NEW YORK ( TheStreet) -- Sirius XM Radio ( SIRI - Get Report)and E*Trade Financial ( ETFC - Get Report) saw the biggest changes in short interest in the first half of May among stocks traded on the Nasdaq Stock Market, according to a published report.

Sirius short interest jumped over 30% to more than 209 million shares, making it the most heavily shorted stock on the Nasdaq, ahead of Level 3 Communications ( LVLT), Reuters reported. Level 3 short interest fell slightly during the period, to about 102 million from just over 107 million April 30.

Short sellers borrow shares in the hope of buying them at a lower price in the future and pocketing the difference. The Nasdaq reports short interest twice a month, about two weeks after the period in which the shorting activity took place.

Since reaching the $1 mark in February, Sirius shares have been unable to hold their gains. The satellite radio provider's shares hit a high of $1.25 on May 3, the day they affirmed they expected to add 500,000 subscribers in 2010. A day later they surprised analysts by reporting a surprise profit of a penny, but missed analysts' revenue expectations. The shares have traded off sharply with the rest of the market since the May 6 flash crash.

E*Trade shares hit their 52-week high of $2.08 in September and have been stuck in a range of about $1.40 to $1.70 for several months. Analysts cited strong April trading numbers for E*Trade in reports last week, and some expressed optimism that the company's exposure to troubled home loans will show surprising signs of improvement. The shares were at $1.50 Wednesday afternoon, up more than 5% on a bullish day for stocks.

Aside from Sirius XM and E*Trade, other Nasdaq stocks that saw a heavy rise in short interest were YRC Worldwide ( YRCW - Get Report), JetBlue Airways ( JBLU - Get Report) and Mylan ( MYL). YRC, a shipping company based in Overland Park, Kan., was the fifth-most-shorted Nasdaq stock in the first half of May, behind Sirius, Level 3, Cell Therapeutics ( CTIC - Get Report) and E*Trade.

The top decrease in short interest on the Nasdaq was seen in Popular. ( BPOP - Get Report), Puerto Rico's largest bank. It has seen its shares rise 50% in the last three months as it has benefited from being among the healthiest institutions in a stressed Puerto Rican banking sector.

-- Written by Dan Freed in New York.