Habitat use of cownose rays (Rhinoptera bonasus) in a highly productive, hypoxic continental shelf ecosystem

Highly productive surface waters and hypoxic (dissolved oxygen, DO < 2.0 mg L-1) bottom waters develop 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, temperature, 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. Shelfwide sampling indicated that rays were associated with regions of high surface chlorophyll and low bottom salinity and DO, conditions characterizing the Mississippi-Atchafalaya plume region. Local sampling in and around the hypoxic zone indicated that rays preferred 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 enhanced productivity and to hypoxia-induced habitat degradation, which are both the products of coastal eutrophication.