STABLE ISOTOPE DIFFERENCES BETWEEN BLUE MARLIN (MAKAIRA NIGRICANS) AND STRIPED MARLIN (KAJIKIA AUDAX) IN THE SOUTHERN GULF OF CALIFORNIA, MEXICO

Stable isotope values of Kajikia audax (Philippi, 1887) and Makaira nigricans (Lacepede, 1802) were analyzed to detect differences associated with trophic segregation influenced by their feeding habits in the southern Gulf of California, Mexico. We sampled the dorsal white muscle of 47 M. nigricans and 35 K. audax collected from 2005 to 2007. No relationship was found between billfish body size and isotopic values. Significant differences in delta N-15 and delta C-13 values were found among genders, years, or areas for K. audax. The significant differences in delta N-13 and delta C-13 values found between areas for M. nigricans suggested differences in foraging preferences or movement patterns. Significant differences were also found in delta N-13 and delta C-13 values between K. audax and M. nigricans, possibly placing these two billfish species at different trophic levels. These patterns appear to correspond to each species' known foraging ecology and suggest that these two billfishes have different trophic preferences or migratory histories.