Kashrut
Among the numerous laws that form "kashrut" are prohibitions on the consumption of certain animals (such as pork and shellfish), mixtures of meat and milk, and the commandment to slaughter mammals and birds according to a process known as "shechita". There are also laws regarding agricultural produce that might impact the suitability of food for consumption.
Kosher animals
Traditionally fins has been interpreted as referring to translucent fins. The Mishnah claims that all fish with scales will also have fins, but that the reverse is not always true. For the latter case, the Talmud argues that ritually clean fish have a distinct spinal column and flatish face, while ritually unclean fish don't have spinal columns and have pointy heads, which would define the shark and sturgeon (and related fish) as ritually unclean. Nevertheless, Aaron Chorin, a prominent 19th-century rabbi and reformer, declared that the sturgeon was actually ritually pure, and hence permissible to eat. Many Conservative rabbis now view these particular fish as being kosher, but most Orthodox rabbis do not. The question for sturgeon is particularly significant as most caviar consists of sturgeon eggs, and therefore cannot be kosher if the sturgeon itself is not. Sturgeon-derived caviar is not eaten by some Kosher-observant Jews because sturgeon possess ganoid scales instead of the usual ctenoid and cycloid scales. There is a "kosher caviar" made of seaweeds.
The salmon roe is also "kosher".