internal seiche in a stratified lake

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1 Notes 187 lakes in Alberta, Canada. Can. J. Fish. Aquat. Sci. 41: STAUFFER, R. E Vertical nutrient transport and its effect on epilimnetic phosphorus in four calcareous lakes. Hydrobiologia 154: , AND G. F. LEE Role of thermocline migration in regulating algal blooms, p Zn E. J. Middlebrooks et al. [eds.], Modeling the eutrophication process. Ann Arbor Sci. STEFAN, H. G., AND M. J. HANSON Phosphorus recycling in five shallow lakes. J. Environ. Eng. Am. Sot. Civ. Eng. 107: , G. M. HORXH, AND J. W. BARKO A model for the estimation of convective exchange in the littoral region of a shallow lake during cooling. Hydrobiologia 174: TAYLOR, W. D., J. W. BARKO, AND W. F. JAMES Contrasting diel patterns of vertical migration in the dinoflagellate Cerutium hirundinella in relation to phosphorus supplies in a north-temperate reservoir. Can. J. Fish. Aquat. Sci. 45: TWINCH, A. J., AND R. H. PETERS Phosphate exchange between littoral sediments and overlying water in an oligotrophic north-temperate lake. Can. J. Fish. Aquat. Sci. 41: WEILER, P. R Littoral-pelagic exchange in Lake Wingra, Wisconsin, as determined by a circulation model. Univ. Wis., Madison, Inst. Environ. Stud. Rep p. WETZEL, R. G The role of the littoral zone and detritus in lake metabolism. Arch. Hydrobiol. 13: Submitted: 26 February 1990 Accepted: 25 June 1990 Revised: 4 September 1990 Limnol. Oceanogr., 36(l), 1991, , by the American Society of Limnology and Oceanography, Inc. Shifts in fish vertical distribution internal seiche in a stratified lake in response to an Abstract- Juvenile sockeye salmon in Quesnel Lake were nocturnally thermoselective and distributed in a narrow vertical band within the thermocline. Differences in vertical position across and between lake transects occurred when an internal seiche was active, as evidenced by a shortterm alteration in the thermal structure of the lake. The results provide qualitative evidence for a direct relationship between Quesnel Lake physics and the vertical distribution of juvenile sockeye. For ectothermic organisms in thermally stratified bodies of water, temperature provides a strong and unambiguous environmental signal. Predictable thermal gradients might be used as orienting stimuli in fish migrations, and there are numerous examples of temperature serving as a proximate stimulus affecting movements and hence distribution of fish populations (Reynolds 1977). The nocturnal vertical distribution of juvenile sockeye salmon (Oncorhynchus nerka) in the limnetic zone of their nursery lakes evidently is controlled by temperature (Levy 1990). Field observations show that Acknowledgments We thank H. Enzenhofer and C. Mueller for assistance during field data collection. An earlier version of the manuscript was reviewed by R. Thomson and J. Stockner. Fig. 1. Depth chart of Quesnel Lake showing location of acoustic transects.

2 Notes TEMPERATURE C TEMPERATURE C PROPORTION OF INTEGRATED PROPORTION OF INTEGRATED VOLTAGE VOLTAGE Fig. 2. Depth distribution of juvenile sockeye across transect 6 as indicated by echo-integrating signals from a 420-kHz echosounder. Fish vertical distribution is plotted adjacent to the temperature profiles. A. 5 August B. 23 August the fish have strongly peaked, monomodal frequency distributions in temperature preference, similar to those observed in laboratory-induced temperature gradients (Neil1 1979). Physical processes that alter the temperature distribution should therefore indirectly influence the vertical position of juvenile sockeye in lakes. Internal waves are common features in large lakes and marine environments and can influence vertical temperature distribution dramatically (Boyce 1974). During routine population surveys ofjuvenile sockeye at Quesnel Lake, British Columbia, we observed anomalies in acoustic records that led us to postulate internal wave-induced shifts in thermal structure. In this report, we present these acoustic observations together with associated temperature data. Quesnel Lake is a large (270 km2), deep (mean depth, 158 m; max depth, 460 m) lake in the central interior plateau region of British Columbia (52 N, 12 low). Stockner and Shortreed (198 3) characterized the lake as oligotrophic by virtue of its high transparency (mean Secchi depth, 10.9 m) and low nutrient concentrations. The lake serves as a nursery for large numbers of sockeye juveniles (N 106-log), which rear in the pelagic zone during their first year of life (Canadian BioSonics Ltd. unpubl. data). In 1982, 1986, 1987, and other years, we undertook quantitative hydroacoustic surveys to estimate juvenile sockeye densities in the lake. A fixed transect design, consisting of 16 hydroacoustic transects (Fig. l), was adopted and systematically surveyed in summer and fall. These same acoustic observations were used to analyze the depth distribution of juvenile sockeye during the present study. For acoustic surveys in 1986 and subsequently, we used a dual-beam 420-kHz BioSonics echosounder coupled to a digital tape recorder. Echo signals were monitored

