Diel Behavior Research Articles

Could beaked whales get the bends?

A mathematical model, based on current knowledge of gas exchange and physiology of marine mammals, was used to predict blood and tissue tension N2 (P(N2)) using field data from three beaked whale species: northern bottlenose whales, Cuvier's beaked whales, and Blainville's beaked whales. The objective was to determine if physiology (body mass, diving lung volume, dive response) or dive behaviour (dive depth and duration, changes in ascent rate, diel behaviour) would lead to differences in P(N2) levels and thereby decompression sickness (DCS) risk between species. Diving lung volume and extent of the dive response had a large effect on end-dive P(N2). The dive profile had a larger influence on end-dive P(N2) than body mass differences between species. Despite diel changes in dive behaviour, P(N2) levels showed no consistent trend. Model output suggested that all three species live with tissue P(N2) levels that would cause a significant proportion of DCS cases in terrestrial mammals. Cuvier's beaked whale diving behaviour appears to put them at higher risk than the other species, which may explain their prevalence in strandings after the use of mid-frequency sonar.

Influence of diel behaviour in the morphology of decapod natantia

Natantia is a group of decapod species showing morphologically convergent adaptations for swimming behaviour (i.e. the shrimps and prawns morphotype). Different categories are identified in relation to water column or seabed zones where the diel behaviour takes place (i.e. pelagic and nektobenthic submorphotypes). In the present study, we hypothesized a morphological convergence in different pelagic and nektobenthic species based on common nocturnal or diurnal behaviour. We analysed the pattern of morphological variation in 56 Atlanto-Mediterranean species by measuring biometric and morphometric (i.e. landmarks) characters. We also extensively reviewed the literature, recompiling information on species diel behaviour and type of displacement. That information was organized in different grouping variables and their categories: Diel-Behaviour (categories: Diurnal and Nocturnal); Sub-Morphotype (categories: Nektobenthic and Pelagic); and Classification-Status (categories: Penaeidea or Caridea). Soft independent modelling of class analogy analysis was performed per each variable to group species in each category and consequently obtain a morphological model of reference. The results obtained indicate that species sharing the same type of diel behaviour present morphological convergence in characters located in the cephalothorax and rostrum areas. Differences in the size of eyes were not related to nocturnal or diurnal rhythms but to the type of displacement (i.e. pelagic and nektobenthic). The results were interpreted assuming that visual predators exert a morphological selection on natantia depending on the presence o absence of environmental light when animals are active. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 96, 517–532.

Diel Movement Behavior of The Stripe-Necked Musk Turtle (Sternotherus Minor Peltifer) in Middle Tennessee

Few studies have used radiotelemetry to focus directly on the diel behavior of turtles. We used radiotelemetry to relocate 10 Stripe-necked Musk Turtles (Sternotherus minor peltifer) every 2 h over a 24 h cycle in Middle Tennessee. Movements during the 24 h cycle were monitored on seven occasions between 27 Jul. and 4 Nov. 2004. Sternotherus minor peltifer were often inactive and remained sedentary throughout a 24 h cycle; 87% of the total relocations revealed no movement. Overall, S. m. peltifer's diel movement behavior was restricted to evening and nocturnal hours. Frequency data suggested a uni-modal distribution of movement during evening and midnight periods (1700–0159 h). Similarly, the mean distance traveled showed a uni-modal distribution with a peak in the evening period of a 24 h cycle. Based on the data set with only light and dark categories, males were found to move more frequently in daylight relative to females. A decrease in movement (frequency and distance) occurred with the changing of the seasons from summer to winter. Probably due to temperature change, nocturnal and crepuscular behaviors were highest in Jul. and Aug., while during the autumn months, there was little preference for daylight or darkness. This phenomenon has been reported in several Sternotherus species; but, to our knowledge, this is the first report for S. minor.

