GIANNA VOLPE PHOTO | A house for sale on Shelter Island.

The Shelter Island real estate market lagged in the third quarter of 2012 behind the same period last year, according the latest quarterly report by Corcoran, a major real estate firm serving the area. The report uses market-wide data to assess East End trends four times a year.

The total sales volume of residential real estate transactions on Shelter Island was down 32 percent in July, August and September compared to last year’s third quarter, from $15.487 million to $10.603 million, according to the report. The number of transactions remained the same at 13.

The median price was down 44 percent, from $1.325 million to $743,000, and the average price fell 32 percent from $1.191 million to $816,000, according to the report.

The numbers for the East End as a whole show a drop in sales volumes of 1 percent from $737,640,000 $731,469,000 with total sales 542, down 8 percent from 588. On the North Fork, sales volume was down 5 percent from $75.873 million to $72.178 million while the number of sales rose 4 percent from 125 to 130. The median price was down 1 percent from $430,000 to $425,000 and the average fell 9 percent from $607,000 to $555,000.

For the area that Corcoran calls “the Hamptons” — and the company includes Shelter Island with the South Fork in its numbers — total sales volume was virtually unchanged at about $660 million while the number of sales was down 11 percent from 563 to 412. The median price rose 13 percent from $775,000 to $879,000 while the average price was up 12 percent from $1.429 million to $1.6 million.

An exception to the lackluster numbers on the North Fork was found in Greenport, where total volume was up 34 percent to $7,683,000 on 16 transactions, up from 14. The average price was $480,000 during the third quarter compared to $409,000 in the same quarter last year.

The most stellar performers on the South Fork were the Quogue-Quiogue area, with a 75-percent increase in the number of sales from 8 to 14 on a total volume of $21,454,000, up 124 percent, and an average price of $1.532,000; the Bridgehampton-Sagponack area, where the numbers of sales were up 57 percent to 36 with a total sales volume of $144,115,000, up 122 percent, and an average price of $1.6 million, up 12 percent; and Southampton, with total sales of 43, up 16 percent, with a volume of $42,590,000, up 25 percent, and an average price of $990,000.

In commenting on the figures, Corcoran indicated that lower numbers in the third quarter may not reflect a long-term trend. It notes there was “pent-up demand” after the national real estate crash of 2008 that produced a “surge in sales activity in the first and second quarters of 2010” and a leveling off early this year. “Decreased activity in the third quarter of 2011,” according to the report, “may indicate a return to more normal seasonal activity.”