Eicosatetraenoic Acid (ETA): ETA is a lesser-known omega-3 fatty acid that also contains 20 carbons, like EPA, but only four bonds instead of five. It is found richly inroe oil and green-lipped mussel and is only recently being recognized for its potent health benefits. Not only is it anti-inflammatory, like the other omega-3s, but ETA can also limit your body’s production of the inflammatory omega-6 fatty acid arachidonic acid (ARA). In fact, ETA redirects the enzyme that normally creates ARA to convert it to EPA instead!
The supplements contain omega-3 fatty acids, the polyunsaturated oils prominent in fatty cold water fish like salmon, sardines and mackerel. In many observational studies, people who regularly consumed fish two or more times a week were less likely to suffer heart attacks, strokes and cardiovascular deaths than those who ate fish infrequently or not at all.
Good points, Miroslav. Focusing on your 4th point, with so many different formulations on the market that contain various preservatives, only looking at the blood levels of omega-3’s as the flag for increased risk for prostate cancer tends to ignore the fact that certain populations in coastal regions maintain a diet high in omega fish oils and don’t have a marked increase level of prostate cancer, pointing to the fact that another agent may be to blame here.
One of the most well-known benefits of omega-3s are the way they positively affect risk factors associated with heart disease. That’s one reason the American Heart Association is very clear about encouraging people to get enough in their diets. (8) Heart disease and stroke are the leading causes of death worldwide, but communities who eat diets rich in fish have remarkably low instances of these diseases, which is at least partially due to their high omega-3 consumption. (9, 10)
Recently another Omega-3 fatty acid, DPA (Docosapentaenoic Acid) has been discussed more frequently in the scientific community, as a new and very potent Omega-3 fatty acid. Previously thought to work in through EPA and DHA we are now learning it has very distinct functions in the body. All three of these polyunsaturated fats play an important role in the functioning of our bodies.
Most U.S. adults fail to consume adequate amounts of foods rich in EPA and DHA on a regular basis (at least 8 ounces of fatty fish per week is recommended), and probably consume too many omega-6 fats in comparison (soybean oil, canola oil, cottonseed oil, etc.). This omega-3:omega-6 imbalance can have a negative effect on inflammation patterns and may also be implicated as a contributing factor to other processes related to cellular metabolism, hormone signaling, and even weight regulation.
Fish oil is effective in reducing inflammation in the blood and tissues. Regular consumption of fish oil supplements, tablets, pills, and capsules is helpful to those who suffer from chronic inflammatory diseases. Fish oil is effective in treating gastrointestinal disorders, Celiac disease, short bowel syndrome and inflammatory bowel disease (IBD) including Crohn’s Disease and ulcerative colitis, which are both typical disorders of the intestine. Patients suffering from Crohn’s disease often find it difficult to absorb vitamins, fats, and essential supplements. Fish oil supplements are an effective diet for such patients.
Human studies also confirm cognition and memory improvement with omega-3 supplementation. For example, a study showed that both fish oil and krill oil enhanced cognitive function in a group of older men by increasing oxygen delivery to their brains. Interestingly, for those taking krill oil this effect was more prominent than those taking fish oil, though both groups were significantly better than placebo.30 As we pointed out earlier, because the omega-3 DHA is bound to phospholipids in krill it may be more effectively incorporated into the critical cell membrane in brain cells.
All people need to consume omega-3 fats regularly. The recommended daily intake for adults is 1.6 grams for males  and 1.1 grams for females, according to the National Institutes of Health. The omega-3 family encompasses numerous fatty acids, but three primary forms are eicosapentaenoic acid, docosahexaenoic acid, and alpha-linolenic acid. The first two forms primarily occur in fish, such as salmon, mackerel, and tuna. The third can be found in plant oils, including flaxseed, soybean, walnut, and canola oils.

Additionally, total polychlorinated biphenyl (PCB) content was measured in every product. All product recorded PCB levels within the Food and Drug Administration’s (FDA) 2 PPM limit for the edible parts of fish/shellfish as well as the stricter standards enacted by California’s Proposition 65, which requires products containing greater than 0.09 PPM of PCB content to bear a cancer warning. The worst offender, Now Foods Ultra Omega-3 Fish Oil, recorded 0.04 PPM of PCB content.
