Glad to find this blog... , I've been eating "walu" for years but never had this problem until I ate an unusually large amount the other day... Found this on Wikipedia...
Escolar is consumed in several European and Asian countries, as well as in the USA, sometimes raw as sushi or sashimi. It may be sold as "white tuna" - a term also used for the albacore - or as "super white tuna" to distinguish it from the albacore.[1] Escolar is also sold misleadingly as "butterfish", "oilfish" and "Hawaiian butter fish"; in Hawaii and Fiji, it is known as walu. Like oilfish, a related species with similar consumption consequences, escolar is also sometimes deceptively sold under the name of an entirely different species of fish, most commonly "codfish" or "orange roughy".
[edit] Effects of consumption
Like its relative the oilfish (Ruvettus pretiosus), it cannot metabolize the wax esters (Gempylotoxin) naturally found in its diet, which accumulates to give an oil content in the muscle meat of 18–21%. These wax esters may rapidly cause gastrointestinal symptoms following consumption; however, these effects are usually short lived.
The gastrointestinal symptoms, called "keriorrhoea", caused by these wax esters may include oily orange diarrhea, discharge, or leakage from the rectum that may smell of mineral oil. The discharge can stain clothing and occur without warning 30 minutes to 36 hours after consuming the fish. The oil may pool in the rectum and cause frequent urges for bowel movements due to its lubricant qualities and may be accidentally discharged by the passing of gas. Symptoms may occur over a period of one or more days. Other symptoms may include stomach cramps, loose bowel movements, diarrhea, headaches, nausea, and vomiting.
To minimize the risk of symptoms, strict control of portion size is recommended as well as preparation methods that remove some of the oil (e.g. grilling). Portions should be no greater than 6 ounces.