Nearly 40 percent of registered investment advisory firms say they plan to add additional staff within the next 6 months, according to the latest Advisor Index Survey released by TD Ameritrade Institutional.

RIAs surveyed reported an average annual revenue growth rate of 15 percent and indicated that 57 percent of new client assets continue to come from full-commission firms, up from 55 percent in 2011.

To maintain momentum, many RIAs say they're making the move to add staff to help sustain this growth. Nine in 10 advisors report the total number of clients has increased or remained steady over the past year. Of those firms looking to hire, 25 percent said that they will look to hire a female advisor to attract and retain female investors.

Also of note: Nearly 80 percent of advisors have experienced no turnover in their offices in the past year.

Hiring and firing staff led the list of top human resources challenges advisors say they face today, while developing and training staff also ranked high. Compared with 2011, more advisors report increased spending on human capital efforts, including professional development (43 percent), staffing (40 percent), and salaries and bonuses (50 percent). 

"Adding staff is a very important process for each advisor," said George Tamer, director, strategic relationships, TD Ameritrade Institutional. "The survey mirrors what we've been hearing from advisors, who say successfully cultivating new staff in an advisor's business can be a difficult task, but in the end, it is essential for growth."

The survey was conducted by St. Louis-based Maritz Inc. on behalf of TD Ameritrade Institutional; a division of TD Ameritrade Inc. Martiz interviewed 502 RIAs in a telephone survey from March 29 to April 9. Independent RIAs who custody with TD Ameritrade Institutional, as well as other independent RIAs from across the country, were queried on the economic outlook for their firms and the advisor market in general. TD Ameritrade Institutional provides brokerage and custody services to over 4,000 fee-based, independent RIAs and their clients.