The mass is not a problem, But the only way a tornado could pick up a shark is that the Shark jumps almost completely out of water like dolphins. Why?
First you need to understand pressure; There is no vacuum, this means that there is no suction. It's always the higher pressure which pushes the things to the lower pressure. This always happens the easiest way, resulting the strong surface winds. But these wind's surely doesn't rip off anything wide below the surface. The impact is very limited at the surface. Thus though you see frogs and alligators been ripped of by tornados, this is mainly caused by the fact that the Air pressure is able to blow under them. If some fishes have ever been blown out, they must have first become airborne through waves or if some small lake is sucked completely dry.
You might want to think this further; lets take the greatest pressure difference ever measured in Tornado 0.194 bars, this means quite exactly a water pressure of 2 meters. So it can suck 2 m of water "away", and thus anything closer to surface than 2 m could be sucked away, Right?
NO. as there is no suction, this pressure difference pushes from outside the Tornado-eye, and therefore it pushes the whole water mass causing a Storm surge. But these Storm Surges are mostly much higher than only 2 m, ie. the highes ever measured was in Australia caused by the Cyclone Mahina 1899; 13 m. It's pressure drop could explain only about 1 m of this. From this can be concluded, that the winds are transferring much more water to the hurricane eye than the hurricane is sucking away, and this must result a downwards pushing flow to the sea surface. Thus anything below the surface, will remain under the surface.
If a shark is ever sucked by a hurricane, it has been living in some small water pool which is sucked dry with the shark. I can't imagine why a shark would ever jump above surface in storm, as it's fin would indicate the heavy winds above the surface before. Even if this would happen, it's size and very good aerodynamical shape would make it almost impossible to be carried out by wind blow to a hights that could be considered as "falling from the sky." As for this lift, the blow of the wind must be higher than the terminal velocity of the object. The Fins would turn the shark very soon on nose down position, and at this position the shark shaped object might need some 500 km/h wind to remain airborne. So you might experience some shark rolling by the wind at the ground, near some zoo. The physical limits goes here.
This does not happen as a result of any gas law. It happens because there is a surprisingly high force from the atmosphere pressing down on the suction cups, and comparatively little force pressing up from the table and partial vacuum below.
Best Answer
I think this picture covers the gist of it. You need rubber or plastic, a mailable material that flattens when pressed against the object, but that wants to return to it's original shape when released.
Think of, as part of measuring the weight of an object, you measure the pressure all around it. If you have more air pressure on the bottom than the top, the object weighs less and the suction cup works by creating a low air pressure pocket on the object it's lifting.
Source: http://www.wired.com/2013/08/how-strong-are-the-suction-wheels-in-bad-piggies/
If the object is on the ground, the air pressure is still there unless it's kind of vacuum sealed, like sink-holes or mud can sometimes create a vacuum seal and make it very difficult to lift things - kinda like this picture. It's not that the boots and mud are that heavy, it's the vacuum seal that makes the boots hard to lift. If air pressure didn't exist under objects sitting on the floor, everything would be enormously heavy.