E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Kettle of Fish.
A fête-champêtre in which salmon is the chief dish provided. In these pic-nics, a large caldron being provided, the party select a place near a salmon river. Having thickened some water with salt to the consistency of brine, the salmon is put therein and boiled; and when fit for eating, the company partake thereof in gipsy fashion. Some think the discomfort of this sort of pic-nic gave rise to the phrase A pretty kettle of fish. (See KITTLE OF FISH.)
The whole company go to the waterside today to eat a kettle of fish.Sir Walter Scott: St. Ronans Well, xii.