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Using Service Argos for Real Time Tracking

Mikael Sjberg, Department of Animal Ecology, Swedish University of Agricultural Sciences, 901 83 Umea, Sweden, Dave Thompson, Phil Lovell, and Ed Bryant, SMRU, University of St. Andrews, St. Andrews, United Kingdom

During summer 1996 we studied the behavioral response of Baltic grey seals (Halichoerus grypus) to acoustic disturbances (airgun detonations). To examine the effects of transient stimuli we required high resolution behavioral (e.g. swim speed, depth and duration) and physiological data (e.g. heart rate and stomach temperature) as well as accurate position estimates. Because grey seals are difficult to catch and often perform large-scale movements (>100 km/24 hr), data collection using retrievable data loggers was not feasible. Instead, we used a real time tracking technique, based on a combination of satellite transmitters (PTTs) and acoustic data transmitters, to follow seals as they traveled at sea. The Argos satellite tracking system was utilized both for positioning the animals and for transmitting depth and swim speed information. However, data transmission rates are limited in the Argos system. Only limited amounts of data can be sent on each transmission, transmission rate is restricted to a maximum of once every 40 sec and previous studies suggested that only 4% of data transmissions from grey seals are received by the Argos system.

In order to overcome these limitations and utilize the Argos system for real time tracking, each PTT was programmed to emulate three different PTTs. The transmitting schedule was designed to change between different pre-programmed messages in order to maximize the transfer of information. The different messages consisted of: Message 1 (during real time tracking) containing a detailed dive description, with data regarding the duration, time at surface, and, within in a dive, depth levels and 21 velocity levels in 20 equal time intervals. Message 2 containing summarized information from 4 dives (duration, surface time and depth). Message 3 (after the real time tracking period) containing information about haul out and technical data. To increase the transmission detection rate during the real time tracking we used a GONIA 400P satellite receiver and direction finder.

Position fixes from the Argos system were relayed to a tracking vessel. When the boat was within 3-4 km of the seal, the GONIA 400P satellite receiver was used to obtain bearings to the transmitter to track its local movements and record all transmissions. This enabled us to receive more or less continuous depth and swim speed records during the real time tracking period as well as getting location and additional data from the Argos system when the tracking vessel was away.

More than 343,000 transmissions was sent from six PTT's during 13,860 hr. The number of received messages was 13,646 by the Argos system and 8,082 by the GONIA 400P. Data were collected from about 16,300 dives, about 23% of which was detailed data and about 77% summarized data.

Within 500 m of the seals we used directional hydrophones and purpose built hydrophone receivers (VEMCO VR60 receivers, Vemco Ltd, Halifax, Canada) to track ultra sonic acoustic transmitters. The acoustic transmitters (SINTEF, Trondheim, Norway) sent either stomach temperature data encoded in the inter-pulse intervals of a 65-85 kHz signal or heart rate data in the form of the seal's ECG as an FM modulation of a constant 90-kHz carrier.

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