BERLIN — A longtime invasive weeds issue at a local pond was believed to be responsible for killing the pond’s fish population, officials announced this week.

“We are taking this very seriously,” town officials said in a statement Wednesday.

Officials are conducting tests to determine the cause of death. Officials did not indicate an estimate of how many fish were killed.

Officials said the town has been trying to address invasive weeds — water chestnut, watermeal, duckweed, phragmites and algae — that started to take over Paper Goods Pond, Railroad Pond and Veterans Park for about a year.

If these weeds, some of the most invasive to be found in a body of water, are left untreated, they can continue to grow at alarming rates, officials said.

With ponds, especially where the water is less than 10 feet deep, these invasive weeds can completely take over and cause stagnant water that could start to smell, officials said. They deprive the water of oxygen, killing off species that need it to survive like fish, officials said.

The town said it weighed many options after conversations with state Department of Energy and Environmental Protection and experts in the field over the last year.

Among the choices was the option to mechanically remove the plants — but it turns out the water is too shallow on one side of the pond to get the needed equipment in place. Another suggestion was to remove the plants by hand, but the infestation is too severe. The only option left was herbicide, town officials said.

The town hired a company in May to apply for all the needed permits to apply herbicide to Paper Goods Pond. Officials said that paperwork was obtained by late August.

“The vendor applied the herbicide by boat at less than half the recommended rate the permit was approved for,” officials said.

The town was notified by the state Department of Energy and Environmental Protection about the fish kill on Monday.

“During the conversation with the DEEP officer we informed him that we had hired a company to apply herbicides to the pond,” officials said. “The officer then contacted the vendor regarding rate of application, temperature of the water and proper posting of signage.”

Officials said DEEP investigated and determined the fish were killed by the “decomposing plants sucking up all the oxygen,” but will still do an autopsy to determine the exact cause.”