Though Pythagoras loaned his name to the meatless diet, some biographers suspect he may have eaten fish as well at some points,[9] which would have made him not a vegetarian but a pescatarian by today's standards.

In Plato's ideal republic, a healthy diet would consist of cereals, seeds, beans, fruit, milk, honey and fish.

[11][12] In 675, the consumption of livestock and wild animals was banned in Japan by Emperor Tenmu, due to the influence of Buddhism and the lack of arable land.

During the 1200 years from the Nara period to the Meiji Restoration in the latter half of the 19th century, Japanese people ate vegetarian-style meals, and on special occasions, seafood was served.

[15] Exceptions were wild fowl served amongst the Heian nobility, [16] and when Europeans arrived in Japan in the 15th century, the Japanese diet included boar meat.

[27][28] In 1336, Pope Benedict XII permitted monks to eat meat four days a week outside of the fast season if it was not served in the refectory.

[28] The anchorites of England ate a pescetarian diet of fish seasoned with apples and herbs, bean or pea soup and milk, butter and oil.

In 2018, Ipsos MORI reported 73% of people worldwide followed a diet where both meat and non-animal products were regularly consumed, with 14% considered as flexitarians, 5% vegetarians, 3% vegans, and 3% pescetarians.

[45] Pescetarianism may be perceived as a more ethical choice because fish and shellfish may not experience fear, pain, and suffering as more complex animals like mammals and other tetrapods do.

[50][51] Some pescetarians may regard their diet as a transition to vegetarianism, while others may consider it an ethical compromise,[52] often as a practical necessity to obtain nutrients that are absent, not easily found, or not readily bioavailable in plants.

[53] It is common for all kinds of meat-abstainers to participate in the "green movement" and be conscientious about global food sustainability and environmentalism;[54] switching to a pescetarian dietary pattern can potentially positively affect both.

[64] A common reason for adoption of pescetarianism may be health-related, such as fish and plant food consumption as part of the Mediterranean diet, which is associated with lowered risk of cardiovascular diseases.

[71] According to a 2018 global consumer survey, the majority of pescetarians, vegetarians and vegans (87% prevalence) reported that their food product choices are influenced by ideological factors, like ethical concerns, environmental impact or social responsibility.

The lack of fins and scales also deems crustaceans (e.g. shrimp, crab, lobster) and molluscs (e.g. oyster, clam, conch, octopus, squid) to be "treif"—non-kosher.

[82] The society has several advocacy interests: public health, promoting healthy eating, praising pescetarianism as "the natural human diet", supporting better animal welfare, bringing awareness to the climate-change crisis, and demanding seafood be sustainable and responsibly-caught.

[88] Though the Rastafari are generally associated with avid vegetarianism and veganism, a large minority of adherents do deem certain kinds[89] of fish to be an acceptable exception in the Ital diet.

Rastafari who permit fish will avoid eating all kinds of shellfish as they are considered to be "unclean" scavengers,[90][91] a belief that stems from biblical teachings.