Stomach temperature of narwhals (Monodon monoceros) during feeding events

Abstract

Background: Information on the feeding rate by free-ranging odontocetes is necessary for assessing potential conflicts with fishing activities. One way of obtaining a proxy for feeding events in homeothermic predators is to measure stomach temperature, which drops during prey ingestion. In this study, stomach temperature pills (STPs) were deployed in eight narwhals (Monodon monoceros) in East Greenland (2012-2013). A coded message with information on the temperature was transmitted from the STP and received and relayed by a satellite-linked radio transmitter attached to the back of the whale. Meal size and prey composition were estimated from samples collected from the Inuit hunt of narwhals. Results: Two STPs provided data for 7.9 and 17.3days and six STPs were rejected within 48h. All whales had their first ingestion event between 20min and 14h after handling and release. The mean duration of the STP deployments was 93h (SD=164) and duration was positively correlated with the time between the deployment and the first ingestion event, but did not seem to be affected by the ingestion rate. The average stomach temperature during non-feeding periods was 35.5°C. During ingestion events, the temperature dropped, on average, to 31.6°C. Ingestion events took place at depths of 13 to 850m with a mean depth of 286m (n=126, SD=195). The mean number of detected ingestion events was 9.9 (SD=4.2) per 24h. The average duration of the ingestion events was 9.6min (SD=4.1) and it was not correlated with the size of the drop in temperature or the depth of the feeding dive (r2=0.03 and 0.004) and there was no diel pattern in the ingestion events. The average mass of the stomach contents was ∼2kg. No effect was detected on narwhal behavior as a result of the instrumentation. Conclusions: Stomach temperature telemetry offers the possibility of directly estimating narwhal feeding rates over periods of weeks. The information obtained, however, would need to be validated to account for mariposa and to gauge whether feeding events could be missed by the STPs.