A decades-long study led by researchers in the U.K. has found a sharp decline in the fertility of male dogs, and scientists say the findings could have worrisome implications for human reproduction.

Researchers from the University of Nottingham used microscopes to examine dog sperm from as many as 97 retrievers, Border Collies and German Shepherds annually between 1988 and 2014.

They found a decrease in forward moving, normal-looking sperm of 2.5 per cent per year on average from 1988 to 1998 and 1.2 per cent per year on average from 2002 to 2014.

The researchers also observed a growing incidence of the birth defect Cryptorchidis (undescended testes) in canines over the course of the study.

Richard Lea, a reproductive biologist at the University of Nottingham's School of Veterinary Medicine and Science, said he believes the decline in sperm quality is caused by environmental contaminants that were detectable in the dogs' sperm and some dog foods.

But it's not only dog-owners who might be worried, considering that humans and dogs share a similar environment, according to lead researcher Lea.

"While further research is needed to conclusively demonstrate a link, the dog may indeed be a sentinel for humans,” he said, according to a press release.

For decades, studies have suggested the quality of human semen is decreasing, while male infertility, testicular cancer, and birth defects like hypospadias (misplaced urethra) and Cryptorchidism appear to be increasing, according to the researchers.

The paper, "Environmental Chemicals Impact Semen Quality in Dogs in Vitro and May be Associated with a Temporal Decline in Quality and Increased Cryptorchidis," is available online in the journal Scientific Reports.