Diel Feeding Cycle Research Articles

Effects of multigenerational rearing, ontogeny and predation threat on copepod feeding rhythms

Marine planktonic copepods frequently exhibit diel feeding cycles coupled with vertical migrations. However, copepod feeding rhythms can be influenced by factors others than different food availability between layers. In this study, we determined the changes in the diel feeding behaviour of two marine copepod species (Centropages typicus and Paracartia grani) through multigenerational laboratory rearing, ontogeny, and upon the exposure to predator chemical cues. The wild females of both C. typicus and P. grani showed marked feeding rhythms with higher ingestion rates at night. The diel rhythms of C. typicus were maintained in the first laboratory-reared generation (F1), suggesting maternal effects, but disappeared in the following generations. The P. grani females of a long-term culture (> 10 years) also showed no differences in their day–night feeding activity. Ontogenetic variations were detected in the F1 generation of C. typicus: feeding rhythms were absent in naupliar stages, but adults fed more intensely at night. In the case of the cultured P. grani, in general none of the stages showed feeding rhythms. Laboratory-reared C. typicus (8–11 generations) did not recover back the natural cyclic feeding when exposed to jellyfish and fish exudates, indicating that either predation risk does not significantly affect their diel feeding activity or predator-induced responses in marine copepods might not involve chemical signalling. Our study confirms that feeding-related functional traits of marine copepods can experience quick multigenerational changes in the laboratory; consequently, cultured copepods might not be good models for studies involving their diel feeding behaviour.

Diel vertical feeding behaviour of Atlantic mackerel (Scomber scombrus) in the Irminger current

The main prey of the Atlantic mackerel undertake diel vertical migrations, however diel and vertical patterns in the feeding by mackerel remains poorly known thus limiting our understanding and quantification of ecosystem trophodynamics and potentially influencing stock assessment and ecosystem based management. By intensive monitoring of adult Atlantic mackerel and its prey during summer in the Irminger Sea, it was found that the diel feeding cycle is of high quantitative importance. Fresh stomach content analysis indicated a marked diel pattern with copepods as the main prey between 16 and 24 (solar time), and euphausiids/myctophids between 22 and 08. Compared with the zooplankton distribution and composition, there were no indications of feeding below the mixed layer. Mackerel selected for the largest and most energy rich prey, namely copepods with prosome lengths >2 mm, and then switched from copepods to euphausiids/myctophids around midnight, despite that the copepods were also aggregated during night. Stomach fullness increased significantly with length and condition, suggesting that individuals that were larger were more effective feeders. The stomach content by weight primarily consisted of copepods (65%, exclusively Calanus finmarchus), euphausiids (18%), and to a lesser extent teleosts (5%, mainly myctophids), hyperiids (4%) and cephalopods (2%). The results are likely widely applicable in the main feeding season and areas (the oceanic parts of the Nordic Seas during summer), because of similarities in the diet of the adult mackerel and in the prey’s diel vertical migration. In conclusion, it is imperative to account for the diel dynamics when quantifying the mackerel diet and its impact on prey populations.

Improved survival, prey selectivity and diel feeding cycle of silver therapon Leiopotherapon plumbeus (Perciformes: Terapontidae) larvae reared in tanks with substrate

Physical substrates in the rearing environment can influence the early survival and feeding patterns of captive-reared fish. In this study, we determined whether substrates affect the survival and growth of hatchery-reared silver therapon Leiopotherapon plumbeus larvae as well as examined their prey selectivity and diel feeding cycle. Newly hatched larvae [1.92 mm total length (TL)] were reared for 40 days in triplicate 4 m3 tanks with or without tropical almond Terminalia catappa leaves as substrate. Prey selectivity of larvae reared in tanks with substrate for 35 days from the yolk-sac stage was measured by the Chesson’s selectivity index (αi). Diel feeding cycle of 3-4 days post-hatch (dph) silver therapon larvae reared in tanks exposed to natural light cycle and fed wild zooplankton was also studied. Larvae reared in tanks with substrate had significantly higher survivorship (48.44 ± 7.85%) than those reared in tanks without substrate (26.73 ± 1.60%). However, total length, specific growth rate and body weight of silver therapon larvae from tanks with or without substrate were not significantly different. Silver therapon larvae are generalist predator, demonstrating a degree of prey selectivity in some prey items during early ontogeny. Prey selectivity of silver therapon larvae varied during larval ontogeny, with higher Chesson’s selectivity index for copepod nauplii among the smaller fish larvae (2–5 dph; 2.94–5.17 mm TL), cladocerans (Moina micrura and Bosmina coregoni) among intermediate (6–11 dph; 5.72–9.60 mm TL), and ostracod, cladoceran and insect larvae among larger fish (12–35 dph; 10.28–20.96 mm TL). Larvae showed a diel feeding cycle where they actively fed during daylight hours, with a peak in the late afternoon, and reached a minimum at dark. Together, these findings advance our understanding of the feeding predatory behavior and efficiency of silver therapon larvae and preference for tanks with substrate that improve their survival.

