Trophic ecology of roosterfish (Nematistius pectoralis) inferred from stomach content and stable isotope analyses in the southern part of Baja California peninsula.

The roosterfish (Nematistius pectoralis) is a highly prized organism in the sport fleet for its fight to catch and the showiness of its dorsal fin. However, despite its popularity, studies on its basic biology are limited. This study aims to understand the feeding behaviours of the roosterfish by analysing the stomach content (SCA) and performing bulk stable isotope analysis (BSIA). The study also seeks to identify ontogenetic changes in its feeding habits and habitat preferences. To achieve this, 505 stomachs and 165 muscle samples were analysed in three locations in Baja California Sur (Cabo San Lucas, La Paz and Puerto San Carlos) between 2010 and 2017. The SCA determined that the diet consisted mainly of fish from the families Carangidae, Engraulidae and Clupeidae, which together constituted almost 50% of the total diet. Statistical differences in diet composition were found by size class but not by sex or the interaction between these variables. Concerning BSIA, there were no significant differences between males and females; however, differences were observed in the 'undifferentiated sex' grouping, which was comprised largely small juvenile individuals. Concerning size, BSIA values were correlated with roosterfish fork length, so the observed differences for the 'undifferentiated sex' category were related to size variation rather than sexual maturity. The findings from this study suggest that roosterfish experience an ontogenetic increase in trophic position, relying less on sardine-like species as they grow. Furthermore, BSIA values could inform on relative residency of the roosterfish to the Baja California peninsula.