Diurnal and nocturnal catchability of Kuwait's commercial shrimps

In descending order of importance, the shrimps Penaeus semisulcatus De Haan, 1844; Metapenaeus affinis (H. Milne-Edwards, 1837); and Parapenaeopsis stylifera (H. Milne-Edwards, 1837) account for over 95% of Kuwait's commercial landings. Throughout its range, P. semisulcatus is nocturnal, but Kuwait trawlers are active 24 h a day during season. Historically, all scientific shrimp surveys in Kuwait have been conducted during daylight because of regional security concerns. In order to more accurately assess shrimp stocks using survey data, comparative trawl tows were conducted during day and night from March 1999 through December 2000. Monthly catches from one to four different stations were standardized to catches (kg and no.) per net hour at 3.3 knots. Catch rates of eight shrimp species ( P. semisulcatus, M. affinis, P. stylifera, Metapenaeus stebbingi Nobili, 1904, Metapenaeopsis stridulans (Alcock, 1905), Megodris granulosus (Haswell, 1879), Trachysalambria curvirostris (Stimpson, 1860), and Solenocera crassicornis (H. Milne-Edwards, 1837) and fish by-catch were analyzed using generalized linear models and tested for significant differences for timing (day vs. night), month, year, region, time × month, time × year, time × region, month × year, month × region, and year × region. Of the 80 comparisons (8 species × 10 variables) for mass and numbers, significant differences were similar for all but 15 comparisons. Significantly higher catch rates (kg or no. net h −1@3.3 knots) during night than during day were found for P. semisulcatus (3.705 vs. 3.198 kg), M. affinis (1.420 vs. 1.077 kg; 135 vs. 95 no.), M. stridulans (0.420 vs. 0.050 kg; 143 vs. 14 no.), M. granulosus (0.437 vs. 0.018 kg; 125 vs. 8 no.), T. curvirostris (0.082 vs. <0.001 kg; 21 vs. 0 no.), S. crassicornis (0.007 vs. 0.003 kg; 1.4 vs. 0.8 no.). Significant differences also occurred between years for P. stylifera (2000 > 1999), M. granulosus (1999 > 2000), and S. crassicornis (2000 > 1999). Monthly and regional differences occurred for five or six of the eight species, depending on mass or no., reflecting recruitment patterns, fishing season, and distance from the Shatt Al-Arab. These differences are believed to be influenced by a combination of water turbidity and predation pressure. With the damming of the Tigris and Euphrates Rivers in Turkey, the silt load as well as water volume discharges of the Shatt Al-Arab will decrease. Consequently, future studies of shrimps’ diel behavior may find changes from the results presented here.

Patterns in vertical movements of black rockfish Sebastes melanops

Black rockfish Sebastes melanops are generally associated with benthic structure, but are also described as semi-pelagic and are frequently found near the surface. We described patterns associated with the vertical movements of 33 black rockfish using acoustic telemetry in the nearshore waters of the Northeast Pacific Ocean off Newport, Oregon (9 to 45 m depth). The fish were moni- tored by an array of 18 moored receivers for a 12 mo period. Black rockfish showed larger, more frequent, and more temporally structured vertical movements throughout the study period than are typically assumed for rockfishes, which may make hydroacoustic surveys difficult to design. Diel vertical movements occurred sporadically, with individuals sometimes being shallower either during day or at night, and the pattern was often maintained for more than a week. Periods of large vertical movements (multiple excursions >5 m not in synch with diel phase) were most common during the months of October and May. The diel behavior of being shallower at night was most common during spring months and showed a slow decline in prevalence throughout the summer months. The reverse pattern (shallower depths during daylight hours) was less common overall, but most prevalent in autumn. These data show that black rockfish make extensive vertical movements, often tightly in phase with sunrise or sunset, and that several behaviors, such as diel vertical migration, may be more prevalent at certain times of the year.

