Summary
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Whales help with ship navigation, and intelligent whales are part of Cetacean Ops.
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Cetacean Ops divisions existed on known Starfleet ships in the Star Trek universe.
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The concept of Cetacean Ops was born from Rick Sternbach's obsession with dolphins going to space.
The crews of Starfleet ships i Star Trek universes are as diverse as they come. Humans of all nationalities and ethnicities serve alongside hundreds of other species from across the galaxy. But humanoid life forms are not the only life forms in Star Trek universe aboard Starfleet ships. Some ships in the fleet also have divisions manned by intelligent animals. One such division is known as Cetacean Ops.
According to the Merriam-Webster Dictionary, the word Cetacean refers to “any of an order of aquatic mammals (such as a whale, dolphin, or porpoise).” As the name suggests, Cetacean Ops is a division of a Starfleet ship manned and operated by such aquatic mammals, usually whales or dolphins. These sea-dwelling Starfleet officers are usually housed in an aquarium aboard the ship they serve on, working alongside a team of marine biologists responsible for their care.

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Why are there whales on Starfleet ships?
In short, the whales aboard Starfleet ships help the humanoid crew members navigate the ship. The fourth Star Trek The Original Series Film, Star Trek IV: The Voyage Home, introduced the idea that cetaceans, especially cetaceans, are highly intelligent species that established contact with alien species long before humans did. But whales were extinct on Earth in the 22nd century. So, Captain James T. Kirk and his crew set out on a mission to save the whales, and the whales survived into the 24th century.
An in-universe reason why Starfleet decided to recruit whales to aid in guidance and navigation is never mentioned. However, in the 24th century, Cetacean Ops (sometimes called Tursiops Ops) was a regular division on Galaxy Class starships, according to Star Trek: The Next Generation Technical Manual. This book says:
“In Galaxy-class starships, ongoing G&N [guidance and navigation] systems research tasks are handled by a mixed consulting crew of twelve Tursiops truncatus and T. truncatus gilli, respectively, Atlantic and Pacific bottlenose dolphins. This herd is watched over by two more whales, Orcinus orca takayai, or Takaya's whale. All theoretical subjects in navigation are studied by these elite specialists, and their recommendations for system upgrades are implemented by Starfleet.”
Several of Starfleet's most famous ships had Cetacean Ops divisions, including Enterprise-D, Enterprise-C, Cerritos, and Voyager-A.
The theoretical obsession that became whales Ops
In the real world, the idea for Cetacean Ops was a collaboration between Star Trek: The Next Generation's senior production illustrator and designer, Rick Sternbach, and the show's scenic art supervisor, Michael Okuda, who later co-wrote Star Trek: The Next Generation Technical Manual.
In an interview with the art magazine 70s Sci-Fi Artrevealed Sternbach that he was obsessed with the idea of dolphins in space. Sternbach grew up loving science fiction and art, and when he entered college at UConn, he decided to pursue a degree in art. At the same time, he discovered a passion for marine biology. His love of sci-fi, aquatic creatures and art culminated in one of his most famous projects: an illustration of a dolphin in a space suit. Sternbach explained that the illustration was his way of answering a question he had become obsessed with – how could dolphins safely go into space with humans?
This question stayed with Sternbach for several years. When he found himself working on TNGhe decided to make his fantasy of dolphins in space a part of Star Trek cannon. He slipped subtle references to the “Tursiops Crew Facility” in the first season episode “We'll Always Have Paris.” Later, the third season episode “Yesterday's Enterprise” and the fifth season episode “The Perfect Mate” contained brief references to Cetacean Ops.
Although Sternbach desperately wanted to bring Cetacean Ops to the screen, the production constraints of the late 1980s and early 1990s meant that Sternbach never got to see his idea in action while working on LPG, or later Deep Space Nine and Voyager.
Cetacean Ops in action
But Sternbach finally got to see his dream of Cetacean Ops on screen in the Season 2 finale of Star Trek: Lower Decks. Although Cetacean Ops was frequently referenced in the first and second seasons of Lower deckthat part of the ship and its crew members were not actually shown until the episode “First First Contact.”
In the episode, Ensign Brad Boimler had to swim to the bottom of the dolphin tank in Cetacean Ops to manually release a lever that was crucial to saving Cerritos. Arriving at Cetacean Ops, fans met Lieutenants Matt and Kimolu, two beluga whales who were instrumental in guiding Cerritos as they traveled at warp speed. The episode explained that without them the ship would not be able to go to warp. After their first appearance, Lieutenants Matt and Kimolu appeared in five more episodes of Lower deckand helped save Cerritos on several occasions.
Cetacean Ops also played a major role in Star Trek: Prodigy. Voyager-A was equipped with a huge Cetacean Ops division that housed a humpback whale named Gillian. Without Gillian, Voyager-A and its crew would not have survived. When all navigation systems on the ship went down, Gillian and Rok-Tahk led the ship through the loom and a wormhole.
Because of Sternbach's dream and Mike McMahan's fascination with every obscure detail of the Trekverse, Cetacean Ops lives on as an important part of Starfleet. What began as one man's obsession with dolphin space travel has become a fundamental part of Star Trek canon, which might be the most Trekkie thing that's ever happened.
Sources: Star Trek: The Next Generation, Star Trek: Lower Decks, Star Trek: Prodigy, Star Trek: The Next Generation Technical Manual, 70s Sci-Fi Art Interview