Diel vertical interactions between Atlantic cod Gadus morhua and sprat Sprattus sprattus in a stratified water column

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 583:195-209 (2017) - DOI: https://doi.org/10.3354/meps12319 Diel vertical interactions between Atlantic cod Gadus morhua and sprat Sprattus sprattus in a stratified water column Niels G. Andersen*, Bo Lundgren, Stefan Neuenfeldt, Jan E. Beyer Technical University of Denmark, National Institute of Aquatic Resources (DTU-Aqua), Charlottenlund Castle, 2920 Charlottenlund, Denmark *Corresponding author: nga@aqua.dtu.dk ABSTRACT: Information about species interactions at a spatial scale comparable to the perceptive abilities of the involved species is crucial for establishment of predictive food consumption models at the population level. Nevertheless, such information is sparse due to methodological constraints. We studied the diel vertical dynamics of species interactions between Atlantic cod Gadus morhua and its major clupeid prey, sprat Sprattus sprattus, at a location in the Bornholm Basin of the central Baltic Sea during late winter. This was accomplished by combining acoustic information on diel vertical fish distribution, time of ingestion of individual sprat estimated from cod stomach content data and observed vertical profiles of salinity, temperature and oxygen content. Predation by cod took place primarily at dusk and dawn during ascent and descent of sprat associated with school dissolution and formation, respectively. Cod resided close to the bottom outside these temporal predation windows. Sprat schools were located at the same depth as cod in the daylight hours, whereas at night dispersed sprat were situated higher in the water column. These vertical dynamics could be explained by fitness optimization using bioenergetics and trade-offs between temperature, oxygen saturation of the water and predation risk. This study forms a first step towards providing a mechanistic background for the predatory impact of cod at the basin scale and beyond. KEY WORDS: Baltic Sea · Stomach content dynamics · Diel feeding cycle · Bioacoustics · Hydrography · Winter situation · Bioenergetics · Fitness optimization Full text in pdf format Supplementary material PreviousNextCite this article as: Andersen NG, Lundgren B, Neuenfeldt S, Beyer JE (2017) Diel vertical interactions between Atlantic cod Gadus morhua and sprat Sprattus sprattus in a stratified water column. Mar Ecol Prog Ser 583:195-209. https://doi.org/10.3354/meps12319 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 583. Online publication date: November 16, 2017 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2017 Inter-Research.

Chronologically sampled flight feathers permits recognition of individual molt-migrants due to varying protein sources.

This is a proof of concept paper based on chronological samples of growing feathers from geese thought to be molt-migrants. When molt-migrant birds initiate molt shortly after migrating to a new isoscape, isotope values measured along the length of their feathers should change continuously. To assess long-term changes and daily cycling in δ15N and δ13C values, we serially sampled a growing primary from three presumed molt-migrant geese. Two showed changing δ15N signatures along the length of their growing primary, indicating they were molt-migrants, while the third, presumably a resident, showed no change. We then resampled these feathers at closer intervals for evidence of the predicted diel cycle in the use of exogenous and endogenous protein for feather growth, generated by the diel feeding cycle of these geese. As predicted, a periodicity of ca. 24 h in δ15N values was found along the primary of the two equilibrating geese, but not in the other goose that was probably a resident. Our results demonstrate that chronological sampling along the length of individual primaries holds great potential for identifying individuals that are molt-migrants.

