Pescatarian is a pretty common restrictive diet, mostly just leaves out land meat. It’s not generally a diet based on the same reasons for being restrictive as vegetarian or vegan. While some people can arrive at it ideologically, like this commercial is displaying, it’s more commonly a conclusion people would choose based on health concerns rather than ideology. And from what I could read from the french I saw, it does seem like despite the depictions in the cartoon, the message is something like " it doesn’t matter how you arrive at a healthy diet, as long as you make the effort" or something like that. So it seems like they are more concerned with health generally than ideology.

But if you want to get creative, here’s an ideological explanation. The forest critters government has spent decades if not centuries grievously 'other’ing fish to the point that their populace doesn’t think of them as animals anymore. And their sex education is so attrocious that until any individual animal sees it for themselves, they have no idea where babies come from. And by then they are so traumatised by what they have already done to and with eggs that they can’t bring themselves to accept the truth, they gaslight themselves into thinking that was a freak occurence.