The efficiency of syringe stomach flushing in diet sampling of salmonids

Background. A sampling of stomach and intestine content is usually performed by dissection of the gastrointestinal tract of fish. To avoid fish sacrificing, various non-lethal techniques have been developed and tested. Such sampling methods are very useful, especially for small populations and protected species. In this study, a modified syringe stomach flushing was applied on Oncorhynchus mykiss (Walbaum, 1792), and Salmo trutta Linnaeus, 1758, to test its efficiency in retrieving stomach content. Materials and methods. Water was injected repeatedly by a syringe through a silicone tube directly into the stomach of live fish to flush the content out. Afterwards, fish were dissected to collect residual stomach content. The efficiency of the method was assessed as the percentage of flushed content by weight and as the percentage of the number of flushed prey items, both in respect to the whole stomach content (100%). The relation between the fish body size (length and weight) and efficiency of flushing (expressed as the weight and number of flushed prey) was tested by linear regression. The sensitivity of the method was tested with respect to 25 designated prey types. The share of each prey type was compared in the pooled sample of flushed and residual content. Results. Collected stomach content was well preserved for identification of ingested organisms. From 25 designated prey types, 17 were 100% flushed, 4 over 90%, one over 80%, 2 were 44% effectively flushed (gastropods and caddisflies in stone cases), and 1 prey type was present only in the residual sample (Gordius sp.). The efficiency of the method assessed as the mean percentage of flushed content by weight was found to be 78.78%, while 91.99% of prey items were effectively flushed. Conclusion. The applied modification of syringe stomach flushing was found to be effective for investigation of stomach content of salmonid fish since (1) collected prey items were well preserved and easily identified; (2) the percentage of flushed prey items was high (91.99%); and (3) the method is easily applicable and inexpensive.