A Novel Acoustic Dissolved Oxygen Transmitter for Fish Telemetry

ate its performance in relation to field biotelemetry. When attached to fishes in the wild, the DO transmitter may be exposed to temporally and spatially changing temperatures. This issue is emphasized by laboratory studies on Atlantic cod (Gadus morhua) demonstrating how this species may decrease the preferred temperature as a response to hypoxia (Petersen and Steffensen, 2003). Accordingly, the transmitter was tested at different temperatures with oxygen saturations ranging from 0 to 191%. Using a tower tank, Claireaux et al. (1995) demonstrated how Atlantic cod voluntarily may perform short foraging excursions into hypoxic water. In order to evaluate the applicability of the DO transmitter to detect and quantify this type of foraging behaviour, the response time (≥ 90% of end value) was tested by transferring the transmitter directly from 100% to 0% DO saturation and vice versa. Methods Transmitter Specifications The DO transmitter is based on an oxygen sensor attached to an acoustic transmitter (e.g. 69 kHz) (Figure 1). Simplified circuits are shown in Figure 2. When the two units are connected (Figure 1), and the transmitter Introduction here are several ways of collecting time series information on fish migration (Lucas and Baras, 2000). In recent studies, the use of telemetry on free-ranging fishes has enabled accurate quantification of changes in the use of space over time on an individual level (Lucas and Baras, 2001). Using a variety of sensors attached to free-ranging animals living in marine and freshwater environments, telemetry studies increasingly include remote measurements of the physiology, behaviour and energetic status of the tagged animals and environmental conditions that are relevant to the organismal physiology (Cooke et al., 2004). This methodology has been termed biotelemetry, and Cooke et al. (2004) predicted that some of the most interesting future findings in ecology will be derived from research that incorporates telemetered or logged field measurements. Owing to its immense influence on aquatic life, the impact of changes in dissolved oxygen (DO) has received substantial attention among researchers working with aquatic animals. Several laboratory studies on marine and freshwater species have shown that changes in DO saturation may cause multiple effects on fish behaviour, physiology, biochemistry, metabolism, swimming performance, growth and mortality (Kutty and Saunders, 1973; Siefert et al., 1973; Tetens and Lykkeboe, 1981; Bushnell et al., 1984; Petersen and Petersen, 1990; Schurmann and Steffensen, 1992; Nilsson et al., 1993; Schurmann and Steffensen, 1997; Dalla Via et al., 1998; Chabot and Dutil, 1999; Brauner et al., 2000). Despite these profound effects, dissolved oxygen measurements are not commonly included in fish biotelemetry field studies (Cooke et al., 2004). Priede et al., (1988a,b) developed and applied a transmitter capable of detecting changes in dissolved oxygen, however to our knowledge no subsequent studies have utilized DO transmitters, presumably reflecting the limitations of conventional oxygen sensor technology and difficulties associated with the method. As indicated by the authors, the polarographic oxygen sensor is vulnerable to drift and thus requires frequent recalibrations, which may be impractical for field work. In addition, polarographic sensors require a stabilized polarising power supply, which may be complicated to obtain in a telemetry transmitter set-up. The objective of the present study was to conduct a controlled laboratory examination of a novel DO transmitter and critically evalu-