The Tasmanian-based salmon and seafood company reported net profit after tax surged by 24.3 per cent to $40.8 million in the six months to December 31, compared to the same period a year earlier.

That was despite a 15.8 per cent drop in revenue to $274.5 million.

Tassal said the reduction in sales was mostly in the lower value export market.

It said harvesting and selling less salmon in the half-year supported greater biomass and salmon size and was expected to lead to increasing harvest and sales in the current half-year and in 2020-21.

Managing director and chief executive Mark Ryan said Tassal had a strong growth outlook.

"Positive market dynamics, growth in salmon biomass and size and the prawn ramp-up is expected to drive increased financial year 2020 earnings and beyond," Mr Ryan said.

He said salmon biomass growth was exceeding expectations in a buoyant market.

"We continue to transition supply towards the domestic market with a focus also on more profitable product lines," he said.

"The export market will continue to be strategically used to support growing and production efficiencies, with surplus stock exported."

He said the company was looking to get the best out of existing leases and working to reduce growing costs.

Tassal bought the Sydney-based De Costi Seafoods business in 2015 and the Fortune Group prawn business in Queensland and New South Wales via De Costi in 2018 for $31.9 million.