Keeping Pace During Early Season

Getting back to temperature, it’s believed that flatheads at the coldest winter temperatures can become so inactive they enter a state of torpor or quiescence, to the point where they might stop feeding for long periods. Anglers fishing mid-winter congregations of flatheads Up North hook them on snagging gear, but flatheads rarely seem to bite traditional presentations then. We’ve had good success fishing for them in winter aggregates during late fall and early winter, when water temperatures are in the mid-40F range; but their interest in feeding seems to bottom out in water down to the low 40s and 30s.

A recent study provides a direct comparison of food consumption by flatheads across a range of temperatures in a controlled laboratory experiment. Dr. Jason Vokoun of the University of Connecticut, along with biologists Sam Bourret, John Hoxmeier, and Kevin Stauffer of the Minnesota DNR, presented preliminary findings of their research on coldwater feeding patterns, metabolism, and homing ability.

Over a 2-week period, flatheads were acclimated to 6 different temperatures ranging from 37F to 73F, fed rations of minnows, and consumption determined over another 2-week period. Consumption at 73F was higher than at the other temperatures. Flatheads rarely ate below 59F, most stopped feeding at 52F, and nothing was consumed at 45F or below. Metabolism, which was determined by placing flatheads in a respirometer and measuring oxygen consumption of the test fish in a resting state, also slowed at 59F and below.

Another aspect of their study included a field experiment in which they sampled flatheads from wintering holes and inserted transmitters. One group of fish was released at the wintering hole, while a second was transported almost a mile downstream and released there. All the fish released downstream moved back to the wintering site within 48 hours and remained there until spring. “It’s impressive that flatheads can ‘wake up’ in cold water and physiologically perform,” Vokoun says. “We were surprised at the ability of the fish to swim and move against current at low temperatures.”

Whether the results of the laboratory experiment, which used smaller flatheads, translates to larger flatheads in a natural environment isn’t known. Angling observations show that feeding slows considerably, possibly even stopping in the coldest water, but it’s hard to say that flatheads completely shut down in water less then 52F in the wild. On several occasions, In-Fisherman editors have caught flatheads from wintering sites on lures, suggesting that the fish at least can be prodded into striking at temperatures down into the lower 40F range—another case of how flatheads can “wake up” and perform in cold water.

Anglers fishing the earliest part of the Spring Coldwater Period might still find concentrations of flatheads in and around wintering holes, with conditions becoming more favorable for catching fish when water rises into the mid- to upper 40F range. In-Fisherman Editor In Chief Doug Stange: “We know anglers catch flatheads when they’re still in winter aggregates, using heavy leadhead jigs and plastics; but I don’t know if there’s increased catchability on lures compared to natural baits early. I’ve caught them under these conditions on cutbait, and if you sit on a hole, they’re catchable and cutbait’s an option.”

Siltbacks Astir

Warming water during the Spring Coldwater Period eventually cues flatheads to vacate wintering holes. The move seems to begin when water temperatures reach the upper 40F to 50F range, typically in early to mid-April in northern regions of the flathead’s range and early to mid-March in the lower Midwest.

Vokoun tracked flatheads in Missouri for his doctoral research. He found that all flatheads tagged in the Grand River, a tributary of the Missouri, moved to the Missouri to overwinter, while of those tagged in the Cuivre River, a Mississippi tributary, some overwintered in the tributary while others moved to the Mississippi. Proximity to favorable wintering spots was the likely explanation for how far flatheads moved.