Laboratory video recordings and underwater visual observations combined to reveal activity rhythm of red-spotted grouper and banded wrasse, and their natural assemblages

The diel activity rhythm of red-spotted grouper Epinephelus akaara was studied both in captivity and in the wild. Behavior of solitary grouper (58 to 397?mm in total length) in a tank was video recorded using infrared illuminators under 11L/10D and two 1.5-h twilight transition periods, and was compared to that of banded wrasse Halichoeres poecilopterus, a typical diurnal fish. Underwater observations using SCUBA were also conducted in their natural habitat to reveal the behavioral activity together with a visual census of adjacent fish and crustacean assemblages. Red-spotted grouper showed a strong nocturnal activity in a tank regardless of body size as opposed to the strongly diurnal banded wrasse. Activity of groupers in natural waters was high at dawn and dusk, low at noon, and only a few individuals were observed at night. Visual census in the habitat revealed that fish abundance and species richness was highest at noon, lowest at night, and intermediate at dawn and dusk. The opposite trend was found in crustacean assemblages. Absence of groupers at night may reflect their nocturnal feeding migration away from the study area. Alternatively, the crepuscular activity of groupers in the wild is suggested to be an adaptation to feed on small fishes that shift between daytime activity and nighttime rest and/or on nocturnal crustaceans that show the opposite activity pattern.     

Foraging rhythm of great crested grebes Podiceps cristatus adjusted to diel variations in the vertical distribution of their prey Osmerus eperlanus in a shallow eutrophic lake in The Netherlands

Summary Great crested grebes Podiceps cristatus only foraged for an hour or two during dawn and again during dusk on Lake IJsselmeer in August-September. During this time of the year the adult birds are in wing moult and temporarily unable to fly. The food of grebes consisted almost exclusively of smelt Osmerus eperlanus, the most numberous pelagic fish. Simultaneous sonar registrations and trawl net fishing showed that smelt moved to the water surface during the twilight periods. During day and night they were concentrated near the bottom. We argue that grebes have the best foraging opportunities during twilight when much of their prey is near the surface, where light intensities allow the fish to be detected and captured. When the smelt are in the upper water layers the distance to the covered to get the prey (i.e. diving time and cost) is also least.     

The fourbeard rockling, Enchelyopus cimbrius (L.), in eastern Newfoundland

A population of fourbeard rocklings, Enchelyopus cimbrius, was studied in shallow water (<10 m) in eastern Newfoundland. They are nocturnal, occupying burrows in muddy substrate during the day. and often retreating into these burrows at night when disturbed. The population consisted mostly of 80–150 mm individuals, aged 2-3 years. Feeding was observed only at night, and food consisted mostly of polychaete worms, especially the scale worm Harmorhoe imbricara . The contribution of polychaetes increased with increasing size of fish.     

Diurnal variation in the abundance of juvenile parasitic gnathiid isopods on coral reef fish: implications for parasite-cleaner fish interactions

 The dynamics of parasitic gnathiid isopod infestation on the fish Hemigymnus melapterus were examined at Heron Island, Great Barrier Reef, by measuring the abundance and feeding state of gnathiids on fish collected between dawn and sunset and by estimating the time required for gnathiids to become engorged on host fluids. A model was developed to estimate gnathiid abundance on fish for any given time of day and host size. Fish at dawn had 2.4 times as many gnathiids compared with fish at sunset, indicating that some gnathiids infest fish overnight. Most gnathiids had engorged guts (72–86%); the proportion of empty guts and engorged guts did not differ in three time periods of collection (<0800 h, 0800 to 1100 h, and >1100 h). In the laboratory, gnathiids fed quickly with 75% of gnathiids exposed to fish for 4 h having engorged guts. The short time required for gnathiids to become engorged and the presence of gnathiids with empty guts throughout the day suggests that gnathiids also infest fish during the day. Thus gnathiids eaten by cleaner fish during the day may be replaced by other gnathiids during the day or night suggesting that interactions between gnathiids and cleaner fish are highly dynamic. Accepted: 15 April 1999     

