NEW YORK ( Stockpickr) -- Professional traders don't just look at a stock's price moves; they also look for big changes in volume.

Major moves in volume can signal unusual activity, such as insider buying or selling -- or buying or selling by "superinvestors."

>>5 Trading Setups to Ride This Rally

Let's take a look at several stocks rising on unusual volume today. They are all recording volume in midday trading that is already at least 50% above their average trading volume for a full day.

99 Cents Only Stores ( NDN) is engaged in the sale of various consumer products through its retail stores at price points of mainly 99.99cent or less. The stocks is trading up 4.4% at $21.39 in recent trading.

Volume: 23.1 million

Average Volume: 785,300

Volume % Change: 2,803.8%

99 Cents Only Stores agreed to be acquired for $22 a share, or $1.6 billion. 99 Cents is one of TheStreet Ratings' top-rated multiline retail stocks.

Superior Energy Services ( SPN) is a provider of specialized oilfield services and equipment focusing on serving the production-related and drilling-related needs of oil and gas companies. The stock is trading up 1.3% at $23.93 in recent trading.

Volume: 5.2 million

Average Volume: 1 million

Volume % Change: 391%

Superior Energy is one of the tiop holdings at David Dreman's Dreman Value Management and was one of Jefferies' four best energy stocks for 2011.

Endocyte ( ECYT) is a biopharmaceutical company developing targeted therapies for the treatment of cancer and inflammatory diseases. The stocks is trading up 5.1% at $11.03 in recent trading.

Volume: 477,036

Average Volume: 207,800

Volume % Change: 129.6%

BigBand Networks ( BBND) develops, markets and sells network-based solutions that enable cable operators and telecommunications companies to offer video services across coaxial, fiber and copper networks.. The stocks is trading up 74% at $2.21 in recent trading.

Volume: 27.6 million

Average Volume: 282,800

Volume % Change: 9,634.8%

BigBand Networks will be acquired by Arris Group ( ARRS) for $2.24 a share. It also shows up on a list of today's Biggest & Gainers.

Synergetics USA ( SURG) is a medical device company that designs, manufactures and markets innovative microsurgical instruments and other devices. The stocks is trading up 7% at $6.10 in recent trading.

Volume: 328,512

Average Volume: 110,800

Volume % Change: 196.5%

Synergetics is set to announce earnings Tuesday after the market close.

MarkWest Energy Partners ( MWE) is engaged in the gathering, compressing, treating, processing and transportation of natural gas; the transportation, fractionation, storage and marketing of NGLs; and the gathering and transportation of crude oil.. The stock is trading up 1.4% at $45.88 in recent trading.

Volume: 1.3 million

Average Volume: 440,700

Volume % Change: 195.6%

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