[TrainedMonkey]

> Note that Diamond also implicates the reluctance of the Vikings to eat fish, which is mystifying

Completely guessing, but maybe eating fish turned out to be a huge evolutionary advantage... so groups that did outcompeted groups that did not.

[AlotOfReading]

No, this is just one of the many cases where Diamond simply wasn't familiar with the basic facts of the situations he wrote about. He's notorious for it. Later studies have indicated that the last couple centuries of Norse Greenlanders ate a diet rich in fish and other marine animals.