Let's take the succinct definition straight from Merriam-Webster online. A trash fish is "a marine fish having little or no market value as human food but used sometimes in the production of raw meal." These are fish that usually get tossed back into the water when they're hooked. Most of the trash fish used in commercial shrimp feed are accidentally caught as by-product of trawling nets.
Due to freshness and excellent nutrient content, trash fish are a suitable and cheap source to put into shrimp food -- especially when it comes to cost. After all, they're sitting there in the nets. The fishermen can either throw them back and make nothing for their trouble or sell them to shrimp meal producers and recoup some of the expense of running the boat. There has yet to be found a better source of nutrition for shrimp growth. Trash fish come naturally equipped with easily digestible essential amino acids. As yet, the shrimp farming industry has come up with neither a manufactured or natural raw material for shrimp meal that works so well.
But there is a problem with the use of trash fish in food for shrimp farmers. Many are afraid to introduce shrimp meal containing trash fish on a large scale due to the risk of pond pollution and disease outbreaks. They're worried. As soon as the trash fish die in the nets or on the docks, the process of deterioration begins when bacteria do what they were put on this earth to do -- decompose formerly living tissue. As quality control techniques and processing methods improve, the shrimp farming industry's goal is to improve the percentage of trash fish included with shrimp meal. Right now the average is 10 to 15 percent. One particular issue with trash fish is their high fat content, which oxidizes quickly and becomes poisonous to the shrimp, thereby slowing growth and production on the farms. Processors have to get trash fish into freezers quickly and process them into aquaculture food products with haste. Once they have a system in place that overcomes obstacles like this, shrimp farmers will be more willing to buy higher concentrations of trash fish in their shrimp food.