cetaceans of any kind (what is basically a large swimming pool can never be a replication of the North Atlantic)
As it has been stated numerous times on ZC: not all of the 90-odd cetacean species have the same husbandry requirements, and not all of them live in the North Atlantic. I'm well aware that keeping cetacea is quite an emotional topic for many people, but sweeping generalisations all too often miss the mark here.
As for the necessity of farm animals in zoos: I tend to agree with Youssarian. One shouldn't forget that the ongoing global urbanisation often goes along with a growing alienation of urban children from agriculture and farm animals (or animals in general). Since most zoos are located in or close to metropolitan areas, their petting farms are often the first (and sometimes only) access of city slicker kids to farm animals. And to be allowed to get close, feed and touch some of them (unlike the aforementioned big cats, bears, otters, elephants, rhinos, monkeys, etc.) makes them a very popular attraction for families with young children, who form the majority of zoo visitors.
However, one shouldn't forget to mention the negative aspects as well. And I do not mean the "They takin' away our space" aspect frequently mentioned before.
All too often, the "farm/barn" exhibits in most western zoos are generic, boring and unoriginal, portray modern agriculture in a very idealistic way far from today's reality and fail to raise public awareness in a proper way. The husbandry requirements of farm animals frequently rank behind visitor expectations and other, "more valuable" animals; after all, they are "just" domestic animals. Just think of all the cattle, donkeys or ponies in their small barren zoo enclosures or the rabbits in outdated wooden rabbit boxes that you've quickly walked past on your way to the latest great ape or elephant exhibit. Thankfully, at least some European zoos have started to improve their situation, but it's still an exception from the rule. Modern zoos should be public role models for modern husbandry standards. In the case of domestic / farm animals, they all too often fail to live up to that, as various dedicated private keepers never tire to point out.
Then there's the aspect of public health, hygiene and animal welfare. As for the latter: for every kept dolphin, there are hundreds if not thousands of petting zoo animals that have been molested, injured, mutilated and killed, often due to feeding them illegally. Dying of gastrointestinal bloating or foreign body obstipation is a horrible way to go.
So should zoos get rid of all farm and domestic animals and leave them to special petting zoos, teaching farms and visitor / model farmsteads? I somehow doubt that despite the aforementioned aspects, any zoo director in his / her right mind would do so and thus eliminate one the respective zoo's most popular attractions.