Importance of Diurnal and Nocturnal Nest Defense in the Energy Budget of Male Smallmouth Bass: Insights from Direct Video Observations

Reproductive behavior has never been considered an important contributor to overall activity costs for fish and is usually ignored in fish bioenergetics models. We provide evidence that parental care by male smallmouth bass Micropterus dolomieu substantially affects both activity costs and food intake. We used timelapse underwater video with infrared illumination to monitor nests of male smallmouth bass continuously for nearly 2 weeks from oviposition to swim-up of larvae, and we discovered that unlike nonnesting bass, nestguarding males were active during both night and day. Guarding males constantly sculled their caudal and pectoral fins (swimming in place) while on nests. We used caudal fin beat frequencies to relate the costs of swimming in place to those of nonguarding swimming. Combining these estimates with predictions of a respiration model for largemouth bass M. salmoides, we estimated that the metabolic rate of guarding smallmouth bass was up to 60% greater than that of nonguarding individuals. Energetic costs of nest guarding are exacerbated by food deprivation; the males we observed were unable to obtain appropriate daily rations while nesting. One of the guarding males disappeared prematurely under circumstances that suggested nest abandonment because of energetic constraints.