Diel vertical movements by lotic mayfly nymphs under variable predation risk

1. The diel foraging periodicities of two grazing mayfly (Ephemeroptera) nymphs, Heptagenia dalecarlica and Baetis rhodani, under variable fish (European minnow) predation risk were examined in a series of laboratory experiments. 2. Heptagenia dalecarlica were almost exclusively nocturnal in their use of feeding areas on stone tops. There was a sharp increase in the proportion of nymphs out of refuge at nightfall, both in the control and fish-odour treatments. In a treatment with freely moving fish, H. dalecarlica decreased their activity during both the day and night. In feeding trials with three freely foraging minnows, nymphs were completely safe when they had access to refuges beneath stones, whereas almost half the nymphs were consumed when no refuges were available. 3. Baetis rhodani nymphs reduced their use of stone tops when exposed to four caged minnows or a freely moving fish, but this occurred both day and night. In feeding trials, B. rhodani were captured only while in the water column, and their mortality risk was independent of refuge availability. 4. It is suggested that because H. dalecarlica lack efficient post-encounter defences, they must rely on pre-encounter mechanisms to reduce the threat of fish predation. It appears that in order to dwell sympatrically with fish, H. dalecarlica have evolved a coexistence by hiding strategy. In contrast, Baetis are vulnerable to fish attacks only if they enter drift in daylight, and are thus able to remain on stone tops both during the day and at night.     

Diel differences in fish assemblages in nearshore eelgrass and kelp habitats in Prince William Sound,Alaska

The importance of a particular habitat to nearshore fishes can be best assessed by both diurnal and nocturnal sampling. To determine diel differences in fish assemblages in nearshore eelgrass and understory kelp habitats, fishes were sampled diurnally and nocturnally at six locations in western Prince William Sound, Alaska, in summer 2007. Abundance of fish between day and night were similar, but species composition and mean size of some fish changed. Species richness and species diversity were similar in eelgrass during the day and night, whereas in kelp, species richness and species diversity were greater at night than during the day. In eelgrass, saffron cod (Eleginus gracilis) was the most abundant species during the day and night. In kelp, the most abundant species were Pacific herring (Clupea pallasii) during the day and saffron cod at night. Diel differences in fish size varied by species and habitat. Mean length of saffron cod was similar between day and night in eelgrass but was greatest during the day in kelp. Pacific herring were larger at night than during the day in kelp. Diel sampling is important to identity nearshore habitats essential to fish and help manage fish stocks at risk.     

Seasonal changes in the nocturnal singing routines of Common Nightingales Luscinia megarhynchos

Male Common Nightingales Luscinia megarhynchos famously sing at night. There are two distinct types of nocturnal singing routine (the dusk-to-dawn pattern of variation in song rate): (1) dusk and dawn choruses, with little or no song during the middle of the night; (2) a rapid increase in song rate after dusk, reaching a broad peak in the middle of the night, declining towards dawn, and followed by a dawn chorus. Males sing different nocturnal singing routines at different times in the breeding season. Earlier in the breeding season, most males sing Type 2 routines. Later in the breeding season, most males sing Type 1 routines, as do birds on the wintering grounds. At least some individuals may also sing Type 1 routines during the first few days after their arrival on their breeding territories, before the arrival of females. The main function of nocturnal song appears to be mate attraction. The patterns of variation in song rate over the course of the night are qualitatively similar to those predicted by stochastic dynamic programming (SDP) models of daily singing and foraging routines, for birds that do not forage at night, in circumstances when birds can pair at night (Type 2 routines), and when they cannot (Type 1 routines). The observed seasonal changes in the types of routine sung are also consistent with the predictions of these models.     

