Habitat use of cownose rays (Rhinoptera bonasus) in a highly productive, hypoxic continental shelf ecosystem J. K. CRAIG, 1,* P. C. GILLIKIN, 2,  M. A. MAGELNICKI 2,à AND L. N. MAY 3 1 Department of Biological Science, Florida State University Coastal and Marine Laboratory, 3618 Highway 98, St. Teresa, FL 32358-2702, USA 2 Nicholas School of the Environment, Duke Center for Marine Conservation, 135 Duke Marine Lab Rd., Beaufort, NC 28516-9721, USA 3 NOAA ⁄ NMFS, Southeast Fisheries Science Center, Mississippi Laboratory, Building 1103, Room 218, Stennis Space Center, MS 39529-0001, USA ABSTRACT Highly productive surface waters and hypoxic (dis- solved oxygen, DO £ 2.0 mg L )1 ) bottom waters de- velop seasonally on the northwestern Gulf of Mexico continental shelf due to nutrient and freshwater inputs from the Mississippi-Atchafalaya River system. We investigated the spatial distribution of the cownose ray (Rhinoptera bonasus), a highly mobile, bentho-pelagic species that is a seasonal resident of the shelf, in relation to surface chlorophyll, bottom-water hypoxia, and other environmental variables (salinity, tempera- ture, depth). We used synoptic trawl and aerial surveys to investigate ray distributions at both shelfwide (100– 1000s km) and local (5–50 km) spatial scales. Shelf- wide sampling indicated that rays were associated with regions of high surface chlorophyll and low bottom salinity and DO, conditions characterizing the Mis- sissippi-Atchafalaya plume region. Local sampling in and around the hypoxic zone indicated that rays pre- ferred habitats where bottom waters were hypoxic but they primarily occupied normoxic (DO > 2.0 mg L )1 ) waters above the bottom hypoxic layer. Stomach fullness and diet composition were similar between rays sampled in habitats with hypoxic versus normoxic bottom waters. These results indicate that cownose rays are strongly associated with riverine-influenced regions of the shelf and preferentially use habitats with hypoxic bottom waters, perhaps for benthic foraging. Collectively, our results highlight the importance of considering the responses of mobile species to en- hanced productivity and to hypoxia-induced habitat degradation, which are both the products of coastal eutrophication. Key words: cownose ray, eutrophication, Gulf of Mexico, habitat selection, habitat use, hypoxia, Mississippi River, scale-dependent, spatial distribution INTRODUCTION Spatial variation in ocean conditions can influence the growth, survival, and production of marine fishes. At regional spatial scales, the distribution of fishes and other upper trophic levels is often positively associated with areas of high primary and secondary productivity, presumably because these habitats provide enhanced foraging opportunities (Valavanis et al., 2004; Bi et al., 2007, 2008; Zainuddin et al., 2008). River plumes originating from the outflows of major river systems onto continental shelves are particularly important in this regard because they provide external nutrients that enhance primary production and also contribute to the creation of frontal zones that concentrate prey organisms (Grimes and Kingsford, 1996; Grimes, 2001; Morgan et al., 2005). Numerous studies have shown that pelagic planktivores preferentially use river plumes (Grimes and Finucane, 1991; Fukuwaka and Suzuki, 1998; De Robertis et al., 2005) but less is known about the distribution of demersal species in continental shelf ecosystems that are heavily influ- enced by freshwater discharge. Some demersal fishes are associated with areas of high surface productivity, possibly due to the increased availability of benthic food resources stimulated by the vertical export of organic matter to the sea floor (Jaureguizar et al., 2003; Ware and Thomson, 2005; Juan-Jorda ´ et al., 2009). Such pelagic to benthic coupling has been suggested as *Correspondence. e-mail: kevin.craig@bio.fsu.edu   Present address: NC Coastal Reserve and National Estua- rine Research Reserve, 101 Pivers Island Rd., Beaufort, NC 28516, USA. à Present address: Galveston Observer Program, National Marine Fisheries Service, 4700 Ave. U, Galveston, TX 77551, USA. Received 13 July 2009 Revised version accepted 8 March 2010 FISHERIES OCEANOGRAPHY Fish. Oceanogr. 19:4, 301–317, 2010 Ó 2010 Blackwell Publishing Ltd. doi:10.1111/j.1365-2419.2010.00545.x 301