Water temperature, light intensity and zooplankton density and the feeding activity of juvenile brook charr (Salvelinus fontinalis)

SUMMARY 1. The objective of this study was to evaluate the effects of zooplankton biomass (as a measure of density), fish biomass, light intensity and water temperature on the attack rate and swimming characteristics (i.e. swimming speed and angle of turn) of juvenile (1+) brook charr (Salvelinus fontinalis) in field enclosures. We used a portable underwater camera system in a series of pelagic enclosures to quantify the feeding behaviour of brook charr over a gradient of natural conditions. 2. In simple linear or non‐linear regression models we found (i) that attack rate and angle of turn were positively related to water temperature, (ii) that attack rate and swimming speed were positively related to zooplankton biomass and light intensity and (iii) that attack rate was positively related to swimming speed. In multiple regression models, fish biomass, light intensity and variance of the angle of turn accounted for 87% of the variation in attack rate. Light intensity and water temperature accounted for 86% of the variation in swimming speed. Fish gut fullness and attack rate accounted for 83% of the variation in the variance of the angle of turn executed by fish. 3. The increase in the number of attacks as zooplankton biomass increases conforms to the general positive functional response observed in other fish species. Our results also support the hypothesis that swimming speed increases with prey biomass. We did not observe a plateau in attack rate as zooplankton biomass increased. As our experiments were performed under natural biotic and abiotic conditions, factors other than zooplankton biomass might affect or limit this response, such as water temperature and light intensity. 4. Because zooplankton biomass was correlated with water temperature and light intensity, it was not possible to evaluate the independent contribution of these factors on the attack rate and swimming characteristics (swimming speeds and angle of turn) of brook charr. However, this study highlighted the impact of these factors on the feeding behaviour of juvenile brook charr when feeding in the pelagic habitat under natural conditions, and their importance in future models of optimal foraging and fish habitat quality.     

Bridge under troubled water: Turbulence and niche partitioning in fish foraging

The coexistence of competing species relies on niche partitioning. Competitive exclusion is likely inevitable at high niche overlap, but such divide between competitors may be bridged if environmental circumstances displace competitor niches to enhance partitioning. Foraging‐niche dimension can be influenced by environmental characteristics, and if competitors react differently to such conditions, coexistence can be facilitated. We here experimentally approach the partitioning effects of environmental conditions by evaluating the influence of water turbulence on foraging‐niche responses in two competing fish species, Eurasian perch Perca fluviatilis and roach Rutilus rutilus, selecting from planktonic and benthic prey. In the absence of turbulence, both fish species showed high selectivity for benthic chironomid larvae. R. rutilus fed almost exclusively on zoobenthos, whereas P. fluviatilis complemented the benthic diet with zooplankton (mainly copepods). In turbulent water, on the other hand, the foraging‐niche widths of both R. rutilus and P. fluviatilis increased, while their diet overlap simultaneously decreased, caused by 20% of the R. rutilus individuals turning to planktonic (mainly bosminids) prey, and by P. fluviatilis increasing foraging on littoral/benthic food sources. We show that moderate physical disturbance of environments, such as turbulence, can enhance niche partitioning and thereby coexistence of competing foragers. Turbulence affects prey but not fish swimming capacities, with consequences for prey‐specific distributions and encounter rates with fish of different foraging strategies (pause‐travel P. fluviatilis and cruise R. rutilus). Water turbulence and prey community structure should hereby affect competitive interaction strengths among fish species, with consequences for coexistence probability as well as community and system compositions.     

Turbidity amplifies the non‐lethal effects of predation and affects the foraging success of characid fish shoals

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Effects of the presence of a predatory fish and the phenotype of its prey (a shredding shrimp) on leaf litter decomposition

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The Influence of Structural Complexity on Fish–zooplankton Interactions: A Study Using Artificial Submerged Macrophytes

Aquatic macrophytes produce considerable structural variation within the littoral zone and as a result the vegetation provides refuge to prey communities by hindering predator foraging activities. The behavior of planktivorous fish Pseudorasbora parva (Cyprinidae) and their zooplankton prey Daphnia pulex were quantified in a series of laboratory experiments with artificial vegetation at densities of 0, 350, 700, 1400, 2100 and 2800 stemsm^–2. Swimming speeds and foraging rates of the fish were recorded at different prey densities for all stem densities. The foraging efficiency of P. parva decreased significantly with increasing habitat complexity. This decline in feeding efficiency was related to two factors: submerged vegetation impeded swimming behavior and obstructed sight while foraging. This study separated the effects of swimming speed variation and of visual impairment, both due to stems, that led to reduced prey–predator encounters and examined how the reduction of the visual field volume may be predicted using a random encounter model.     