Diel variation of phytoplankton functional groups in a subtropical reservoir in southern Brazil during an autumnal stratification period

A knowledge of diel variation and the vertical distribution of phytoplankton communities may contribute to a better understanding of the driving factors of key species. Applying functional-group classification provides important information on the causes of species selection in the pelagic community. The diel variation of phytoplankton functional groups was analysed during an autumnal stratification period with the aim of understanding their changes in the vertical position related to light, mixing regime and grazing pressure. Phytoplankton and zooplankton communities were sampled every 4 h during a 24-h period in a vertical profile in a subtropical meso-eutrophic reservoir. Strong stratification during a 24-h cycle and a mixed clear epilimnion with partial atelomixis marked the autumn season in the Faxinal reservoir, southern Brazil. The highest phytoplankton densities and biomass were found during the second part of the day, a general pattern reported in the literature, and may be explained by zooplankton dynamics. During the 24-h cycle, phytoplankton functional groups lacking a self-regulating capacity and those able to regulate their vertical position were vertically segregated in the lake. The diel behaviour of both groups was driven by the mixing regime (including atelomixis), light and zooplankton grazing pressure.

Reproductive Asynchrony and the Divergence of Hawaiian Crickets

AbstractAsynchrony in reproductive behavior may contribute to reproductive isolation among sympatric species. While the 38 cryptic species of the genus Laupala are primarily distinguished on the basis of variation in pulse rate of male calling songs, additional phenotypes, such as asynchrony in reproductive behavior, may contribute to reproductive isolation in this genus. Here we document similarities and differences in the diel timing of two reproductive behaviors, male singing activity and insemination events. Asynchrony in the diel timing of male singing behavior was observed between two sympatric species, Laupala cerasina and Laupala paranigra, in the field. An interpopulational comparison within L. cerasina did not reveal variation in diel behavior patterns of singing between two locations. Asynchrony in the timing of copulation and sperm transfer between L. cerasina and L. paranigra was documented in the laboratory. The observed pattern of asynchrony in both the field and laboratory could have arisen in a number of ways. One possibility is that species diverged in sympatry because of interspecific interactions, producing a pattern of reproductive character displacement. Alternatively, the observed asynchrony in reproductive behavior may have played a role in the process of community assembly within this recently diverged cricket genus. The presence of interspecific variation and the absence of intraspecific variation revealed by our study do not support a pattern of reproductive character displacement for diel reproductive behavior, suggesting that the differences seen between species were not caused by recent species interactions.

Diel diving behavior of sperm whales off Japan

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 349:277-287 (2007) - DOI: https://doi.org/10.3354/meps07068 Diel diving behavior of sperm whales off Japan Kagari Aoki1,*, Masao Amano2, Motoi Yoshioka3, Kyoichi Mori4, Daisuke Tokuda3, Nobuyuki Miyazaki1 1Ocean Research Institute, The University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164-8639, Japan 2 Department of Animal Sciences, Teikyo University of Science & Technology, 2525 Yatsusawa, Uenohara, Yamanashi 409-0193, Japan 3Graduate School of Bioresources, Mie University, 1577 Kurimamachiya, Tsu, Mie 514-8507, Japan 4Ogasawara Whale Watching Association, Aza Higashi, Chichijima, Ogasawara, Tokyo 100-2101, Japan *Email: kagari-bw@ori.u-tokyo.ac.jp ABSTRACT: Diel changes in the diving behavior of cetaceans have been documented only in some shallow divers, such as smaller toothed whales and some baleen whales. In this study, we attached data loggers to sperm whales Physeter macrocephalus, one of the deepest and longest divers among all marine mammals, to investigate the possible diel patterns in the species. We conducted the experiments in 2 areas off Japan (the Kumano Coast and the Ogasawara Islands). Obvious diel patterns of diving behavior were found off the Ogasawara Islands, where the whales dived deeper and swam faster during the day than at night (day-time depth: mean 853 ± 130 (SD) m, n = 35; night-time depth: mean 469 ± 122 m, n = 31; day-time velocity: mean 2.0 ± 0.3 m s–1, n = 20; night-time velocity: mean 1.5 ± 0.3 m s–1, n = 19). On the other hand, off the Kumano Coast, whales showed no diel rhythm in diving depths or swimming speed (day-time depth: mean 646 ± 212 m, n = 83; night-time depth: mean 561 ± 229 m, n = 44 and day-time velocity: mean 1.9 ± 0.5 m s–1, n = 57; night-time velocity: mean 1.6 ± 0.5 m s–1, n = 33). We suggest that the environmental differences between these 2 areas contributed to the difference in diel behavior of the whales, which was mediated by the diel behavior of their prey. KEY WORDS: Sperm whales · Cetacean · Diving behavior · Diel patterns · Foraging · Thermocline Full text in pdf format PreviousNextCite this article as: Aoki K, Amano M, Yoshioka M, Mori K, Tokuda D, Miyazaki N (2007) Diel diving behavior of sperm whales off Japan. Mar Ecol Prog Ser 349:277-287. https://doi.org/10.3354/meps07068 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 349. Online publication date: November 08, 2007 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2007 Inter-Research.

