The ranges of smallmouth and largemouth bass overlap in most of North America, but their behavior is anything but the same. To truly take advantage of areas that hold both species, you need to understand how each moves as the seasons change.


We know that both species exhibit strong spawn site fidelity, returning every year to the same spot to reproduce in the spring. Bass begin staging around spawning habitat as soon as the water temperature hits 10˚C (50˚F). In the Great Lakes and larger lake systems, harbors are a relatively small portion of nearshore habitat but are incredibly important for black bass spawning because their water temperature fluctuates much less and they offer protection for adults and fry.


After parental care is complete, both species of bass establish their summer range over an area with a variety of depths and structure. The three most important factors in determining this range are ambient water temperature, food quality, and cover availability. Bass thermoregulate (maintain a safe internal temperature) by moving to where the water is at their optimal temperature for growth. Generally, the optimal temperature for largemouth bass is 26˚C (79˚F) while the optimal temperature for smallmouth bass is 22˚C (72˚F). But how much does water temperature really affect bass movement in summer and fall?


A recent telemetry study performed by researchers at the University of Illinois Urbana-Champaign analyzed the movements of smallmouth and largemouth bass in a Lake Michigan harbor. Adult bass were caught out of the 3.5m (11.5 feet) deep harbor in early summer, measured and weighed, and then anesthetized and fitted with a radio or an acoustic transmitter. The scientists also deployed thermal loggers inside and outside of the harbor to track water temperature hourly. Their goal was to determine where and why smallmouth and largemouth bass move as things cool down and heat up.


The first thing the scientists noticed was that the summer ranges of both smallmouth and largemouth bass are much larger than their overwintering areas or nesting sites. Largemouth bass may travel up to 50 hectares, while smallmouth regularly inhabit home ranges of 100-300 hectares. Smallmouth bass were often found near offshore structure, but the largemouth bass tended to stay much closer to shore. Each species had a different response to water temperature as well.


The water within the harbor remained warmer over the course of the summer, with its temperature remaining more consistent than in the lake’s main body. In fact, the water temperature outside the harbor never reached the optimal temperature for largemouth. Unsurprisingly, the largemouth never left the harbor, as there was no thermal incentive to stray from the safety of the harbor.


The smallmouth bass, however, left the harbor as soon as the outside water temperature reached 18.5˚C (65.5˚F), which was notably cooler than their optimal temperature. This behavior could be explained by the larger home ranges of smallmouth bass in general; smallmouth have a drive to traverse greater areas of water even if it is energetically costly. Perhaps the two species evolved these traits – staying nearshore vs travelling as much as possible – to avoid competing against each other. But that’s a question for another study.


The only other likely explanations for these movements are that the smallmouth were travelling to follow food or to seek out better cover. However, the researchers found that neither the food nor structure availability changed over the summer, so they concluded that changing water temperature was the cause. In fact, a similar study performed on Lake Erie showed the same results; fish were unlikely to leave the warm effluent of a power plant until their optimal temperature was met in the surrounding water.


As the weather cools down again, it pays to focus on water temperature. Except in the hottest of conditions, largemouth bass will associate with nearshore structure. Smallmouth, though, are more likely to roam and to stay offshore until colder temperatures force them into warmer water.


Carter, M. et al. Movement patterns of smallmouth and largemouth bass in and around a Lake Michigan harbor: The importance of water temperature. Journal of Great Lakes Research: 38(396-401)


Originally published in Outdoor News.