More than 20,000 anglers are expected to turn out tomorrow morning for Opening Day of trout-fishing season, the largest sporting event in the state. They will be pursuing about 70,000 brown, rainbow, and brook trout stocked in scores of streams and ponds by the state’s Division of Fish and Wildlife.

A handful of Opening Day anglers may also fish for native brook trout — actually a kind of char — that still occurs naturally in a few streams. Ancestors of the stocked rainbows came from the western states and provinces of Canada. The first brown trout came to America from Germany generations ago.

For the first time, probably since the dawn of the Industrial Revolution, anglers will be able to pursue trout in the town of West Warwick tomorrow. The Pawtuxet River, once the filthiest stream in the state, has two new trout-fishing spots, developed by the Department of Environmental Management and the Pawtuxet River Authority.

One site is in the village of Natick at the Providence Street Bridge, and the other is near Veterans Square at the intersections of Routes 117, 33 and 115, said Robert Nero, chairman of the river authority.

The Pawtuxet now has 14 trout-fishing sites, he said. The most popular is probably at the dam in the village of Hope, just downstream of the river’s headwaters, Scituate Reservoir. In Hope, the river runs clear and bright.

Mild weather allowed many of last year’s stocked trout to survive through the winter.

This week’s hint of warm spring weather stimulated aquatic insects such as mayflies to emerge and become prey for the trout. Fishermen call the insects by the names of the artificial flies they tie.

Jack Riley, who lives on the banks of the Wood River, said, “The river is loaded with both stocked trout and holdovers. I saw fish rising for Black Quills and flashing as they burrowed for nymphs. If the weather holds up and the river stays down it should be a great opening day.

Forecasters have been calling for patchy fog and possibly rain, with highs in the 50s, typical Opening-Day weather, but warmer than usual.

Many tackle stores will be open straight through tomorrow morning to sell bait, lures, licenses and Trout Conservation Stamps that serve as permits to take trout. This year’s stamp depicts a sleek rainbow.

Trout-fishing season officially begins tomorrow at 6 a.m.

Freshwater-fishing regulations, limits, and a list of stocked-trout waters are available at tackle shops, town clerks’ offices, and online at http://www.dem.ri.gov/pubs/regs/regs/fishwild/fish0809.pdf

tmeade@projo.com