Effect of body size and body mass on δ13C and δ15N in coastal fishes and cephalopods

Carbon and nitrogen isotopes have been widely used in the investigation of trophic relations, energy pathways, trophic levels and migrations, under the assumption that delta C-13 is independent of body size and that variation in delta N-15 occurs exclusively due to ontogenetic changes in diet and not body size increase per se. However, several studies have shown that these assumptions are uncertain. Data from food-webs containing an important number of species lack theoretical support on these assumptions because very few species have been tested for delta C-13 and delta N-15 variation in captivity. However, if sampling comprises a wide range of body sizes from various species, the variation of delta C-13 and delta N-15 with body size can be investigated. While correlation between body size and delta C-13 and delta N-15 can be due to ontogenetic diet shifts, stability in such values throughout the size spectrum can be considered an indication that delta C-13 and delta N-15 in muscle tissues of such species is independent of body size within that size range, and thus the basic assumptions can be applied in the interpretation of such food webs. The present study investigated the variation in muscle delta C-13 and delta N-15 with body size and body mass of coastal fishes and cephalopods. It was concluded that muscle delta C-13 and delta N-15 did not vary with body size or mass for all bony fishes with only one exception, the dragonet Callionymus tyro. Muscle delta C-13 and delta N-15 also did not vary with body size or mass in cartilaginous fishes and cephalopods, meaning that body size/mass per se have no effect on delta C-13 or delta N-15, for most species analysed and within the size ranges sampled. The assumption that delta C-13 is independent of body size and that variation in delta N-15 is not affected by body size increase per se was upheld for most organisms and can be applied to the coastal food web studied taking into account that C. lyra is an exception. (C) 2011 Elsevier Ltd. All rights reserved.