The costs of becoming nocturnal: feeding efficiency in relation to light intensity in juvenile Atlantic Salmon

1. Most animals are active by day or by night, but not both; juvenile salmonids are unusual in that they switch from being predominantly diurnal for most of the year to being nocturnal in winter. They are visual foragers, and adaptations for high visual acuity at daytime light intensities are generally incompatible with sensitive night vision. Here we test whether juvenile Atlantic Salmon Salmo salar are able to maintain their efficiency of prey capture when switching between diurnal and nocturnal foraging.
2. By testing the ability of the fish to acquire drifting food items under a range of manipulated light intensities, we show that the foraging efficiency of juvenile salmon is high at light intensities down to those equivalent to dawn or dusk, but drops markedly at lower levels of illumination: even under the best night condition (full moon and clear sky), the feeding efficiency is only 35% of their diurnal efficiency, and fish will usually be feeding at less than 10% (whenever the moon is not full, skies are overcast or when in the shade of bankside trees). Fish were unable to feed on drifting prey when in complete darkness.
3. The ability of juvenile salmon to detect prey under different light intensities is similar to that of other planktivorous or drift-feeding species of fish; they thus appear to have no special adaptations for nocturnal foraging.
4. While winter drift abundance is slightly higher by night than by day, the difference is not enough to compensate for the loss in foraging efficiency. We suggest that juvenile salmon can nonetheless switch to nocturnal foraging in winter because their food requirements are low, many individuals adopting a strategy in which intake is suppressed to the minimum that ensures survival.     

Minnows become nocturnal at low temperatures

Juveniles of several species of salmonid become almost exclusively nocturnal in winter. Their diel activity schedule is determined mainly by temperature, with the fish seeking shelter increasingly during the day as the temperature drops. Through controlled laboratory experiments, it is demonstrated that a comparable response is found in another stream-dwelling species, the minnow Phoxinus phoxinus . Daytime observations showed that fish were found hiding in refuges on only 20% of occasions at 13.2° C but on over 75% of occasions at 6.6° C. In contrast, the minnows remained in the water column and used the refuges rarely at night irrespective of water temperature. As a consequence their activity became increasingly nocturnal as temperature dropped. It is suggested that this may be an adaptation to avoid diurnal predators.     

A telemetry study on the diurnal distribution and activity of adult pikeperch, Sander lucioperca (L.), in a riverine environment

The diurnal movements and spatial distribution of adult pikeperch, Sander lucioperca, in the Elbe River, Czech Republic was observed using radio telemetry. The hypothesis that light intensity, within four different intervals (dawn, day, dusk, night), would determine the spatial distribution of pikeperch in a riverine environment were tested across a time span of 12 months. During the day, fish were located in deep water of the main channel, moving towards shallower waters during twilight and residing in the littoral zone, closest to the riverbanks, at night. Movement activity followed the behavioural pattern in a drainage canal with maximum at twilight and minimum at night. This suggests that nocturnal positions of adult pikeperch in the shallows were not associated with hunting but more likely with resting. Handling editor: J. Cambray     

Sex-specific timing of mate searching and territory prospecting in the nightingale: nocturnal life of females

Formal models have shown that diel variation in female mate searching is likely to have profound influence on daily signalling routines of males. In studies on acoustic communication, the temporal patterns of the receivers'' signal evaluation should thus be taken into account when investigating the functions of signalling. In bird species in which diel patterns of signalling differ between males singing to defend a territory or to attract a mate, the diel patterns of mate and territory prospecting are suggested to depend on the sex of the prospector. We simulated newly arriving female nightingales (Luscinia megarhynchos) by translocating radio-tagged females to our study site. The mate-searching females prospected the area mostly at night, visiting several singing males. The timing of female prospecting corresponded to the period of the night when the singing activity of unpaired males was higher than that of paired males. In contrast to females, territory searching males have been shown to prospect territories almost exclusively during the dawn chorus. At dawn, both paired and unpaired males sang at high rates, suggesting that in contrast to nocturnal singing, dawn singing is important to announce territory occupancy to prospecting males. In the nightingale, the sex-specific timing of prospecting corresponded to the differential signalling routines of paired and unpaired males. The temporal patterns in the behaviour of signallers and receivers thus appear to be mutually adapted.     