Seasonal and Diel Changes in Behaviour,Microhabitat use and Preferences by Young Pool-dwelling Atlantic Salmon,Salmo salar,and Brown Trout,Salmo trutta

There was a pronounced decline in activity of young pool-dwelling Atlantic salmon, Salmo salar, and brown trout, Salmo trutta, as the water temperatures dropped in the autumn and early winter, and the fish switched from a predominantly diurnal towards a nocturnal activity pattern. Such a switch in activity pattern has previously been observed in young brown trout, but the present study is the first documentation for juvenile Atlantic salmon under natural conditions. Juvenile fish fed actively even when water temperatures were below 0°C, although foraging behaviour at near-freezing temperatures was recorded exclusively during night surveys. This indicates that other proximate factors, in addition to water temperature, affect the activity of young salmon and trout in rivers. Trout kept feeding positions significantly higher above bottom than salmon in August and September, but both species reduced the height above bottom at the onset of winter, possibly due to reduced swimming performance and lowered food availability in the upper part of the water column.     

A model for switching between particulate-feeding and filter-feeding in the common bream,Abramis brama

Synopsis A model has been developed to describe the process of switching between particulate- and filter-feeding in common bream, Abramis brama, in relation to fish size and zooplankton density. The model assumes that the encounter rate of fish and zooplankton is determined by the density of zooplankton and the swimming speed of fish. However, if zooplankton density is so high as to allow at least one prey to be engulfed per random snap, the encounter rate is determined by the volume of the buccal cavity and by zooplankton density, but is independent of swimming speed. The snapping frequency will be maximal at the time of switching, decreasing with increasing zooplankton density because of the extra time needed for intra-oral prey handling. The model predicted switching from particulate- to filter-feeding only for bream> 15 cm standard length at zooplankton densities < 500 l^-1. The snap frequency of six size classes of bream (7.5, 10.4, 12.5, 15, 24 and 29.5 cm) was measured at varying densities of Daphnia. The model predictions for snap frequencies of all size classes corresponded to the highest values observed. The average of the observed snap frequencies was only 50% of the predicted values, probably because the calculated average distance between prey animals assumed an ideal swimming route of the fish and error-free vision for particulate-feeding, and the handling time was ignored.     

Consequences of temperature and temperature variability on swimming activity,group structure,and predation of endangered delta smelt

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Why do electric fishes swim backwards? An hypothesis based on gymnotiform foraging behavior interpreted through sensory constraints

Synopsis Fishes producing high-frequency wavelike electrical discharges maintain a relatively rigid body axis and swim forwards and backwards with equal ease. Using stop-action videotape filming we have observed the gymnotiform Apteronotus albifrons feeding on zooplankton and oligochaete annelids. Here it is reported that reverse swimming is characteristic of two foraging behaviors: searching for prey and assessing it. In assessing a potential prey item, fish typically scan it from tail to head by swimming backwards, then ingest it after a short forward lunge. A scan in the opposite direction-from head to tail by forward swimming-would have the prey located near the tail and out of position for the final lunge. Food choice experiments indicate that these electrosensing fish feed equally well, and take larger rather than smaller zooplankton, under light and dark conditions. Furthermore, electric fish take normal (light) colored and darkened prey (Daphnia) in a 50: 50 ratio under both dark and light conditions. These results are consistent with the interpretation that electrosensory cues are being used to detect zooplankton and other prey. Together, our observations support Lissmann's (1958, 1974) and Lissmann & Machin's (1958) assertion that backwards swimming is a component of a locomotory pattern guided by the constraints produced by an active electrical sense.     

