Yes. The basic laws of Kashrut (keeping kosher) are in the Torah (see Deuteronomy ch.14), which according to tradition was given in 1312 BCE. Many of the finer details of kashrut are in the Talmud (especially tractate Hullin), which was handed down orally, and finally put in writing 1500 years ago.
Bacalhau can be kosher if it's made following the rules of kashrut. If it's commercially made, it would have to be certified kosher by a recognized kashrut organization.
Because the rules of food preparation (kashrut) as originated in the Torah were followed.
Kashrut refers to the rules of food preparation and consumption, any food that is produced according to these rules is kosher. Kosher is not a style of cooking.
Kashrut, or "Keeping Kosher"
Kashrut is the name for the system of rules for what food is kosher and what food is not. These rules include such things as a strict prohibition on the mixing of milk and meat (or things derived from milk and meat) in the same meal, rules for which animals may be eaten and how to slaughter them, and special rules that apply during Passover to products made with bread grains (wheat, oats, barley, rye, spelt) or that may be confused with bread grains.
Kosher refers to the process of food production. Anything prepared according to the rules of kashrut is kosher.
The rules of haram (in Islam) and kashrut (in Judaism) are religious in nature, not scientific.
It's cheese made following the rules of kashrut. Kosher cheeses do not contain enzymes derived from animals (rennet and lipase).
There's no exact number, but it's mostly Orthodox Jews who follow the dietary laws.
Mixing dairy and fish is fine according to the laws of kashrut. However, some Jewish groups do not combine fish and dairy as a tradition.
In order for an animal to be considered permissible for eating according to the laws of kashrut, it has to chew its cud and have split hooves (Deuteronomy ch.14). Examples of animals that meet this requirement are cows, sheep (lamb), deer, and buffalo. It also must be slaughtered and processed following the rules set out in the laws of kashrut.
The core rules that Jewish dietary laws are based on are found in the Torah. The actual dietary laws, called 'kashrut', are found in the Talmud.