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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^sup 13^C=-21.3[per thousand]±0.2, n=20) for a period of 56 days and then shifted to a diet of pinfish, Lagodon rhomboids, (mean δ^sup 13^C=-16.6[per thousand]±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^sup 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^sup 13^C value. We observed a significant shift in the δ^sup 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^sup 13^C values. The turnover rates for muscle tissue were 0.005[per thousand] day^sup -1^, and for gonad tissue was 0.009[per thousand] day^sup -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.[PUBLICATION ABSTRACT]