Diel Behavior of Adult Striped Bass Using Tailwater Habitat as Summer Refuge

AbstractGeneral patterns of summer diel distribution and movement were identified for adult striped bass Morone saxatilis using tailwater habitat influenced by the diel operation cycle of a hydroelectric dam during the summers of 2003 and 2004. Striped bass distribution within the tailwater was similar during each diel‐tracking event and across both summers. The majority of fish remained within the tailwater the entire summer; however, some made periodic excursions to and from the tailwater throughout the summer. Further, most striped bass were located within 0.5 km of Richard B. Russell Dam during all stages of operation on all occasions—probably because of the constant availability of optimal habitat during all three stages of operation on all diel‐tracking events. The diel cycle of dam operation, which included pumped storage during each summer, did not degrade tailwater habitat below optimal conditions, according to summer habitat suitability index values for inland adult striped bass. Movement was significantly higher during hydroelectric generation operations than during no‐generation and pumped storage periods in summer 2003; this difference was not apparent during summer 2004. Mean absolute movement peaked during hydroelectric generation on six of eight diel‐tracking events. During both summers, movement was directed up‐reservoir during no‐generation and generation periods and down‐reservoir during pumped storage. Mean total daily movement rates ranged from 0.59 to 4.04 km/d and were greater than those previously estimated from bimonthly sampling for this population. Total daily movement rate peaked during the first tracking event each summer and then declined as summer progressed. These findings suggest that hydroelectric discharges affect adult striped bass behavior, but the effects are not adverse as long as habitat is not degraded by hydroelectric facility operations.

Ontogenetic changes in vertical migratory rhythms of benthopelagic shrimps Pasiphaea multidentata and P. sivado

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 335:167-174 (2007) - doi:10.3354/meps335167 Ontogenetic changes in vertical migratory rhythms of benthopelagic shrimps Pasiphaea multidentata and P. sivado Jacopo Aguzzi*, Joan Baptista Company, Pere Abelló, José Antonio García Institut de Ciències del Mar (CSIC), Passeig Marítim de la Barceloneta 37-49, 08003 Barcelona, Spain *Email: jaguzzi@cmima.csic.es ABSTRACT: Vertical migratory rhythms of the western Mediterranean congeneric benthopelagic glass shrimps Pasiphaea sivado and P. multidentata were studied at depths of 400 to 430 m in relation to light intensity cycles (i.e. light regimes) in 2 different seasons: close to the autumn equinox (October) and to the summer solstice (June). Catches were considered a proxy of animal density close to the bottom and sampling was performed with consecutive epibenthic trawl hauls. These were continuously repeated over 4 d in order to determine the frequency and timing of species presence on the bottom. The density of animals present on the seabed at the time of the haul was estimated. In both P. sivado and P. multidentata marked diel differences in density were found which were correlated to variability in diel light intensity. While P. sivado was mostly captured at daytime in both seasons, regardless of size, P. multidentata only showed this pattern in the case of juveniles, i.e. individuals with carapace length <30 mm. Adult P. multidentata were mainly present on the bottom during the night, thus showing an apparent phase inversion in their movement patterns. This modification of diel behaviour can be interpreted as the result of differential habitat use with size/age. The comparison of present results with previous field studies illustrates that while P. sivado and juvenile P. multidentata perform actual vertical migrations, adults of the latter species undertake epibenthic movements along the bottom. KEY WORDS: Pasiphaea sivado · Pasiphaea multidentata · Ontogeny · Vertical migratory rhythms · Continental slope · Light intensity · Photoperiod length · Behavioural activity Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 335. Online publication date: April 16, 2007 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2007 Inter-Research.

