For some reason, my mermaids have gone vegetarian - ethics, the decline in fish species, or they’re just no longer in the mood for meat. The main issue: will they survive?
They live in coastal saltwater areas, rather than being deep-sea creatures, and can only be in air for the same length of time as a human can be underwater (probably 1-2 minutes; you’ll get the odd ‘freediving’ mermaid who can go for much longer, but that’s an anomaly), and they really struggle to move around when not in water, so gathering anything from the land is probably going to be limited. There’s also no trade between merfolk and humans, so they can only rely on what they find, forage, or grow themselves.
What could they eat that will give them enough calorific intake to survive? (Assuming the same 2,000-2,500 kcal intake advised for normal humans). I’m not fussed about geographic location, so if there’s a region of (present-day) Earth better suited to support their existence, that’s fine.
I’ve pictured them as wanderers rather than settled, so a hunter-gatherer approach is more what I had in mind. However, I’m happy to be persuaded if an agricultural society would be more conducive to their new way of living.
Ideally, I’d like my merfolk not to need to spend 14 hours of every day eating (like pandas) just to get what they need, while also having a nutritionally complete diet. If there’s something that’s calorie dense but isn’t necessarily balanced, then I’ll take that. And if this vegetarian lifestyle isn’t going to work, I’ll just have to convince them to go back to omnivorousness.
Sorry, forgot to include this: I’ve had a look at this question, but while it mentions herbivorous merfolk, it seems to be less interested in the constitution of their diet.