Trading in cryptocurrencies, which along with cannabis-related stocks helped fuel TD Ameritrade Holding Corp.'s (AMTD)  49% trading growth in its most recent quarter, has fallen off since the start of the year as volatility depressed bitcoin valuations and demand fell, the brokerage company's chief executive said.

"We've seen a slight curtailing in cryptos as being a percentage of our trades just in the first couple of weeks versus December, which seemed to be the peak," Chief Executive Timothy Hockey said during a Jan. 23 conference call with analysts.

The Omaha, Neb.-based company, which reported solid quarterly results Monday evening, was among the first to make the CBOE Bitcoin futures product available for trading for qualified retail clients. The quarter ended in December is TD Ameritrade's fiscal first quarter.

"Media attention on cryptocurrency was at a frothy level, and it coincided with market highs," Hockey told TheStreet in an interview after the earnings call. "Once the number starts to come down a little bit, it became less frothy."

To be sure, overall trading activity in January has been even stronger than the first quarter, with about 975,000 trades a day on average through Jan. 17, compared with 726,000 trades a day in the last quarter

"If you zeroed out all of the cannabis and the blockchain related symbols, you'd still have a number that's just shy of 900,000 trading," Hockey told analysts on the call.

The company with a market capitalization of $31 billion reported an uptick of 72% year-over-year in new business among investors under 35 years old. Hockey said that TD Ameritrade has "consistently seen good growth in millennials, but it is disproportionately high" at this time.

While Hockey said the company limited its cryptocurrency products to a small number of experienced future traders, the underlying technology in cryptos, blockchain, garnered the interest of millennials.

"People trade what they know," Hockey said. "As things like blockchain become interesting, which is skewed toward a younger audience," they will enter the market and begin investing, he said. He pointed to Snap Inc. (SNAP) , the parent company of social media platform Snapchat, as a stock that millennials flocked to last year.

Likewise, baby boomers trade what they know, with an orientation toward tech and consumer goods, "things like General Electric Co. (GE) and Apple Inc. (AAPL) ," Hockey said. He also said that Netflix Inc. (NFLX) , which reported a blow-out quarter, always generates lots of trades.

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Taking a broader look at the U.S. market, Hockey said, "It certainly feels like this bull market is long in the tooth." Even though it has been one of the longest bull markets in history, Hockey said, there are a lot of reasons it could continue, such a de-regulation and low unemployment.

"There doesn't seem to be a catalyst for change," Hockey said.

Shares of TD Ameritrade rose by 0.2% on double the average trading volume to $55.98 at 1:30 p.m. EST on Tuesday. The stock jumped as high as $57.74 following the conference call but pared some of the gains in early afternoon trading.

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