Since 2004, scientists have been suggesting that the omega-3 index be used as a way to measure a person’s risk of cardiovascular disease, in a similar way to how cholesterol levels are used today (1). A recent study funded by the National Institutes for Health even indicated that the omega-3 index could be a better predictor of death risk than serum cholesterol levels (2).
Scientific studies have found that fish oil can help to prevent and kill various cancers, including colon, prostate and breast. (13a) Not only has research proven that it makes conventional cancer drugs more effective, but it’s also an effective stand-alone therapy in natural cancer treatment. Intravenous fish oil lipid emulsions, in particular, are rich in omega-3 polyunsaturated fatty acids, which exhibit anti-inflammatory and immunomodulatory effects. (13b)
Kremer, J. M., Lawrence, D. A., Petrillo, G. F., Litts, L. L., Mullaly, P. M., Rynes, R. I., Stocker, R. P., Parhami, N., Greenstein, N. S., Fuchs, B. R., and . Effects of high-dose fish oil on rheumatoid arthritis after stopping nonsteroidal antiinflammatory drugs. Clinical and immune correlates. Arthritis Rheum. 1995;38(8):1107-1114. View abstract.
One day I was cooking pasta when the kitchen started to fill with the odor of fish. I happen to hate fish, so this was not a pleasant experience. It was also a mystery, since I never cook fish. A little detective work discovered that the offensive odor was coming from the pasta. Apparently I didn’t notice the “Now with Omega 3” label on the box when I purchased it. My daughter and I still refer to this as the “fish pasta incident”.

Grigg, L. E., Kay, T. W., Valentine, P. A., Larkins, R., Flower, D. J., Manolas, E. G., O'Dea, K., Sinclair, A. J., Hopper, J. L., and Hunt, D. Determinants of restenosis and lack of effect of dietary supplementation with eicosapentaenoic acid on the incidence of coronary artery restenosis after angioplasty. J Am Coll Cardiol. 3-1-1989;13(3):665-672. View abstract.

Omega−3 fatty acids, also called ω−3 fatty acids or n−3 fatty acids,[1] are polyunsaturated fatty acids (PUFAs).[2][3] The fatty acids have two ends, the carboxylic acid (-COOH) end, which is considered the beginning of the chain, thus "alpha", and the methyl (-CH3) end, which is considered the "tail" of the chain, thus "omega". One way in which a fatty acid is named is determined by the location of the first double bond, counted from the tail, that is, the omega (ω-) or the n- end. Thus, in omega-3 fatty acids the first double bond is between the third and fourth carbon atoms from the tail end. However, the standard (IUPAC) chemical nomenclature system starts from the carboxyl end.
^ Jump up to: a b Casula M, Soranna D, Catapano AL, Corrao G (August 2013). "Long-term effect of high dose omega-3 fatty acid supplementation for secondary prevention of cardiovascular outcomes: A meta-analysis of randomized, placebo controlled trials [corrected]". Atherosclerosis. Supplements. 14 (2): 243–51. doi:10.1016/S1567-5688(13)70005-9. PMID 23958480.
“The review provides good evidence that taking long-chain omega 3 (fish oil, EPA or DHA) supplements does not benefit heart health or reduce our risk of stroke or death from any cause.  The most trustworthy studies consistently showed little or no effect of long-chain omega 3 fats on cardiovascular health. On the other hand, while oily fish is a healthy food, it is unclear from the small number of trials whether eating more oily fish is protective of our hearts. 
Jump up ^ Kwak SM, Myung SK, Lee YJ, Seo HG (May 2012). "Efficacy of omega-3 fatty acid supplements (eicosapentaenoic acid and docosahexaenoic acid) in the secondary prevention of cardiovascular disease: a meta-analysis of randomized, double-blind, placebo-controlled trials". Archives of Internal Medicine. 172 (9): 686–94. doi:10.1001/archinternmed.2012.262. PMID 22493407.

