Question: How do seniors benefit from eating oily fish?
About the article "Seafood", section "Health benefits".

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Keriorrhea (oily diarrhea, oily orange diarrhea, anal leakage, orange oily leakage) is the production of greasy, orange-colored stools which results from the consumption of indigestible wax esters found in oil fish and escolar. 
The most widely available dietary source of EPA and DHA is cold water oily fish, such as salmon, herring, mackerel, anchovies, and sardines.
Oils from these fish have a profile of around seven times as much omega-3 as omega-6.
Other oily fish, such as tuna, also contain "n"-3 in somewhat lesser amounts.
Consumers of oily fish should be aware of the potential presence of heavy metals and fat-soluble pollutants like PCBs and dioxins, which are known to accumulate up the food chain.
After extensive review, researchers from Harvard's School of Public Health in the "Journal of the American Medical Association" (2006) reported that the benefits of fish intake generally far outweigh the potential risks.
Although fish are a dietary source of omega-3 fatty acids, fish do not synthesize them; they obtain them from the algae (microalgae in particular) or plankton in their diets. 
The most widely available dietary source of EPA and DHA is cold water oily fish, such as salmon, herring, mackerel, anchovies, and sardines.
Oils from these fish have a profile of around seven times as much omega-3 oils as omega-6 oils.
Other oily fish, such as tuna, also contain omega-3 in somewhat lesser amounts.
Although fish is a dietary source of omega-3, fish do not synthesize them; they obtain them from the algae (microalgae in particular) or plankton in their diets.
For comparison, here are the omega-3 levels in some common non-fish foods. 
Fatafat is taken after having eating too much oily and spicy food and the pills help in bringing quick relief from indigestion.
It makes bowel movements nice and easy and smooth.
They have been part of Indian culture for ages, and historical records show that during the Mahabharata, Bheem used these to ease his defecation after copious amounts of feasting. 
Some species of fish, notably the puffer fugu used for sushi, and some kinds of shellfish, can result in serious poisoning if not prepared properly.
These fish always contain these poisons as a defense against predators; it is not present due to environmental circumstances.
Particularly, fugu has a lethal dose of tetrodotoxin in its internal organs and must be prepared by a licensed fugu chef who has passed the national examination in Japan.
Ciguatera poisoning can occur from eating larger fish from warm tropical waters, such as sea bass, grouper, and red snapper.
Scombroid poisoning can result from eating large oily fish which have sat around for too long before being refrigerated or frozen.
This includes scombroids such as tuna and mackerel, but can also include non-scombroids such as mahi-mahi and amberjack.
The poison is odourless and tasteless.
A lot of fish eat algae and other organisms that contain biotoxins (defensive substances against predators).
Biotoxins accumulated in fish/shellfish include brevetoxins, okadaic acid, saxitoxins, ciguatoxin and domoic acid.
Except for ciguatoxine, high levels of these toxins are only found in shellfish.
Both domoic acid and ciguatoxine can be deadly to humans; the others will only cause diarrhea, dizzyness and a (temporary) feeling of claustrophobia.
Shellfish are filter feeders and, therefore, accumulate toxins produced by microscopic algae, such as dinoflagellates and diatoms, and cyanobacteria.
There are four syndromes called shellfish poisoning which can result in humans, sea mammals, and birds from the ingestion of toxic shellfish.
These are primarily associated with bivalve molluscs, such as mussels, clams, oysters and scallops.
Fish, like anchovies can also concentrate toxins such as domoic acid.
If suspected, medical attention should be sought.
The toxins responsible for most shellfish and fish poisonings, including ciguatera and scombroid poisoning, are heat-resistant to the point where conventional cooking methods do not eliminate them.