Effect of digestion on the d13C and d15N of fish-gut contents

Gut contents of sand goby Pomatoschistus minutus showed higher C and N isotope values than the food before consumption. This enrichment was more pronounced in the hindgut than in the foregut, probably because of preferential assimilation of 12C and 14N along the gastro-intestinal tract. The results indicated that the shift towards higher values in the alimentary canal occurs in the first 2 h after feeding. # 2008 The Authors Journal compilation # 2008 The Fisheries Society of the British Isles Key words: assimilation; carbon and nitrogen stable isotopes; Pomatoschistus minutus; stomach content analysis; trophic enrichment. Stable isotope techniques are routinely used in the study of trophic relationships and animal migrations in aquatic and terrestrial ecosystems (Gannes et al., 1998; Hobson, 1999; Thompson et al., 2005). Use of stable C and N isotopes to eluci-date trophodynamics requires a priori estimates of the alteration in d15N and d13C values between prey and predator (known as trophic fractionation). The stable isotopic composition of a consumer was commonly accepted to be en-riched in 13C by c. 0–1 % and in 15N by c. 34 11 % (mean S.D.) relative to its diet (DeNiro & Epstein, 1978; Minagawa & Wada, 1984; Owens, 1987). Considerable variation in the trophic shift for d13C and d15N, however, has been observed between and within trophic groups or even among conspecifics. Multi-ple potential sources for this variation have been recognized, such as taxon iden-tity (Minagawa & Wada, 1984), main biochemical form of the nitrogenous waste (Vanderklift & Ponsard, 2003), dietary differences (Adams & Sterner, 2000