Aspects of the trophic ecology of the mesopelagic fish Lampanyctus alatus (family Myctophidae) in the eastern Gulf of Mexico

AbstractLampanyctus alatus is an abundant myctophid in subtropical waters of the eastern Gulf of Mexico. The population has been estimated at 85–140 × 103 fish per km2 of sea surface, with most of the population biomass (>60%) in individuals larger than 35 mm. This species concentrates in the 80175 m zone at night and at 550–750 m during the day. The diet is essentially crustacean (>98%), with over 60 prey species having been identified. Seventy-three percent of the number and 44% of the biomass of the diet are copepods. Euphausiids rank first in biomass and are particularly important in the diet of larger fishes. Lampanyctus alatus is selective for prey >1 mm and for the copepod genus Pleuromamma.A diel feeding cycle is apparent, stomachs being fullest in the period 0100–0500 h. Daily ration is estimated at 2–4% of body weight and decreases proportionately with increasing fish size. It is estimated that one-fifth of respiration and less than one-tenth of the daily ration is utilized in diel vertical migr...

Temporal changes in feeding habits and daily rations of Hoplostethus mediterraneus in the bathyal Ionian Sea (eastern Mediterranean)

Data on the diet, feeding habits and daily rations of Hoplostethus mediterraneus Cuvier, 1829 in the bathyal eastern Ionian Sea (Mediterranean Sea) are presented. A total of 430 specimens collected by bottom trawls at depths ranging from 473 to 603 m during four 24-h day–night sampling cycles covering the four annual seasons was examined. H. mediterraneus diet consisted of pelagic and vagile epibenthic prey, mainly crustaceans, and was dominated by benthopelagic natantian decapods (83.35% IRI, index of relative importance). Seasonal changes in diet were apparent and related to seasonal fluctuations in suprabenthic and zooplanktonic prey in the environment. Diel patterns in stomach fullness and trends in diel feeding cycles are discussed in relation to the vertical migratory movements of available prey (i.e. suprabenthos and zooplankton). Daily-ration estimates were determined by evacuation-rate models and ranged from 0.143% to 0.397% WW/WW. Overall, daily-ration estimates were within the range of the daily consumption of other deep-sea fish. Deduced from diet contents, we found a constant gross energy intake (305–316 kcal g–1) during all seasons. As a possible response to the reproductive peak of mature females observed in summer, H. mediterraneus increases its food consumption, which, in turn, is coupled with an increase in food availability.

Diel spatial distribution and feeding activity of herring ( Clupea harengus) and sprat ( Sprattus sprattus) in the Baltic Sea

We analysed Baltic Sea pelagic fish (herring and sprat) spatial and temporal distribution, size distribution at different depths and time of the day and diel feeding pattern. In 1995 the study area was investigated by acoustic survey for 3 d, 3, 4 and 11 October, to investigate spatial and temporal distribution of pelagic fish. The area was divided in four different transects forming a survey quadrate of 15 nautical miles of side. The survey quadrate was ensonified each day four times in the 24 h. In 1997 the acoustic survey was conducted in the same area and in the same week of the year to analyse the diel feeding cycle of herring and sprat and their size distribution by depth and time of the day using pelagic trawls. Fish abundance, from 1995 survey, was statistically different among days and survey quadrates. However, from our data it is not clear whether the variation stems from random dispersion or directed movements occurring at the temporal small-scale. Pelagic fish were dispersed during the night at the surface and aggregated during the day at the bottom. They aggregated at dawn and dispersed at dusk at the surface. For herring this distribution pattern coincided with peaks of stomach fullness analysed in the 1997 survey, while sprat seemed to continue feeding during the whole day time. Larger herring were deeper in the water column than smaller individuals. Diel vertical migrations (DVM) of pelagic fish likely mirrored zooplankton diel vertical movements and it was reasonably in response to optimal predation conditions in the sea and possibly intertwined with predation avoidance and bioenergetic optimisation.

