The behaviour of each fish was videotaped by an underwater
camera SONY VIDEO 8 — TR805 (10×) for a total of 10 min (5 min of
silence and 5 min of playback). Before starting the field video
recordings we set a distance, near enough to get sufficient
information, overcoming the problem of poor water visibility, but
not too close to disturb the fish. As we worked in a protected area
where fish are quite acquainted with SCUBA-divers, this shortened
their flight-distance. Eventually a distance of 1.3–1.5 m from the nest
was the best range. Ten minutes of acclimatization with camera and
loudspeaker turned off occurred before each playback trial; this
allowed the fish to habituate to the apparatus. In addition, previous
observations indicated that the whole recording and projecting
system did not have an over-riding effect on the fish behavioural
response (Picciulin, 2004).