Effect of Hypoxia on Diet of Atlantic Bumpers in the Northern Gulf of Mexico

Cultural eutrophication is a global problem that often leads to hypoxic conditions in coastal systems. Although improving, our understanding of the impacts of hypoxia on trophic interactions in pelagic and benthopelagic food webs is limited. Toward this end, we evaluated diet composition of and mass-specific consumption by the Atlantic Bumper Chloroscombrus chrysurus, a numerically dominant planktivorous fish in the northern Gulf of Mexico, relative to dissolved oxygen concentration and fish size. Atlantic Bumper CPUE was similar in hypoxic and normoxic areas. Mean mass-specific consumption by small Atlantic Bumpers in hypoxic areas was greater than that of both small and large individuals in normoxic areas. The most commonly ingested prey type for both large and small Atlantic Bumpers was shrimp larvae. Large quantities of fish larvae were consumed by adult Atlantic Bumpers in hypoxic regions. These findings demonstrate that hypoxic conditions can alter feeding of dominant fishes in the northern Gulf of Mexico, which may influence energy flow in the region. Hypoxia (dissolved oxygen [DO] ≤ 2 mg/L) is an environmental stressor that has grown in prominence in aquatic ecosystems worldwide (Rabalais et al. 2009). Hypoxia occurs naturally in many marine systems that are characterized by phenomena such as coastal upwelling, high productivity, and stratification (Rabalais et al. 2010). However, human activities, such as intensive agriculture practices and land use changes, have caused the magnitude and extent of hypoxia to increase (Turner et al. 2008; Bianchi et al. 2010). Within the United States, there are many systems that experience seasonal hypoxia, including Long Island Sound, Chesapeake Bay, and the northern Gulf of Mexico (NGOMEX; Anderson and Taylor 2001; Rabalais et al. 2001; Hagy et al. 2004). The NGOMEX is the site of one of the world’s largest human-caused coastal hypoxic zones, with an area exceeding 20,000 km in some years (Rabalais et al. 2001; Turner et al. 2008; Bianchi et al. 2010). It also supports profitable commercial and recreational fisheries for the Spotted Seatrout Cynoscion nebulosus, Red Snapper Lutjanus campechanus, Red Drum Sciaenops ocellatus, King Mackerel Scomberomorus cavalla, Sheepshead Archosargus probatocephalus, Dolphinfish Coryphaena hippurus, and shrimp species, including white shrimp Litopenaeus setiferus and brown shrimp Farfantepenaeus aztecus (NMFS 2016). The effects of hypoxia on these commercial and recreational fisheries are of particular interest given recent evidence for hypoxia-related declines in productivity for brown shrimp (O’Connor and Whitall 2007; Huang et al. 2010). Hypoxia holds great potential to alter food web interactions (Pihl et al. 1991; Diaz and Rosenberg 1995; O’Connor and Whitall 2007). Reduced oxygen levels, which typically form in the bottom layer of the water column (Diaz and Rosenberg 2008), have been shown to both increase (Pierson et al. 2009; Craig 2012; Roman et al. 2012) and decrease (Taylor and Rand 2003; Ludsin et al. 2009; Zhang et al. 2009) the spatial overlap between predators and prey. When prey habitat is compressed into the layer of water adjacent to the hypoxic zone, predators may be able to take advantage of dense prey resources, thus increasing trophic transfer (Prince and Goodyear 2006). However, several field studies have reported evidence that zooplankton use hypoxic bottom water as a refuge from predation (Vanderploeg et al. 2009; Zhang et al. 2009), which may interrupt trophic interactions. In addition, several studies have used information on species distribution shifts to measure the potential impact of hypoxia on fish growth (Costantini et al. 2008; Arend et al. 2011; Brandt et al. 2011; Zhang et al. 2014). Although these studies have no doubt improved our ability to assess the potential impact of hypoxia on food webs, few studies in the NGOMEX have actually quantified the impact of hypoxia on diet composition, especially for pelagic species (Rabotyagov et al. 2014). We explored how hypoxia affects the diet of the Atlantic Bumper Chloroscombrus chrysurus, a numerically dominant pelagic species in the NGOMEX (S anchez-Ram ırez 2003; Lewis et al. 2007; Craig and Bosman 2013). The Atlantic Bumper is an important food source for a variety of commercially and recreationally important fishes (Shaw and Drullinger 1990; Leffler and Shaw 1992). Although the Atlantic Bumper is a common member of the NGOMEX pelagic community, little is known about its diet either in the presence or absence of hypoxia. Therefore, we also focused on quantifying differences in diet based on Atlantic Bumper size. We hypothesized that consumption by Atlantic Bumpers is lower in hypoxic areas than in normoxic areas because fish experience a spatial mismatch with prey such as zooplankton and pelagic larvae, which exhibit vertical migration to avoid predators (Hopkins 1982; Nielson and Perry 1990) and can use the hypoxic zone as a refuge from predation (Breitburg et al. 1999; Ludsin et al. 2009). In addition, we expected diet composition to vary (1) between hypoxic and normoxic areas due to speciesspecific differences in zooplankton responses to hypoxia (Kimmel et al. 2009; Elliott et al. 2012; C. N. Glaspie and colleagues, unpublished manuscript) and (2) between small and large Atlantic Bumpers due to ontogenetic shifts in 2 GLASPIE ET AL.