Feeding preferences, seasonal gut repletion indices, and diel feeding patterns of the sea urchin Tripneustes gratilla (Echinodermata: Echinoidea) on a coastal habitat off Toliara (Madagascar)

The feeding biology of the echinoid Tripneustes gratilla from a seagrass bed off Toliara, on the south-west coast of Madagascar, was investigated during the summer and winter seasons. Repletion indices of the gut of T. gratilla varied seasonally, in such a way that higher amounts of food occurred in the gut in winter than in other seasons. Conversely, a more detailed analysis of one summer and one winter sample of the gut tissue dry weights revealed higher weights in summer (February) than in winter (August). This was interpreted as a varying capability of nutrient storage and closely related to the feeding activity of the echinoid. This inverse relationship between feeding activity and nutrient storage was suggested to be dependent on gonadal growth. Indeed, higher food consumption was observed during the post-spawning period (August), that is, when most of the energy accumulated was used up for gonadal growth. Low feeding activity occurred when a sufficient amount of energy was accumulated in gut tissues (February), the time corresponding to the initiation of gonadal growth. Investigations on the feeding habits, using Ivlev's electivity index, revealed preferential feeding of T. gratilla on Syringodium isoetifolium, the dominant seagrass species found in the gut. This was partly related to the higher availability of that particular seagrass species in the field. However, its soft and terete leaf blade's morphology also makes it easier to manipulate and ingest. Studies on the feeding behaviour indicated the presence of a diel feeding cycle in T. gratilla individuals in both summer and winter seasons. Yet, its pattern varied between the two studied months (one summer and one winter) as the cycle was less contrasted during period of active feeding, i.e. in winter compared to summer. This study provides novel information on how intrinsic factors, such as physiological state, can govern the feeding activity of T. gratilla living in a seagrass habitat.

Vertical distribution and feeding of larval blue whiting in turbulent waters above Porcupine Bank

In April 1995 a patch of blue whiting larvae Micromesistius poutassou was found at low illumination levels below 20 m depth near Porcupine Bank, west of Ireland, together with high densities of copepod nauplii and reduced turbulence rates, suggesting that larval blue whiting vertical distribution was determined by prey concentration, illumination and turbulence. Most (83·8%) larvae (2·0–7·5 mm Ls) had food in their guts. Feeding incidence and feeding intensities increased with increasing larval length. Only larvae >5·5 mm reduced numerical in favour of weight‐based feeding intensity, indicating a shift in dietary composition. Maxima of the diel rhythms of feeding incidence and intensities occurred at 1800 and 2100 hours and minima at dawn (0600 hours). Proportionately, more nauplii were eaten by day but more copepod eggs and tintinnids at night. The distinct diel pattern in larval blue whiting feeding suggests that any analysis of factors mediating feeding must take into account diel feeding cycles. Larval feeding was significantly affected by wind speed. The larvae ate more and larger items at low than at higher turbulence levels. The data suggest that the maximum level of turbulence was not beneficial for larval blue whiting, but more moderate wind speeds could have had an enhancing effect on larval feeding success.

Diel feeding of the chaetognath Sagitta enflata in the Zanzibar Channel, western Indian Ocean

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 193:117-123 (2000) - doi:10.3354/meps193117 Diel feeding of the chaetognath Sagitta enflata in the Zanzibar Channel, western Indian Ocean Vidar Øresland* Institute of Marine Research, National Board of Fisheries, PO Box 4, 453 21 Lysekil, Sweden *E-mail: v.oresland@imr.se ABSTRACT: The diel feeding cycle of Sagitta enflata, the dominant chaetognath in the Indian Ocean, was investigated, using gut content analyses, between the monsoon periods on 2 occasions, October 1995 and May 1996. Zooplankton samples were collected every second hour during a 24 h period close to a drifting buoy in the middle of the Zanzibar Channel. S. enflata made up 64% (October) and 77% (May) of all chaetognaths in this study. S. enflata and all chaetognaths contributed as much as 30 and 33%, respectively, of the total 90 µm net zooplankton wet weight in October (May samples could not be weighed due to occurrence of phytoplankton). S. enflata fed most actively at night. Small copepods with a mandible width <0.07 mm (prosome length <1 mm) were its predominant prey. Feeding rate and mean number of small copepods found in S. enflata were 3 times higher in October than in May. In contrast, the estimated daily predation impact on the standing stock of small copepods (by number) was around 1% for both periods. This may be a negligible predation effect considering the short life-span of tropical copepods. It is suggested that predation effects are related to temperature, since a similar predation impact may be important in cold polar waters where copepod life-spans are much longer. Daily cannibalistic predation impact (by number) was estimated to be between 1 and 4% (October) and between 2 and 9% (May) using assumed maximum and minimum digestion times of 8 and 2 h. Cannibalism, therefore, may be an important source of mortality in S. enflata populations. KEY WORDS: Zooplankton · Chaetognatha · Sagitta enflata · Flacisagitta enflata · Copepods · Feeding · Predation · Cannibalism · Indian Ocean Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 193. Publication date: February 28, 2000 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 2000 Inter-Research.

