The Gulf of Mexico’s dead zone, where nutrient pollution from farms in the Midwest has chocked off fish life, is bigger this year than ever before, according to university researchers. Stretching from the Mississippi River delta to Texas waters, the 8,000-square-mile, low-oxygen area is forcing crabs and other bottom feeders to the surface. Environmental groups are struggling to get the Bush administration to act on the recommendations made by a Clinton-era task force to reduce fertilizer and animal-waste runoff into the Mississippi River.