Diet of black drum (Pogonias cromis) based on stable isotope and stomach content analyses
Mendenhall, Kathryn S.
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The Black Drum (Pogonias cromis) is a large-bodied sciaenid species occurring throughout warm-temperate to subtropical estuaries including the Gulf of Mexico. This sportfish is economically important to Texas having generated $1.6 million in landings in 2013, second only to Louisiana in the U.S. Within Texas’ bays and estuaries, Baffin Bay supports the highest relative abundance of Black Drum. In late 2012, the Texas Parks and Wildlife Department reported an emaciation event among the Black Drum population of Baffin Bay, represented by underweight fish with jelly-like fillets. Due to a lack of environmental and biological data collected in the Baffin Bay ecosystem during this time period, it was difficult to determine the proximate causes. Although a number of factors may have been responsible, I sought to determine whether trophic dynamics could play a role. In this study, I characterized: 1) the distribution and abundance of benthic food resources, and 2) the diet of Black Drum using a combination of stomach content analysis and stable isotopes. Macrofauna (<500 μm) abundance, diversity and biomass were examined in conjunction with hydrological parameters throughout Baffin Bay from March 2014 to March 2015. The diet of Black Drum during this period was evaluated using gut content analysis and stable isotopes of C and N. Of 21 different food categories identified, bivalves and polychaetes were the two most frequently occurring prey items in the 264 drum stomachs analyzed. Vacuity indices reflected the proportion of empty stomachs, indicating relatively full stomachs (0≤VI≤20) in all Baffin Bay regions except Baffin Bay proper (VI=63.16). The Strauss Index (L>0) indicated that Black Drum were positively selecting gastropod and bivalve prey over other available benthic invertebrates. Isotopic compositions of macrofauna were -24.89 to -13.79‰ for δ13C, and 4.36 to 11.36‰ for δ15N. Sediment organic matter ranged from -19.04 to -22.11‰ for δ13C, and 4.1 to 10.19‰ for δ15N. Isotopic composition of Black Drum stomach contents ranged widely for δ13C, from -26.7 to - 12.8‰, and from 4.7 to 11.4‰ for δ15N. Black Drum muscle tissue ranged from -21.13 to - sus, 1/25/2012 13.68‰ for δ13C and 7.93 to 12.43‰ for δ15N. These results indicate Black Drum rely on a wide variety of benthic invertebrates, including those that depend on both benthic (e.g. annelids) and pelagic (e.g. bivalves) food sources. My study demonstrates the benefits of using stable isotopes as a complement to traditional stomach content analysis in characterizing fish diets. Such an approach provides key information to fisheries resource managers so they may better understand, plan, and respond to future emaciation events.