Diel Feeding, Daily Food Intake, and Daphnia Consumption by Age-0 Gizzard Shad in Oneida Lake, New York

Abstract Young gizzard shad Dorosoma cepedianum can have dramatic effects on ecosystem structure because their high consumptive demand can eliminate crustacean zooplankton populations. The objective of this study was to examine diel feeding chronology of age-0 gizzard shad (25–89 mm total length), as well as their daily ration and consumption of Daphnia spp., in Oneida Lake during 1991 and 1992. Gastric evacuation titres, treasured experimentally, were faster for fish fed continuously (0.8–1.8 h) than for fish fed single meals of zooplankton (4.0–7.2 h) and were influenced by fish size. In Oneida Lake, young gizzard shad exhibited a diel feeding cycle; gut fullness gradually increased to a maximum during 1100–1900 hours and declined during the night. Juvenile gizzard shad shifted from a diet of exclusively zooplankton in July to a combination of zooplankton, algae, and detritus in August. Estimates of total daily ration ranged from 1.2 to 21.3% of wet body weight, and total daily ration of young gizzard s...

Day-night feeding by decapod crustaceans in a deep-water bottom community in the western Mediterranean

Changes in the composition of the diet, foregut fullness, and the degree of digestion of ingested food were taken into account in determining diel feeding activity of the numerically dominant species of decapod crustaceans dwelling on the Catalan Sea Slope (western Mediterranean). Two 24-h periods were sampled at two different stations on the upper middle slope (between 400 and 710 m) using bottom trawls. Additional foregut fullness data forAristeus antennatusandAcanthephyra eximiawere recorded below 1000 m. The influence of the relatively shallow-living mesopelagic fauna (Pasiphaeidae,Sergestes arcticus, euphausiids, and fishes) over the 24-h cycle apparently had a large effect on the feeding activity rhythms in the deep-sea decapods studied. Species can be classified into two different groups according to their feeding patterns. Thus, species whose diet was based on pelagic prey (Plesionika edwardsi, Plesionika martia, andA. eximia) exhibited a feeding pattern conditioned to the availability of this type of prey. In contrast, in those species in which pelagic prey contributed only a small portion of the diet (A. antennatus, Plesionika acanthonotus, Polycheles typhlops, andGeryon longipes) no variations in the foregut fullness or in the percentage of undigested prey in foreguts during the diel feeding cycles were recorded. The decrease in the influence of the abundance of mesopelagic fauna with depth, with a commonly accepted boundary at around 1000 m would be responsible for the progressive flattening out of activity rhythms among the species dwelling on the lower slope. Foregut fullness values for certain species with broad depth distributions in the Catalan Sea (A. antennatus, A. eximia), would support this hypothesis.

Predator-induced changes in the diel feeding cycle of a planktonic copepod

An in situ manipulation experiment examined the effect of a diurnally foraging, zooplanktivorous fish, the three-spine stickleback Gasterosteus aculeatus Linnaeus, on the diel feeding cycle of the planktonic marine copepod Acartia hudsonica Pinhey. Copepods in the presence of fish had a significantly lower level of daytime gut fullness, and consequently a sharpened diel cycle of grazing, than did copepods in the absence of fish. The reduced level of copepod feeding could not be attributed to reduced availability of phytoplankton, such as has been previously suggested for vertically migrating animals residing at depth during the day. Rather, these changes in the feeding behavior of the copepods stemmed directly from the presence of predators. Due to reductions in motion and gut pigmentation, reduced daytime feeding may have adaptive value in reducing the copepods' risk of predation by making them less conspicuous to visual predators. Our results, combined with those of earlier studies, indicate that diel cycles of feeding and vertical migration can be separate, independent behaviors, although both likely have the same adaptive significance-predator evasion.

