Niche Differentiation and Prey Selectivity among Common Bottlenose Dolphins (Tursiops truncatus) Sighted in St. George Sound, Gulf of Mexico

Abstract

Two groups of common bottlenose dolphins (Tursiops truncatus) have been identified within St. George Sound, Florida, USA: high site-fidelity individuals (HSF) which are individuals sighted multiple times in the region (i.e., ≥2 months, ≥2 seasons, and ≥2 years), and low site-fidelity individuals (LSF), which are individuals sighted fewer than 2 months, in 2 different seasons among 2 different years. Our goal was to determine whether differences in foraging behaviors were correlated with differences in sighting frequency and overall usage of St. George Sound by the two groups. We used carbon, nitrogen, and sulfur stable isotopes and niche hypervolume metrics to model the foodweb of St. George Sound. Mixing model results indicated that croaker, mojarra, pigfish, pinfish, and silverperch were the most important prey items for dolphins. The hypervolume metrics demonstrate niche partitioning between HSFs and LSFs, with the HSFs relying more heavily on pinfish, pigfish, and mojarra, while the LSFs relied more on silverperch. Plankton, benthic diatoms, seagrass, and epiphytes all contributed to secondary production within St. George Sound. This diversity of source utilization by seagrass-associated consumers supported by a high rate of total production likely sustains high secondary productivity despite the potential for competition in this system. Zooplankton was the most important basal source to the system, followed by seagrass and benthic primary production (as indicated by a sanddollar proxy). The reliance of dolphins on seagrass-dependent prey indicates that alteration of seagrass habitat would significantly impact the dolphin community foraging in St. George Sound and suggests that preservation of seagrass habitat is an important component of an effective management strategy for dolphin populations in the region.

Trophic niche size and overlap decreases with increasing ecosystem productivity

“…Models were run in three chains with 1 000 000 iterations and a burn-in of 500 000 to allow for adequate model convergence. Fractionation factors of 1.3 (±0.3), 3 (±1) and 0.6 (±0.2) were used for C, N and S, respectively (Vander Zanden and Rasmussen 2001, Hussey et al 2014a, Nelson et al 2015, Wilson et al 2017. Concentration-dependent mean percent contributions of each source were calculated for each individual O. chrysoptera and L. rhomboides sample.…”

The effects of lipid extraction on δ13C and δ15N values and use of lipid‐correction models across tissues, taxa and trophic groups

“…These large, aquatic mammals occupy different trophic levels. Dolphins are carnivorous, feeding on lipid‐ and protein‐rich fish (Wilson et al, 2017). Manatees are herbivorous, feeding on lipid‐ and protein‐poor seagrasses and aquatic vegetation (Mignucci‐Giannoni & Beck, 1998).…”

Individual consumer movement mediates food web coupling across a coastal ecosystem

“…Models were run in three chains with 1,000,000 iterations and a burn-in of 500,000 to allow for adequate model convergence. Trophic fractionation factors of 1.3 (AE 0.3), 3.3 (AE 0.26), and 0.5 (AE 0.54) were used for C, N, and S, respectively (McCutchan Jr et al 2003, Wilson et al 2017. Using the source contributions determined by the mixing model, we calculated the trophic position of each fish according to the following formula:…”

“…Models were run in three chains with 1,000,000 iterations and a burn‐in of 500,000 to allow for adequate model convergence. Trophic fractionation factors of 1.3 (± 0.3), 3.3 (± 0.26), and 0.5 (± 0.54) were used for C, N, and S, respectively (McCutchan Jr et al 2003, Wilson et al 2017). Using the source contributions determined by the mixing model, we calculated the trophic position of each fish according to the following formula:where Δδ 15 N = 3.3‰ (McCutchan et al 2003), Δδ 15 N ind is the nitrogen value of an individual consumer, Δδ 15 N source is the nitrogen value of each particular food source, and mean %cont source is the mean proportional contribution of each source to the consumer diet (Wilson et al 2010, Nelson et al 2015).…”

“…The means of these posterior distributions from each prey base to each snook along with its calculated trophic position were z‐transformed before hypervolume analysis to allow for standardized, comparable axes in n‐dimensional space (Blonder et al 2014). We used the Hypervolume R package (v 2.0.12) to seed a Gaussian kernel density estimation, that generated a cloud of points based on the distribution of the z‐scored values along the 4 axes that define the multidimensional trophic niche of the species (Blonder et al 2014, Wilson et al 2017, James et al 2019). The quantile threshold used was 0.05, so that each hypervolume included 95% of the total probability density (Blonder et al 2014).…”