Summary
Laboratory-based studies of locomotory performance in many taxa have noted that individuals form stable hierarchies of organismal performance. Though laboratory studies of teleost fishes have consistently demonstrated individual repeatability of swimming performance, this phenomenon has rarely been studied in the field and never across multiple years. Using a whole-lake acoustic telemetry array with submetre accuracy, we assessed the individual repeatability of two metrics of swimming performance (daily distance traveled and mean daily swimming speed) within four seasons during a year (fall, winter, spring, and summer), among these seasons, and between winters of 2 years. Largemouth bass (Micropterus salmoides (Lacepède, 1802)) formed stable performance hierarchies within seasons except spring and no sex-specific differences in rankings were noted. Individual swimming performance was not repeatable among seasons during 1 year or across multiple winters. Seasonal changes in environmental and intrinsic biological conditions appear to result in a reshuffling of performance hierarchies, perhaps reflecting individual differences in organismal physiology.
Methodology
Study site
This study was carried out on data collected during the 2004–2005 calendar years in Warner Lake (8.3 ha surface area; 44°31′N, 76°20′W), eastern Ontario, Canada. Warner Lake is equipped with the only whole-lake acoustic telemetry system in the world and can position fish with submetre accuracy at fine temporal scales (seconds) year-round, including under the ice (Hanson et al. 2007a). Detailed activity of individually tagged largemouth bass could therefore be monitored across multiple time scales, providing a unique opportunity to evaluate performance hierarchies in the wild. To do so, four time periods were chosen to represent various temperature regimes common during the annual cycle. To represent stable temperature regimes during winter and summer, data collected during 27–31 December 2004 (4.1 ± 0.4 °C; mean ± SD) and 20–24 June 2005 (21.5 ± 1.1 °C) were analyzed. To represent changing temperature regimes during the transitional fall and spring time periods, data collected during 2–6 November 2004 (8.8 ± 0.4 °C) and 14–18 April 2005 (10.1 ± 1.4 °C) were analyzed. These dates corresponded with changes in water temperature associated with winter cooling (before ice on) and spring warming (after ice off). Additionally, to determine if performance hierarchies are stable across multiple years, data were collected on the same six individuals experiencing similar seasonal temperature regimes in 16–20 December 2005 (3.8 ± 0.6 °C) and 16–20 December 2006 (3.6 ± 0.4 °C). Further details on the lake structure and community can be found in Suski (2000) and Hanson et al. (2007a). Briefly, Warner Lake is a small, shallow (maximum depth of 8 m) eutrophic lake consisting of two basins characterized by emergent and submergent aquatic vegetation and coarse woody debris. 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)) in addition to a population of ~700 adult largemouth bass (total length >250 mm).
Study animals
Largemouth bass were collected by angling in Warner Lake between 14 and 16 October 2004. In total, 11 males and 9 females (male total length: 391 ± 23 mm (mean ± SD); female total length: 408 ± 30 mm) were implanted with code division multiple access (CDMA) temperature–pressure sensing acoustic transmitters (Lotek CTP-M16-25, 16 mm × 25 mm, signal transmission rate 10 s, depth resolution ±0.7 m, temperature resolution ±0.5 °C, life expectancy of 1 year, weighing 10.0 g in air). A second collection and implantation event took place between 5 and 6 October 2005, during which 4 males and 6 females (male total length: 398 ± 20 mm; female total length: 396 ± 18 mm) were implanted with longer life expectancy CDMA transmitters (Lotek MA-TP16-25, 16 mm × 25 mm, signal transmission rate 60 s, depth resolution ±0.7 m, temperature resolution ±0.5 °C, life expectancy of 3 years, weighing 23.9 g in air). Implanted transmitters weighed less than 2%–3% of the body mass to avoid an effect of the tag on individual behaviour (Winter 1983; Brown et al. 1999), and transmitters were implanted into the intraperitoneal cavity. Indeed, previous research on adult largemouth bass in Warner Lake has validated that acoustic transmitters used in the current study had negligible impacts on organismal health or condition relative to untagged individuals (Caputo et al. 2009). In both surgery sessions, all surgeries followed methods described in Cooke et al. (2003) and Hanson et al. (2007a) and were conducted by the same experienced individual to eliminate variance associated with multiple surgeons (Cooke et al. 2003). Briefly, prior to surgery, individual fish were anesthetized in a 60 ppm induction bath of clove oil (emulsified in ethanol, a clove oil to ethanol ratio of 1:9) and ethanol (Anderson et al. 1997). Upon equilibrium loss, fish were measured (total length to the nearest millimetre) before being moved to a foam-lined surgery table where a recirculating maintenance dose of anesthetic (20 ppm of clove oil) in lake water was used to irrigate the gills. During transmitter implantation, the sex of individuals was determined. The surgical incision was closed by two simple interrupted sutures (3/0 PDS II, absorbable monofilament sutures; Ethicon Inc.). Additionally, all individuals were marked with a passive integrated transponder (PIT) placed within the surgical incision to allow for future identification. Following surgery, fish were placed in a recovery tank containing fresh lake water until equilibrium was regained (usually within 5 min). Fish were then released into the lake at a central location.
Telemetry array
Fish movements were recorded by a fixed station acoustic telemetry array originally installed in Warner Lake in November of 2003. The physical structure of the array consists of two multi-port MAP_600 receivers (Lotek Wireless, Inc., Newmarket, Ontario, Canada) connected by cabling to 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. (2007a). Briefly, the system relies upon CDMA technology that encodes transmissions from each telemetered individual on the same frequency eliminating issues associated with signal collision and data loss associated with monitoring multiple transmitters on the same frequency in a discrete area. Submetre accuracy of 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, and triangulation by as few as four hydrophones have submetre precision within the footprint of the array and precision greater than 1 m outside the footprint of the array (Niezgoda et al. 2002). All received data are immediately logged on flash storage cards prior to transfer to a personal computer for data processing.