You can use the Multiply Blending Mode if you don't need an Alpha Channel for the fishes. If you want an Alpha Channel OR if there are white areas within your fish then you'll need to create a matte for the fish.
You can try the Color Key and use the colorpicker on the white area. a value of 33 on my end did a decent job. Then use Matte Choker to clean up the edges. The default setting for Matte Choker did a decent job.
Another method is to apply
Effect>Stylize>CC Threshold. A value of 245 for threshold was used to create a silhoutte effect for the fish. Then apply
Effect>Channel>Shift Channels and set
Take Alpha From to Luminance. Then apply Simple Choker with a setting of -3 to clean up the edges.
Use this layer as a track matte for the original fish image.
For both methods, the Paint tools; eraser and brush may come in handy to clean up certain spots.
It's good to have more than one technique handy as one may work better than the other on specific issues. I keep thinking there's another useful method but I just can't recall it right now. Hopefully these will work for you.
When dealing with stills, remember to render out the images after keying and BEFORE animating so that you're not keying more than the one frame that you need for stills.
HTH
RoRK
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