It is believed that regular consumption of fish oil aids in boosting your immune system, thereby enabling you to resist the occurrence of common diseases like colds, cough, and the flu. Omega-3 fatty acids present in fish oil bolster the immune system by affecting the activity and amount of cytokines and eicosanoids present in our body. Researchers have also studied the effect of a fish meal and fish oil on the immune system of pigs and found that fish oil aided in the growth of the animals. Similar research conducted on mice at Taichung Veterans General Hospital, Taiwan, also gave positive results.
A 2009 metastudy found that patients taking omega-3 supplements with a higher EPA:DHA ratio experienced fewer depressive symptoms. The studies provided evidence that EPA may be more efficacious than DHA in treating depression. However, this metastudy concluded that due to the identified limitations of the included studies, larger, randomized trials are needed to confirm these findings.[40]
Several small studies have shown that combination therapy with fish oil and HMG CoA reductase inhibitors is safe.56–61 The largest trial to date, the JELIS trial,32 was an open label trial of 18,645 Japanese adults with hypercholesterolemia who were randomized to a standard statin regimen or a fish oil formulation containing 1.8 g of EPA added to a statin medication. The cohort was made up mostly of postmenopausal, nonobese women with a 15% to 20% incidence of diabetes, tobacco use, or CAD. The primary outcome of any major cardiovascular event, at a mean of 4.6 years, was moderately reduced by a relative risk reduction of 26%. Both unstable angina and nonfatal MI were reduced, but no change was seen in sudden death. Overall, the findings were remarkable because at baseline approximately 90% of Japanese consumed at least 900 mg of EPA and DHA per day.62 The rates of cancer, joint pain, lumbar pain, or muscle pain were similar in the 2 groups. There was a similar rate of increase in measures of creatine phosphokinase, but more patients had an increase in aspartate aminotransferase levels (0.6% vs. 0.4%) in the fish oil group. The rate of bleeding was 1.1% in the fish oil combination group versus 0.6% in the HMG–CoA reductase inhibitor group.
I've done a lot of shopping and comparing of fish oil softgels and have reached the conclusion that these are these best you can buy. Prior to seeing these in my chiropractor's office I scanned the labels and specs on many brands at Mothers, Vitamin Shoppe, Sprouts and Amazon vendors. These have 430 mg of EPA and 290 mg of DHA per softgel, with a recommended dose of two.. If you compare as well, you will find most other brands, including those sold as premium products at health food stores at premium prices don't have the same potency.Especially among Krill Oil products. My chiropractor shared a clinical study that showed taking fish oil containing levels of EPA and DHA consistent with these supplements caused participants to say it had the same favorable affect as taking ibuprofen. I make no claims. I am not a doctor, am not associated ... full review

Unsafe for children. Fish oil supplements are not considered safe for children. Too much of this fat in their system can lead to a chemical imbalance in the brain which could stunt healthy growth and development. Because of this, and the possible exposure to polychlorinated biphenyls, methylmercury and other toxins which are found in some sources of fish, it is not recommended that women who are pregnant take excessive amounts of fish oil. Servings of fish should be limited to six ounces per week and they should refrain from using fish oil or other fatty acid supplement pills. Children should not consume more than two ounces of fish per week to avoid overexposure to these chemicals. Now you know although fish oil can benefit human beings a lot, fish oil side effects should never be neglected for the sake of your safety. 
These low levels are especially bad when compared to the numbers from the Japanese population. In Japan, the average omega-3 index level is more than double that of the average American, with some surveys showing Japanese men consume over 100 g (approximately 3.5 oz) of fish every day. These radically different dietary habits help explain how even those with omega-3 indexes in the lowest 5th percentile of the Japanese population have higher omega-3 index averages than most Americans (8).
Many studies show that eating fatty fish and other types of seafood as part of a healthy eating pattern helps keep your heart healthy and helps protect you from many heart problems. Getting more EPA or DHA from foods lowers triglyceride levels, for example. Omega-3 dietary supplements can also help lower triglyceride levels, but it is not clear whether omega-3 supplements protect you from most heart problems.

