Trophic position and individual feeding histories of fish from Lake Okeechobee, Florida

We used C and N stable isotope measurements to identify trophic position and feeding histories of fish from Lake Okeechobee, one of the largest lakes in North America. When δ15N values were used to estimate trophic levels, analyses of about 500 individual fish collected in two seasons at five sites showed that trophic level varied from 2 (herbivore) to 4.3 (second-level carnivore) among the 29 fish species examined. Lower trophic levels predominated among small fish caught at a littoral marsh site, and highest trophic levels were found among larger offshore fish. Marsh fish showed about three times the C isotope variation than did offshore fish, consistent with the wider array of foods in the marsh versus offshore food web. In addition to these community-level results, there were often unexpected large isotopic differences between individuals in almost all fish species examined, suggesting large differences in individual feeding behavior. Isotopic analyses indicate long-term differences in feeding among individuals, in addition to the short-term differences often observed in stomach content studies. Long-term individual variation in fish foraging may be important in understanding variation in other biochemical parameters measured in fish, such as Hg or PCB contents.