The effect of artificial light on the community structure of reef-associated fishes at oil and gas platforms in the northern Gulf of Mexico

The northern Gulf of Mexico (GOM) contains almost 2300 oil and gas platforms. While some of these platforms are actively lit, allowing for normal working conditions throughout the night, many are unlit, illuminated only by small navigation beacons to warn vessels of their presence. In addition to producing oil and gas, these structures inadvertently serve as artificial reef habitat for a variety of GOM fish and invertebrate species. Though many fish species have been reported aggregating near these structures, this study is the first to examine changes in fish community structure in response to the structures’ artificial light regimes. Baited remote underwater video (BRUV) surveys were conducted at two lit and three unlit oil and gas platforms off the coast of Louisiana to examine the effect of platform artificial lighting on fish community structure during the day, at night, and during crepuscular periods. Seventeen recreationally harvested species were observed, with red snapper (Lutjanus campechanus) being the most abundant. Statistical analysis indicated that fish community varied significantly with depth, season, type, and season by type interactions. More fish were seen at lit platforms than at unlit platforms, with the majority of individuals identified near the surface. While fish abundance at the surface remained high during the day, the numbers declined at night. This suggests that though fishes are attracted to the vertical relief of the structure, they may be avoiding the artificial light field at the surface either to escape nocturnal predation or to forage away from the platform.     

Predator-dependent diel migration by <Emphasis Type="Italic">Halocaridina rubra</Emphasis> shrimp (Malacostraca: Atyidae) in Hawaiian anchialine pools

Diel migration is a common predator avoidance mechanism commonly found in temperate water bodies and increasingly in tropical systems. Previous research with only single day and night samples suggested that the endemic shrimp, Halocaridina rubra, may exhibit diel migration in Hawaiian anchialine pools to avoid predation by introduced mosquito fish, Gambusia affinis, and perhaps reverse migration to avoid the predatory invasive Tahitian prawn, Macrobrachium lar. To examine this phenomenon in greater detail, we conducted a diel study of H. rubra relative abundance and size at 2-h intervals in three anchialine pools that varied in predation regime on the Kona-Kohala Coast of Hawai‘i Island. We found two distinct patterns of diel migration. In two pools dominated by visually feeding G. affinis, the abundance of H. rubra present on the pool bottom or swimming in the water column was very low during the day, increased markedly at sunset and remained high until dawn. In contrast, in a pool dominated by the nocturnal predator M. lar, H. rubra density was significantly lower during the night than during the day (i.e., a pattern opposite to that of shrimp in pools containing fish). In addition, we observed that the mean body size of the shrimp populations varied among pools depending upon predator type and abundance, but did not vary between day and night in any pools. Our results are consistent with the hypothesis that H. rubra diel migratory behavior and size distributions are influenced by predation regime and suggest that diel migration may be a flexible strategy for predator avoidance in tropical pools where it may be a significant adaptive response of endemic species to introduced predators.     

Effect of light intensity on the shoaling behaviour of the guppy (Poecilia reticulata)

It has been shown that the tendency of fish to shoal decreases as night falls. Much is known about shoaling in the daytime, however, little is known about the social behaviour of fish at night. Although the nocturnal disintegration of shoal structure is the conventional expectation for most diurnal marine fish, it has not yet been investigated for diurnal freshwater fish. This possibility has been investigated using guppies (Poecilia reticulata), collected from the wild, as an experimental model. Three preference tanks were used, one of which permitted only visual cues, another only olfactory cues and the other both visual and olfactory cues. Shoaling tendency was observed at four different light intensities (8 wt/m, 0·05 wt/m, 0·025 wt/m, 0·003 wt/m). These light intensities were chosen to mimic daylight, dawn/dusk, clear night and cloudy night conditions, respectively. Trials were carried out on randomly selected male guppies. Results indicated that with both modalities present fish significantly preferred the stimulus shoal at all light intensities. However with only one modality to indicate the presence of the shoal, fish showed no significant shoaling tendency at any of the diminished light intensities. A test of shoal cohesion at the four different light intensities was carried out on freely interacting fish. This test condition was chosen to mimic the situation of guppies in the wild. The results to date suggest that guppies continue to shoal during dusk (at low light intensities) but not during the night. These findings make an important contribution to our understanding of the social behaviour of fish at night and deserve further investigation.     