Daphnid morphology deters fish predators

Spine and helmet production in zooplankton are thought to provide protection from invertebrate rather than vertebrate predators. We examined selectivity for Daphnia lumholtzi, a species that exhibits extreme cyclomorphosis with a large helmet and long tail spine (total length can exceed 5 mm), by juvenile bluegill (15–80 mm) in the laboratory and field. Bluegill consumed more D. pulex than D. lumholtzi when the species were presented alone. When the daphnids were offered together in equal numbers, bluegill selected against D. lumholtzi. Bluegill foraging behavior helped explain the observed nonrandom feeding. Bluegill capture efficiency foraging on D. pulex was high (85–100%) and handling times were low (usually too short to detect), whereas efficiencies were lower (40–96%) and handling times were longer (1–3 s) when foraging on D. lumholtzi, particularly for fish <50 mm. As they gained experience, bluegill <50 mm that oriented towards D. lumholtzi rejected them more often than striking. In addition, more D. lumholtzi were rejected and expelled than were D. pulex. From these experiments, we conclude that larger bluegill (>50 mm) are able to forage more successfully on D. lumholtzi than smaller fish. Selectivity by bluegill collected from a reservoir infested with D. lumholtzi verified our laboratory conclusions. Smaller bluegill selected against D. lumholtzi, whereas it was a preferred diet item for bluegill >50 mm. These results show that the morphology of D. lumholtzi interferes with predation by small planktivorous fish, posing foraging constraints for these fish more similar to those of piscivores, where handling time is important, than to those of planktivores, where prey density is of primary importance. Received: 13 August 1998 / Accepted 21 August 1998     

Diet overlap of non‐native common carp and native Nile tilapia: A potential cause for population reduction of Nile tilapia stock in Lake Ziway,Ethiopia

Lake Ziway harbours indigenous and exotic fish species including Nile tilapia (Oreochromis niloticus) and common carp (Cyprinus carpio). Nile tilapia was the dominant and preferred fish. However, its contribution to total catch has dramatically declined from 89.3% in 1994 to 27% in 2014 while the introduced common carp has increased from 0% before 2012 to 25% in 2014. Common carp potentially compete with Nile tilapia for available resources and could be a cause for the decline. Thus, the study explored the dietary overlap of the two species from April to August 2017. Schoener's overlap index (α) revealed significant dietary overlap between the two species (α = 0.84, between juveniles and α = 0.63, between adults). Juveniles fed mainly on animal origin (zooplankton and insects), while adult Nile tilapia have consumed plant origin (macrophyte and phytoplankton), and adult common carp fed on macrophytes and detritus. The presence of significant dietary overlap between the two species, particularly due to intense competition among juveniles, might cause the reduction of Nile tilapia stock because the native species has shown a competitive disadvantage for food in the presence of common carp. This study provides baseline information to researchers and decision makers working towards the sustainable resource utilisation of the system.     

The importance of zooplankton in the diets of three native fish species in floodplain waterholes of a dryland river,the Macintyre River,Australia

The zooplankton of freshwater systems has been recognized as an important energy resource for fish of small body size that, in turn, provide energy to piscivorous fish consumers higher up the food web. This study evaluates the importance of zooplankton to the diets of three species of fish living in floodplain waterholes of an Australian dryland river. The species selected for study represent different trophic categories in waterhole food webs: Ambassis agassizii is a microcarnivore, Leiopotherapon unicolor is an omnivore, and Nematalosa erebi is a detritivore. Dietary differences among size classes of each species were also evaluated to understand possible ontogenetic shifts in zooplankton consumption. Ambassis agassizii fed primarily on zooplankton (99.9%, made up mostly of 81.6% Calanoida and 17.4% Moinidae), regardless of the size of individual fish. Leiopotherapon unicolor fed on zooplankton (47%, mostly Daphniidae and Moinidae) and aquatic insects (46.7%). Smaller individuals of Leiopotherapon unicolor (30–49 mm TL—total length) were responsible for 36.1% of the plankton consumed by the species. Nematalosa erebi fed on detritus (84.6%) with zooplankton (Calanoida, Moinidae, and Cyclopoida) contributing only 13.7% of the mean diet. Smaller individuals (40–69 mm TL) were responsible for 98% of the plankton consumed by Nematalosa erebi, and individuals of 40–49 mm (TL) fed exclusively on zooplankton (53.8% Moinidae and 46.2% Calanoida). Although the three fish species had different diets, reflecting differences in species-specific and ontogenetic morphological and behavioral characteristics, zooplankton formed the basis of the diet of all species when young. These results confirm the importance of zooplankton as a major food resource for three fish species and smaller size classes of these species in floodplain waterholes of the Macintyre River, Australia. Guest editors: U. M. Azeiteiro, I. Jenkinson & M. J. Pereira Plankton Studies     

