National Shark Research Consortium
Florida Program for Shark Research - Florida Museum of Natural History
Featured Projects
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NSRC Featured Project:
Validation of Fatty Acid Signatures in Elasmobranch Diet Reconstruction
In recent years there has been a worldwide decline in commercially exploited fish stocks. This declination has especially been noted in the large coastal shark fisheries in the northwest Atlantic Ocean. Proper characterization of trophic relationships is one component of fish ecology that is essential to successful management of fish populations. Sharks, as apex predators, have significant impacts on many bony fish populations, which comprise the major proportion of their diets. Overexploitation of bony fish stocks can, in turn, have negative effects on shark populations. Therefore, it is paramount that potential predator-prey interactions are accurately assessed. Improved understanding of these interactions will allow fisheries to be better managed on a multi-stock level, taking into account the impacts each fishery may have on others.
Caribbean reef shark (Carcharhinus perezi)
© George Ryschkewitsch
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The traditional method for delineating food habits and trophic webs is the examination of a predator's gut to identify and quantify recently eaten prey or indigestible parts of prey. This method, while providing some data on prey selection, is subject to bias and limitations. For example, the method of capturing a predator might be biased such that only "hungry" fish are caught. Studies have shown that as many as 50-70% of sharks caught by baited hook methods have empty stomachs and provide no diet data. Alternatively, sharks may evert their stomach due to the stress of capture. Recently eaten prey also provide only short-term data on food habits, giving no indication as to what a predator has been eating during the more remote past. In addition, prey items that do not have hard, indigestible parts and/or have low retention time in the gut could be underrepresented in diet reconstruction. Moreover, excluding the few exceptional cases where gut extrusion or lavage is feasible, the predator must be killed to extract the gut contents.
Recently eaten prey items from the digestive tract of a shark.
Courtesy FPSR
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A modern approach to diet analysis has focuses on evaluating fatty acid composition. Studies have confirmed that the biochemical properties of many dietary fatty acids result in their incorporation into organisms with little to no modification. Analysis of the types, number, and proportions of fatty acids in an organism results in a characteristic "fatty acid signature." A prey item has a fatty acid signature that reflects its recent dietary habits. When a predator consumes that prey item, the fatty acid profile in the storage tissues of the predator will reflect the fatty acid composition of the prey. Comparisons of fatty acid signatures in predators and potential prey items can provide data for diet reconstruction without direct examination of stomach contents. Furthermore, fatty acid analysis is not subject to the biases associated with traditional gut content analysis.
Many studies have shown fatty acid composition to be useful in determining trophic relationships at both the bottom and top of food webs. Fatty acid signatures have been used to characterize prey selection in fish, marine mammals, and terrestrial carnivores. However, to date no study has used this innovative approach to characterize the diet of any elasmobranch (sharks, skates, and rays) species. Furthermore, although the technique has been widely used in studies of feeding ecology of marine organisms, it has rarely been validated experimentally with known diets.
Our long term goal is to be able to take a tissue sample from a live shark and determine its recent diet history based on the fatty acid signature of its tissue. The objective of this study is to validate the use of fatty acid composition analysis in elasmobranch species. To this end we are performing a lab experiment with Atlantic stingrays (Dasyatis sabina) to determine if fatty acid profiling is a viable method for characterizing the diet of cartilaginous fishes.
Left: Atlantic stingray (Dasyatis sabina)
Right: Stingrays in holding tank
Courtesy FPSR
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This stingray is a suitable experimental surrogate since it is closely related to sharks and is readily obtained in Florida. Additionally, this ray can be maintained in captivity for extended periods due to its small size and general hardiness. Stingray specimens for this study were collected via a trot-line from Lake Jesup, Florida. Five randomly chosen specimens were euthanized to serve as field-caught controls. Live stingrays were held in three 75 gallon tanks at Mote Marine Laboratory and after a week long fast, were fed one of three specific diets (shrimp, fish, or shrimp and fish). After a month of feeding we euthanized the rays and collected liver and muscle samples to analyze their fatty acid profiles. The results will be compared to the fatty acid profiles of the prey items (shrimp and fish).
Left: Removal of muscle sample
Right: Removal of liver sample
Courtesy FPSR
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Validation of this technique will provide a useful tool for better characterization of the diets of commercially important sharks and will produce a more detailed understanding of the impact prey availability has on the populations dynamics of over-exploited elasmobranch species. In addition, characterization of shark feeding habits will provide better identification of predators of the commercially important bony fishes and invertebrates. These studies will prove useful for a variety of fisheries management applications on a worldwide scale. Funding for this project was provided by Florida Sea Grant.
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