This is one of my favorite questions because for me it forces me to consider the environment and sustainability. There is a good deal of hype surrounding the use of krill oil as the superior source of omega 3 fatty acids. In my opinion, I don't feel the science proves that yet. Of larger concern to me is environmental sustainability.
To clarify, the healthful benefit of "omega 3's" are not from the whole oil but from 3 very specific fatty acids found in foods. These would be EPA (eicosapentaenoic acid), DHA (docohexaenoic acid) and ALA (alpha-Linolenic acid). DHA and EPA are the fatty acids of importance found in fish oil and krill oil. ALA is the omega 3 fatty acid found in flaxseed. Mammals can metabolize ALA into EPA and DHA which is how many vegetarians can successfully consume enough omega 3 fatty acids.
Krill are crustaceans (like shrimp) that are a major food staple for many marine animals such as whales, fish, and birds. Krill oil contains approximately 7 - 20% EPA and DHA. In addition, krill oil contains an antioxidant carotenoid pigment called astaxanthin that gives it the characteristic reddish color. The fatty acids of krill oil are assembled as phospholipids. This is in contrast to marine fish oils that contain approximately 30% EPA and DHA and are assembled as triglycerides and ethyl esters. While the evidence is very strong to suggest that the phospholipid assembly is more bioavailable, the issue lies in the concentration of EPA and DHA. In a 2011 study (Schuchardt et. al.) had to supply 14 krill oil capsules per day, compared with 4 fish oil capsules per day to reach similar concentrations of EPA and DHA. Another 2011 study conducted by Ulven et. al. had to provide 6 krill oil capsules to reach 543 mg of DHA and EPA while it took 3 fish oil capsules to reach 864 mg of DHA and EPA. At those doses, there were no differences in metabolic effects. The bottomline is to pay attention to the amount of DHA and EPA listed on the bottle. In other words if you decided to replace your 2 - 3 fish oil capsules daily with 2 - 3 krill oil capsules per day, you will be getting at least 30% less EPA and DHA.
The larger concern for me is definitely the issue of sustainability. That goes for fish and krill. I do believe we can make a huge difference when it comes to how foods are manufactured whether that be fish and krill harvesting or meat processing. There are sustainable and proper ways to obtain these precious commodities for ourselves and for our beloved pets. We need to make sure that we don't overfish our oceans and lose the biodiversity that supports our planet. While I do believe that aquaculture farms have their own sets of limitations, we need to make careful decisions about what we chose to buy. Don't just buy products...take a look at how those products are manufactured and make sure they are coming from sustainable sources.
Schuchardt et. al. 2011. Lipids in Health and Disease. 10: 145
Ulven et. al. 2011. Lipids. 46: 37