Come the era that we have a daily diet of cats killing cats in fictional stories on the TV, you may reverse that feeling.
My very large and authoritive book on sayings and their origins suggests that this comes from ye dayes of olde when archers would practice shooting at a cat tied to a length of rope and swung around by a serf. That image should cause Joe some quite conciderable distress.
Here's one explanation. Don't know how true it is http://www.****e.org/cgi-bin/pooclub.cgi?p=chat&c=phrases&i=kettleoffish
That different-kettle-of-fish link doesn't appear to work, so here's another: http://www.worldwidewords.org/qa/qa-ket1.htm
There is nothing like a rainbow trout freshly tickled from a tributary to the Test and taken home and grilled with butter and some almonds and eaten with fresh Hampshire field mushrooms and fresh Hampshire watercress. It is a superb meal and if you were very naughty like my poacher of a little brother a meal that cost absolutely nothing but the cost of the energy for the grill. Andover Saint might know what I am talking about.
Is that where the saying "holy mackerel" comes from then........or is it the more likely from the feeding of the 5000!!