Static Stability and Swim Bladder Volume in the Bluegill Sunfish (Lepomis macrochirus)

Abstract

The static stability of a fish in the water is determined by the relative locations of its center of mass and center of buoyancy. These locations may not be constant, but may depend on the volume of the swim bladder. Changes in swim bladder volume affect both the distribution of mass and the overall volume of the fish, thus affecting the location of the center of mass and center of buoyancy, respectively. To determine the static stability and to examine the influence of the swim bladder on static stability, we used micro-computed tomography to estimate the locations of the center of mass and center of buoyancy in bluegill sunfish (Lepomis macrochrius). In fish oriented head up in the scanner, we found that the center of buoyancy is located 0.441 body lengths from the snout and 0.19 body lengths above the ventral surface of the pelvic girdle, and that the center of mass is 0.0012 BL posterior to and 0.00045 BL ventral to the center of buoyancy. Swim bladders from our specimens ranged in size from 1.9% to 7.6% total body volume, and we found no correlation between swim bladder volume and the distance between the center of mass and center of buoyancy. However, in fish scanned in a head-down orientation, the center of mass is anterior and dorsal to the center of buoyancy--the opposite configuration of that found in fish oriented head up. This change in center of mass and center of buoyancy seems to be caused by changes in the location of the swim bladder in the body: the centroid of the swim bladder is located more posteriorly in fish oriented head-down. The air in the bladder "rises" while heavier tissues "sink," driving a change in tissue distribution and changing the location of the center of mass relative to the center of buoyancy. We conclude that while static stability does not change with swim bladder volume, pitch angle could have a marked effect on static stability.