Quote Originally Posted by MikeM View Post
All I would add is that any type of fish meat being fed may be fine as a treat or supplement, but not as a well-balanced, complete diet. In Nature, a large fish eating a small fish is consuming the whole fish, organs, muscle, bones, scales and all. If you slop a bunch of fish guts into the pond, you'll have a mess. Consider using the mackerel as an ingredient in a homemade food that uses gelatin as a binder. If you use the forum search function, you'll find some recipes. (And, don't feed the bones! ... Bones are OK when pulverized into a powder and used as an ingredient in a pellet. Fishbones are just as dangerous to koi as to people.)
Very good points Mike. I supplement with whole anchovies (small ones as there are large anchovies also). The reason I do is because the koi food I use uses plenty of soy meal, which I personally dislike but have to live with as my choices are limited given my budget. Also, live food from the sea, which as mentioned by Dick is safer to use for freshwater fish such as koi, balances out the blended, rendered, and manufactured nature of koi pellets. Generally, pellets give us the advantage of convenience but lose out on their wholeness. They lack fats and are heavy on carbs, and the protein has gone thru heat and possibly chemical processing that makes it suitable for animals but not humans. Which is why we prefer fast foods over pellets, if convenience is what we look for. Furthermore, raw food comes complete with enzymes and probiotics, which can't be said of pellets. Better assimilation of nutrients contribute greatly to the koi's development. Better digestion means less waste production. Pellets are loaded with carbohydrates, which are not easily digested and form a large part of solid wastes.

Mackerel is an even better than anchovies, in that the higher fat content provides koi with more energy for use in basal metabolism. Less protein used for basal metabolism means less ammonia production, which can only be good for water quality since less biofiltration is needed. More protein is used for growth and for other functions such as to strengthen immunity and to develop koi coloration.

I would get smaller sized fish of the same family as mackerel and tuna. Scad, which we know as galunggong in the Philippines, would be ideally sized. Although I've only toyed with the idea, I would chop off the head along with the innards, and use the remainder of the scad to feed koi. To make it easy to chop, I would chill the scad in the freezer where it is just firm enough to be sliced into steaks (as opposed to filleting it). Since scads are small, you're looking at mini-steaks for koi. It's much much smaller than halibut steaks, just in case you're thinking "Oh my, the koi is eating my dinner." As for the head and innards, the cat will readily warm up to you.