Advisors are optimistic about the growth of their firms in 2016, but worry about their aging clients, according to a recent study.

Seventy-nine percent of RIAs project continued growth in assets under management in 2016, predicting that firm assets will increase by 17 percent on average this year, according to the TD Ameritrade Institutional RIA Sentiment Survey released on Wednesday. 

The survey indicated 2015 was a good year for RIAs: 63 percent of those surveyed added clients in the second half of the year, reporting an average growth rate of 13 percent. Half of the firms TD Ameritrade surveyed increased their revenues during the same period, reporting an average rise of 14 percent.

The survey found that advisor’s primary concerns are shifting as 2016 unfolds, from the legal, compliance and regulatory worries that topped the list of concerns since 2012 to their clients’ pending generational wealth transfers.

For the first time, the transfer of wealth from aging clients to the next generation of investors topped advisors’ concerns, followed by investors choosing to manage their own finances online.

While the survey’s respondents were optimistic about their firms, they’re less certain about the direction of the stock market: 40 percent expected U.S. stocks to rise in the first half of 2016, and 64 percent of the advisors are shifting their clients to less volatile assets.

The respondents seemed underwhelmed by the recent digital advice craze, with just one percent saying they are "extremely concerned" about the threat of robo-advisors to their businesses, and 51 percent reporting that they are "not at all concerned."

Yet the percentage of RIAs embracing online investing services as part of their growth plans remains small, with 14 percent of respondents developing new online advice tools for their clients. Of these advisors, 85 percent plan to launch their robo-advisor services by the end of 2016.

TD Ameritrade also asked advisors about how they are using technology for future growth. More than half of RIAs said they are targeting new client niches in 2016, and 47 percent said they will spend more on marketing and advertising this year in hopes of fueling new client growth.

But advisors are also developing their firm’s workforce—nearly one-third of those surveyed plan to hire a junior advisor in 2016, and more than one-in-four will add back-office staff.