Distribution,movements and diet of nocturnal fishes on temperate reefs

We counted nocturnal fishes both day and night, and monitored the position of tagged individuals on temperate reefs in New South Wales, Australia. Pempheris affinis and P. multiradiata were the most abundant nocturnal planktivores on Sydneys rocky reefs and showed great differences in diel migration behaviour. Both species were observed in deep shelter sites during the day (5–10m), and most emerged into the water column at night. P. multiradiata was found to undergo extensive vertical and horizontal migrations. In contrast, P. affinis remained within daytime depth strata, with tagged individuals often moving less than 20m at night. Tagged adult P. affinis returned to tagging sites for up to 7weeks, indicating high site fidelity. Dietary analysis demonstrated that small and large pempherids differed in diet and the timing of foraging, suggesting a size-based transition from diurnal to nocturnal foraging. Stratified sampling of planktonic assemblages at different depths during the day and night showed spatial variation in the availability of prey items at different times of the day. Amphipods, the main prey of large fish, were only available during the night, and concentrated in shallow water, whereas decapod larvae, consumed mainly by small fish, were abundant day and night. Large P. affinis also fed on polychaetes, which were never found in the stomachs of P. multiradiata, suggesting that these species may have different prey requirements, or that these polychaetes are only found in deep water where foraging P. affinis were abundant. We found no general model for the Pempheridae. The movements and behaviour of nocturnal fishes varied greatly by species, and this may be due to differences in body size, and/or physiological (e.g. visual ability) and ecological constraints.     

Understanding the function of nocturnal song in ovenbirds: males do not countersing at night

Many diurnal bird species vocalize at night, however the function of nocturnal song is, generally, still poorly understood. Previous research has suggested that nocturnal song may serve a social function and is influenced by environmental factors. To test whether males attend to the nocturnal song of conspecifics, we experimentally exposed ovenbirds Seiurus aurocapilla to nocturnal flight songs, and recorded their response both during the night and during the following dawn chorus. We compared latency to song and vocal output before and after playback exposure to determine if males altered their vocalizations in response to exposure to flight songs from an unknown male. We found no evidence of counter singing or change in nocturnal song output, nor a change in vocal output during the dawn chorus following playback exposure. Our results suggest that, in ovenbirds, nocturnal song does not serve as an intraspecific social function. Nocturnal song, through rare, may be significant in the mating systems of some diurnal bird species, and requires additional study.     

Diel Habitat Partitioning by Bull Charr and Cutthroat Trout During Fall and Winter in Rocky Mountain Streams

We used underwater observation to determine diel habitat partitioning between bull charr, Salvelinus confluentus, and cutthroat trout, Oncorhynchus clarki, during fall and winter (0.1–8.3°C) in two Rocky Mountain streams that differed in habitat availability. The majority (>70%) of both species emerged from concealment cover at night, though bull charr exhibited a greater tendency for nocturnal behavior than cutthroat trout. Differences in day and night counts were most pronounced at temperatures <3°C, when very few fish of either species were observed in the water column during the day, but both species were common at night. Both species used concealment cover of large woody debris and boulder substrate crevices in deep pools during the day. At night, fish emerged from cover and habitat use shifted to shallow water with low cover. Microhabitat partitioning among species and size classes occurred at night, cutthroat trout moving into shallower, faster water that was farther from cover compared to bull charr. Smaller fish of both species occupied focal positions in slower, shallower water closer to the substrate than larger fish. Large, mixed-species aggregations also were common in beaver ponds both day and night. High variation in diel and site-specific winter habitat use suggests the need for caution in developing habitat suitability criteria for salmonids based solely on daytime observations or on observations from a few sites. Our results support the need to incorporate nocturnal habitat use and partitioning in studies of salmonid ecology.     

