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Fishery Bulletin 111(2) 
Discussion 
Effects of tissue type, fractionation, and fish size 
Both 5 13 C and 5 15 N SI values can differ between tis- 
sue types (Pinnegar and Polunin, 1999). Differences 
between the isotopic values of fish liver and muscle 
tissue have been ascribed to differences in tissue turn- 
over rates of liver and muscle (Sweeting et al., 2005), 
and to seasonal differences in growth rates, feeding 
behavior, and prey availability (Perga and Gerdeaux, 
2005). However, in subtropical waters, such as Missis- 
sippi Sound, fish growth likely occurs throughout the 
year (Vetter, 1982), and, in this study, 5 13 C values of 
liver and muscle were generally very similar, indicat- 
ing that the pattern reported by Perga and Gerdaeux 
(2005) at more northern latitudes was not observed 
in this study. The isotopic values for 5 15 N did show a 
negative bias in liver compared with muscle. Because 
all sampling occurred during periods of probable high 
growth (summer and fall), this result may be related to 
differences in isotopic fractionation and turnover rates 
between tissue types. 
The increase in both 8 13 C and 5 I5 N with fish size 
