Asian carp diets
I am a fisheries biologist for the USGS working with Asian carp. You may have seen me on CBS evening news on Wednesday night. The original post asked why people catch Asian carp on various baits or lures, when they are filter feeders. First off, there are four different kinds of Asian carp introduced to the US (six if you count common carp and goldfish). Only two of these fish are filter feeders, the bighead carp and the silver carp. Adult grass carp are primarily vegetarians, although they will eat nearly anything if it is easy to catch - they won't turn up their noses at a worm under a bobber. Black carp, which to our knowledge do not yet have a self-sustaining population in the US, eat primarily molluscs. Bighead and silver carp are primarily filter feeders. I have cut open hundreds of silver and bighead carp, and I have never found anything in the gut that would be identifiable without a hand loupe, if not a microscope. So almost all of what they eat is filtered. That said, I once caught a 4-pound silver carp on a three-inch diving plug, and I have heard of many people catching silver carp and bighead carp on a variety of baits. One theory is that the fish essentially snag themselves on the hooks, or become snagged. These fish are incredibly numerous in some areas, and they pump a lot of water through those mouths, so it is possible that they just suck in a lot of things that they don't really want. The fish also sometimes use a behavior called ram-feeding, which is swimming throught the water with their mouth open to push the water the water through their filtration apparatus. They may sometimes just swim into trotlines or baits hung below bobbers. There are so many fish out there that you would expect it to happen regularly. And that silver that I caught trolling a diving plug may have been minding its own business, filtering away, when my plug came by and got it (in the mouth, though). Another possibility includes the thought that most fish are opportunists, and if the bigheads or silvers see something that is just too good to pass up, they wolf it down. They might not be excellent predators, and are not likely to expend a lot of energy trying to chase down minnows, but if it is just hanging there looking like a free lunch..... Lastly, there is the issue of anger or aggressive strikes against something they don't really mean to eat. Salmon and shad don't really eat while migrating, but you can catch them on hook and line. Probably all of these different explanations are true at different times. But silver and bighead carp rarely eat anything of any size; I can verify that from hundreds of guts I have examined. In fact, the throat is fairly small and it would be hard to get anything the size of my diving plug through it. And if they did succeed in getting a spiny fish down the throat, the gut (they don't really have a stomach, since they don't need one to eat plankton instead of larger objects) is not very robust, and the fins would likely puncture the gut, resulting in septicemia and likely death.