Try http://www.kashrus.org/kosher/fish.html for an explanation of the kashrut status of fish, and a stunningly comprehensive list of kosher and non-kosher fishes.
The Jewfish, incidentally, is kosher (naturally ;j).
Rick
Thanks for the link, RickG. However, by looking at the list, I find that the porkfish is kosher! :eek: ;j
Zev Steinhardt
There’s a pretty good book on this subject, * Cows, Pigs, Wars and Witches, * but the author’s name escapes me at the moment.
About the original question, I remember reading a few years ago an interesting article in a Discover magazine that had a theory about that. An anthropologist had studied various Amazonian tribes each one with food taboos. The anthropologist could not find a single health/nutrition reason why a certain food was taboo for one tribe since it was perfectly all right for all the neighbouring tribes.
If I remember correctly, each tribe had a word for its members that meant ‘human’ or ‘people’ and another word for everybody else that meant ‘non-people’ or ‘others’. When members of a tribe were asked why they didn’t eat a certain food they answered something like “People don’t eat this”. When asked why members from other tribe ate it they simply answered: “They are not people, so they eat this”.
The conclusion of the anthropologist was that food taboos, not unlike facial paint, body piercing, hair style, clothing, etc. helped people identify to a group, strenghten their sense of community, and define outsiders to the group.
This theory is repeated by Stephen Pinker in his recent book “How the Mind Works”. But first he quotes (and dismisses out of hand) learned Jewish opinion on the dietary laws - I paraphrase:
Aristeas (1st century BC): Abstention from eating blood tames man’s instinct for bloodshed.
Isaac ben Moses Arama: These foods defile and pollute the soul and lead to confusion and destruction.
Nahmanides: The “fins and scales” rule relates to the habits of finny scaly fish, which live in clear water near the surface, while the others live in deep cold muddy water.
He believes the real reason is as a tribal marker, exploiting the psychology of disgust. If you don’t eat something, you’ll find the idea of eating it disgusting, and therefore will be revolted by those who do (sheep’s eyes, anyone?)
This serves to prevent defection to the neighbouring tribe and also the opportunity of treasonable or disloyal behaviour that would arise from dining with the neighbouring tribe.
[QUOTE **A similar controversy exists about the sturgeon as well.
Zev Steinhardt **[/QUOTE]
True, but imagine the Jewish mother of 2001 B.C.E…
"Oye, such a boy to make his mother proud, he’s a world famous sturgeon !!! ". 
Cartooniverse
<---- Bar Mitzvah’d in the Oldest Ashkenazic Temple in North America.
[clarification]
Shark scales are also called dermal denticles (the word I meant rather than dentin, in my previous post).
From Sharks of the World by Rodney Steel (Facts on File - 1985 Blanford Press) pages 50-51 (of the 1994 reprint).
Placoid Scales
The skin of sharks is covered by placoid scales - backwardly projecting tooth-like structures…that make the hide intensely abrasive if it is rubbed the wrong way, i.e. from back to front. These scales vary in shape from one region of the body to another. They may, for example, be simple and rounded on the snout, shield-like on the belly, keeled along the flanks (to facilitate the shark’s passage through the water), diamond-shaped along the front edge of the fins, where a firm sharp edge is required, and tiny quadrangular shaped structures where they line the pharynx (throat). The denticles of young sharks may be of a different pattern to the denticles of the same species when fully mature…(Physiology of shark scale and comparison to teeth)…In very ancient fishes, a heavy external dermal armour was present, with a surface layer of bony denticles. The scales and teeth (modified scales) seem to be all that is left of these original denticles, the bony substratum having been lost early on in the evolutionary development of the group.
The pen-and-ink illustrations on page 50 show the tooth-like structure of a cross-sectioned denticle, and denticles of the whale shark (Rhincodon) [looks like a sharp-edged leaf) and the grey shark (Charcharhinus) [looks like a ridgy potato chip].
[/clarification]