I have wondered about that myself, and just looked into it. The requirement is not to eat fish, or whether it is connected to the crucifixion. The idea is that Catholics should "partially fast", or "abstain" from meat. This seems to be a carry over from the pagan religions which it replaced.

Interestingly the Catholic Church has just opened a window for disaffected Anglicans (there is much distress there over gay bishops. For the Catholics in the audience, the Anglican church is "episcopal", or bishop led. As there is no Pope, that is as high as it goes. In protest, several Massachusetts churches have placed themselves under the administration of an African Bishop). In any case the Catholics require of them "fasting" or "abstinence" in accord with the Book of Common Prayer. I never noticed any mention of it there.

The laws of Kashrut have undergone a similar transformation over time. Somehow a prohibition against boiling a goat in its mother's milk turned into waiting an hour for ice cream after chicken when the rabbis finished playing telephone with the Talmud. My brand of originalism hasn't been well received in the religious community, although my parents did abandon many of the practices when they grew tired of fighting with me. It's been suggested to me that the value of the traditions in fact lies within the arbitrariness of them. I don't subscribe to that viewpoint, but I maintain a healthy respect for those who do.

Eating fish in no longer a sacrifice but a luxury,since fish is now an expensive food item.