A dolphinarium is an aquarium for dolphins. The dolphins are usually kept in a large pool, though occasionally they may be kept in pens in the open sea, either for research or for public performances. Some dolphinariums consist of one pool where dolphins perform for the public, others are part of larger parks, such as marine mammal parks, zoos or theme parks, keeping other animals and having other attractions as well.While cetaceans have been held in captivity since the 1860s, the first commercial dolphinarium was opened only in 1938. Their popularity increased rapidly until the 1960s.Since the 1970s, increasing concern for animal welfare lead to stricter regulation, which in several countries ultimately resulted in the closure of all dolphinariums in the past decades. Despite this trend, dolphinariums are still widely spread in Europe, Japan and North America.The most common species of dolphin kept in dolphinariums are Bottlenose Dolphins as they are relatively easy to train and have a long lifespan in captivity.