Summary
A whole-lake acoustic telemetry array was utilized to monitor the three-dimensional position of 20 largemouth bass (Micropterus salmoides). Code division multiple access (CDMA) technology enabled the simultaneous monitoring of the 20 transmitters (equipped with pressure and temperature sensors) at 15 s intervals with sub-meter accuracy. Fish were monitored between November 2003 and April 2004 to evaluate the behaviour of fish across different temporal and spatial scales. The distance moved by largemouth bass, assessed both on a daily and hourly basis, varied by season and was positively correlated with water temperature. For example, daily movement rates were 2.69 ± 1.45 km/day in mid November (average daily water temperature 5.9°C), 2.24 ± 0.73 km/day in early January (5.1°C), and 7.28 ± 2.62 km/day in mid April (7.7°C). Interestingly, daily movement rates varied by as much as 25 fold among individual fish. Visualization of fish swimming paths revealed that whereas some fish occupied discrete areas and made only localized movements, other individuals made lengthier journeys covering much of the lake in periods of as little as one day. Analysis of fish behaviour at a finer temporal scale revealed that during the winter, fish spend more than 95% of their time swimming at speeds less than 0.1 m/s (0.07 ± 0.24 m/s). During late fall, and especially in spring, swimming speeds were higher with mean swimming speeds of 0.11 ± 0.27 m/s and 0.19 ± 0.29 m/s, respectively. When the telemetry dataset was queried to simulate 24 h manual tracking intervals, it was clear that manual tracking data would not have been representative of actual daily movement rates, underestimating daily movement and swimming speeds by at least 75 fold. This study identifies the importance of evaluating fish activity at multiple spatial (whole lake to sub-meter position) and temporal (seasonal to seconds) scales and illustrates the potential of CDMA telemetry to yield such data.
Methodology
Study site
In 2003 a freshwater lake (Warner Lake, Queen’s University Biological Station [QUBS]) was instrumented with a CDMA acoustic telemetry system as the backbone of an aquatic “ecological observatory”. The observatory extends traditional telemetry studies involving manual triangulation of a few representative animals to a fully automated all-season community and species-interactive multi-dimensional view. Warner Lake is a private research reserve located in eastern Ontario (44°31′ N, 76°22′ W) that is wholly enclosed on QUBS property, enabling the deployment and field testing of equipment in an undisturbed setting. The small lake (18.2 hectare surface area) is composed of two basins; a smaller shallow basin (max depth = 2 m) and a slightly larger deeper basin (max depth = 7 m). The entire shallow basin and the near shore regions of the deeper basin have extensive littoral zone characterized by emergent and submergent vegetation. The deeper areas have dense weed beds with the majority of the lake bottom covered by Chara spp. The entire shoreline has substantial amounts of fallen timber. Warner Lake is a closed system for fish and has been the focus of a long-term ecological study on the reproductive dynamics of largemouth bass (see Suski, 2000). Beginning in 1996, researchers have conducted underwater nesting surveys of largemouth bass and have implanted nesting males with passive integrated transponders (PITs) to monitor the growth, survival, and reproductive activity of individual fish over time, providing ample background data for this current research activity. Other documented species include white sucker (Catostomus commersonii), pumpkinseed (Lepomis gibbosus), yellow perch (Perca flavescens), brown bullhead (Ameiurus nebulosus) and golden shiner (Notemigonus crysoleucas).
Telemetry array
The underwater acoustic telemetry array was installed in Warner Lake during November 2003 (Detailed description of the equipment, configuration, and system performance is provided in Cooke et al., 2005). To meet the acoustic telemetry requirements of the observatory, a CDMA-based telemetry system (MAP_600, Lotek Wireless, Newmarket, ON) was installed at the observatory. The telemetry equipment consists of two multi-port MAP_600 receivers monitoring a total of 13 hydrophones distributed in such geometry as to provide coverage of the entire lake, including the littoral zones. Equipment was configured to monitor eight hydrophones (large basin) on one receiver, with the remaining five hydrophones (small basin) on the other receiver. A CDMA temperature-pressure sensing tag (burst rate 15 s) was also placed at each moored hydrophone location. Cabling (i.e., connecting hydrophones to shore-based receiving equipment) was routed along the bottom of the lake to one point on the shoreline and brought out of the water through conduit. Hydrophones were moored from fixed posts (steel piping driven into the lake bottom) at an approximate depth of 2 m from the water surface to ensure that lake ice conditions would not damage or move the hydrophones. To facilitate sub-meter positioning of instrumented fish, all hydrophones were surveyed using differential GPS (±0.2 m). Data were transferred from each receiver to a personal computer for further processing by use of flash storage cards.