ST. PAUL, Minn. (AP) — The Minnesota Department of Natural Resources is adding more canine inspectors to its enforcement arsenal to help stop invasive zebra mussels from spreading in waterways.

The agency began with two dogs four years ago, and it's adding two more dogs this year.

The new dogs are German Shorthair Pointers. Shelby, a female, will patrol the Twin Cities, and Storm, a male, will be based in northwest Minnesota.

K-9 unit coordinator Jason Beckmann says it cost $13,000 to buy and train the two new dogs. The expense was covered by a $25,000 federal grant.

The canines are trained to smell zebra mussels on boats. Beckmann says the dogs sit down to indicate that they've found an odor of zebra mussels, and then they get a reward.