SALTWATER FISHING came to a screeching halt this week, thus this will be the final Fish Lines of the 1990 season.

Chilly nights caused water temperatures in the Chesapeake to dip below 55 degrees; bluefish, weakfish, flounder, porgy and sea bass are leaping over each other to get out of the bay and into the ocean where the water's warmer. The only saltwater action you can hope for this weekend is to hook a few straggler blues or find a school of white or yellow perch. Freshwater angling should remain good to excellent through the winter.

Fish Lines will return with the mackerel run next spring.

A 28-MINUTE video about the Izaak Walton League's program "Save Our Streams" tells how to become a stream monitor to help fight pollution. The tape is available for $15 by writing to: SOS Video, Izaak Walton League of America, 1401 Wilson Blvd., Level B, Arlington, VA 22209. WHAT'S THE CATCH? WASHINGTON AND VICINITY

POTOMAC RIVER --

Guide Glenn Peacock is catching 1- to 3-pound largemouths at Blue Plains. The fish are hitting plastic worms and smoke-colored grubs fished slowly close to the bottom. He discovered a few shallow lumps just below Wilson Bridge where he's catching largemouths on a pig and jig combination. Other hotspots include Fox Ferry and Nanjemoy Creek, especially during low and incoming tides. Guide Ken Penrod reports good catches of largemouths in the Washington Channel, where the water is relatively clear and the bass are cooperative. Upriver, at Brunswick and White's Ferry, the water is clear but still high. Smallmouth bass action runs fair to good, but the best news is sizes have increased substantially, to 2- and 3-pounders.