Keywords

Stable isotope, feeding ecology, dolphin, turnover rate, discrimination, bottlenose, delphinidae

Abstract

Prey availability, directly or indirectly, affects all aspects of a predator’s life history and is a primary factor influencing habitat selection and movements. This is especially true for delphinid species where it has been documented that the behaviors and movements of dolphins are strongly influenced by food availability. Unfortunately, the feeding ecology and habitat use patterns of many of these species are poorly understood. Many methodologies that have been employed to explore these facets of dolphin ecology have limitations and constraints or are logistically infeasible. Stable isotope and fatty acid signature analyses have been used extensively on a wide variety of species and have been shown to be methodologies that overcome some of these limitations. These approaches can provide information on feeding habits and the geographic origin of the prey thereby giving tremendous insight into habitat usage patterns. The present study applied stable isotope and fatty acid signature methodologies to gain insight into the feeding ecology and habitat usage of various dolphin species to improve upon our understanding of these important facets of their life histories. The application of stable isotope analysis in ecological studies relies on both species and tissue specific measurements of parameters such as diet-tissue discrimination factors, the difference in stable isotope ratio between a consumer and its prey, and turnover rates, the change in tissue isotopic composition attributable to growth and tissue replacement. Initially, controlled studies were conducted and animals were switched from one isotopically distinct diet to another which allowed for the calculation of these values in bottlenose dolphin skin. Diet-tissue discrimination factors for dolphin skin averaged 2.20‰ for nitrogen and 0.82‰ for carbon. Average turnover rates (expressed in half-lives) in dolphin skin were 17 days for nitrogen and iv 16.5 days for carbon. The present study represents the first reported diet-tissue discrimination factors and turnover rates for carbon and nitrogen in the skin of any cetacean. Next, skin samples were collected from net-entangled and free-ranging dolphin species off the coast of South Africa and analyzed for stable carbon (δ13C) and nitrogen (δ15N) isotope ratios. The Indo-Pacific bottlenose dolphin (Tursiops aduncus), the common dolphin (Delphinus capensis), the striped dolphin (Stenella coeruleoalba), and the humpback dolphin (Sousa chinensis) all occur off the southeastern coast of South Africa with overlapping distributions. Isotopic signatures revealed resource partitioning among these four species of dolphins with differences in diets, as well as differences in the use of habitat. Mean values for δ 15N ranged from 11.92 ± 0.11‰ (n=3) for striped dolphins to 14.95 ± 0.19‰ (n=27) for humpback dolphins, indicating that these species are feeding at different trophic levels. Striped dolphin carbon isotope signatures were consistent with evidence that they typically forage further offshore (- 17.94 ± 0.14‰) and the carbon isotope values of the humpback dolphins reflected their use of inshore habitats by comparison (-15.16 ± 0.12‰). Common and bottlenose dolphins for nitrogen (13.66 ± 0.08‰, 14.35 ± 0.07‰ respectively) and carbon (-15.48 ± 0.07‰, -15.76 ± 0.06‰ respectively) fell in between these two extremes. Analyses also revealed that males and females have differences in their diets. On average, males were enriched in δ 15N by 0.74‰ compared to females suggesting some dietary differences in prey composition. Isotopic niche width has been compared to traditional measures of niches used by ecologists and was measured for these South African dolphins. Humpback and bottlenose dolphins had the largest standard elliptical area (SEA), striped dolphins had the smallest SEA, and the SEA for common dolphins was intermediate. Larger SEA values reflect a broader trophic diversity, while smaller SEA values reflect a narrower trophic diversity or a more specialized niche. v Finally, a resident group of bottlenose dolphins (Tursiops truncatus) in the Indian River Lagoon (IRL) in east central Florida were sampled and explored for differences in isotopic signatures based on sex, age category, season, and location within the IRL. In addition to stable isotope analysis, fatty acid analysis was also used to compare and contrast the findings between the two techniques. Comparison of stable isotopic signatures revealed differences among age categories and among locations. Fatty acid analysis was able to discern further and found differences in the signatures between male and female dolphins. The combination of both techniques allowed for an extensive examination into the feeding ecology and habitat utilization of these resident dolphins. The Bayesian mixing model (Stable Isotope Analysis in R- SIAR) was validated using controlled study data and was found to be accurate when inputting isotopically distinct prey items (sources). The mixing model was then used to estimate the proportions of prey items that make up the diet of Indian River Lagoon bottlenose dolphins. Two models were run in which dolphins were grouped together (model 1) and dolphins were separated by year (model 2). Results of the model reaffirm stomach content analysis results previously obtained. Stable isotope techniques were applied to various dolphin species to gain better understanding of their feeding ecology and habitat utilization. Resource partitioning was suggested for four South African dolphin species which gives crucial insight into the ecology of both at-risk and data-deficient species. These discernments will provide much needed data to conservationists and managers and contributes to our general understanding of these species. This is the first study of its kind to undertake controlled diet studies with bottlenose dolphins which determined diet-tissue discrimination values and turnover rates for carbon and nitrogen isotopes in the skin of any cetacean. The current study is also the first of its kind to attempt to vi model bottlenose dolphin diet in the Indian River using stable isotopes. Food, being a primary driver for many species, can lend explanation of things like movement patterns, habitat usage, competition, reproductive success, survival, and the spread of diseases, which has been an issue in Indian River dolphins in recent years. Dietary information modeled in this study provided new data for the relative contribution of a suite of potential prey to an apex predator in the Indian River. Data produced through the current study contributes towards a large, unprecedented step forward in understanding dolphin ecology and the roll of cetacean stable isotope ecology.

Notes

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Graduation Date

2013

Semester

Summer

Advisor

Worthy, Graham

Degree

Doctor of Philosophy (Ph.D.)

College

College of Sciences

Department

Biology

Degree Program

Conservation Biology Ecology and Organismal Biology

Format

application/pdf

Identifier

CFE0004823

URL

http://purl.fcla.edu/fcla/etd/CFE0004823

Language

English

Release Date

August 2013

Length of Campus-only Access

None

Access Status

Doctoral Dissertation (Open Access)

Subjects

Dissertations, Academic -- Sciences, Sciences -- Dissertations, Academic

Included in

Biology Commons

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