Sainsbury's has reported it has given away six tonnes of sustainable fish as part of its Switch the Fish initiative, with rainbow trout emerging as the UK's favourite 'alternative fish'.

Under last week's sustainable fish drive, Sainsbury's offered free portions of sustainable fish to shoppers asking for cod, haddock, tuna, salmon or prawns at its fish counters.

Of the six alternative species offered, rainbow trout accounted for 22% of all free fish given to shoppers, closely followed by hake (21%). Despite recent high-profile campaigns to boost mackerel consumption in the UK, mackerel was chosen least often, accounting for just 11% of free fish ­given away.

The initiative, which took place on 17 July, also boosted fish sales in Sainsbury's more generally, with fish counter sales on the day up 12% week-on-week. A Switch the Fish van is currently on tour around the UK.