Backwashing In Fish

Most studies have traditionally sacrificed relatively large numbers of fish to examine their stomach contents. However, sacrificing fish for food habit studies may cause public relations issues. They may not be a convenient option if study fish which are threatened, endangered, economically valuable, or come from a low density population. Additionally, lethal methods may significantly alter the population structure of fish. Accordingly, a number of nonlethal methods were developed to obtain and analyse the gut content of fish. They are based on the stomach contents extraction by mechanical means, or emetic substances (Hyslop 1980; Bowen 1996; Kamler & Pope 2001 and many references there; Elosegi & Sabateri 2009). Nonlethal methods include the

This technique could obtain stomach contents by anal backwashing for fish species with no pyloric sphincter, or by stomach flushing through the esophagus for species with a pyloric sphincter. A short tube can be inserted through the anus into the intestine of the fish. Water was injected through the tube with a hypodermic needle and the gut and stomach contents is flushed out through the mouth. Other way how to use syringes is to inject water directly into the stomach of fish through the mouth and esophagus. The stomach contents is flushed back through the esophagus and mouth, through a funnel into a container. More complicated device uses two sizes of syringes, a plastic tube, and interchangeable rubber tubes. Water was forced into the stomach through the esophagus by the smaller syringe through a plastic tube which is encased within a rubber tube that led to a larger syringe for the collection of the stomach contents. The diameter of the tubes and the volume (capacity) of syringes depend on size of the fish. These methods have been evaluated as very effective removing up to 100 % of the stomach contents. Although syringes may be effective for some species of fish, they may be ineffective and even fatal in other species. Water pressures associated with stomach flushing may probably harm swim bladders and causing other internal injuries in small or juvenile fish. Softer material (intramedic tubing) and careful handling may perhaps reduce the mortality (Kamler & Pope 2001 and many references