Importance of diel behaviour when studying habitat selection: examples from female Scandinavian brown bears (Ursus arctos)

The goal of habitat selection studies is to identify important habitats for a particular species. However, most studies using radiotelemetry have focused on habitat-selection patterns using daytime positions only. We used 24 h data from six female brown bears ( Ursus arctos L., 1758) equipped with GPS–GSM collars and activity loggers to analyse variations in habitat selection related to diel variations in activity (foraging and resting). We found that the bears rested mainly during the daylight hours and foraged mainly during the crepuscular and nocturnal hours. The bears selected habitats differently when they were resting than when they were foraging. We found no selection for tall coniferous forest using all data, but this habitat was selected by resting bears and avoided by foraging bears. Thus, for studies of habitat selection, our results show the importance of obtaining data from all 24 h and dividing these data into relevant categories based on the diel activity pattern of the studied species.

Preliminary telemetry data on the movement patterns and habitat use of European catfish (Silurus glanis) in a reservoir of the River Ebro, Spain

Abstract – Knowledge of the movements and diel behaviour of the European catfish (Silurus glanis), the largest European freshwater fish, is limited to anecdotal information. In a preliminary telemetry study of European catfish, the spring diel movement patterns of five adult catfish were examined. After intraperitoneal insertion of the acoustic tags, the positions of the fish were recorded automatically in the Flix Reservoir (River Ebro, NE Spain). A marked nocturnal mobility pattern was observed throughout the study. During daytime, the catfish were consistently located in the littoral zone and spent extended periods of the day hidden in concealed habitats. Catfish movements were in a radial pattern, with upstream and downstream excursions followed by returns to a previously occupied location. Significant individual variations in movement pattern were observed among the tagged fish and within the 24 h cycle for each fish. Mean instantaneous swimming speed was 0.17 body lengths per second (BL·s−1) at night but 0.09 BL·s−1 during the daytime.

Seasonal dynamics and ecosystem impact of mesozooplankton at station ALOHA based on optical plankton counter measurements

Abundances of mesozooplankton‐sized particles were measured at 45‐m depth at station ALOHA (22.75°N, 158°W) during 18 cruises from February 1995 through December 1996 with an optical plankton counter (OPC). Mesozooplankton were also sampled with oblique net tows to 155 m depth. Vertical OPC profiles showed uniform total abundance in the upper mixed layer, usually &gt;45 m. OPC and net data agreed with respect to total abundance, size composition, abundance of individual size classes, and seasonal cycle of abundance. We found no evidence for significant contributions to OPC particle counts by diatom aggregates, Trichodesmium spp., or detritus. Variations in OPC estimates of abundance are well explained by diel behavior and seasonal cycles of species that dominate mesozooplankton abundance, of which 80% are copepods. The summer maximum in mesozooplankton abundance is due primarily to the increase of the six smallest OPC size classes (&lt;1.15 mm equivalent spherical diameter), dominated by 14 nonmigrating copepod species that account for more than 95% of average copepod abundance. Seasonal cycles of zooplankton egestion estimated from OPC measurements were highly correlated, and comparable in magnitude, with observed sinking flux measurements of both C and N. Sinking flux at the base of the euphotic zone was 0.67 and 0.77 mol C m−2 yr−1 and 81 and 87 mmol N m−2 yr−1, for 1995 and 1995, respectively. The potential contribution of mesozooplankton egestion in the mixed layer, based on OPC measurements, accounts for 95% and 90% of C and 86% and 81% of N, respectively.

