Patterns of stable carbon isotope turnover in gag, Mycteroperca microlepis, an economically important marine piscivore determined with a non-lethal surgical biopsy procedure James Nelson & Jeffrey Chanton & Felicia Coleman & Christopher Koenig Received: 14 December 2009 / Accepted: 27 October 2010 # Springer Science+Business Media B.V. 2010 Abstract To determine the feasibility of using stable isotopes to track diet shifts in wild gag, Mycteroperca microlepis, populations over seasonal timescales, we conducted a repeated measures diet-shift experiment on four adult gag held in the laboratory. Fish were initially fed a diet of Atlantic mackerel, Scomber scombrus, (mean δ 13 C= -21.3‰±0.2, n =20) for a period of 56 days and then shifted to a diet of pinfish, Lagodon rhomboids, (mean δ 13 C= -16.6‰±0.6, n = 20) for the 256 day experiment. We developed a non- lethal surgical procedure to obtain biopsies of the muscle, liver, and gonad tissue monthly from the same four fish. We then determined the δ 13 C value of each tissue by isotope ratio mass spectrometry. For the gonad tissue we used the relationship between C/N and lipid content to correct for the influence of lipids on δ 13 C value. We observed a significant shift in the δ 13 C values of all of the tissues sampled in the study. Carbon turnover rates varied among the three tissues, but the shift in diet from mackerel to pinfish was clearly traceable through analysis of δ 13 C values. The turnover rates for muscle tissue were 0.005‰ day -1 , and for gonad tissue was 0.009‰ day -1 . Although it is generally thought that tissue turnover rates in ectotherms are driven primarily by growth, we found that metabolic rate can be a major factor driving tissue turnover in adult gag. Keywords Biopsies . Surgery . Lipid correction . Trophic shift Introduction For most animals, the menu is always changing. Permanent changes in diet typically occur with ontogenetic habitat or diet shifts (Mullaney and Gale 1996; Herzka et al. 2001; Bosley et al. 2002; Post 2003; Post et al. 2007; Logan and Lutcavage 2008). Other changes in diet are temporary, occurring either during annual migrations or with seasonal changes in prey availability (Hobson 1999; Herzka 2005). De- termining the timing and magnitude of diet changes can provide information on an animal’ s life history, food web interactions, and energy flow within and among ecosystems. Stable isotope analysis has become an important tool for ecologists in addressing a range of topics from food web dynamics to migration patterns (DeNiro and Epstein 1978; Fry and Parker 1979; Peterson and Fry 1987; Chanton and Lewis 2002; Fry 2006). Stable isotopes themselves (e.g., those of carbon, sulfur, nitrogen) occur naturally in biological Environ Biol Fish DOI 10.1007/s10641-010-9736-4 J. Nelson (*) : J. Chanton Department of Oceanography, Florida State University, Tallahassee, FL 32306, USA e-mail: nelson@ocean.fsu.edu F. Coleman : C. Koenig FSU Coastal and Marine Laboratory, 3618 Coastal Highway 98, St. Teresa, FL 32358-2702, USA