Fish and Meat
Although modern-day medicine and science may beg to differ, the sages of the Talmud were under the impression that eating fish and meat together is extremely dangerous to one’s health. In fact, in those days, there was actually a dermatological condition that was believed to be caused by eating fish and meat together. The rabbis, therefore, prohibited the consumption of such mixtures, a practice which continues to this day. The ban applies to eating fowl and fish together, as well. When intending to eat both fish and meat at the same meal, the fish should be served first as it is considered to be a more digestible food.
Clearly, however, today this is not the case and eating fish and meat together is harmless. Some try to reconcile this medical contradiction by suggesting that what was unhealthy in those days may not be unhealthy in ours. In fact, in Talmudic times it was even believed that rotten fish was good for you! Without delving into the commentary, it is worth noting that Rabbi Avraham Maimonides, son of the renowned Rambam, advises us not to rely on rabbis for medical advice. Others suggest that the health concern only applied to a specific fish and not to all varieties.
It is important to note that the prohibition on eating fish and meat together is unlike the one forbidding mixing milk and meat. Here, it is merely forbidden to eat fish and meat at the exact same time, or in immediate succession. A waiting period, however, is not necessary. This would logically extend to restrict baking dishes with these two types of foods in the oven at the same time, unless both pots are well covered, lest any tastes, via the steam, spread from one to the other. The dishes used for eating them, however, may be interchanged, and hence there is no need for special “fish dishes”, though some authorities do recommend keeping pots specifically for cooking fish. With regard to the use of knives for chopping foods, it is considered acceptable, for example, to use a clean meat knife to slice onions that will be cooked with fish. Even if fish and meat were accidentally cooked together in the same pot, although the food may not be eaten, the pot retains its kosher status.
Some communities are incredibly strict about this practice, and even require the washing of hands between fish and meat courses, as well as the mouth. The washing of the mouth is considered accomplished by merely swishing some scotch or other beverage (with the exception of plain water) around in one’s mouth. Indeed, it is required for one to somehow “cleanse” the mouth in between fish and meat, even by merely eating some other food in between. Common custom requires one to use a clean or different piece of silverware when switching from fish to meat at the same meal, due to the residue that may remain on the utensil. Both foods, however, may be on the table at the same time though some authorities insist that fish and meat dishes should not be on the table together.
All of these rules pose serious concerns to lovers of Worcestershire sauce, a popular steak sauce that includes anchovies in its ingredients. The resolution of this problem is actually quite interesting. You may be aware that in Jewish law, anything less than one-sixtieth of a mixture is considered insignificant and nullified. For example, if a tiny drop of milk fell into a big pot of chicken soup, the chicken soup will remain permitted, as the milk content would be less than one-sixtieth. So too, there are Worcestershire sauces in which the anchovy content is so small that it is considered insignificant and permitted to be used with steaks. Sadly, Lea and Perrins brand Worcestershire sauce contains large anchovy content and may not be used for steaks!
On a related topic, there are also some Sefardic and Chassidic communities which avoid eating fish and milk together (e.g. lox and cream cheese). This is because mixing fish and milk was once considered to be dangerous to one’s health, as well. This is, of course, no longer true and all mixtures of fish and milk are permitted, including frying fish in butter. Avoiding fish and milk is more of an act of piety rather than a halachic requirement. In any event, one should be sure to follow one's family custom regarding this issue. It is interesting to note that tuna or salmon that come from the Atlantic Ocean will not taste the same as those from the Pacific Ocean.
While there may be certain customs or rituals which were enacted for health measures that make little sense in today's world of modern science and medicine, it is worth recalling that our sages teach us that “a danger to life [no matter how small] is even stricter than a Torah prohibition.”
 Pesachim 76b. Note: the Rambam mysteriously omits any mention of a prohibition on eating fish and meat together.
 Y.D. 116:2, Kitzur Shulchan Aruch 33:2
 Pitchei Teshuva Y.D. 116:2.
 Kitzur Shulchan Aruch 32:12
 See Bereishit Rabba 34:13, 60:10.
 Magen Avraham O.C. 173:1, Aruch Hashulchan Y.D. 116:10.
 Tosafot;Mo'ed Katan 11a.
 Cited in the introduction to Ein Yaakov. See also: Tosfot;Mo'ed Katan 11a
 Chatam Sofer Y.D. 101
 As with any pareve food. See Taz;Y.D. 95:3 for more on this.
 Taz Y.D. 95:3
 O.C. 173:2.
 Kaf Hachaim;Y.D. 116:3.
 Tosfot;Mo'ed Katan 11a, Aruch Hashulchan Y.D. 116:10
 Rama Y.D. 116:3
 Y.D. 95:3;Taz, Yabia Omer, Y.D. 6:9.
 Kaf Hachaim O.C. 173:6
 Although it may be suggested that there remains a prohibition of "ein mevatlin issur l’chatchila" (one may not intentionally cause a forbidden food to become permitted through nullification), some authorities permit one to consume such a food when a) all products being combined are inherently kosher on their own and b) the bittul is being done by a non-Jew. A full discussion of this issue is beyond the scope of this work.
 Beit Yosef Y.D. 87:3; Shach Y.D. 87:5; Pitchei Teshuvah Y.D. 87:9.
 Pitchei Teshuva Y.D. 87:9; Darkei Teshuva 116:43. See Darkei Moshe to Beit Yosef Y.D. 87.
 Shach, Y.D. 87:5; Taz;Y.D. 87:3, Aruch Hashulchan Y.D. 87:15, Yechave Daat 6:48
 Kaf Hachaim O.C. 173:3
 Rashi, Bereishit 1:10
 Y.D. 116:5