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The Spawning Urge
What Scientists Say

By Russ Bassdozer

Is it based on temperature? Whereas many anglers often ask each other if they think the bass are ready to spawn on this or that full moon, most biologists do not look up at the night sky, but rather look at the seasonal thermometer.

I do think the moon phase means something, but it's just not something bass biologists are prone to experiment with or study. True, the moon the sun, other planets and their relationship to our spinning earth can and do exert forces that influence life here, but bass biologists are often in laboratories, and even in the field, they typically focus on more down-to-earth causes and results, such as trying to determine what influence the spring weather, water temperatures and water levels will have on the bass spawning in this lake or that region.

So yes, I do think "more" bed fish will be found on the moons after a certain water temperature is achieved, but like I said, scientists do not care about the moon, they care about other more measurable factors to which they can attribute the year's spawning success or failure.

Are you having your photoperiod yet? There are also occasional discussions of photoperiod (length of daylight and angle of sun in the sky) among anglers, as if there is a magic day on the calendar when bass wake up that morning and begin spewing milt and eggs. True, I have occasionally heard of the photoperiod cited as a factor in many fish movements (spawning, mustering, migration) for a number of differnet species, however, it was not in my notes of the studies that I came across when preparing this article for black and brown bass spawning. However, since seasons, temperatures and weather are all results of earth's orbit, I do believe photoperiod is probably mixed in there somewhere. It's kinda like when my wife asks me why I love her: "No big reason, honey, but many countless small ones that all add up!". That seems to satisfy her.

But...most studies see spawning primarily as a function of water temperature - the stability and duration (taken together, the persistence) of the average daily water temperature over time, the velocity of the season's overall warming water trend over time, the average delta of daily  low and high temperature extremes, the frequency of sudden changes in water temperature. All these temperature factors have measurable effects on spawning success.

And while temperature seems to be the biological controller here, studies also indicate that oxygen, PH, salinity and other factors can measurably affect spawning. This article will tell you a bit about all these factors. Are you interested? Please read on!

Green versus brown. In the field, you may find smallmouth and largemouth spawning in different places and at different times. In general, smallmouth nest building may start a few degrees colder on average, and may be in slightly rockier areas on average than largemouth. These factors are different - but not different enough - to warrant much special mention in this article. Most studies have not indicated a major difference between largemouth and smallmouth spawning in terms of temperature, nest substrate, dissolved oxygen, PH, etc.

North versus South. Studies do not show dramatic differences based on water temperature, areas used, oxygen, PH, etc. In general, bass in the deep southern ranges may get their urges a few degrees warmer than their far northern counterparts.

Nest Building. Studies show that the urge to build nests occurs in males at lower temperatures than when females are ready to lay eggs. Most studies indicate bachelor males will begin to build nests in water temperatures as low as 54 - 57 and surely by 60. There is a "magic number" above 60 when females begin to reveal their interest in the boys and the nests that they've built!

Males dig nests by dishing out the softer top layers of sediment with their tails to ideally get down to harder ground. After sweeping it out vigorously, the bottom of the nest may be scoured down to clean chunk rock, gravel, roots, etc. Where the bottom is sand or dirt, the sweeping will tend to remove all the finer granules, leaving behind a slightly raised floor of pebbles, twigs, shells, rubble, etc.

The nest-building urge in males is thigmotrophic. Given a choice, they will build nests that are protected on one or more sides by "things" - logs, rocks, pilings, stumps, ledges, etc. This may provide partial protection from predators and egg robbers, or a break from wind or water current.

Click here to visit our fishing art galleryBass can spawn in main lakes and rivers or ascend tributaries to spawn. They typically migrate up the tributaries on periods of high water levels. This makes navigation easier, and allows bass to claim the highly-preferred gravel, stone and hard sand bottoms that have been flushed clean of silt by the high water's passage.

Both smallmouth and largemouth bass nests are commonly in shallows, backwaters or tributaries of either streams or lakes. Nests are commonly close to shore in protected bays and creeks, or on the sides and tops of mid-water shoals. Nests are usually in areas of quiet water. Nests are usually in areas of very slow current. Nests are usually on the leeward shore or sheltered from prevailing winds.

