That particular photo was a combination of both. I used a rear curtain flash setting which drops the shutter speed down and panned the fish as it was swimming. Then I duplicated the layer in photoshop and added speed line blur. Then i used the histtory brush to uncover some of the head. I really like the effect and plan on experimenting more with the sem technique.
You can get a similar effect by just panning. Problem there is that you have to exactly match the speed and motion of the fish with your pan...otherwise the image will be blurred.
Pete...that video has been downloaded more than 50,000 times. Lame yes. Interesting, of course. A 56K dial up choker...as planned. heh heh