Volatile fatty acids and anaerobic fermentation in temperate piscivorous and omnivorous freshwater fish

Endproducts of anaerobic fermentation, volatile fatty acids (VFAs), occur in the intestines of fish but no direct comparisons exist between VFA levels in fish with differing feeding ecologies during different seasons. We measured intestinal concentrations of six types of VFA in the upper and lower intestines of two freshwater omnivores (Cyprinus carpio and Dorosoma cepedianum), and one piscivore (Micropterus salmoides) during the spring, summer and autumn. Acetate occurred in all species, and was highest in M. salmoides. In all species, concentrations were similar between upper and lower guts and higher during the summer. All three species contained anerobic bacteria and C. carpio and D. cepedianum contained cellulolytic types. Scanning electron microscopy revealed extensive colonization and suggested microbial breakdown of digesta in M. salmoides. In radio-tracer experiments, C. carpio dosed orally with [2-¹⁴C] acetate contained label in liver, muscle and blood tissues. Amounts of intestinal VFA did not appear to increase in species with refractile diets, and low VFA in D. cepedianum suggests fermentation plays a minimal role in the nutrition of this species. Low levels of intestinal VFA during cool seasons are consistent with the hypothesis that temperature limits fermentation in these species.