Nesting smallmouth bass

I finally got back in the water for some early summer snorkeling. The surface of the lake was rather murky from all the pine pollen that’s in the air right now, but the water was relatively clear below and so visibility was somewhat decent. And now that the water’s above 60º, the smallmouth bass have all come up for the spawn and so I got to spend some time with my old friends!

Around mid-May when temperatures in the lake hit the upper 50s, smallmouth bass come up from deeper water to spawn in the rocky shallows along Lake Champlain (they also spawn throughout our river systems). The males create nests by fanning their tails against the substrate and sending up a blast of water that clears off all the silts, clays, and other detritus that has accumulated over the winter (for visual reference, check out female Atlantic salmon clearing out nest sites: video). The exposed rocks of the nest are a bright contrast against the dull brown backdrop of the rocky lake bottom, which makes these nest sites visually obvious (and attractive) to female bass and intrepid naturalists alike.

After the female deposits her eggs in the male’s nest, the males become the sole caregiver for the young, fastidiously defending the nest with constant vigilance from egg to fry. The below 37 second timelapse video was taken over about 15 minutes of real time, and shows the males circling over and over again (about 30x), occasionally leaving the nest to chase away potential threats. The males do this hour after hour, day after day, night after night for weeks.

While most males came up to investigate me, one larger male (~20″ long) actually made physical contact with me and repeatedly harassed me until I left the area (video of the aggressive male). It’s hard to draw any firm conclusions from today, but the larger males tended to be more aggressive around nests than smaller males, as has been noted in the more detailed observations that indicate that females seem to prefer larger males for this reason (source).

The energetic costs of defending nests is significant. Defending males are active day and night, and their metabolic rate is around 60% higher than non-defensive males (source). Not only does nest defense take more energy, but it also precludes males from feeding. As a result, nest guarding males take in almost no energy during the 2-4 weeks they spend defending their young! Some males will abandon nests when the energetic costs become to severe a cost to maintain.

I snorkeled the stretch of shoreline between Little Eagle Bay at Rock Point and the Thrust Fault (see map below), which is about a third of a mile, and did a pretty thorough survey of all the nest sites along the way. I tallied 39 active nests (sites that had eggs and/or a defending male) and another 6 sites that had been cleared out but that didn’t have eggs and/or weren’t occupied. This density (~80 nests/km) seems to be roughly on par with other estimates of nest density in lakes (river ecosystems seem to have higher densities, up to 150 nests per kilometer of river: source). All the nests were in water less than about 8′ deep and the shallowest one was in just 2′ of water and was easily visible while I was standing in the water (video below)! 

Map showing my somewhat scientific transect. About 35 of the 39 nests were in the top off of the transect.