It’s that time of the year. The dogwoods are in bloom, the water is warming, the days are getting longer, and the bass are building nests and spawning. But what triggers the bass spawn? I doubt the blooming dogwoods put the bass in a romantic mood. Is it photoperiod? Temperature? 

Photoperiod — the length of daylight — is a powerful force. It determines when plants grow, flower, and become dormant. In the animal kingdom, photoperiod regulates migrations and mating seasons. Photoperiod probably affects largemouth bass spawning but exactly how is poorly understood. I say poorly understood, because most bass spawn on an increasing photoperiod. The exception is in Florida where Florida bass have been observed (and caught) spawning in December while the days are still getting shorter. It’s not an effect of Florida bass — Florida bass in Mississippi spawn at the same time as (northern) largemouth bass.

Water temperature appears to be the dominant environmental variable controlling the bass spawn. Based on observations of fisheries biologists and information from hatchery managers, largemouth bass spawn at 64-68 oF. Yes, I’ve heard numerous reports of bass spawning at cooler and warmer temperatures. I do not doubt that some are valid observations, but realize how much water temperature in the shallows where bass spawn varies from one day to the next, and even from morning to afternoon.

I think the “spawn at 64-68 oF” rule works quite well in most of North America. But, again, bass in Florida break this rule. First, those December-spawning bass spawn when temperature is decreasing. Second, the largemouth spawn lasts up to four months in south Florida and two to three months in north Florida. Certainly the water temperature isn’t staying in the 64-68 oF range for that long under the bright Florida sun.

How long the bass spawn lasts in individual largemouth bass populations is also without clear explanation. What I have observed is the duration of the largemouth spawn decreases from south to north. The four-month spawning period in some Florida lakes diminishes to about two months in some deep south lakes, tapers off to about one month in Midwestern bass fisheries, and lasts only a couple weeks in many northern bass fisheries. Again, I’m referring to how long the largemouth spawn lasts in a single fishery — a single lake or reservoir — not how long it lasts throughout a state or region.

Lacking an agreed-upon explanation by biologists, I offer a couple thoughts. Certainly, size and depth of the water body affects how fast it warms. It might be 65 oF in a shallow cove but only 55 oF in the main lake, so the shallow bass spawn early, the bass in the main lake may move up to spawn later. But this breaks down because there are big lakes up north where the spawn is “on and over” in a couple weeks.

Alternatively, it may be adaptation to survival in the conditions that have persisted for the 25 million years bass have been around. It takes a minimum temperature for bass eggs to develop properly and hatch. Then, the young bass need a rich supply of zooplankton and invertebrates, followed by the young of minnows, sunfish, and shad to survive and grow. These resources are available shortly after the bass spawn at 64-68 oF. 

Unlike in Florida and the deep South where bass have almost year-round growing conditions, bass growing season in the north is much shorter and winter is much longer. Late-spawning bass in the northern waters would have a very short growing season during which they can pack on enough body size and energy to make it through a long winter. And those smaller, late-spawned fingerlings would be easy pickings for the numerous predators that feed through the long winter. Advantage: early spawning bass. Over time, the genes for late spawning would be filtered out of the population.

One rule about temperature that largemouth bass do follow — whatever water temperature the bass spawned at last year in your favorite lake is the temperature that they will very likely spawn at this year.

A final trigger for the spawn — and one a lot of anglers have strong opinions about — is moon phase. Maybe it has an effect, but bass hatchery managers — biologists who observe bass day after day, year after year — are in agreement that the largemouth spawn is triggered by temperature and is independent of moon phase.