Diet of black drum (Pogonias cromis) based on stable isotope and stomach content analyses
Abstract
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.
Description
A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Fisheries and Mariculture from Texas A&M University Corpus Christi in Corpus Christi Texas.