Diet Analysis

Stomach Eversions

Photo Credit: Matthew D Potenski

 

To investigate the diet of the lemon shark, obtaining the stomach contents is essential. Gastric evacuation is a non-lethal method which takes advantage of the carcharhinid sharks’ unusual ability to evert their stomachs.

The sharks are lightly immobilized by placing them in a padded box containing 25l of sea water and approximately 1g of tricaine (MS 222). After about 4 minutes, the sharks become immobile and are removed from the box and positioned vertically along the arm of the holder, snout facing downwards. A pair of long rounded forceps are inserted inside the buccal cavity and pushed upwards into the upper stomach, the wall is pinched and gently pulled downwards into the mouth. A collecting tray is positioned just underneath to obtain any of the stomach’s contents. To replace the stomach, the shark is positioned vertically, snout up, and the lower jaw manipulated to simulate the rhythmic pumping of the mouth. Occasionally, the forceps are required to gently push the stomach back down. The shark is then placed into a steady stream of sea water for revival.   

The samples are then weighed and identified as accurately as possible, any unidentifiable parts are sent to the genetics laboratory at the School of Ocean and Atmospheric Sciences, Stony Brook University, New York for genetic analysis. 

Isotope Analysis

Knowledge of diet, trophic position and movement patterns of species have long been recognized as critical factors, the ‘tenets of ecology’, required for the successful conservation and management of species and the environment they inhabit. New technologies offer a rapidly expanding toolbox for ecologists to examine these factors.

Since the development of bulk stable isotope analyses (SIA) and reduced analysis costs, the application of stable isotopes, in particular those of carbon and nitrogen, has rapidly grown as a standard tool for ecologists to examine and link processes at multiple scales, from individual cells to ecosystem. Specifically for animal ecology, the marked fractionation of nitrogen isotopes(15N:14N) between prey and consumer have been used to examine diet, foraging ecology, trophic position and food web structure, while the conservative fraction of carbon (13C:12C) between primary consumers and predators provides a tool to examine animal habitat use and movement/migration patterns. For large, highly mobile animals that inhabit environments where they are difficult to observe, SIA has provided a unique tool to unveil previously unknown behaviours and interactions

The application of SIA to examine animal origin and movements has grown in recent years, revealing detailed residency, movement and migration patterns that have proven valuable for investigating cryptic and highly threatened species. The use of SIA to track animal movements is based on the retention of stable isotope signatures in an animal’s tissues that reflect the local food web where the animal resides for an extended period of time. Because the stable isotope values of food webs vary spatially depending on localized biogeochemical processes, the stable isotope values of an animal’s tissues from within a given food web will reflect those particular habitats or environments.

Red blood cell and plasma tissues can be sampled easily in the field through the minor invasive puncture of the caudal vein and spinning down and separation of blood component. Fin clips, which are easier to obtain, also provide a valuable source material. All samples are freeze dried and shipped to Dr. Nigel Hussey at the University of Windsor for processing and analyzing.

Close content
Open content