Hierarchical foraging by giraffe in a heterogeneous savannah,Tanzania

Understanding foraging decisions made by wildlife at different spatio‐temporal scales is important for wildlife management and conservation. We tested whether foraging decisions by Masai giraffe (Giraffa camelopardalis tippelskirchi Matschie) differed with scales; habitat, patch and tree in a heterogeneous savannah. We collected data from Arusha National Park, Tanzania, in March–May and August–October 2013. Visual observations were used to collect data on foraging behaviour. Measurements of tree height and stem height and scoring of accumulated browsing were made in 133 patches around trees where the giraffes had been seen browsing, and in a corresponding number of available patches. Giraffes selected Acacia shrub and Dodonea shrub habitats but not for water availability or predator avoidance. For patch use, giraffe selected high quantity of Acacia xanthophloea and Dodonea viscosa. Between plant species, A. xanthophloea was the most preferred and within plant species, tree quality was enhanced by tree height and high score of accumulated browsing. Generally, giraffes selected for A. xanthophloea at all scales.     

Vertical distribution of pelagial zooplankton in a middle-sized dimictic valley reservoir

Our observations indicate the vertical distribution of zooplankton and its seasonal changes in Dubník II reservoir (Slovakia) are determined mainly by the thermal regime of the reservoir, by transparency, and by fish and invertebrate predation. During periods of circulation, zooplankton vertical distribution in the whole water column was more homogeneous, whilst during summer temperature stratification zooplankton concentrated in the epilimnion — rotifers in higher layers than crustaceans. During summer stagnation a steep thermal gradient occurred at the boundary of the epi-and hypolimnion and low temperature and low dissolved oxygen in hypolimnion offered a refuge for Chaoborus flavicans larvae against fish, enabling coexistence of vertebrate and invertebrate predation. This evidence supports our previous findings concerning dominance of rotifers in zooplankton and representation of crustaceans by small-bodied species in the study reservoir. Steep thermal gradient and the presence of Chaoborus larvae caused very low zooplankton abundance in the lower part of the water column and a reduction of cladocerans refuges against fish to layers of thermocline or closely under thermocline where Daphnia cucullata and Daphnia parvula were found. Our previous assumptions about the high density of zooplanktivorous fish in Dubník II reservoir are supported by the fact that these small cladocerans are represented by smaller individuals in the upper layers and bigger individuals in deeper layers.     

Rainbow trout <Emphasis Type="Italic">Oncorhynchus mykiss</Emphasis> energetic responsesto pulsed flows in the American River,California, assessed by electromyogram telemetry

Although rainbow trout Oncorhynchus mykiss within the American River, California, apparently exhibit minimal upstream or downstream movements in response to hydroelectric-power-generation-related pulsed flows, the associated energetic costs are unknown. We implanted rainbow trout (n = 9, ≥30 cm SL) with electromyogram (EMG)-sensor-equipped radio transmitters to assess the swimming behavior and associated energetic costs associated with their responses to pulsed flows. Using laboratory calibrations in a Brett-type swimming respirometer, the trouts’ swimming speeds and oxygen consumption rates were estimated for their in-river EMG data, through a complete hydroelectric power-generation river pulsed-flow sequence (pre-pulse, increasing flow, peak, and decreasing flow stages), on several (mean: 3.2) sampling dates. Using a mixed-linear model, we found that fish swimming speed estimates increased during the increasing flow stage, while the associated mean oxygen consumption rates also increased at this stage. At river flows near the usual peak (>44 m³s⁻¹), swimming speeds and movement rates decreased, possibly due to the fish using the river’s habitat complexities as hydraulic cover. We conclude that rainbow trout incur increased swimming-related energetic costs during increasing flows and, potentially, decreased foraging opportunities at high flows.     

