Tetra Tech Inc. late Wednesday lowered its guidance for the remainder of the year due to expected slower demand for its construction management services in the slower economy.

Shares on Thursday closed down $4.32, or 17 percent, to $20.75 on the Nasdaq. In a note to investors, analyst Debra Coy at Janney Capital Markets downgraded shares from “buy” to “neutral.” Coy said that while the company had reported good first-quarter results, it lacked near-term visibility on new projects.

After the markets closed, the Pasadena engineering company reported 15 percent higher fiscal first quarter net income of $18.7 million (30 cents per share), compared with $16.3 million (27 cents) a year earlier. Revenue fell 15 percent to $542 million.

Analysts surveyed by Thomson Reuters on average expected per-share earnings of 29 cents on revenue of nearly $340 million.

The company’s backlog of work at the end of the quarter was $1.61 billion, up 1.8 percent from the end of the fourth quarter, but down 8.9 percent from same quarter a year earlier.

Updating its guidance for fiscal 2010, Tetra Tech now expects earnings per share in the range of $1.08 to $1.18, lower than its earlier estimate, and net revenue to range from $1.4 billion to $1.5 billion. Analysts on average expect earnings of $1.32 per share on revenue of $1.50 billion.

Explaining the more conservative guidance, Chief Executive Dan Batrack said, “We now believe that back-end construction management services will weaken in the second quarter and remain soft for the remainder of the fiscal year, driven primarily by the commercial market downturn and the continued slow pace of stimulus spending.”.