Effects of light and prey availability on nocturnal, lunar and seasonal activity of tropical nightjars

Nightjars and their allies represent the only major group of visually hunting aerial insectivores with a crepuscular and/or nocturnal lifestyle. Our purpose was to examine how both light regime and prey abundance in the tropics, where periods of twilight are extremely short, but nightjar diversity is high, affect activity across different temporal scales. We studied two nightjar species in West African bush savannah, standard‐winged nightjars Macrodipteryx longipennis Shaw and long‐tailed nightjars Caprimulgus climacurus Vieillot. We measured biomass of potential prey available using a vehicle mounted trap and found that it was highest at dusk and significantly lower at dawn and during the night. Based on direct observations, both nightjars exhibit the most intense foraging behaviour at dusk, less intense foraging at dawn and least at night, as predicted by both prey abundance and conditions for visual prey detection. Nocturnal foraging was positively correlated with lunar light levels and ceased below about 0.03 mW m^?2. Over the course of a lunar cycle, nocturnal light availability varied markedly, while prey abundance remained constant at dusk and at night was slightly higher at full moon. Both species increased twilight foraging activity during new moon periods, compensating for the shorter nocturnal foraging window at that time. Seasonally, the pattern of nocturnal light availability was similar throughout the year, while prey availability peaked shortly after onset of the wet season and then slowly decreased over the following four months. The courtship and breeding phenology of both species was timed to coincide with the peak in aerial insect abundance, suggesting that prey availability rather than direct abiotic factors act as constraints, at least at the seasonal level. Our findings illustrate the peculiar constraints on visually orienting aerial nocturnal insectivores in general and tropical nightjars in particular and highlight the resulting nocturnal, lunar and seasonal allocation of activities.     

Diel activity of the pink shrimp Penaeus duorarum

Twelve juvenile pink shrimp, Penaeus duorarum Burkenroad, were tested individually for 3-day periods in electronic shuttleboxes to determine their diel patterns of locomotor activity, in relation to a natural summer photoperiod. Nocturnal activity was twice that exhibited during the daytime; however, a bimodal pattern was evident with crepuscular peaks occurring at dawn and dusk. The dusk peak was more pronounced, with activity increasing markedly before sunset (indicative of an endogenous circadian rhythm component), and continuing during the initial hours of darkness, gradually declining later during the night with a secondary peak at dawn falling off to minimal activity during daylight. Crepuscular activity (mean of dawn and dusk) was twice the nocturnal average.     

Ontogenetic shifts in habitat use and activity in a stream-dwelling isopod

We examined ontogenetic changes in diel patterns of habitat use and activity of the lotic isopod Lirceus fontinalis in relation to differences in predation risk from green sunfish, Lepomis cyanellus. Juveniles are ≤5 mm total length (TL), individuals 6-9 mm TL are mature females (c. 90%) and immature males, and mature males are ≥10 mm TL. Isopods of all sizes were far more dense in Cladophora , a filamentous green algae, than in open silt/sand substrates in fish pools. Densities of each size did not increase in open areas at night in fish pools when risk of predation presumably decreases. Few individuals of any size class occurred in open silt/sand areas in fish pools. However, habitat use did not depend on fish presence. Densities and relative abundances of all sizes were similar between algal and open habitats in fishless pools both day and night, except for mature males which were more abundant in open areas during daylight. All sizes of isopods were significantly more dense in fishless versus fish pools, and density of a size class was influenced by fish presence. Densities of size classes were typically juveniles > individuals 6-9 mm TL > mature males. Green sunfish are size selective feeders on isopods, thus demography of isopods within pools may be partially influenced by the size structure of the fish population.
Isopods reduced their activity, as measured by captures in benthic traps, in the presence of caged green sunfish only at night. This behavior coupled with relatively high activity levels during daylight regardless of fish presence suggests other predators or factors may influence diel activity. Mature males were overrepresented in traps relative to their occurrence in fish runs, indicating high activity levels. Mature males were also more active than gravid and nongravid females and juveniles in the presence of fish in laboratory aquaria. Number of male encounters with nongravid females depended on activity level.