Joseph E. Petrillo has resigned as executive officer of the California Coastal Conservancy, effective Aug. 15. Petrillo, 45, said he was leaving his $50,000-a-year post to enter private law practice in San Francisco specializing in coastal matters. Petrillo has been executive officer of the conservancy since 1977. The conservancy is responsible for acquiring coastal lands for public use and restoring degraded coastal wetlands and access ways. Before joining the conservancy, Petrillo was a legislative aide and chief counsel to the California Coastal Commission and was one of the primary drafters of the California Coastal Act.