Little is known about the feeding habits of large free ranging fish, due in
large part to lack of an appropriate technique for quantifying feeding
variables. A previous study demonstrated that changes in gastric pH can be
used as a proxy for feeding events in free-ranging sharks. Here we describe
the development of a new acoustic pH transmitter to remotely measure gastric
pH in sharks in the field. The transmitter consists of a dual sensor (pH and
temperature) continuous pinger, and was tested in captive adult blacktip
reef sharks (Carcharhinus melanopterus). The transmitter was retained in the shark's stomach for between 5–12 days. The empty stomach had a low pH (1.6 ± 0.2) and
feeding induced a rapid increase in gastric pH, which was clearly
distinguishable from baseline levels. Meal size showed a significant linear
relationship with the magnitude of the pH changes. Measurement accuracy of
the pH transmitter ranged from 0.05–0.9, although resolution of the VR100
receiver that decoded the transmitter signal was 0.1 units. The pH transmitter can be used to determine when free-ranging sharks in the field are feeding and hence quantify feeding
chronology, frequency and daily ration.