loan grarantee program and federal insurance program for the industry and set in motion the preparation of a national aquaculture development plan to expand the commercial potential of certain aquatic species.

Carter said there is a need to support the aquaculture industry but that there is already an array of federal activities " in this area. He said he was particularly concerned about beginning major new government subsidies such as loan guarantees.

A "pocket veto" involves the refusal of a president to sign a bill after Congress has adjourned. The measure automatically dies and is not sent back to the Congress for an override attempt.

Meanwhile, the president has signed two more bills, the White House announced. They are:

The Ocean Shipping Act of 1978, which gives the FederalMaritime Commission broad authority to regulate the rates of carriers owned or controlled by foreign countries.

A $27 billion appropriations measure that contained a compromise public works program and funding for a variety of federal activities including public broadcasting and several drug abuse and health projects.

The original public works program, containing a number of water projects Carter objected to, was vetoed by the presdent. Congress gave in to most of Carter's demands in enacting the compromise version.