The influence of hydrous Mn–Zn oxides on diel cycling of Zn in an alkaline stream draining abandoned mine lands

Many mining-impacted streams in western Montana with pH near or above neutrality display large (up to 500%) diel cycles in dissolved Zn concentrations. The streams in question typically contain boulders coated with a thin biofilm, as well as black mineral crusts composed of hydrous Mn–Zn oxides. Laboratory mesocosm experiments simulating diel behavior in High Ore Creek (one of the Montana streams with particularly high Zn concentrations) show that the Zn cycles are not caused by 24-h changes in streamflow or hyporheic exchange, but rather to reversible in-stream processes that are driven by the solar cycle and its attendant influence on pH and water temperature ( T). Laboratory experiments using natural Mn–Zn precipitates from the creek show that the mobilities of Zn and Mn increase nearly an order of magnitude for each unit decrease in pH, and decrease 2.4-fold for an increase in T from 5 to 20 °C. The response of dissolved metal concentration to small changes in either pH or T was rapid and reversible, and dissolved Zn concentrations were roughly an order of magnitude higher than Mn. These observations are best explained by sorption of Zn 2+ and Mn 2+ onto the secondary Mn–Zn oxide surfaces. From the T-dependence of residual metal concentrations in solution, approximate adsorption enthalpies of +50 kJ/mol (Zn) and +46 kJ/mol (Mn) were obtained, which are within the range of enthalpy values reported in the literature for sorption of divalent metal cations onto hydrous metal oxides. Using the derived pH- and T-dependencies from the experiments, good agreement is shown between predicted and observed diel Zn cycles for several historical data sets collected from High Ore Creek.

Seasonal and Diel Activity Patterns of Italian Wall Lizards, Podarcis sicula campestris, in New York

Abstract Lizard behavior can be influenced by ultimate forces such as adaptation and phylogeny, and proximate forces such as temperature and rainfall. Italian wall lizards (Podarcis sicula campestris) were successfully introduced into two locations in the USA, both at latitudes similar to their probable sources in Italy. Behavioral differences between native and introduced populations are likely due to proximate forces. From 1999–2000, we documented the seasonal and diel behavior of wall lizards in New York. We observed a bimodal activity pattern during the summer and a unimodal activity pattern in spring and fall, which has been reported for native populations in Italy. Unlike Italian populations, New York lizards were completely inactive during winter months, which is probably due to the much lower minimum winter temperatures in New York.

Diel behavior of rare earth elements in a mountain stream with acidic to neutral pH

Diel (24-h) changes in concentrations of rare earth elements (REE) were investigated in Fisher Creek, a mountain stream in Montana that receives acid mine drainage in its headwaters. Three simultaneous 24-h samplings were conducted at an upstream station (pH = 3.3), an intermediate station (pH = 5.5), and a downstream station (pH = 6.8). The REE were found to behave conservatively at the two upstream stations. At the downstream station, REE partitioned into suspended particles to a degree that varied with the time of day, and concentrations of dissolved REE were 2.9- to 9.4-fold (190% to 830%) higher in the early morning vs. the late afternoon. The decrease in dissolved REE concentrations during the day coincided with a corresponding increase in the concentration of REE in suspended particles, such that diel changes in the total REE concentrations were relatively minor (27% to 55% increase at night). Across the lanthanide series, the heavy REE partitioned into the suspended solid phase to a greater extent than the light REE. Filtered samples from the downstream station showed a decrease in shale-normalized REE concentration across the lanthanide series, with positive anomalies at La and Gd, and a negative Eu anomaly. As the temperature of the creek increased in the afternoon, the slope of the REE profile steepened and the magnitude of the anomalies increased. The above observations are explained by cyclic adsorption of REE onto suspended particles of hydrous ferric and aluminum oxides (HFO, HAO). Conditional partition coefficients for each REE between the suspended solids and the aqueous phase reached a maximum at 1700 hours and a minimum at 0700 hours. This pattern is attributed to diel variations in stream temperature, possibly reinforced by kinetic factors (i.e., slower rates of reaction at night than during the day). Estimates of the enthalpy of adsorption of each REE onto suspended particles based on the field results averaged +82 kJ/mol and are similar in magnitude to estimates in the literature for adsorption of divalent metal cations onto clays and hydrous metal oxides. The results of this study have important implications to the use of REE as hydrogeochemical tracers in streams.

