Autonomous measurement of ingestion and digestion processes in free swimming sharks

Abstract

Summary Direct measurement of predator feeding events would represent a major advance in marine trophic ecology. To date, devices available for empirically quantifying feeding in free-swimming fishes have relied on measuring stomach temperature, pH or physical motility, each of which has major, practical limitations. We hypothesized that the considerable physical changes which occur in the stomachs of carnivorous predators during the processes of ingestion and digestion should be quantifiable using Bulk Electrical Impedance measured across paired electrodes. We used a prototype archival data logging tag (Wildlife Computers Inc. Redmond, WA) to record changes in impedance inside the stomachs of captive, free swimming tiger and sandbar sharks over multiple, successive feeding events. Feeding and digestion events produced characteristic changes in electrical impedance of the stomach contents identifiable as 5 successive phases: (1) Pre-ingestion (empty stomach), (2) Ingestion, (3) Chemical 'lag' phase, (4) Mechanical 'chyme' phase, and (5) Stomach emptying phase. The duration of the chyme phase was positively related to meal size.