Loading Herring at Ullapool in 1967

This picture shows men loading herring onto a lorry in Ullapool, Scotland and was taken sometime in 1967.

The title written on the slide index card by the photographer is 'Loading Herring at Ullapool' and when I read that I expected to see a boat unloading fish. In this picture however, there is no boat but two lorries, so it made me think about what is actually happening here.

On the lorry in the foreground it has a name, 'D.A. Macre Ltd, Fraserburgh & Hull' and from what I can make out, that company was a big fish and seafood company based in Fraserburgh, and is still a running company, although it's now been taken over by another company. I assume, from the picture, that the fishing boat unloads on the dock to a local processing plant which has loaded the first lorry, and then this second lorry is being loaded from the first? There are certainly men on both vehicles, and boxes are passing from one to the other - although the boxes seem to be empty. Possibly, this is not a picture of Loading herring at all but a collection of empty boxes to be returned to the processor? Anyone who can give more details on this process please let me know in the comments below.

Although I know the date is 1967, there are no details regarding the month or day in 1967. It looks fairly grey and damp, but truthfully in the UK that could be any time of year.