Nonlinear foraging response of a large marine predator to benthic prey: eagle ray pits and bivalves in a New Zealand sandflat

Abstract

The density-dependent foraging response of eagle rays ( Myliobatis tenuicaudatus) to infaunal bivalves ( Macomona lilliana) was measured in a New Zealand sandflat. Disturbance pits provided unequivocal indicators of ray feeding activity, and pits were counted on a plot (250 m×500 m) which had prey density mapped in a 200 cell (25 m×25 m) grid. Although foraging response increased significantly with prey density treated as a nominal (class, ANOVA-type) variable, treating bivalve density as a ratio scale (continuous, regression-type) variable provided more information about characteristics of the response. Eagle rays exhibited a nonlinear segmented response to prey density, in which ray foraging activity was low and independent of prey density at low Macomona densities, while foraging increased sharply above a threshold density of prey but did not reach satiation at the highest prey densities in our site. By counting ray pits repeatedly over a 31 day period, we showed that the levels and slope of the foraging response (no. of ray pits per 707 m 2 per 4 days) varied temporally during the season, but the nonlinear characteristic and the threshold of prey density were consistent. Correlation analysis showed that the distribution of bivalve prey and ray foraging was spatially constant during the season. Comparison of 3 estimators of prey density showed that a fitted polynomial density was the best predictor of ray foraging, and indicated that rays were responding to prey patches on a scale of 75–100 m. The temporal features of the response to prey density were incorporated into a nonlinear segmented model and integrated with respect to time for each cell of the study grid. The impact of ray foraging estimated from the integral indicated that only about 1.6% of the Macomona population was consumed and 5.0% of the total plot was disturbed by rays during one month of study. However, the nonlinearity of response indicated that foraging impacts were concentrated disproportionately on high density patches of prey, which suffered up to 4% mortality and 13% disturbance. Macomona gained a refuge from predation and disturbance at low density, which would stabilize prey populations and even out prey distribution.