Male bass instinctively prefer not to build nests wherever turbidity may be a concern. Not only may a soft bottom composition (mud, silt, clay) be avoided if possible, but areas that are prone to have wind disturbance or water flow are also avoided, since both wind and water action can induce fluctuating temperatures, raise turbidity and deposit silt that can suffocate eggs.

Bass may spawn on depth breaks (edges of pools, cliffs, ledges, etc.), provided these areas should have minimal wind and current exposure, and at a depth sufficient so that wave action will not destroy the nest.

Water levels. Some studies indicate that a high water period may trigger bass migrations towards the spawning grounds. Once they've arrived on the scene, then relatively stable water levels are preferred before bass dig nests, drop eggs and hatch fry. Dropping water levels can result in poor spawns for various reasons including desertion by males, wind-driven wave destruction, and nests ending up high and dry. Rising water levels (barring floods) usually do not have major negative consequences, although quickly rising waters may usher in cold or turbid water that stalls egg or fry development.

Depth. Most studies indicate highest spawning success off nests covered in from 1 to 3 feet of water, but deeper nests may occur in very clear warm water. Although uncommon, studies done on clear, deep-sided impoundments report males still maintaining nests from 20 to 27 feet deep that were covered with slowly but steadily rising warm water. I do believe there is a concept of bass becoming "committed" to a nest that is covered by rising water, and so long as the bass feel the nest has a chance to draw females or hatch eggs, they'll stick with it.

More depth. Although shallow nests are "scientifically" more successful, nests down to 6 or more feet are not that uncommon, particularly for smallmouth in deep, clear impoundments. Rationales for deeper nests include that:

1) Deep clear lakes simply have very little littoral benthos (shoreline bottom) in the 1-3 foot range. Often just a small rim that smallmouth in such lakes do not normally live in anyway, except to visit for brief feeding periods at those magic moments of half-light at dawn and dusk. If there is any livable littoral benthos...other species will usually dominate it if they exist - largemouth, panfish, pike, pickerels, muskies, etc.) Smallmouth cannot compete against largemouth or even panfish for food in such areas, and they'll be heavily predated by the water wolves.

2) Deep clear lakes characteristically have more wind surface (which causes stronger waves and more forceful underwater turbulence) that would upset shallower nests. So, the added depth provides stability.

3) Deep clear lakes often have rock sides and bottoms which amplify any heat gain/loss as sun (or lack of it) is transmitted easily into the nearby water. Hence, deeper nests are buffered better from some of this daily temperature fluctuations in such areas. So, the added depth provides stability, but also causes the incubation and hatching process to take longer than in shallower water.

Mating. The actual laying and fertilizing of eggs can range higher or lower, but it usually takes place when the water temperature is stabilized above 60 and rising slowly between 60 and 70. Dropping water temperature will tend to keep females off the nests, and rapidly rising temperatures have been reported to delay spawning until the warming trend slows down and stabilizes too.

Sharp drops in water temperature, followed by increases, will  cause repeated waves of mating, but that doesn't necessarily mean multiple crops of viable eggs or fry. Sharp drops in temperature will also kill eggs, and studies report increased frequency of males deserting eggs in water dropping below 60.

I had no notes of studies detailing how long females lay, but I have seen plenty of them doing it in the wild. The act itself is beautiful and fleeting, often under ideal environmental conditions...water like glass, pleasant day, flowers blooming on shore and all that. Maybe I'm romanticizing here, but both bass seem to appear to have heightened body colors...Dare I say an aura? She lays on her side and shudders with the male also, then moves off the nest. It's not long! She'll often do some inspecting and tidying up the nest when she comes onto it and she often acts more aggressive to intrusions by nearby egg-robbers like sunfish than will the male at that moment. The male seems more intent on keeping her there, and will often circle her and positon himself to cut her off from leaving him. She'll do a lot of enticing lingering near the nest both before and after, usually at the nearest weedline or slope, sometimes slipping back up for another quickie or two or three. She'll usually be present in the area for days, especially so if there are several males with nests nearby. I might add she'll often "smoke" a smoke-colored finesse bait (tube, etc.) after the act when she returns to the weedline, and she'll often snap it right off the nest before the male if you interrupt them during the act.

Incubation. Fresh eggs need time to "harden" and become acclimatized after fertilization. Studies show eggs will become temperature tolerant after 12-15 hours. Then the males will become fathers and caretakers of the egg clutches. Males defend their clutch from predators and fan eggs with their tails to keep a small flow of aerated water circulation and to keep sediment from settling and suffocating the eggs. Clutches can also get infected by fungus that destroys them, and fanning also prevents fungus from getting into the eggs.

