A subtidal-zone (<10 m) system for sound transduction, acquisition, and archival was developed to study and monitor harbour seals. In developing the transduction system - a multi-channel hydrophone array - several engineering challenges were solved: cable armour was applied and smooth low-noise hydrophone cages were designed to protect against animal bites. Helical anchors were used to attach the cable to the sandy bottom; they are adjusted as necessary when sand is moved by currents. The cable was weighted so that it sank into the sand to prevent cable strum from wave action. The acquisition and archival system had several engineering goals: simultaneous multi-channel sound acquisition; changeable sampling rate; high data rate (200 Kbyte/s); large storage volume to allow continuous long- term monitoring; configurable recording times. The system developed was a PC with a multi-channel data acquisition board (SignaLogic), custom software that allows automatic reconfiguration (channels recorded, sampling rate, etc.) at preset times, and a high-capacity (4 Gbyte uncompressed) data DAT drive. The tape drive software allows acquired data to be accessed directly as computer files, obviating a separate re-acquisition step. The acquisition system compares favourably to multi-track tape recorders in price and very favourably in recording capacity.