Another recent study shows that fatty fish consumption can cut the risk of eye-diabetes complications. The researches tracked the seafood consumption of about 3,600 diabetic men and women between the ages of 55 and 80 for nearly five years. The researchers found that people who regularly consumed 500 milligrams each day of omega-3 fatty acid in their diets (equal to two servings of fatty fish per week) were 48 percent less likely to develop diabetic retinopathy than those who consumed less. (23)
for canned sardines i noticed the omega 3 EPA/DHA levels (written on the can) varied between the different company brands (sometimes by a lot!) , and also, the EPA/DHA amounts varied depending on what was added in the can with the sardines (sunflower oil, olive oil, brine, spring water, etc --- little note: there's more fat in the oily fish, than found in the brine/spring water)
The most extensive data of the effect of fish oil on lipoprotein subfractions are based on trials performed before the widespread use of statins. This data were aggregated over a decade ago in a meta-analysis of 16 randomized trials including over 1500 patients.17 In this analysis, low-density lipoprotein (LDL) was increased by an average of 5% and high-density lipoprotein was marginally changed. Although a shift toward less atherogenic, larger and more buoyant LDL particle composition has been shown,74 this has been offset by the observation that the number of apolipoprotein B 100 particles increases and may be more susceptible to oxidation.75 Increased conversion of remnant particles (intermediate density lipoprotein) to LDL has also been observed.76
However, since the dosage of fish oil required for an ideal effect in the improvement of a patient is unknown, the Arthritis Center in the Department of Rheumatology at John Hopkins University considers including omega-3 fatty acids and fish oil in the treatment of arthritis as controversial. The University also cautions that arthritis patients must be wary of all the other side effects that can come from using fish oil. You can read more about arthritis on the web page of the Arthritis Foundation and the Arthritis Center.
Evidence linking fish oil and cancer has been all over the map. Some research suggests diets high in fatty fish or fish oil supplements might reduce the risk of certain cancers, including prostate cancer. Other research shows just the opposite, a  link between eating a lot of oily fish or taking potent fish oil supplements and a 43% increased risk for prostate cancer overall, and a 71% increased risk for aggressive prostate cancer.
Heart rate variability, a possible surrogate outcome for the risk of sudden death, was assessed in a randomized trial of myocardial infarction (MI) survivors with an ejection fraction of 40%. In the 49 patients that were randomized to either fish oil or olive oil, Holter monitor recordings showed an increase in heart rate variability in the fish oil group.31 In a larger cohort assessed in the Japan EPA Lipid Intervention Study (JELIS),32 however, no difference in heart rate variability could be attributed to fish oil.
Finally, in order for AA to be converted into inflammatory products it must be released from phospholipids (part of the cell membrane) using the enzyme phospholipase A2 and then converted by the enzyme cyclooxygenase. EPA utilises both of these enzymes, so if EPA levels are increased in the diet, it attracts enzyme away from AA to EPA – again giving rise to anti-inflammatory products instead of inflammatory ones.
Omega-3s are important components of the membranes that surround each cell in your body. DHA levels are especially high in retina (eye), brain, and sperm cells. Omega-3s also provide calories to give your body energy and have many functions in your heart, blood vessels, lungs, immune system, and endocrine system (the network of hormone-producing glands).
Additionally, total polychlorinated biphenyl (PCB) content was measured in every product. All product recorded PCB levels within the Food and Drug Administration’s (FDA) 2 PPM limit for the edible parts of fish/shellfish as well as the stricter standards enacted by California’s Proposition 65, which requires products containing greater than 0.09 PPM of PCB content to bear a cancer warning. The worst offender, Now Foods Ultra Omega-3 Fish Oil, recorded 0.04 PPM of PCB content.
Weak bones (osteoporosis). Research suggests that taking fish oil alone or together with calcium and evening primrose oil slows the rate of bone loss and increases bone density at the thigh bone (femur) and spine in elderly people with osteoporosis. But taking fish oil does not slow bone loss in older people with osteoarthritis in the knee but without weak bones.