Once acclimatized to moderate temperature fluctuations, desertion of the male is the factor most harmful to eggs.

Hatching. Studies usually indicate optimum incubation and hatching temperatures to be from 66 to 72. More eggs will hatch, and they will incubate quicker in this temperature range. For instance, almost all eggs will hatch in 3 to 4 days in this temperature range. That seems to be the ideal. Far fewer eggs will hatch and will take much longer to do so at lower temperatures. 

Dissolved Oxygen. DO must exist in all life-giving waters. Normally, there is a saturation point - how much oxygen can remain dissolved - which rises or falls based on water temperature, water/air pressure, altitude, depth, PH and other factors.

Some lab studies have pumped up DO levels far above what's reasonably expected in nature. These lab results have not seemed very stressful to adult bass, although eggs and fry clearly thrive best when oxygen stays near nature's normal level - the "saturation point".  In fact, normal, healthy levels of DO can reduce the bad effect of other adverse conditions. For example, if water temperatures rise rapidly and get too hot, it is not as harmful if DO also climbs higher. The difficulties arise when there's not enough oxygen. As other survival factors worsen (too low/high temperature, PH, etc.), DO becomes critical. Low DO will dramatically worsen the effects of other bad conditions. There is also a problem anytime when DO is reduced suddenly. For instance, there is a marked difference (called the "diurnal DO flux") between oxygen levels from day to night. If this daily change is severe, it can hamper the ability of fry to hatch or grow.

In summary then, studies show bass (adults, fry, eggs) better able to handle other stressful factors when DO remains high or increases. A higher percentage of eggs will hatch and fry will grow quicker in optimum, well-oxygenated water. Keep in mind that well-oxygenated water usually occurs as a result of when all other conditions (temperature, depth, etc.) are also optimum for egg incubation and fry development. On the other hand, when DO is unfavorable, eggs will take much longer time to incubate, many eggs will not hatch, and fry will develop more slowly or not at all. This problem is compounded in that low levels of DO are usually associated with other unfavorable factors (too low/high temperature, PH, etc.).

PH. Yes, some of us may have learned about PH back in school and biologists are always talking about it as one of the factors for life. If PH is too high (alkaline) or too low (acidic), an environment cannot support life easily if at all.

As we mentioned about oxygen above, there is a normal range that can be tolerated by adults, eggs and fry. PH that is too low or too high can be avoided by non-spawning adults who will move to better conditions. But spawning adults, eggs and fry will be stressed by PH above or below the normal range. As we also mentioned above, there can be a diurnal flux in PH levels from day to night. In areas where vegetation becomes far too abundant, the PH can become very high in daytime as plant photosynthesis peaks. The dramatic daily change, plus the daily peak in PH can both badly affect the urge to spawn, and affect the survival of bass eggs and fry.

Salinity. Although adult bass may live in brackish water environments, studies show it is not always the best environment for adult bass to prosper. As for the more vulnerable bass eggs and fry, studies indicate that even low levels of salinity (far below an adult's tolerance) will greatly impair the survival of bass eggs and fry. Therefore, it is presumed that adult bass in brackish water will seek out fresher tributaries and headwaters for spawning purposes.

Turbidity. Clear water is preferable for spawning. Studies have shown that there is limited survival and success in moderately turbid water, and eggs may not hatch in highly turbid waters.

Hatchlings. Fry are usually better able to survive temperature changes that would destroy eggs. Fry become independent from their father and optimal growth metabolism for fry is achieved during early summer at water temperatures between 78 - 85.

Predation. The above are just a few of the more commonly-recognized factors that have measurable effects on bass spawning. Many studies also show that predation on bass eggs and fry can be critical to the year's spawning success.

Food availability. As a final factor, studies also show that the timing and availability of a food supply is also critical to fry.

Preferences. This article, and the studies it is based on, deal with preferences and optimal factors. Keep in mind that preferences may not necessarily be limiting factors. For example, most studies indicate that bass prefer to spawn and live in clear water, however, studies also show that although turbidity is not preferred, it is not necessarily a limiting factor in bodies of water where it is the only option. That's what's so great about bass! Between largemouths and smallmouths, you can usually find them everywhere!

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