Changes in the metabolic rate (oxygen uptake) of damselfish juveniles were measured and compared among six experimental treatments crossing the presence of a mesopredator (2 levels: dottyback, goby) with a top-predator (3 levels: coral trout, thicklip wrasse, empty tank). Behavioural observations were recorded both on the damselfish juveniles and the mesopredators (dottybacks/gobies). Eight to nine replicate trials were undertaken for each treatment.
- Metabolic and behavioural assessment
Measures of metabolic rate (oxygen uptake; mg O2 h-1) and activity (line crosses) were estimated for each damselfish juvenile. Routine metabolic rates were estimated as the mean level of oxygen uptake in the 1 h before and after exposure to the predator cues. Rates during each closed phase were calculated using linear least-squares regression, excluding the first and last minute of each closed phase. In total, 6 measures of oxygen uptake were collected per hour and used to calculate the oxygen uptake of each prey damselfish. Activity was measured by quantifying the number of times the damselfish juvenile crossed five equidistant lines that transversally divided the respirometer chamber into six 1.8 cm-width sections. Line crosses were only assessed during the first 10 min of the pre- and post-stimulus 1 h periods selected for metabolic analysis. Pilot observations showed this sample period (10 min) was representative of the activity of the damselfish during the correspondent hour.
Three behavioural attributes of the mesopredators (dottybacks/gobies) were quantified from the 10 min post-stimulus period: (a) time spent inside shelter (min); (b) time spent near the respirometer containing the prey (min); and (c) number of strikes to the respirometer. The time near the respirometer included all of the time the mesopredator was closer than one body length from the chamber, while the number of strikes considered all of the attacks in which the mesopredator hit the chamber with its mouth.