Bioenergetics Insight into Black Bass Distribution Shifts in Ozark Border Region Streams

Population densities of smallmouth bass Micropterus dolomieu (SMB) have declined in streams of the Missouri Ozark border region since the 1940s while replacement by largemouth bass M. salmoides (LMB) has occurred in some cases. A recent field study found that two habitat variables, known to have been influenced by human activities, largely explained present densities of SMB and LMB throughout streams in this region. Densities of SMB declined with increasing maximum summer temperature (range, 23–33°C) and percent pool area while LMB densities increased with these variables. To explore these correlations from a bioenergetics perspective, we determined maximum consumption rates of SMB and LMB at 18, 22, 26, and 30°C. Consistent with the field study's findings about temperature, maximum consumption results indicated that SMB scope for growth becomes progressively restricted at temperatures higher than 22°C, whereas this does not occur until 26°C for LMB. Maximum consumption rates of SMB also averaged twice those of the LMB, indicating a much greater per capita demand for prey biomass by SMB and suggesting SMB have a lesser capacity to tolerate prey base decline than LMB. The higher consumptive demand by the SMB may relate to the field study's finding that SMB density declined with increasing pool area. Increases in pool quantity are accompanied by reduced food production and the loss of prey types known to be of importance to SMB. Findings indicate that changes in growth conditions for SMB and LMB may be a proximate cause for shifts in distribution of black bass Micropterus spp. in Ozark border region streams.