The States police are now investigating following the incident, involving two Manx Loaghtan lambs that grazed on land owned by the National Trust near Devil's Hole.
Shepherd Aaron Le Couteur, who said that he put his 'heart and soul' into looking after the large flock, said that the incident was 'incredibly frustrating'.
'Because there is an eye-witness the police are investigating, using the leads that they have available, and there is visual evidence,' he said.
Mr Le Couteur said that about 90 per cent of dog owners were responsible and had 'exactly the right attitude', but he added: 'There is also the ten per cent who are a lot more difficult to get through to.'
He added: 'It is pretty obvious. There are signs on the gates as you walk in warning you that there are lambs on the site and to tell people to put their dogs on leads.
'We are trying to keep this open as a public space rather than restrict people from using it.
'What we have got to try to get across is that these are living, breathing animals and rearing them is no mean feat.'
Under the Dogs (Jersey) Law 1961, it is an offence for dogs to chase or worry livestock or to not be under proper control. Anyone in breach of the law is liable to a fine of up to £1,000.