Comparison of fish and phantom midge influence on cladocerans diel vertical migration in a dual basin lake

Diurnal vertical migrations (DVM) behaviour of cladocerans was investigated in two mesotrophic Irish lakes connected by a canal, characterised by interesting differences in the presence of zooplanktivorous predators. In Doon Upper, fish (mostly juvenile perch and roach) and a little-studied phantom midge Mochlonyx fuliginosus (Chaoboridae) were found, but Doon Lower was solely inhabited by fish. As the presence of diverse predators may alter spatial avoidance behaviour of zooplankton prey in different ways, the aim of this study was to determine whether and how two predator types, fish and phantom midge larvae, have changed DVM pattern of cladocerans during day and night in Doon lakes. Two sampling series of phytoplankton, zooplankton, fish, and water physical analyses were conducted on 09–10 June and 19–20 September 2007 in both lakes. In the study conducted in June, under a similar distribution of M. fuliginosus and juvenile fish in Doon Upper, a reverse migration of Daphnia galeata was observed as a strategy allowing them to avoid both types of predators. However, in September, when M. fuliginosus lived in a 24 h refugium below the oxycline as a response to increasing predation risk posed by YOY fish penetrated the upper strata of water during day and night, reverse migrations of D. galeata were not clear. In Doon Lower, normal migration was observed as an advantageous behavioural response against visual predators (fish), in both large and small Cladocera species: D. galeata, Diaphanosoma branchyurum and Bosmina sp. Thus, our results indicate dissimilar migration patterns of D. galeata depending on the presence of one (Doon Lower) or two predators with different predation behaviour (Doon Upper).     

Length–weight and length–length relationships of three fish species from the Yarlung Zangbo River in Tibet,China

Length–weight relationships and length–length were evaluated for three fish species (Schizopygopsis younghusbandi Regan, 1905; Ptychobarbus dipogon Regan, 1905 and Oxygymnocypris stewartii Lloyd, 1908) from the Yarlung Zangbo River in Tibet. Specimens were captured monthly using floating gillnets (mesh size 7.5 cm), bottom gillnets (mesh size 6.5 cm), and trap nets (mesh size 1.5 mm) from August 2008 to August 2009, March to August 2012, and March to April 2013. Regression coefficient (b) values of length–weight relationships (LWRs) ranged from 3.045 for P. dipogon to 3.193 for O. stewartii, whereas the a values ranged from 0.0040 to 0.0168 for O. stewartii and P. dipogon, respectively.     

Nile perch,Lates niloticus,predation on the freshwater prawn,Caridina nilotica,in the Nyanza Gulf,Lake Victoria,East Africa

Synopsis The percentage of Nile perch, Lates niloticus, containing the prawn, Caridina nilotica, declined from 60–85%, for fish between 5 and 40 cm total length, to less than 10% for fish larger than 80 cm in length. The maximum number of prawns eaten by a Nile perch increased steadily, from 10 prawns for a fish in the 5–9.9 cm length group, to 913 prawns in a fish in the 60–69.9 cm length group, and then fell sharply for larger fish. The mean number of Caridina eaten by fish that were foraging on this animal followed a similar trend. Gill raker spacing increased in direct proportion to fish length and this may prevent fish larger than 70 cm in length from being effective prawn predators.     

Impact of live food on survival and growth of hatchery‐reared sea trout (Salmo trutta trutta L.) parr in the wild

Survival rates and growth parameters of hatchery‐reared sea trout (Salmo trutta trutta L.) fry were determined after stocking in the wild. The larvae were hatchery‐reared for 12 weeks in two groups: fry were fed either on live zooplankton and live chironomidae larvae (LFG), or fed a pellet diet (PFG). The survival rate and specific growth rates were higher in the LFG than in the PFG group. Most effective for hatchery‐reared fish intended for stocking was the natural, live feed. The mean number of chironomid larvae found in the stomachs of fish that were initially captured in the wild was significantly higher in the LFG than in the PFG group. The live diet supplied in the rearing period had a positive impact on the foraging skills of the sea trout fry and their survival in the wild after their release on 24 April 2010.     

Variation in the summer diet of age-0 brown trout in a regulated mountain stream

We describe the age-0 brown trout (Salmo trutta) collected from a mountain stream downstream from a reservoir in June, July, and August 1990. Sample sites were 1, 12, and 20 km downstream from the reservoir at elevations of 2840, 2600, and 2230 m above mean sea level. The mean length of fish in the samples increased with downstream progression and from June through August. Zooplankton predominated the diet of fish collected in June from the most upstream sampling site, while Diptera larvae predominated the diet of fish from more downstream sampling sites in June. In July and August, the diet of fish from the upstream site was predominated by Diptera larvae, while the diet of fish from the other two sites was predominated by Ephemeroptera nymphs. Variation in diet among sampling sites and sampling months was related not only to increasing fish sizes over the summer but also to the availability of zooplankton flushed from the reservoir.