Under-ice feeding and diel migration by the planktonic copepodsCalanus glacialis andPseudocalanus minutus in relation to the ice algal production cycle in southeastern Hudson Bay, Canada

The marine planktonic copepodsCalanus glacialis Jaschnov andPseudocalanus minutus (Kroyer) typically dominate the copepod biomass in spring under the ice in southeastern Hudson Bay, Canada. Females of both species exhibited significant diel feeding cycles, as measured by gut pigment content, throughout a bloom of ice algae at the ice-water interface in 1986. Periods of grazing correlated well with a nighttime vertical migration by females to within 0.2 m of the ice-water interface, suggesting that feeding took place at or just below the thermohaline boundary between seawater and the interfacial layer containing the ice algae. Seasonal melting of the ice bottom in mid-May resulted in freshening of the surface layer and release of the ice algae into the water column. FemaleC. glacialis andP. minutus responded by ceasing migration to the interface. Gut pigment content, and by reasonable assumption, feeding activity in the water column, increased substantially immediately after this event. In mid-May, the water column phytoplankton consisted of flagellates, sedimenting ice algal cells, and diatoms (Navicula pelagica andChaetoceros sp.) previously found at the interface and then growing in the water column. We conclude that algae growing at the ice-water interface, and sedimenting or actively growing algae derived from this interfacial layer, are a regular and principal source of nutrition for these pelagic copepods during and immediately after the ice algal bloom.

Food and feeding dynamics of the larval Antarctic fishNototheniops larseni

The feeding dynamics of larvae of the Antarctic fishNototheniops larseni were analyzed from data collected over three years in Bransfield Strait and adjacent waters (Antarctica). Seasonal feeding was examined from 1977/1978 (November–March). The diel feeding cycle was investigated during a 96 h station established in February 1976, while food selection was analyzed using larvae and zooplankton samples collected in February 1982. Hatching occurs in early spring, and larvae fed on eggs of calanoid copepods and on cyclopoid copepods. Copepod eggs were the principal food near the pack ice, and cyclopoids in open waters. Cyclopoids were the staple food in summer. Eggs of the Antarctic krillEuphausia superba were ingested selectively and formed major portions of the larval summer diet in neritic (Joinville Island) and oceanic (Elephant Island) spawning areas ofE. superba. In the fall, copepods predominated in the diets. Most abundant and most frequently ingested prey in summer and fall wereOncaea spp. Feeding commenced at dawn and continued at least until dusk. Krill eggs were taken chiefly during morning hours and egg incidence declined during the day, suggesting that eggs were ingested soon after spawning. Prey size at the onset of feeding was estimated as 0.130 to 0.330 mm. Size-selective feeding was evident in small larvae, while in larger larvae median prey length remained constant. High feeding incidence among yolk-sac larvae in spring, high overall feeding incidence in summer, and size-selective foraging of small larvae suggested favorable feeding conditions in the 1977/1978 season. Yolk-absorption times in Antarctic fish larvae vary on a scale of weeks and may be further retarded due to early feeding. Hence, year-to-year variability of yolk incidence inN. larseni indicated variable biotic environments of early feeding larvae rather than temporal shifts of hatching periods. As hatching periods are constant between years in contrast to the variable retreat of the pack ice and subsequent onset of the production cycle in space and time, maternal yolk reserves are probably utilized to compensate for such variations.

Diel Feeding Cycle and Some Effects of Light on Feeding Intensity of the Mississippi Silverside, Menidia audens, in Clear Lake, California

A study of the diel feeding cycle of 940 Mississippi silversides, Menidia audens Hay, in two 24-hour sampling periods in Clear Lake, California, showed a predominance of crustacean zooplankton in the diet. Ceriodaphnia sp., Daphnia pulex, and emerging dipterans were the most important food items. A pronounced morning feeding peak indicated visual feeding. Evidence of visual feeding is further given by statistically significant ingestion of larger food items at night and decreased feeding during the one moonless night. Occurrence of emerging midges in the diet during the second period indicates possible selective predation.