3 Notes 189 B SEQUENCE NUMBER i-== 15 :, \ 21 i ' Fig. 3. A. Echogram showing nocturnal depth distribution of juvenile sockeye across transect 6 on 23 September B. Mean depth of acoustic targets within echo-integrator sequences. in the field with a dual-channel oscilloscope and a chart recorder. Transducers were suspended in a towed body connected to a 7-m fiberglass boat operating at 7.4 km h-l. Juvenile sockeye undertook diel vertical migrations, alternating between m depths during the day and m depths at night (Clark and Levy 1988). Fish occur in schools during the day and disperse as individuals at night, making nocturnal periods better for acoustic enumeration. During midwater trawl tows (Enzenhofer and Hume 1990) conducted concurrently with our acoustic surveys, juvenile sockeye composed 99.8% of the catch in 45 samples since Thus the acoustic information from the lake pertains directly to the juvenile sockeye population. Other fish species present in the pelagic zone in relatively low numbers include rainbow trout (Oncorhynchus mykiss), lake char (Salvelinus namaycush), Rocky Mountain whitefish (Prosopium williamsoni), and kokanee (nonanadromous 0. nerka). In the laboratory, taped echo signals were integrated with a BioSonics digital echo integrator. Because our objective was to determine relative fish abundance in different depth layers, values of 1 were adopted arbitrarily for the equipment scaling factors required by the instrument. Mean depth of the fish population, for a given sequence of sound pulses along a transect, was computed by scaling the integrator voltage in successive depth strata by the total integrated voltage within the water column (formula given by Levy 1990). Temperature was measured with a YSI- SCT meter at l-m depth intervals from the surface to 50 m. Surface temperatures also were measured with a mercury thermometer to correct for inaccuracies (<0.5 C) of the SCT meter. Hydroacoustic observations indicated that juvenile sockeye concentrated nocturnally in a narrow band of the thermocline, particularly in August when the lake was strongly stratified (Fig. 2). The narrow nocturnal depth distribution may reflect thermoselection by juvenile sockeye; when the thermocline descended (seasonally), the fish were situated in deeper strata (Fig. 2). Nocturnal thermoselection by juvenile sockeye also has been observed in other lakes (Levy 1990) and is indicated by the nocturnal distribution patterns of a vertically migrating freshwater sculpin (Wurtsbaugh and Neverman 1988). In virtually all of the surveys in Quesnel Lake, as well as hydroacoustic surveys over the past decade in 13 other sockeye lakes in the Fraser River system, echograms typically have indicated discrete, horizontal layers of echo targets of varying thickness. Such was not the case for transect 6 on 23 September 1986 (Fig. 3A) when we observed a substantial tilt in the juvenile sockeye vertical distribution across the transect.

4 190 Notes Transect 4 Transect 6 A SbWNlCe Sequence Transect 6 Seauence SEQUENCE NUMBER 1 N L -g 14-0" 16-6 r" 18- Fig. 4. As Fig. 3, but across transects 4, 5, and 6 on 12 August Echo-integration results (Fig. 3B) showed a 4-m difference in mean vertical position of sockeye juveniles across the transect. All of the other 15 transects undertaken during the two-night survey produced horizontal echograms typical of sockeye juveniles in their nursery lakes. Because the nocturnal vertical depth position of juvenile sockeye is evidently controlled by temperature (Fig. 2), the distribution of fish targets (Fig. 3) implies that the temperature distribution in this region of the lake was tilted on the night of 23 September Wind-induced thermocline seiches occur in large temperate lakes and, where morphometry and bottom topography permit, can take the form of internal (baroclinic) Kelvin waves propagating cyclonically around the lake (Hamblin 197 8; Goldman and Home 1983). Tilting of the thermocline can occur across the main lake axis as a function of time due to counterclockwise propagation (northern hemisphere) of large- scale, baroclinic Kelvin waves. The thermocline position across a lake with an active Kelvin wave would, at a certain stage of propagation, be qualitatively similar to the tilted juvenile sockeye distribution observed on 23 September 1986 (Fig. 3). This idea was tested in August The day before the start of the survey ( 10 August 1987) a strong storm passed through the area, causing minor damage to shoreline dock structures. The population of sockeye juveniles was much less dense in 1987 than 1986; echograms taken during the 1987 survey (Fig. 4A) have been resealed to facilitate comparisons of vertical distributions. Differences in vertical distributions of juvenile sockeye were evident both between and within transects (Fig. 4). On transect 4, sockeye juveniles were centered at 15-m depth- several meters shallower than sockeye on transect 6 (sequences 1 and 2; Fig. 4B). Sockeye distributed across transect 6 again showed a tilted vertical distribution,

