TOLEDO, Ohio (AP) — The operators of three agriculture businesses have been told to pay more than $30,000 for three large fish kills that Ohio’s natural resources department says were caused by livestock manure spread on fields.

Investigators think ammonia-laden manure put onto the fields in northwestern Ohio ahead of rainstorms in August washed into creeks and caused the fish kills.

An Ohio law put in place to combat algae in Lake Erie prohibits farmers from putting manure on fields before heavy rains because the manure also contains phosphorous that feeds algae.

State officials say the manure spills killed close to 67,000 fish — including minnows and sunfish — in creeks in Williams, Allen and Hardin counties. They say the biggest spill killed 37,000 fish near Delphos in early August.

The Ohio Department of Natural Resources said it finished its investigations into the fish kills and sent letters last week to three people it found responsible.

The fines were issued to Dan Wagner, of Kenton; David Youngpeter, of Spencerville; and Mike Bockey, of Delphos. Messages seeking comment were left with all three on Wednesday.

The state said in the letters that it could seek higher penalties if the three parties challenge the fines in court.

Officials have said that the manure spills did not cause any long-term damage in the creek and that fish had returned within weeks.

The fish kills again put a spotlight on questions about how farmers are disposing of manure and the impact that has on waterways, especially in the Lake Erie region.

Ohio, along with Michigan and the Canadian province of Ontario, have agreed to sharply reduce the amount of phosphorus from farm fertilizers, livestock manure and sewage treatment plants that flows into the lake’s western end within the next 10 years.

But many environmental groups argue that the state needs stricter rules on disposing manure and that voluntary efforts are not enough. Agriculture organizations in the state have been leading efforts to educate farmers about how to properly use fertilizer and manure.