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The beginning of October not only ushered in the fourth quarter, but a time of the year when the markets can be volatile. The bull run approached nine years long, and yet the markets continued to reach new highs. Judging by the most shorted stocks traded on the Nasdaq between the September 29 and October 13 settlement dates, short sellers were growing selective. While the short interest moves among these stocks were mixed, the upswings tended to be more robust than the downswings.

Comcast, Intel and Mattel led this trend, each with double-digit percentage growth in the numbers of its shares sold short earlier this month. On the other hand, short sellers shied away somewhat from those stocks that top this list yet again: Sirius XM and AMD.

Note that still just three Nasdaq stocks had more than 100 million shares short as of the middle of this month.

Sirius XM

The more than 227.00 million Sirius XM Holdings Inc. (NASDAQ: SIRI) shares short after the first two weeks of this month were almost 6 million less than on the previous settlement date, and that  was the lowest level of short interest so far this year. The latest reading represented 15.8% of the total float. The average daily volume rose from a year-to-date low, so the days to cover slipped from more than 17 to less than 16.

During the period, Sirius hiked its dividend by 10%. The stock ended the short interest period more than 3% higher than where it began it, though most of that gain came in the first couple of days of the month. The Nasdaq rose less than 2% between the settlement dates. Sirius shares closed most recently at $5.72 apiece. That is less than the 52-week high of $5.89 seen near the end of July. The 52-week low of $4.05 was reached almost a year ago.

AMD

By the middle of October, Advanced Micro Devices Inc. (NASDAQ: AMD) had more than 148.60 million shares short. That was a decline of more than 7% from the total on the previous settlement date, which was the highest level of short interest in at least a year. The most recent reading represented 16.8% of the company’s float. It would take a little more than three days to cover all short positions.

Despite the run up in the industry, AMD appeared during the period to still have upside potential. The October 13 share price was more than 11% higher than on the previous settlement date, though the shares have retreated a bit since then. The stock closed on Tuesday at $14.25, which is more than 8% higher than six months ago. Shares have changed hands between $6.22 and $15.65 apiece in the past year.

Intel

Approximately 131.85 million Intel Corp. (NASDAQ: INTC) shares were sold short as of the latest settlement date. That was the greatest level of short interest in at least the past year, as the figure grew by almost 16 million shares, or more than 13%. And it represented 2.8% of the company’s float. As of the latest settlement date, it would take more than five days to cover all short positions.

This semiconductor giant was among the stocks pushing markets to new highs earlier this month. Its shares ended the short interest period with more than a 4% gain, and they have continued to climb since then. The stock closed most recent at $40.95 a share, after hitting a 52-week high of $41.05 earlier in the day. The 52-week low is $33.23. The share price is nearly 13% higher than at the beginning of the year.

Mattel

Mattel Inc. (NASDAQ: MAT) saw the number of its shares short grow for the eight consecutive period, most recently by nearly 14% to the middle of this month. The more than 71.86 million shares reported represented 21.1% of the toymaker’s total float, and it was the highest it has been in the past year. At the posted average daily trading volume, it would take almost 10 days to cover all short positions.

The toy maker named a new chief financial officer during the settlement period. In the initial two weeks of this month, Mattel shares inched up less than 2%, though it gave up that gain afterward. The most recent closing stock price was $15.40, which is still down more than 28% in the past six months. In the past year, the shares have traded between $14.35 and $33.23 apiece.

Opko Health

Opko Health Inc.’s (NASDAQ: OPK) short interest retreated about 2% from in the previous period to more than 71.35 million shares on the most recent settlement date. That was the fewest shares short since back in January, and it was 21.6% of the total float. The daily average volume tumbled from a multi-month high in the previous period, and the days to cover number grew from around 11 to more than 19.

For well more than a year, the CEO frequently has bought batches of Opko shares. The stock closed most recently at $6.78 a share, in a 52-week trading range of $5.85 to $12.15. By the end of the two-week short-interest period, the share price was a little more than 1% higher, despite being in the red a couple of times in the interval. It is now more than 10% lower than it was six months ago.

Synergy Pharmaceuticals

Short interest at Synergy Pharmaceuticals Inc. (NASDAQ: SGYP) totaled over 66.83 million shares on October 13, after a gain of more than 6% from on the prior settlement date. That represented a whopping 29.8% of the total float, and it erased a similar decline in the prior period. The days to cover dropped from more than 14 to less than 10 during the latest period as the average daily volume increased.

This biopharmaceutical company is focused on treatments for constipation and irritable bowel syndrome. Short sellers watched the stock rise more than 19% but then end up less than 7% higher by the conclusion of those two weeks. The most recent closing share price was $2.65, which is more than 35% lower than six months ago. The shares have traded hands between $2.56 and $7.15 apiece in the past year.

And Others

Rounding out the top 10 were Comcast Corp. (NASDAQ: CMCSA), Micron Technology Inc. (NASDAQ: MU), Cisco Systems Inc. (NASDAQ: CSCO) and Microsoft Corp. (NASDAQ: MSFT). As mentioned, Comcast led this group, as it had a more than 10% rise in short interest, though the shares short at Microsoft grew nearly as much.

Lingering just outside the spotlight of the top 10 most shorted Nasdaq stocks were Ascena Retail Group Inc. (NASDAQ: ASNA), Novavax Inc. (NASDAQ: NVAX) and Oclaro Inc. (NASDAQ: OCLR), all with short interest that declined modestly, as well as BlackBerry Ltd. (NYSE: BB), which has since begun trading on the New York Stock Exchange.