Foraging Ecology of Bottlenose Dolphins (Tursiops Truncatus) in Galveston Bay, Texas

The overall goal of this research was to develop a better understanding of the trophic ecology of the Galveston Bay common bottlenose dolphin (Tursiops truncatus) stock that would provide critical data needed to manage this species. The specific objectives of this study were to: (1) estimate areas used by dolphins for foraging, (2) estimate factors contributing to foraging behaviors of dolphins, and (3) estimate proportions of different prey consumed by bottlenose dolphins in Galveston Bay. From 2015-2017, two survey methods (behavioral data from photo-identification surveys and stable isotope data from biopsy surveys) were used for objective one and two, while stable isotope (δ13C and δ15N) mixing models were used for objective three. Dolphins were observed foraging for 62.3% of all photo-ID sightings. Dolphins were observed foraging most often in the channel (52.3%), followed by open bay (41.4%), and nearshore (6.3%) but there is no evidence that the odds of observing foraging behavior was different between the habitats. For the sightings where foraging behavior was observed, 68.7% of the time a trawler was present. It is estimated that when approaching a shrimp trawler during a survey, the probability of observing dolphins patrolling is 60.8% of the time (95% CI: 55.6% to 100.0% , one-sided, one-sample porportion test, p-value <0.05). Foraging significantly decreased as time passed throughout the day in sightings from 2015-2017 (beta regression: pseudo R2= 0.8726, p-value <0.05). Potential prey of dolphins were collected in 2015 and 2016 for stable isotope analysis. Data from those sampling events and select nekton from Barcenas (2013) were used to model proportions of prey consumed by dolphins using A Bayesian isotope mixing model, Stable Isotope Analysis in R (MixSIAR version 3.1.10). Ward’s hierarchical cluster analysis was used to group 19 nekton species into six groups based on their mean C and N isotopic values. Overall, group six which contained only one species, White Mullet (Mugil curema), was estimated to contribute to the highest proportion of nekton prey consumed by dolphins (median: 25.3%) based on MixSIAR analyses. The second highest proportion consumed by dolphins overall was group two (Atlantic Brief Squid [Lolliguncula brevis], Hardhead Catfish [Ariopsis felis], and Striped Mullet [Mugil cephalus]) at 21.0%. There was a significant difference between the δ15N (‰) values in Upper Galveston Bay (UGB) and Lower Galveston Bay (LGB) (Wilcoxon Rank Sum Test, W=105.5, p-value≤0.05). This difference may suggest that dolphins in UGB and LGB are foraging on different prey or may support the notion that the upper portions of the bay are more heavily influenced of elevated anthropogenically produced δ15N (‰). This research contributes to baseline data that can be used for further analysis in future studies. The results from the stable isotope analysis may be used in combination with mercury and organochlorine contaminant analysis to examine trophic level biomagnification in the Galveston Bay ecosystem.