Vol. 482: 169–179, 2013 MARINE ECOLOGY PROGRESS SERIES Published May 22 doi: 10.3354/meps10285 Mar Ecol Prog Ser Effects of scale and habitat distribution on the movement of the southern stingray Dasyatis americana on a Caribbean atoll Alexander Tilley1,2,*, Juliana López-Angarita2, John R. Turner1 1School of Ocean Sciences, Bangor University, Menai Bridge LL58 8AB, UK 2Environment Department, University of York, York YO10 5DD, UK ABSTRACT: The structure of animal movement paths at varying spatial scales allows insight into the importance of habitat distribution and their response to scale in heterogeneous landscapes. Home-ranging animals typically exhibit constrained random movements at large spatial scales, with small-scale orientation reflecting responses to sensory stimuli. The southern stingray Dasy- atis americana is an abundant demersal elasmobranch found in coastal systems throughout the Caribbean, yet very little is known of its movement ecology. Twelve southern stingrays were man- ually tracked at Glovers Reef (Belize) for up to 32 non-continuous hours to evaluate movement structure and activity space. Response to spatial scale was analysed using fractal analysis, and domains of scale were compared to habitat spatial characteristics. Mean stingray activity space was relatively small (<0.5 km2) with daytime activity space significantly larger than nighttime activity space. Movement paths showed significant straightening correlated with increasing size in females. Stingray movement structure exhibited 2 distinct domains: at scales of <100 m, paths were more dispersed than a correlated random walk (CRW), and at scales >100 m, paths were more constrained than CRW, indicating directed movement at scales up to 100 m. Nearest neigh- bour spatial analysis of lagoon patch reefs showed mean spacing of 100 m (±4.5), equivalent to ori- entation distance seen in rays. Random walk movements at large scales are consistent with a home-ranging animal; however, larger scale orientation than expected and patch reef spacing equalling 100 m suggests that patch reefs represent important spatial and ecological networks, strongly influencing stingray movement, habitat use, and dispersal. KEY WORDS: Acoustic telemetry · Correlated random walk · Fractal analysis · Movement · Spatial ecology · Spatial scale patterns Resale or republication not permitted without written consent of the publisher INTRODUCTION key insights into their influence on biodiversity (Til- man 1994), population viability (Laidre et al. 2004) Spatial ecology can drive population level pro- and metapopulation dynamics (Hanski 1998, Turchin cesses with a similar effect to that of mortality, preda- 1998). The mode of movement utilised by a searching tion and competition (Pickett & Cadenasso 1995, animal can significantly affect dispersal distance and Hanski 1998), hence the search strategy employed distribution, with directed or orientated movements by an animal is crucial to success and efficiency in resulting in highly heterogeneous distributions com- terms of overall fitness. Furthermore, examining ani- pared to random and correlated random walks mal movement patterns with respect to scale and (CRW) (Hein et al. 2004). Without a priori information structure of heterogeneous landscapes can provide regarding the goal of a moving animal, discovering *Email: [email protected] © Inter-Research 2013 · www.int-res.com 170 Mar Ecol Prog Ser 482: 169–179, 2013 orientated movement is complex. Benhamou (2004) commute time between foraging patches, thereby developed a statistical test based on the idea that the conserving energy and lessening predation risk squared net displacement of a moving animal will (Papastamatiou et al. 2011). increase linearly with number of steps if the move- Despite the influential role demersal batoids can ment mode is a correlated random walk, and qua- have in driving prey population dynamics (Dale 2011) dratically if it is a directed walk (orientated move- and structuring benthic ecosystems through bioturba- ment). The ability to orientate over large distances tion (Thrush et al. 1991, Lohrer et al. 2004, O’Shea et has been observed for centuries in marine organisms al. 2011), very little is known about their movement such as turtles, whales and fish that return to natal or patterns and response to landscape scale and struc- spawning grounds. However, the analysis of move- ture. Directional and non-directional movement modes ment structure in relation to spatial scale and envi- have been detected in bat rays Myliobatis californica ronmental homogeneity is relatively novel in the moving along a linear embayment (Klimley et al. marine environment, having increased with tech- 2005), thought to be a behavioural thermotaxic re- nological advancement and greater precision of sponse to water temperatures inside and outside the acoustic telemetry (Lowe & Bray 2006). bay (Matern et al. 2000). Additionally, the manta ray Animal search patterns represent a combination of Manta alfredi displays straighter movement paths movement processes (Bartumeus et al. 2005) that re- within patches of high plankton prey abundance com- flect foraging within, and at scales greater than the pared with more tortuous movements at larger spatial range of sensory perception (Papastamatiou et al. scales (Papastamatiou et al. 2012). 