first, you need to make sure your animation lasts that long... if your dolphin is only keyed to move between frames 1 and 48, then even if you render 1 million frames, it'll only move those first 48.

48 frames at 24 fps (film) should be 2 seconds... so, you'd need 2880 frames for 2 min.

So, you'd need to change the animation of your dolphin to last that long and you'll need to change the render globals end frame to 2880 or whatever you decide to use.