New Hampshire Lake Region September is peak time for targeting NHs largest native, the lake trout laker, togue, or namaycush (dweller of the deep to Native Americans), if you prefer. In the central Lakes Region, lakers spawn on shallow, windswept, cobble shoals, typically each side of Halloween, with water temperatures near the 50°F mark - while seemingly distant, in fact, peak pre-spawn feeding activity is well underway and will continue until the season ends on September 30th on landlocked salmon/lake trout managed lakes.
Standard trolling techniques in the thermocline will certainly take some lakers, but anglers specifically targeting lakers will attest, more sluggish, deeper summer presentations can really increase catches sometimes all day long. Simply dropping trolling speeds (with spoons maintaining appropriate action at such speeds, e.g., Sutton #5) can pay dividends, as well as kicking the boat into neutral often to simulate dropping, wounded baitfish (with spoon wobbling downward)
shifting back into forward can produce savage strikes as the bait seemingly recuperates and tries to struggle upward and away (obviously hits will also occur on the fall).
When calm enough conditions permit, vertical presentations such as old-school hand jigging copper or wire line with wrecking spoons (a somewhat lost art, at least in New Hampshire) can really pay off, especially for larger- sized lakers. However, with most average-sized lakers present, vertical jigging gear can be better matched to the quarry utilizing modern, medium/light weight spinning outfits - hooking up jigging and battling straight under the boat on these outfits will have you quickly re-assessing the average lakers fighting prowess vs. towing in the proverbial boot with 10 colors of leadcore line
For the not faint of heart, dragging downrigger balls along appropriate bottoms, with a short lead just above the ball off bottom, can really draw the attention of lakers as the bottom sediment is stirred up and lakers are quickly called into the set however, with the granitic rock-and-shoal strewn nature of most NH large lakes, know your trolling runs and know them well - before employing this method!
Pre-spawn concentrations of lakers can currently be found in the deepest basins and holes, down 60-120+ feet depending on the exact conditions, time of day, and basin. That said, by investigating with the fish finder and experimenting, you can still find a surprising number of shallower (and thus easier to target) lakers along reefs and other structure, and even moderately deep flats. As has been noted accurately, not all fish of one species are necessarily doing the same thing at the same time
With surface temperatures still currently well into the mid-70°F range after the recent hot weather, immediate and proper release of lakers (and all other coldwater gamefish) is imperative for those not harvested/kept. Immediate release with a sharp plunging motion will typically get most lakers well on their way back to the depths (in fact you can often observe them streaking back downward to bottom on the fish finder); if unsuccessful, often a slight burping motion by gently pressing on the belly can help deflate the air bladder all must be accomplished in a very timely manner. Finally, if all resuscitation methods are unsuccessful, harvest these specimens (within legal limits) while allowing others more likely to survive the chance to do so.
As mentioned, lakes managed for landlocked salmon/lake trout are closed to the taking of salmonids after September 30 (this is the last legal day), so get out and enjoy these last couple weeks.