Well, the Mayo Clinic has a long list of contraindications of eating too much O-3s. The list is much too long to summarize, but bleeding risk (3 grams of O-3s per day), and pregnancy and baby problems are on the list: http://www.mayoclinic.org/drugs-supplements/omega-3-fatty-acids-fish-oil-alpha-linolenic-acid/safety/hrb-20059372.
Fish farm fish get their O-3s from fish meal and fish oil in farmed fishmeal. But because they have over fished the top 19 of 20 forage fish stocks (some are poorly managed) around the world, O-3s are half the level than they used to be, because fish farms have had to substitute more vegetable protein and energy. (This would include more O-6s, found in plants).
So it may be a good thing to have less O-3, but this is the result of over fishing for reduction industries, ie, fish meal, oil and silage.
The down side here is that farmed fish have as much as ten times the carcinogenic, persistent organic pollutants (commonly called POPs), PCBs, dioxins and so on as any other meat product, again from the pollution from fish feed. Not to mention fluoroquinolones from the chicken feathers they put in feeds.
Look at this graph:
It turns out that it may be wrong that O-3s do any good, because a study on Inuit people showed that they have a gene that regulates O-3s, which Europeans and most people on the globe don't have, so O-3s don't do much good, meaning don't eat farmed fish.
A New York Times article on the research said this:
"In the 1970s, Danish researchers studying Inuit metabolism proposed that omega-3 fatty acids found in fish were protective. Those conclusions eventually led to the recommendation that Westerners eat more fish to help prevent heart disease and sent tens of millions scrambling for fish oil pills.
The NYT, 2015 reference is: http://www.nytimes.com/2015/09/22/science/inuit-study-adds-twist-to-omega-3-fatty-acids-health-story.html?_r=0.
And this next article on the amount of the various chemicals, including brominated flame retardants showed that much of the worst chemicals gets taken out of the fish eaten and ends up in the body, rather than being passed through. (Note that bromine has been associated with cancers. A common example is the bromine in hot-tubs). See: https://www.ncbi.nlm.nih.gov/pubmed/21284993.
Read the abstract:
"Carry-over of dietary organochlorine pesticides, PCDD/Fs, PCBs, and brominated flame retardants to Atlantic salmon (Salmo salar L.) fillets.
AbstractInformation on carry-over of contaminants from feed to animal food products is essential for appropriate human risk assessment of feed contaminants. The carry-over of potentially hazardous persistent organic pollutants (POPs) from feed to fillet was assessed in consumption sized Atlantic salmon (Salmo salar). Relative carry-over (defined as the fraction of a certain dietary POP retained in the fillet) was assessed in a controlled feeding trial, which provided fillet retention of dietary organochlorine pesticides (OCPs), dioxins (PCDD/Fs), polychlorinated biphenyls (PCBs), and brominated flame retardants (BFRs). Highest retention was found for OCPs, BFRs and PCBs (31-58%), and the lowest retentions were observed for PCDD/Fs congeners (10-34%). National monitoring data on commercial fish feed and farmed Atlantic salmon on the Norwegian market were used to provide commercially relevant feed-to-fillet transfer factors (calculated as fillet POP level divided by feed POP level), which ranged from 0.4 to 0.5, which is a factor 5-10 times higher than reported for terrestrial meat products. For the OCP with one of the highest relative carry-over, toxaphene, uptake and elimination kinetics were established. Model simulations that are based on the uptake and elimination kinetics gave predicted levels that were in agreement with the measured values. Application of the model to the current EU upper limit for toxaphene in feed (50 μg kg(-1)) gave maximum fillet levels of 22 μg kg(-1), which exceeds the estimated permissible level (21 μg kg(-1)) for toxaphene in fish food samples in Norway."
And those words add up that farmed fish should not be eaten by people. Much like the Hites article from Science Jan 9, 2004 that raised this spectre more than a decade ago in Socttish farmed salmon. It resulted in the industry neutralizing Hites, and where I came into this debate as I could no longer trust anything that a fish farm said in its spin.
The original article that explained the cloak and dagger approach by fish farms was the David Miller one on spinwatch.org, almost one of the first posts I put on this site: http://fishfarmnews.blogspot.ca/2011/10/key-document-fish-farm-tactics.html.