Seasonality and Ontogenetic Habitat Partitioning of Cownose Rays in the Northern Gulf of Mexico

Abstract

The seasonal and spatial heterogeneity of highly mobile mesopredators may play a large structuring role in estuarine dynamics. With a hypothesized relaxation of predation pressure from large sharks, growing populations of cownose rays (Rhinoptera bonasus) have been implicated in negatively affecting shellfish beds across multiple estuaries of the Atlantic coast; however, the pervasiveness of these potential impacts remains poorly understood elsewhere due to a lack of information on cownose ray distribution and seasonality across the species’ range. To better predict cownose ray dynamics in estuaries of the northern Gulf of Mexico, we conducted a multi-scale study using a combination of aerial and gillnet surveys. Cownose ray abundance was highly seasonal along the Mississippi-Alabama shelf, and a significant along-shelf gradient (west-east) in nearshore ray density was observed. This trend was best explained by changes in salinity, with higher abundances best correlated with more estuarine (i.e., lower salinity) conditions in nearshore areas. From north-south across the Alabama shelf, cownose rays displayed strong spatial and seasonal distributional patterns with ontogeny: adults (in particular, females) were primarily restricted to barrier islands and Gulf waters, whereas juveniles and young-of-the-year (YOY) exploited fresher waters of inshore bays protected from large predators. Among inshore locales, cownose ray catch was lowest over oyster habitats and only consisted of YOY rays, suggesting that foraging impacts may be reduced in these structurally complex substrates.