2011). Higher resolution spatial data and fractal ana - The southern stingray Dasyatis americana (Hilde- lysis (Nams 2005, 2006) allow us to identify changes in brand & Schroeder 1928) is a large bodied, demersal movement mode in response to landscape and scale, stingray commonly found throughout the Western known as spatial domains (Wiens 1989). The precise Atlantic from New Jersey to Brazil (Bigelow & structure of movement paths and the spatial scales at Schroeder 1953, Aguiar et al. 2009). D. americana is which animals change between directed to random an opportunistic mesopredator with a wide trophic walks provide important insight into drivers of move- niche (A. Tilley unpubl. data), preying predomi- ment and dispersal (Nams 2005, 2006). nantly on crustaceans and polychaetes (Randall 1967, Animals have been seen to orientate to specific Gilliam & Sullivan 1993). We used manual acoustic habitat types (Zollner & Lima 1999a); towards stimuli telemetry to explore diel movement patterns and from sensory receptors (smell, taste; Gardiner & activity space of D. americana. We predicted high Atema 2010); or towards certain locations, such as a site fidelity while utilising relatively large activity burrow or den (Jamon & Benhamou 1989). The spaces, due to limited resource availability in low attraction to such a focal point or resource formed the relief soft substratum (Lowe & Bray 2006). We hypo - basis of mechanistic theories of home range forma- thesised that movement structure would exhibit tion (Börger et al. 2008). Animals orientate at small home-ranging characteristics of random walks with scales when goals are within sensory detection small scale orientated movements reflecting prey range, whereas animals exhibiting orientated move- searching using sensory mechanisms. We used CRW ments at large spatial scales are likely to be locating models and fractal analysis of movement path struc- patches (areas of greater energetic return or reduced ture to answer the following questions: (1) Do south- predation risk), through memory and experiential ern stingrays exhibit orientated movements, and if so learning (Van Moorter et al. 2009). to what scale? (2) Does southern stingray movement Searching organisms increase the chance of structure change according to spatial scale? (3) How encountering and capturing prey, and not being prey do movement patterns relate to habitat distribution, themselves, through the selection of foraging and what can this tell us about stingray habitat use grounds or habitat (Stephens et al. 2007). As such, an and search strategy? animal will move within the smallest area that will satisfy its energetic and reproductive requirements (Mitchell & Powell 2004); this area, or a defined per- MATERIALS AND METHODS centage over various temporal scales (e.g. 95%), is considered to be the home range (Grubbs 2010). A Study area cognitive map of a home range, or of key features within it, is theoretically likely to increase foraging Glovers Reef Atoll (16.8° N, 87.8° W) is the south- efficiency (Van Moorter et al. 2009) by reducing ernmost of 4 coral atolls in the Mesoamerican Barrier Tilley et al.: Southern stingray movement structure 171 Reef System, situated approximately 40 km east of and rays were released at the capture site and moni- the Belizean coast, and 20 km east of the main barrier tored in-water after release for periods of up to reef. The atoll covers approximately 254 km2, most of 30 min to ensure recovery. At Site B, 4 rays were which is made up by the lagoon, surrounded by a tagged in-water using a modified dart tag applied by reef crest. The lagoon is approximately 27 km long pole spear into the dorsal musculature in the saddle and 10 km wide, with a maximum depth of 20 m, and area. These rays were located by boat or with swim- fringed by a sand apron extending inward from the mers; a snorkeler then approached from directly reef crest to a depth of ~2 m, and a flattened shallow behind the ray to insert the dart tag with the pole lagoon comprised of sand, seagrass meadows and spear. Size measurements were estimated post- algal fields amongst rugose patch reefs to a depth of capture by measuring the width of feeding pit