The darkening skies and distant rumble of thunder strongly suggested that our morning trip on the river might come to a premature end.

Still, as I slipped the net under yet another quality-sized smallmouth bass, I was pleased Dan Brown, my client for the day, was making the most of his opportunity.

During that recent post 4th-of-July rainy morning, Dan put on a clinic from his end of the boat, catching two to three bass for every one I caught. As a guide, I’m always happy when my guests catch fish, but naturally I couldn’t help but ponder what the difference was.

Dan was fishing in front of me, so that could have been it. And the impending storms might have had them racing for his lure. We were both using suspending jerkbaits, but maybe his cadence was more to the fish’s preference that day. Then it occurred to me that Dan was using a slightly different jerkbait, one that ran three to four feet under the surface, whereas mine dove down to around two feet. Perhaps this placed his bait at a better depth for the five-to-six-foot pool we were fishing.

On my next guide trip the following day, I made sure to experiment with a deeper running lure. As the boat drifted into a pool of moderate depth, I picked up a rod rigged with a size 12 Rapala RipStop, a lure that runs a couple of feet deeper than the size 8 Rapala X-Rap, the “normal” lure.

Within a couple casts, I hooked into a nice smallmouth bass, shortly afterward another. It didn’t take long to rig a couple of the clients’ rods with RipStops, and they experienced similar results during the trip.

As I’ve mentioned in past columns, details can make a big difference in fishing success. While it’s possible when one person is out-fishing another, it’s coincidence, chances are there’s more to it than that. In this case I’m convinced it was the subtle difference in lure depth.

Lure depth can be particularly critical when fishing lakes during the summer and early fall. From mid-summer on, many lakes stratify into distinct layers of water temperature: A warmer upper layer, and cold (often oxygen poor) lower level, and sandwiched between the two a layer known as the thermocline. It’s within the thermocline that the water temperature begins to make a significant plunge.

There’s often not enough dissolved oxygen below the thermocline to support fish life. In this case, food fish and gamefish will often use the upper reach of the thermocline as yet another edge to relate to. Most modern sonar units will show a thermocline, displaying the plankton, baitfish, bug matter and decaying weed litter, that often suspends within this thermal edge.

It may be necessary to increase the gain (also called sensitivity or brightness, depending on fishing finder brand and sonar type) to get a good view of the thermocline.

An exception to the thermocline rule is nearby Keystone Lake, which does have dissolved oxygen below the thermocline, likely produced by the deep submerged weedgrowth the lake features. In this case, fish can be taken from 25 to 35 feet of water, though I fish those depths with some trepidation as they can be tough to release when coming from deeper water, crappies in particular. Bass and walleyes tend to fare well when immediately released.

Whether you’re trolling for suspended walleyes in Pymatuning Lake, or hovering a jig over Lake Arthur crappies, pay attention to what your sonar screen is showing you in relation to water temperatures, baitfish, and gamefish location.

Use this information as a starting point to determine the proper depth to present baits that particular day.

Jeff Knapp is an outdoors columnist for the Butler Eagle