Whatever good reasons there might be for eating fish, or for abandoning vegetarianism, "they eat each other" is a bad one, a confabulation.
No, that isn't implied. There are all sorts of coherent value systems which make ethical distinctions between killing things that kill other things and killing things that don't kill other things. Maybe Franklin was confabulating, but again, that moral does not inspire me. In most cases the reasoning is sound and does move the values a step towards coherency.
There is a difference between dastardly rationalisation and updating your ethical position by eliminating obviously poor thinking.
Fish and other animals are not capable of reflecting ethically on their actions, so they are ethically blameless for whatever they do.
A lot of people are good at not reflecting ethically too, and it does help them get away with stuff (via more effective signalling). This is not a feature of the universe over which I rejoice and nor is it one that I encourage via my ethical signalling.
Maybe Franklin was confabulating
His comment on the matter suggests he thought he was.
The context does not record whether he returned to vegetarianism once away from the temptation.
This is our monthly thread for collecting these little gems and pearls of wisdom, rationality-related quotes you've seen recently, or had stored in your quotesfile for ages, and which might be handy to link to in one of our discussions.