(ShareCast News) - Investment manager Rathbone Brothers posted an increase in third quarter operating income, though it warned the economic environment remained challenging.In the three months to 30 September, net operating income rose 13.4% year-on-year to £55.6m, while funds under management, a crucial driver of earnings, declined 2.4% to £27.6bn.The FTSE 250 group said fees received rose 19.3% from the corresponding period in 2014 to £36.4m, while net interest income edged 1.7% higher to £2.7m and commissions fell 8.5% year-on-year to £8.6m."Although our outlook remains cautious in light of recent markets, our agreement in early October to purchase the remaining 80% of the Vision group is an example of the investment we continue to make to take advantage of future growth opportunities," said group chief executive Philip Howell.Rathbone shares were down 1.31% to 2,115.00p at 0856 BST on Thursday.