Diel pattern with abrupt crepuscular changes of zooplankton over a coral reef

We studied zooplankton abundance and emergence patterns over coral reefs in the Gulf of Aqaba using highresolution acoustics (multibeam sonar, FTV, 1.6 MHz, and WH600 ADCP, 614 KHz), emergence traps, and an underwater pump, supplemented with field observations on feeding behavior of zooplanktivorous fish. The zooplanktonic community over coral reefs is a complex and highly dynamic mixture of pelagic plankton advected into the reef, larvae and eggs released by benthic animals, and demersal plankton. At sunset (0 ±4 min, mean 695% confidence interval) acoustic back‐scattering intensity and zooplankton biomass started to increase rapidly. In contrast with this ascent, the predawn decline began much earlier during the dark (82 ±5 min before sunrise) and terminated before sunrise. This light‐dependent diel behavior was highly consistent throughout the year, regardless of seasonal and environmental changes. Smaller zooplankters (500–700 µm) ascended first and demersal zooplankton accounted for most of the increase in zooplankton soon after sunset. Surprisingly, the emergence of zooplankton occurred while large schools of diurnal zooplanktivorous fish were still foraging in the water column. However, at that time, prey‐capture efficiency by the fish is greatly reduced and corals had not yet expanded their tentacles, producing an optimal window of opportunity of low risk for emergence.

Diel behavior of iron and other heavy metals in a mountain stream with acidic to neutral pH: Fisher Creek, Montana, USA

Three simultaneous 24-h samplings at three sites over a downstream pH gradient were conducted to examine diel fluctuations in heavy metal concentrations in Fisher Creek, a small mountain stream draining abandoned mine lands in Montana. Average pH values at the upstream (F1), middle (F2), and downstream (F3) monitoring stations were 3.31, 5.46, and 6.80, respectively. The downstream increase in pH resulted in precipitation of hydrous ferric oxide (HFO) and hydrous aluminum oxide (HAO) on the streambed. At F1 and F2, Fe showed strong diel cycles in dissolved concentration and Fe(II)/Fe(III) ratio; these cycles were attributed to daytime photoreduction of Fe(III) to Fe(II), reoxidation of Fe(II) to Fe(III), and temperature-dependent hydrolysis and precipitation of HFO. At the near-neutral downstream station, no evidence of Fe(III) photoreduction was observed, and suspended particles of HFO dominated the total Fe load. HFO precipitation rates between F2 and F3 were highest in the afternoon, due in part to reoxidation of a midday pulse of Fe 2+ formed by photoreduction in the upper, acidic portions of the stream. Dissolved concentrations of Fe(II) and Cu decreased tenfold and 2.4-fold, respectively, during the day at F3. These changes were attributed to sorption onto fresh HFO surfaces. Results of surface complexation modeling showed good agreement between observed and predicted Cu concentrations at F3, but only when adsorption enthalpies were added to the thermodynamic database to take into account diel temperature variations. The field and modeling results illustrate that the degree to which trace metals adsorb onto actively forming HFO is strongly temperature dependent. This study is an example of how diel Fe cycles caused by redox and hydrolysis reactions can induce a diel cycle in a trace metal of toxicological importance in downstream waters.

