Movements, foraging and habitat selection of southern stingrays (Hypanus americanus) in a tropical ecosystem altered by an invasive seagrass

Abstract

This study provides some of the first evidence of how a common Caribbean fish species that relies on seagrass and sand habitats interacts with an invasive seagrass. The invasive seagrass Halophila stipulacea, first documented in the Caribbean in 2002, has rapidly expanded its range, displacing native seagrasses, and overgrowing bare sand. The southern stingray (Hypanus americanus) uses shallow seagrass and sand habitats for foraging. This paper examined the impacts of the invasive seagrass, H. stipulacea, on southern stingray behavior, foraging and movement patterns using acoustic telemetry and visual observations. From 2015 to 2018, 15 southern stingrays (14 juveniles of unknown sex and 1 female) were tagged with acoustic transmitters and passively monitored within an acoustic array in Brewers and Perseverance Bays, St. Thomas, United States Virgin Islands. The residence time, rates of movement and activity spaces for 50% and 95% utilization distributions (UD) were calculated for day and night periods. Tagged southern stingrays were detected within the array on average for 260 d (range 11–801 d) and residency for all individuals averaged 53% over the monitoring period (1079 d). Mean rates of movement per individual ranged from 40 to 150 m h⁻¹, was lowest during the nighttime and were significantly higher during crepuscular periods. Activity spaces during day and night periods were nearly identical and averaged 0.05 km² and 0.25 km² for 50% and 95% UD, respectively. Distances between day and night 50% UD activity spaces was 148 m (range 8–409 m) and mean percent overlap was 38%. In addition to acoustic monitoring, benthic cover of available habitats and visual surveys of stingray behavior and habitat preferences were conducted along fixed transects. Monotypic stands of the invasive H. stipulacea seagrass represented 42% of benthic cover, followed by bare sand (27%), mixed patches of native and invasive seagrasses (23%) and the native Syringodium filiforme (7%). A habitat selection index (HSI) comparing the percent cover of available habitats and habitats occupied by stingrays found a strong preference for the native seagrass S. filiforme (HSI = 2.30) and bare sand (HSI = 1.79) compared to H. stipulacea (HSI = 0.70). Visual surveys also revealed that 50% of foraging stingrays were in S. filiforme, while 28% and 17% were foraging in H. stipulacea and bare sand, respectively. Our results are an important baseline for examining how further expansion of H. stipulacea may affect southern stingray movement patterns and foraging preferences.