Here is what Dr. Mercola has to say about Fish Oil compared to Krill Oil.
I prefer krill oil compared to all other animal-based omega-3's because while the metabolic effects of krill oil and fish oil are "essentially similar," krill oil is as effective as fish oil despite the fact that it contains less EPA and DHA.8 This is because krill oil is absorbed up to 10-15 times as well as fish oil due to its molecular composition (Its EPA and DHA fatty acid chains are phospholipid bound), and is less prone to oxidation (rancidity) because it is naturally complexed with the potent fat-soluble antioxidant astaxanthin.
Fish oil is in a triglyceride molecule that has to be broken down in your gut to its base fatty acids of DHA and EPA. About 80-85 percent is never absorbed and is eliminated in your intestine (this is why fish oil can cause you to experience burp back and why about half of all people cannot tolerate fish oil). Then once the fatty acids are absorbed into your bloodstream, your liver has to attach it to phoshphatidyl choline for it to be used by your body.
The beauty of krill is that all of it is in the correct form in the original pill, so your body uses virtually 100 percent of it. As a result, most people only need two to three 500-mg capsules of krill oil per day (each capsule typically contains about 50 mg of DHA and 90 mg of EPA) to support optimal brain, and overall, health.