Catfish have always been déclassé, a poor peoples fish... until they began showing up as "blackened catfish" in fine restaurants. Many fishermen still scorn them, but perhaps it is in part because they never learned and efficient skinning technique. In part it was because they populate polluted water, surviving in low-oxygen conditions other fish cannot survive. It is certainly about where you catch them.
Sharks have always been summarily thrown back, along with other trash fish. Then mako started showing up at restaurants alongside swordfish and tuna.
Cod have always been the staple of New England fishing: mild in flavor, easy to catch, and plentiful. The cod fishery was temporarily closed when the regulatory folks decided that commercial fishing had brought them to commercial extinction. The fish-&-chips people quickly learned that dogfish (sand sharks) tasted very much the same, were plentiful, and set about catch as many of those as possible. The fisheries people concluded that the dogfish too were under great pressure also and must be protected. Commercial catches of both cod and dogfish remain heavily restricted and populations have rebounded, although both still bare watching. It is a success story from what I can see; when visiting Cape May this summer, dogfish were all anyone could catch close to shore or in the backwaters, and there were hordes of them!
Well, I am converted. In the past I always threw them back; this year Jessica questioned me on that., after I threw 2 back, one after the other. "Why not try them?" We did try them, Cajun style that first time, and she was right and I was wrong; fine eating with no bones.
Even more than catfish, they have an incredibly tough hide and must be skinned. If anything the operation is simpler, though, because they are not slippery but rather like 150 grit sandpaper. In fact, they are quite easy to handle, as they have no teeth, no dangerous spines, and are easy to hold on to. Skinning instructions can be found all over the web, better than what I can relate; youtube is best (http://www.youtube.com/watch?v=dye0aK4FG6A). Perhaps their best characteristic, at least in preparing smaller ones, is that they have no bones! No filleting step is needed, minimizing waste. A cartilaginous spine is easily removed leaving a long trunk of solid meat that is easily prepared in any number of ways.
Bleed them as soon as possible after catching. They will taste better. Like catfish, they do not die quickly out of water, so clubbing may be involved. Just taking the head off will not kill them, strangely.
Skates are next on my target list. They show up in restaurants occasionally, fish markets regularly, and occasionally, as fake scallops (cut round with a cookie cutter). There is, of course, the venomous spine to contend with. I haven kept one yet, but I will keep the next one and I will report in!
For more information (regulatory and background) on dog fish and atlantic cod, visit the NOAA information sites: http://www.nmfs.noaa.gov/fishwatch/species/atl_spiny_dogfish.htm and http://www.nmfs.noaa.gov/fishwatch/species/atlantic_cod.htm.
For information on skinning: http://www.youtube.com/watch?v=dye0aK4FG6A