"Admiral, there be whales here!" Scotty's [James Doohan] famous phrase would not have worked if producer Harve Bennett had gotten his way for Star Trek IV: The Voyage Home. In a very early idea for the story, according to Startrek.com, Bennett wanted to focus on a tiny creature called the "snail darter." Formerly listed as a endangered species until 2022, this fish was no bigger than 3 1/2 inches long, and that's why Bennett wanted the probe to be focused on them.
Bennett liked the idea of something so small being so vital to the salvation of Earth. Of course, it would have also been a great money-saver, but one couldn't imagine there would be much drama or humor trying to capture snail darters. There would certainly have been no need for the huge tank inside the Klingon Bird of Prey as the fish could have been carried aboard the ship in a cup.
Fortunately, director Leonard Nimoy, who wanted whales, got whales. Fortunately for us all. Can you imagine Scotty saying, "Admiral, there be snail darters here?" The size of the whales played a huge part in the movie, with Nimoy's character, Spock, swimming alongside one of them so he could mind-meld. The sheer size of the whales alone set the stage for some tension aboard the Klingon ship when the Enterprise crew was trying to get back to their time in the 23rd century.
Nimoy wanted the movie to focus on biodiversity and how losing a species could affect the planet whereas Bennett liked the notion that a big impact could be made by something so small. But I can't imagine The Voyage Home without George and Gracie, Without them and the humor associated with bringing them back to modern-day Earth, the movie would have crumbled. Thank goodness Nimoy was able to call the shots on this one!