Fish has become the protein of choice for the health-conscious. Those who have to lose weight or modify their diet because of heart disease or other ailments are told by their doctors to eat fish instead of other flesh. Fish has just two fifths the fat of chicken, and just one tenth the fat of red meat. As for vitamins, unless you're out in sunlight, it's hard to get natural vitamin D, but you get it by eating fish. Many fish are also rich in those omega acids so good for the heart, or in calcium and other important nutrients.

From a kashrus point of view, too, fish would seem to have much more going for it than meat or fowl. So much can go wrong in the process of ensuring that meat is kosher. Besides being of kosher species, animals or fowl may be slaughtered only by an expert certified shochet known for his religious piety and knowledge of detailed Torah rules. His knife must be razor-sharp without the slightest nick, and the slaughtering process can go wrong in numerous ways. The carcass has to be carefully checked for disqualifying factors such as lung-perforations, then properly salted to remove its blood.

So much can go wrong with meat, in fact, that conscientious Jews eat it only when it is certified by a rabbi or kashrus agency with whom they are closely familiar and know to be absolutely reliable. That's why, at catered affairs, even when supervised by reliable kashrus certifiers, guests very particular about kashrus often ask for fish instead of meat. In comparison to meat, the kosher requirements for fish seem tame. Other than the two signs the Torah specifies for kosher fish, fins and scales (Leviticus 11:9-10), fish needs nothing else to be kosher. Fish needs no ritual slaughter or salting; even its blood is permitted (although we don't consume it unless other fish-parts are visible in it).

To give some idea of the difference in Halachic detail between meat and fish: In the Shulchan Aruch (standard code of Jewish law), rules concerning meat, its slaughter, checking carcasses for disqualifications, salting, what happens when meat gets mixed with milk, and related subjects, take up close to a hundred detailed chapters. Compared to that, the rules for kosher fish are all in one chapter!

Why, then, does fish need kosher supervision?

Here we get to the crux of the problem with today's kashrus realities. If you take your kosher fish home from the fish market while it's still whole, then sure, little can go wrong. But most fish today is processed in some way before it gets to your kitchen.

Even when you buy fish fresh, you may ask the store first to bone and slice it. Most fish markets use the same knives or machines to cut both kosher and non-kosher fish, which creates major kashrus problems. If possible, then, buy your fresh fish from a store selling only kosher fish, preferably whose owner is a conscientious Torah-observant Jew or at least personally committed to eating only kosher.

One complicating factor in kosher supervision of fish is that the Torah requires the kosher signs of each fish to be ascertained before any Jew may eat it. For this we cannot rely even on stringent government certification of a plant as one that uses only kosher species, because the requirement to ascertain the kosher signs is from Torah law itself, with no room for loopholes. So proper supervision of fish requires the constant presence of at least one conscientious mashgiach (kashrus supervisor) throughout the process of any kosher run to check each fish before it is cut up and processed.

Problems arise when fish are caught thousands of miles away. Tuna, for example, is caught by gigantic trawlers in the Pacific Ocean. Some of it is canned on-board; otherwise it is kept in the hold and brought ashore for canning in lands like Peru, Thailand or the Philippines. It's not easy to put a Torah-observant Jew on a trawler among rough and ready fishermen, who may not even speak his language. Besides other difficulties, such a Jew needs to have kosher food to eat.

Before a canning plant can be certified, even for a run of a few days, the usual case for reliably supervised canned fish, it must be made kosher. First it is thoroughly cleaned. Non-kosher fish is totally excluded from the plant or at least kept under sealed lock and key for the duration of the run. Saw-blades used for cutting the fish are replaced or at the very least cleaned with an abrasive cleanser. The gigantic retorts used for steaming cans after they are sealed must be kept empty for 24 hours, then made Kosher by steam. Of course, if other ingredients such as oil are added, these too must come only from reliable kosher sources.

Rabbi David Steigman, a rabbinic coordinator for OK Laboratories, gave us examples of practical problems that often arise. A Pacific-rim company paid for mashgichim to travel out and ready its plant for a kosher run of tuna. For the next few days, however, no trawlers arrived in port, or else they came in empty. After a few days of the mashgichim sitting around with nothing to do, a trawler finally came in on Friday, with enough fish for a three-day run. But now the mashgichim had to leave for Shabbos. They insisted that the plant either be closed during their absence, or else, since the plant would not be supervised over the weekend, that it be koshered all over again before the kosher run would start. The plant's generally cooperative owner explained that financially it would not be worth closing the plant or wasting valuable time to clean it all over. Despite the considerable expense of flying the mashgichim out there and the elaborate process of koshering the plant, that entire run could not be supervised, and the mashgichim flew home after Shabbos with nothing accomplished. . .

All this helps explain why proper kosher supervision often makes products more expensive. Many such hidden expenses are involved, which would never be realized by those who have no direct experience in kosher supervision. Kosher hotels, restaurants and caterers need hefty supplies of boned and sliced fish for cooking, baking, broiling and frying. Until recently, many kosher certifiers allowed hotels, restaurants and caterers under their supervision to use flounder fillets from a European plant supervised by a local rabbi. Last year, however, OK Laboratories discovered that the rabbi provides no constant kosher supervision to check every fish, because he relies on government certification of the plant as one where only flounder is processed. Of course, all kosher certifiers who allowed use of this flounder were informed immediately. OK Laboratories later arranged for a special run of this flounder under constant stringent supervision, costing, though, much more than the usual price for the fish because of the far greater complexity of the supervision.

In another instance, OK Laboratories was asked not long ago about whiting fillets imported from South America to Israel. Since the local rabbi who certifies it there apparently visits the plant only occasionally, OK Laboratories tried to arrange proper supervision with another local rabbi known to be very particular in all details of kashrus. However, his plan for supervision, requiring essential new measures such as the constant presence of mashgichim to check each fish - unavoidably totaled five times the previous price for supervision, and therefore was rejected by the plant's owner. Possibly this fish is still being used by some Israeli hotels, restaurants and caterers usually known for high kashrus standards.

Smoked fish is a separate department. Most people don't realize that smoked whitefish, for example, is first cooked before smoking, creating other kashrus problems. Lox, however, is not cooked before smoking and its supervision presents less problems.

The bottom line is that kosher supervision is rarely simple, even when relevant laws seemingly take up no more than one chapter in the Shulchan Aruch. Soon, however, we will get to eat the most illustrious fish of all the fabled Leviathan, that will be served up, as the Talmud says, at the great national feast after the revelation of the Mashiach. But we can be sure that that fish will have no kashrus problems, for it will be under the strict supervision of the Almighty Himself!