5 Notes 191 Temperature (C) Temperature A ( C) 4 a with a 3-m difference in mean vertical position across the transect (Fig. 4B). There was evidently an active seiche within Quesnel Lake at the time of the acoustic survey. Temperature profiles taken at a monitoring site on the south side of transect 6 (Fig. 5A) showed a 10-m ascent of the thermocline position over 25.5 h between 11 and 12 August. Comparison of the temperature distribution across transect 6 at 0500 hours on 12 August (Fig. 5B) suggested a nonuniform temperature distribution across the transect. The thermocline position was 5 m deeper in the middle part of the transect than at either the north or south sides (Fig. 5B), suggesting an active Kelvin wave. There was no obvious correspondence between the tilted sockeye distribution on transect 6 (Fig. 4) and the deeper thermal distribution at the middle of the transect. This discrepancy possibly relates to the 4-h difference between the acoustic transect and the temperature measurements. Although juvenile sockeye altered their nocturnal vertical positions by several meters in response to an internal seiche, this displacement probably had only minor or insignificant effects on their growth and survival in the lake. The wave-induced nocturnal displacement was small compared to the m vertical amplitude of diel vertical migration by juvenile sockeye within the system. While there is a need for further quantification of this phenomenon, the findings emphasize the importance of temperature controlling the nocturnal distribution of juvenile sockeye. This thermoselection behavior has important metabolic implications for the fish (Brett 197 1; Levy 1990). In the marine environment, Farmer and Smith (1980) used echo-sounding observations from Knight Inlet to observe physical processes associated with tidal mixing and advection. Likewise, Haury et al. ( 1983) c Fig. 5. Temperature profiles over 25.5 h on 1 l-l 2 August 1987 at a single measuring point close to the southern edge of transect 6 (A) and at three measurement points across transect 6 on 12 August 1987 at hours (B).

6 192 Notes used acoustic devices to observe tidally gen- BRETT, J. R Energetic responses of salmon to erated internal wave packets in Massachu- temperature. A study of some thermal relations in the physiology and freshwater ecology of sockeye setts Bay. Planktonic organisms, physical salmon (Oncorhynchus nerka). Am. Zool. 11: 99- microstructure, and turbulence were all po tential sources of acoustic reflections in CLARK, C. W., AND D. A. LEW Diel vertical coastal marine environments (Farmer and migrations by juvenile sockeye salmon and the Smith 1980). Distributional patterns of maantipredation window. Am. Nat. 131: ENZENHOFER, H.J., AND J.M.B. HUME Simple rine organisms may also provide insight into closing midwater trawl for small boats. N. Am. J. physical oceanographic processes (Bratko- Fish. Manage. 9: vich 1988). In future freshwater work, ju- FARMER, D. M., AND J. D. SMITH Tidal intervenile sockeye in the pelagic zone might serve as acoustic markers to delineate thermocline position and provide a means for easily monitoring internal wave dynamics within sockeye salmon lakes. action of stratified flow with a sill in Knight Inlet. Deep-Sea Res. 27: GOLDMAN, C. R., AND A. J. HORNE Limnology. McGraw-Hill. HAMBLIN, P. F Internal Kelvin waves in a fjord lake. J. Geophys. Res. 83: HAURY, L. R., P. H. WIEBE, M. H. ORR, AND M. G. BRISCOE Tidally generated high-frequency internal wave packets and their effects on plankton David A. Levy Robert L. Johnson Jeremy M. Hume2 in Massachusetts Bay. J. Mar. Res. 41: LEW, D. A Sensory mechanism and selective Department of Fisheries and Oceans advantage for diel vertical migration in juvenile West Vancouver Laboratory sockeye salmon, Oncorhynchus nerka. Can. J. Fish Marine Drive Aquat. Sci. 47: West Vancouver, B.C. V7V IN6 References BOYCE, F. M Some aspects of Great Lakes physics of importance to biological and chemical processes. J. Fish. Res. Bd. Can. 31: BRA&OVKH, A The use of planktonic organism distribution as an indicator of physical variability in marine environments, p In D. F. Soule and G. S. Kleppel teds.], Marine organisms as indicators. Springer. 1 Present address: Canadian BioSonics Ltd. No. 204, Main St., Chilliwack, B.C. V2P 4M9. * Present address: Department of Fisheries and Oceans, Cultus Lake Laboratory, Cultus Lake, B.C. VOX 1HO. NEILL, W. H Mechanisms of fish distribution in heterothermal environments. Am. Zool. 19: REYNOLDS, W. W Temperature as a proximate factor in orientation behavior. J. Fish: Res. Bd. Can. 34: STOCKNER, J. G., AND K. S. SHORTREED A comparative limnological survey of 19 sockeye salmon (Oncorhynchus nerka) nursery lakes in the Fraser River system, British Columbia. Can. Tech. Rep. Fish. Aquat. Sci p. WURTSBAUGH, W.A.,ANDD. NE~ERMAN Postfeeding thermotaxis and daily vertical migration in a larval fish. Nature 333: Submitted: 17 January 1990 Accepted: 26 July 1990 Revised: 21 August 1990