TESTS OF THEORY FOR DIEL VARIATION IN SALMONID FEEDING ACTIVITY AND HABITAT USE

For many animals, selecting whether to forage during day or night is a critical fitness problem: at night, predation risks are lower but feeding is less efficient. Habitat selection is a closely related problem: the best location for nocturnal foraging could be too risky during daytime, and habitat that is safe and profitable in daytime may be unprofitable at night. We pose a theory that assumes animals select the combination of daytime and night activity (feeding vs. hiding), and habitat, that maximizes expected future fitness. Expected fitness is approximated as the predicted probability of surviving starvation and predation over a future time horizon, multiplied by a function representing the fitness benefits of growth. The theory's usefulness and generality were tested using pattern-oriented analysis of an individual-based model (IBM) of stream salmonids and the extensive literature on observed diel behavior patterns of these animals. Simulation experiments showed that the IBM reproduces eight diverse patterns observed in real populations. (1) Diel activity (whether foraging occurs during day and/or night) varies among a population's individuals, and from day to day for each individual. (2) Salmonids feed in shallower and slower water at night. (3) Individuals pack more tightly into the best habitat when feeding at night. (4) Salmonids feed relatively more at night if temperatures (and, therefore, metabolic demands) are low. (5) Daytime feeding is more common for life stages in which potential fitness increases more rapidly with growth. (6) Competition for feeding or hiding sites can shift foraging between day and night. (7) Daytime feeding is more common when food avail- ability is low. (8) Diel activity patterns are affected by the availability of good habitat for feeding or hiding. We can explain many patterns of variation in diel foraging behavior without assuming that populations or individuals vary in how inherently nocturnal or diurnal they are. Instead, these patterns can emerge from the search by individuals for good trade- offs between growth and survival under different habitat and competitive conditions.

Consistency of diel behaviour and interactions of stream fishes and invertebrates during summer

We examined the consistency of diel patterns observed in fish and aquatic invertebrates over three 24-h cycles within a 10-day period in summer (28–29 July, 4–5 and 7–8 August 1997) in the River Lee, Hertfordshire, England. We tested for temporal variations in epibenthos density and population size structure, age-specific fish density, microhabitat use and contagion as well as for correlations between fish length and prey number (in the gut) per millimetre fish standard length, a surrogate of foraging success, and fish density in point samples (an estimator of shoal size). Consistent patterns were observed in invertebrates and fish, though variability was greater in the former. Epibenthic invertebrates (except Chironomidae) demonstrated few or no diel differences in density, but some differences in size (based on head widths) were found in larvae of Chironomidae and Ephemeroptera (mainly Baetis rhodani (Pictet) and Ephemerella ignita Poda) between times of day. Fish density varied consistently and diel variations were observed in fish shoaling behaviour, habitat use, prey size and prey composition. Prey numbers in the gut generally decreased with increasing shoal size, but this was significant in Gobio gobio L. only. In some fish species/size classes, prey numbers correlated either positively or negatively with habitat variables, in particular with the distance fish were captured from the bank. Despite relatively low epibenthic densities, Chironomidae larvae were a prominent and preferred food item in the diet of all fish species, followed by Ephemeroptera in some (0 + Barbus barbus (L.), 0 + and 1 + Barbatula barbatula (L.), 1 + and ≥ 2 + Cottus gobio (L.)) and Simuliidae in others (0 + and ≥ 1 +G. gobio, 0 + Leuciscus cephalus (L.), 0 + and 1 + Phoxinus phoxinus (L.)), with Trichoptera larvae, Asellus sp., Ostracoda and Annelida as tertiary prey taxa. The relatively consistent diel patterns observed suggest that data from single 24-h cycles in late summer can be representative of daily patterns. Day-time only surveys of European riverine fishes may give unreliable estimates of mean fish population size and abundance. Copyright © 2005 John Wiley & Sons, Ltd.