Because fish are poikilothermic, water temperature is regarded as a primary factor influencing their activity and behaviour. Rarely have field studies been conducted with the spatiotemporal resolution to enable rigorous quantitative assessments of that relationship. Furthermore, there have been few studies that have considered the influence of sex on the seasonal behaviour of fish. Twenty largemouth bass (Micropterus salmoides (Lacepède, 1802)) were implanted with coded acoustic telemetry transmitters and remotely tracked in near real time in a small lake in Ontario, Canada, via a whole-lake hydrophone array between 1 November 2004 and 30 September 2005. Fish inhabited the deepest waters and were least active during the winter months under ice. During the warmest months, fish were most active and inhabited the littoral zone. Sex-specific differences were noted year-round. Reproductive males were less active and inhabited shallower depths during the spawning and post-reproductive periods. Reproductive males inhabited the deepest depths during winter and fall, with nonreproductive males at the shallowest depths. Throughout the year, the behaviour of nonreproductive males and females was similar. While differences in behaviour of bass are primarily driven by water temperature, sex and reproductive status play important roles year-round, especially during and after the reproductive period.
This study was carried out from 1 November 2004 through 30 September 2005 in Warner Lake, eastern Ontario, Canada (44°31′N, 76°20′W), and is wholly enclosed on Queen’s University Biological Station property, thereby enabling the monitoring of individual fish implanted with transmitters without disturbance from unauthorized personnel. Further details on the lake structure and community can be found in Suski (2000) and Hanson et al. (2007). Briefly, Warner Lake is a small lake (8.3 ha surface area) comprising a shallow basin (maximum depth 2 m) and a deep basin (maximum depth 7 m). The lake is characterized by homogenous habitat consisting of extensive littoral zone featuring both emergent and submergent macrophytes as well as large amounts of fallen timber. Other documented fish species include white sucker (Catostomus commersonii (Lacepède, 1803)), pumpkinseed (Lepomis gibbosus (L., 1758)), yellow perch (Perca flavescens (Mitchill, 1814)), brown bullhead (Ameiurus nebulosus (Lesueur, 1819)), and golden shiner (Notemigonus crysoleucas (Mitchill, 1814)).
Largemouth bass were collected by angling between 14 and 18 October 2004. In total, 11 males and 9 females (mean ± SD: males, 948 ± 220 g; females, 925 ± 210 g) were implanted with code division multiple access temperature–pressure sensing acoustic transmitters (Lotek CTP-M11-25, 11 mm × 25 mm, signal transmission rate 15 s, depth resolution ±0.7 m, temperature resolution ±0.5 °C, life expectancy of 1 year, weight 10.0 g in air; Lotek Wireless, Inc., Newmarket, Ontario) following methods described in Cooke et al. (2003a) and Hanson et al. (2007). Prior to surgery, individual fish were anesthetized in an induction bath of clove oil (0.6 mL/L (60 ppm) emulsified in ethanol, clove oil – ethanol, 1:9) and ethanol (Anderson et al. 1997). Following loss of equilibrium, fish were measured (total length to the nearest millimetre) and weighed (mass to the nearest gram). During surgery, a recirculating maintenance dose of anesthetic (20 ppm clove oil) in lake water was used to irrigate the gills. The sex of individuals was determined during transmitter implantation via inspection of internal anatomy. Two simple interrupted sutures (3/0 PDS II, absorbable monofilament sutures; Ethicon, Inc.) were used to close the incision. Additionally, all individuals were marked with a passive integrated transponder to allow for future identification. All surgeries were conducted by the same experienced individual to eliminate variance associated with multiple surgeons (Cooke et al. 2003a). Following surgery, fish recovered in coolers containing lake water until equilibrium was regained (usually within 5 min) and were released in the lake at a central location.
Fish movements were recorded by a fixed-station acoustic telemetry array installed in Warner Lake in November of 2003. The array consists of two multiport MAP_600 receivers monitoring 13 hydrophones configured in an optimal geometry to provide coverage throughout the entire lake. Details on system performance and accuracy can be found in Niezgoda et al. (2002) and Hanson et al. (2007). Briefly, the system relies upon code division multiple access technology that encodes transmissions from each telemetered individual, thereby eliminating issues associated with signal collision and data loss associated with monitoring multiple transmitters on the same frequency in a discrete area. Positions calculated by triangulation with as little as four hydrophones have submetre precision within the footprint of the array and precision greater than 1 m outside the footprint of the array. Submetre positioning of instrumented fish results from the geometry of the implemented hydrophone array, which was surveyed by differential GPS (±0.2 m) (Niezgoda et al. 2002). Error decreases significantly as more hydrophones receive the transmissions and are added to triangulation calculations (Niezgoda et al. 2002). All received data were logged on flash storage cards and routinely transferred to a personal computer for subsequent processing.