The trailer was taken from a fish processing firm in Damhead Circle in Peterhead, Aberdeenshire, between 10pm and 11pm on Sunday.
It has since been recovered, with the shellfish missing, in Bolton, Lancashire.
The white Peterhead Transport trailer was stolen using an HGV unit believed to be white and blue with a grey stripe, Police Scotland said.
Detectives are appealing for witnesses and are also looking for anybody with information regarding a white or silver car which they believe could be connected to the incident as it was seen in the area at the time and in convoy with the lorry.
Detective Constable Julie Wilson said: "If anybody recognises the description of the trailer or the tractor unit and believes they may have seen it travelling in and around the Aberdeenshire area late on Sunday night and possibly into the early hours of Monday morning, they are urged to get in touch with us.
"It is understood that both the tractor unit and the white or silver Volkwagen Golf may have travelled from England and are likely to have used service stations both north and south.
"If anybody believes they have seen either of these vehicles in convoy or believes they may have seen them in the Aberdeenshire area at the time of the theft, then we would ask that they contact us on 101 or anonymously via Crimestoppers on 0800 555 111."