MAYVILLE — The county’s investment portfolio remains strong in spite of havoc throughout the U.S. financial sector. As of December, the county earned $4.6 million off its investments in 2008, money that offsets property taxes. That was an $866,000 increase over the previous month as the county’s investment portfolio remains relatively shielded from the financial turmoil that has left so many investors with huge losses following Wall Street’s meltdown. The county’s investment insulation is largely thanks to a decision made years ago to invest in a certain type of security, according to Darin Schulz, county finance director. ‘‘One thing that’s...