Seen in an April photo, the Calgary Humane Society's Lindsay Jones holds Shadow, a dog that was given up by its owner. (CBC)

Calgary's animal shelters are overrun with an unusually high number of cats and dogs.

The already crowded Calgary Humane Society is currently housing more than 470 cats and 90 dogs when it usually has about 300 cats and 60 dogs at this time of year.

Given the lack of space and the small number of adoptive families, the animals will have to be euthanized.

"Right now, unfortunately, the cats have about a 10-day to two- week time period in adoptions before we need to make room for the hundreds of cats that come to us on a weekly basis," said the society's Lindsay Jones on Tuesday.

Jones said people are dropping off whole litters at one time instead of just one pet, because they didn't bother to spay or neuter them, and now there are too many mouths to feed. It is also peak season for kittens.

Some owners are also abandoning animals after struggling to cover the costs of keeping a pet.

The City of Calgary's animal services shelter is taking care of 70 cats and 44 dogs right now — an increase of 133 per cent and 233 per cent, respectively.

The summer is often a bit busier because that's when people are on holidays or have pet sitters and their pets go missing. But Bill Bruce, the city's director of animal and bylaw services, said it still doesn't explain this year's spike.

"Typically, in the summer vacation time, we do go up a bit, but what we're seeing is an inordinately high number," he said. "And we're seeing a real slowdown in that return to owner. We're not finding the owners, and we're usually very successful at [that]."

He said it's never too late to look for a lost pet through the city's website, by calling 311 or at the shelter facilities themselves.

Pets that are not claimed can become available for adoption after about four days.

Bruce hopes the city's new free spay and neuter clinic — set to open on Aug. 26 — will help reduce the numbers of strays.

"To make a difference in a city this size, we'll have to be doing literally thousands of animals a year to bring that population down," he said.

Bruce reminded pet owners to get their animals licensed, not only because it's city bylaw but also because it helps owners locate lost pets.