Prey Switching and Optimal Foraging
Alan Reeve and Dr. Samuel H. Gruber
Observing the activity
from the platform
Abstract - The degree of selectivity exhibited by juvenile lemon sharks towards a prey species was determined by presenting individual sharks with differing relative abundances of two prey species and recording the prey choice. Determining the selectivity of an organism is essential for understanding the interactions between a predator and its prey. The majority of studies on elasmobranch diet have reported the prey items found in the stomach of a number of sharks and have formulated predictions on the foraging strategy of the animal. However, the extent to which an elasmobranch specialises on one prey type has never been demonstrated. Similarly it is unclear whether sharks display periodicity when feeding or are asynchronous and consume prey at all times of the day or night.
Using the semi natural conditions of pens constructed in a lagoon, individual sharks were exposed to differing relative abundances of two prey species. An electivity index (E*) and a chi squared goodness of fit test were used to elucidate the selectivity of the sharks. Further analysis enabled the sharks functional response curve towards the prey items to be determined and any size selection in prey items identified. Stomach contents of the sharks were analysed to identify the times that feeding took place.
Examining a young lemon
shark after a feeding trial
Results from the investigation demonstrated that at all abundances of prey items the sharks were highly species selective towards yellowfin mojarra (mean E* 0.326) whilst displaying almost complete negative selection for grey snapper (mean E* -0.939). A type II functional response curve (r2= 0.9121) supported evidence that sharks are specialist predators. No size selection of prey items was observed. Stomach content analysis indicated that sharks are asynchronous feeders; however a more precise method of determining the times of feeding events is necessary for strong conclusions to be made on this matter.
The selectivity of sharks arises as a result of the behavioural and morphological characteristics of the prey species. Mojarras probably provide a higher net energy gain than snappers as they appear easier to perceive and catch.
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