This Canadian animation about a plucky little beluga whale in a hurry to grow up feels less like all-ages family entertainment, and more of a cartoon aimed solely at very small kids. It’s a fairly tepid and gentle-going adventure, except for some bizarrely PG-plus polar bear v killer whale action near the end.

Our pint-sized hero is Katak, a young male beluga living with his pod in a river in the Arctic. A late developer, Katak should have turned white and swum off to live with the adult males. Instead, the poor little guy is stuck at home, teased for his stubbornly grey skin. To prove himself, he runs away to find his famously brave grandfather up north. Perhaps belugas don’t have the looks for screen work, but there is something a bit unnerving about the way the animators have had flap their mouths open to speak; it’s more creepy than cute. The wacky supporting characters are more fun: a sturgeon with a large schnozz called Cyrano and a vegan orca – whose dad is the villain of the piece, killer whale Jack Knife.

As an under-the-sea adventure, the comparison with Finding Nemo is inevitable – and possibly a bit unfair. Still, there’s no avoiding the fact that Katak’s comedy is less witty than Nemo’s, the oceanic world less dazzling. The strongest images here are some beautifully wintry shots of the Canadian Arctic. There’s woozy elemental undersea music, too, that sounds as if Iceland band Sigur Rós have made a yoga album.

The environmental message at the heart of the story is – of course – climate-based extinction. The water is getting warmer, and the females in Katak’s pod have stopped giving birth. Noise from boats – “floaters” the whales call them – is deafening. But the eco story never comes together properly with Katak’s odyssey. As a result, the storytelling sags – until it perks back up with that polar bear on killer whale fight to the death, which may have a Watership Down effect on tiny ones.

• Katak: The Brave Beluga is released on 11 August in UK cinemas.