The Disney-owned animation studio Pixar has taken the rare step of changing the plot of its forthcoming sequel to the 2003 blockbuster Finding Nemo, according to a New York Times report, following claims about a US marine park in the critically acclaimed Sundance documentary Blackfish.

Finding Dory will feature the voice of Ellen DeGeneres, reprising her award-winning role as the friendly but forgetful fish Dory from the first film. The original co-director Andrew Stanton is taking the reins once more, with the movie due in cinemas in 2015.

According to the New York Times, Finding Dory's screenplay features an ending involving a marine park. It is here that the storyline has been changed following claims made in Blackfish, directed by Gabriela Cowperthwaite, that SeaWorld, which has parks in Orlando, Florida, San Diego, California and San Antonio, Texas, was responsible for the deaths of three people, owing to its policy of keeping killer whales in captivity. It is understood that the new ending for Finding Dory retains the marine park, but makes it clear that the movie's aquatic heroes are free to leave at any time.

Blackfish, which was released in the US last month, centres on an orca named Tilikum which, it claims, has been involved in three fatalities since being captured in 1983 off the coast of Iceland. These include the death of animal trainer Dawn Brancheau at SeaWorld Orlando in 2010 after she was pulled into a tank and mauled. SeaWorld has denied the accusations and accused Cowperthwaite of producing an "inaccurate" and "misleading" film which "exploits a tragedy that remains a source of deep pain for Dawn Brancheau's family, friends and colleagues".

Finding Nemo took more than $800m (£516m) worldwide on its original release and was Pixar's highest-grossing film prior to Toy Story 3 in 2010.

Disney has made no public comment on the New York Times's claim about the forthcoming film's altered plotline.