like sardines do and mackerel don’t…
don’t you feel that there is so much intra-species variation in most other species, but not in humans? all we have diff are skin colour, hair, eyes -colour and shape and nothing much else of significance.
whereas look at all the amazlingly different kinds of fish there are, all the kinds of birds…
I don’t know the answer to your question, but I would like to point out that fish are not one species, nor are birds one species.
Um…“fish” isn’t a species. Neither is “birds”. There are thousands, at least, of different species of fish and birds.
As for the variation: note that you are comparing large groups to our single species. There are lots of species of birds, but individual species don’t necessarily exhibit more variation than we do (if anything, quite the reverse, since humans are widely variable in outward appearance). Same with fish: many species, each of which has its own range of variation.
Do fish without scales have anything in common? If I remember my Old Testament, God said that eating fish without scales wasn’t kosher. Are they more likely to be scavengers?
Leviticus 11:9 "Of all the creatures living in the water of the seas and the streams, you may eat any that have fins and scales. "
I guess this is more about excluding dolphins, lobster, shrimp, etc.
Dolphins aren’t fish, but you knew that right?
And bats aren’t birds, but don’t tell Leviticus. 
Catfish don’t have scales. And one of the nearby rivers is so chock-full that a mouse won’t go within three miles of it.
And dolphins are fish or mammals, depending upon which kind you’re talking about.
Well, there is a dolphinfish, but it’s not a dolphin. Is that what you’re talking about?
He’s talking about the fact that “dolphin” is, confusingly, another name for the mahi mahi or dorado, most decidedly a fish. People sometimes use “dolphinfish” to attempt to avoid the confusion, but it is not uncommon for the creature to be refered to simply as a “dolphin”, as in this article:
Not going to argue the point, but the fact is “dolphin” has been ascribed to a species of fish since the 16th century, although the same word applied to the mammal long predates that usage.
Depends what type of catfish you’re talking about as to whether they have scales; there are hundreds of species that do. Fish types and names often get screwed up, so a person has to be very specific when they say XXX group has this characteristic.
ThreeGumpy:
A common translation mistake. The Hebrew word used in Leviticus is “Of”, which means “winged creatures” or “flying things”, (a noun form of the root “to fly”) not necessarily birds, which in Hebrew would be “Tzippor”.
And Taxonomy was created before the bible, right?