Image processing: Image recognition and classification on fish
How can I get the specific key points?


I'm not sure but I think you are talking about Feature matching using SIFT.


Extract keypoints using SIFT from reference images of what you are trying to match.
Extract keypoints using SIFT from the test images.
Try to match the keypoints from reference images to test images using algorithms like RANSAC to get the position of the features(in your case eye,dorsal and pelvic fin).


You can find more information here.