I created this fantasy race called Seadominians, they are basically merfolk. They are mostly bipedal, have scales covering all over, and live all throughout the Atlantic and Pacific Ocean, also the Antarctic Ocean but that has a population under 1 million. Once that live closer to shore have darker skin (In many colors, mainly green and blue but also come in pink, yellow, orange, red and such) But ones that live in the trenches have very light to almost white skin...well more so very thin scales. They also have toxic barbs at the side of their feet, that can leave a blue whale unconscious for 1 hour. And Talons that have grips on them to hold on to their pray. They hunt in small family groups and bring back whatever they catch back to their town. (Towns have roughly 100,000+ individuals at any given time and send out multiple hunting parties a week) They can go one month max without food, before they slowly die within a few weeks to starvation, though they prefer to eat daily, about on to two meals a day, but if needed (food shortage) they can happily go on week with a few meals. They have at least 4 eggs a clutch and 2 of four die due to a common childhood disease that makes their gills constrict to the point no air gets to their lungs. Families normally have 10 clutches on average. The eggs have a very tin and somewhat flexible shell that is easily broken with cornered objects. Around 8/10 children who don't get the deadly disease (that I mentioned earlier) reach age 18 and roughly 6/10 of those live long enough to reproduce on their own. Tiger like stripes on the face or arms are favorable in partners due to it being a sign of high fertility (Except for the Trench subspecies since their partially blind [to the point they only see the outlines of shapes and features and mainly focus on smell]). Trench subspecies mainly focus on the smell of their partners, for feminine genders it's favorable to have a sweet sent, for masculine genders its favorable to have an ash and sulfuric sent. (Most Trench Subdominants are lower in society and work in ore/coal mines that are manufactured into good and traded with other seafolk and sold within the shops and higher cities. As a result, are greatly valued in the society but highly looked down upon and scoffed at their needs by many emperors. So, they often get whatever trickles down to the depths of society [Hehe see what I did there]. So they get a lot less food and resources than the higher ups.) They tend to weigh 400 pounds on average and be 10 foot tall. Their lifespan is up to 3000 years at max, average is 2000. (20 human years= 1 subdominant year). If there's any more information you need to determine their suitable diet
If they are of human-like intelligence and social, I think they would start as hunter-gatherers and eventually develop farming.
I think fish would be harder to keep from escaping than something slower like a crab, lobster or sea cucumber. The boon of agriculture would stimulate technological development, and eventually they would find a way to farm fish or squid. The species would likely vary by region, but there would also be trade of exotic delicacies across oceans, like the human Spice Road.
There are all sorts of sea cucumbers littering the ocean and the Seadominians eat them all though not all at once; one at a time (except for dudes showing off). Having the main food be sea cucumbers makes hunting expeditions less taxing because sea cucumbers are very slow and even little kids and very old people participate. They basically bring bags and fill them up with sea cucumbers they pick up. Sea cucumbers are not very calorically dense and so the Seadominians are munching on them most of the time, often without using hands; these sea cucumbers appear like living holothurian cigars and are munched as these ocean dwellers swim about.
Seadominians also worship the sea cucumber as their main food source, and their god is a sea cucumber. Not an immense sea cucumber but just a regular one with no special properties except being a god. The Seadominians also have sea cucumbers on their flags and when they wear clothes it is with a sea cucumber print although mostly they go naked and everyone is cool with that.