As a reaction to lagging attendance, tumbling shares and a backlash of negative publicity in the wake of the controversial documentary Blackfish, Seaworld has announced multi-million dollar expansion plans for its habitats housing killer whales.

The park’s significant drop in share value last week has seemingly been the final straw for the operator, which has been increasingly a target for animal rights activists following the release of a 2013 documentary recounting the violent behaviour of an orca “driven to madness” in captivity, leading to the death of trainer Dawn Brancheau in 2010.

The orca tanks will be upgraded at three SeaWorld theme parks, starting in San Diego, with the new enclosures almost doubling in size from the existing 5 million gallon (19 million litre) tank, increasing in depth by an additional 50ft (15.2m).

SeaWorld would not reveal exact expansion costs, but did say the work would cost “several hundred million dollars”. The company has also pledged a further US$10m (€7.5m, £6m) in matching funds for research into killer whales.

Construction will begin at SeaWorld San Diego in 2015, with a completion date of 2018 and similar upgrades to follow at parks in Orlando, Florida and San Antonio, Texas.

The San Diego facility will include an innovative new “water treadmill” – a system which allows the whales to swim against a current, allowing more exercise while opening the door for research into how they burn energy.

In a statement, animal rights group PETA dismissed the plans commenting: “A bigger prison is still a prison”, while SeaWorld CEO Jim Atchison rebuffed the comments, stating: “Unfortunately there are some people who want nothing more than to see the end to the relationship between humans and animals, and that would be a sad outcome."

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