Passive- and active-acoustic properties of a spawning Atlantic cod (Gadus morhua) aggregation

A spawning aggregation of Atlantic cod (Gadus morhua) was observed at depths of 40-50 m with passive- and active-acoustic sensors at the Bar Haven grounds in Placentia Bay, Newfoundland, in April 2003. A hydrophone was positioned on the seabed beneath the aggregation, while a 38-kHz split-beam echosounder was moored at the sea surface above it for 18.5 h. Ten grunts were recorded with peak frequencies ranging from 30 to 250 Hz and durations of nearly 300 ms. These grunts are similar to the sounds recorded in the presence of captive, spawning cod from the same substock. The echogram reveals that cod exhibit diel, vertical-migratory behaviour, densely aggregating near the seabed by day and forming columns that extend approximately halfway to the surface at night. This is the first study to demonstrate that cod produce sounds and form columns while migrating vertically during night-time spawning.