Guernsey Police said they want to speak to anyone who saw what happened, in the area around the reservoir at 10:30 last Friday morning, 4 January.
The two dogs which attacked the other were grey Schnauzer types, and they were with a man in his 60s.
Witnesses should contact PC Hopkins on 725111 or via Crimestoppers anonymously on 0800 555 111.
Pictured: St Saviours Reservoir.