Movement and diving behaviour as predictors of foraging success in a large marine predator: the grey seal, Halichoerus grypus.
Austin, Deborah*,1, Bowen, W.D.2, McMillan, J.I.2, 1 Biology Department, Dalhousie University, Halifax, NS, Canada2 Bedford Institute of Oceanography, Department of Fisheries and Oceans, Dartmouth, NS, Canada
ABSTRACT- Establishing where and when predators forage is essential to understanding trophic interactions, yet remains poorly understood in large marine carnivores. We studied the spatial and temporal scales of foraging by grey seals in the Northwest Atlantic using simultaneous deployments of satellite transmitters, time depth recorders and stomach temperature loggers. Our goal was to establish characteristics of diving and movement that would predict foraging success. We linked diving behaviour to foraging success at two temporal scales: trips (days) and bouts (hours). At the scale of trips, trip duration (GLMM; b = 0.45, P <0.0001, n = 16) and accumulated bottom time/d (ABT) (b = 0.003, P = 0.004) were significant predictors of feeding. Feeding bouts had significantly longer ABT (260.2 ± 43.0 min vs. 73.5 ± 14.7 min), more dives/h (9.5 ± 0.9 vs 12.5 ± 0.9) and less traveling dives (4.6 ±1.1% vs. 15.2 ± 2.8%) compared to non-feeding bouts. The best predictors of feeding at the scale of bouts were mean depth (GLMM; b = 0.02, P <0.0001, n = 16) and mean bottom time/h (b = 0.03, P = 0.0001). Successful foraging frequently occurred in flat-bottomed dives, suggesting benthic feeding. Secondly, we tested if characteristics of movement were predictive of feeding. Angular variance, and therefore path tortuosity was greater in successful feeding trips (0.52 ± 0.04) compared to non-feeding trips (0.17 ± 0.52; t = 5.01, P = 0.04) as was mean travel rate (0.3 ± 0.08 m/s vs. 0.07 ± 0.02 m/s; t = -2.7, P = 0.02) and distance traveled/d (24.5 ± 4.8 km vs 11.1 ± 1.3 km; t = -3.4, P = 0.015). Significant predictors of feeding within trips were angular variance (GLMM; b = 2.2, P = 0.03, n = 13) and distance traveled per d (b = 0.03, P = 0.009). These findings suggest that grey seals modify their diving and movement behaviour corresponding to feeding. Finally, we examined how diving varied with type of movement (determined by fit to a correlated random walk model). Diving behaviour varied individually, corresponding to movement type (MANOVA, F = 2.6, P = 0.02, n = 30), with resident seals having the longest bout duration (F = 4.7, P = 0.02), ABT (F = 5.8, P = 0.008) and shallowest dive depth (F = 4.2, P = 0.03) than wider ranging individuals. Behavioural variation may ultimately be linked to characteristics of prey and have implications for the way we model predation in marine ecosystems.
Key words: large marine predators, temporal scale, diving, movement
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