AITKIN, MN (AP) -

The number of zebra mussels in Lake Mille Lacs has dropped this year for the first time since they were found in 2005.

Minnesota outdoors officials say a survey this week found about 1,070 zebra mussels per square foot. That's down by about 200 from a year ago.

Department of Natural Resources treaties manager Tom Jones says it's typical for an invading species to ramp up in big numbers and then fall back a bit. Jones says in other places invasives have often dropped by as much as a third after initial expansion.

Jones tells Minnesota Public Radio that researchers will need more data next year to see if this year is the start of a decline or a one-time thing.