Cold weather is rapidly changing the fishing scene, so an understanding of what's happening biologically may help make these last few weeks of the season productive. Here's a quick roundup of how I'd change my methods.

In the lower Chesapeake Bay, spot and alewives are the food supply for bluefish, the main attraction in the Bay. Cold weather has forced the spot to leave and the alewives will soon follow. This signals an imminent decline in the excellent surf fishing for blues that has prevailed through early October on the beaches between the mouth of the Patuxent and the mouth of the Potomac. The best bets for blues from now through the end of November will be in deeper water, where trolling hoses or chumming cut bait is best. Dedicated surf fishermen will probably do best to look for warm, calm days that will heat up the shallow water along the beaches, thus attracting the departing alewives, which the blues will follow inshore. Concentrate on times when the tide is moving, since that stirs up the bottom and provides food for the baitfish that attract the blues.

On the freshwater side, bass are feeding heaviy now to put on fat for the winter. You can expect to find largemouths working the shallows and feeding voraciously in Occoquan, Anna and Manassas in Virginia, and Liberty, Loch Raven, Tridelphia and Rocky Gorge in Maryland. Although slow-moving summer lures like plastic worms will still take some fish, I'd lean more to baitfish imitators, spinnerbaits and crankbaits, all of which say "meat" to big old bucketmouths. Look for fish along the shorelines and on the points in three to eight feet of water from mid-morning to mid-afternoon, when the water is warming and the bass' metabolic rate is increasing. Remember, though, that a bright day will often send the fish a bit deeper, where they feel safer.

In the trout streams, the cold weather means that you can expect to see very little insect activity. Next year's fly hatches are too immature as aquatic nymphs now to interest trout, so about this time of year I get out a longer and stiffer fly rod and start working weighted streamer and bucktail patterns - like matukas, muddler minnows, sculpins and white and black marabous - that imitate baitfish. Since baitfish face the current that carries their food, I prefer slowly jerking and teasing these patterns upstream through the holes below riffles and along undercut banks where baitfish and trout hide. On warm days the dryfly enthusiast will continue to take some fish on ant patterns, since ants are about the last of the insects recognize the threat of winter. If you see a trout rising downstream from an overhanging bush, wind up your casting arm and throw and an at him as a final salute to the summer of '78.