Effect of species-specificity in auditory-visual intermodal matching in a chimpanzee (Pan troglodytes) and humans

The goal of this study was to compare the performance of a chimpanzee and humans on auditory-visual intermodal matching of conspecifics and non-conspecifics. The task consisted of matching vocal samples to facial images of the corresponding vocalizers. We tested the chimpanzee and human subjects with both chimpanzee and human stimuli to assess the involvement of species-specificity in the recognition process. All subjects were highly familiar with the stimuli. The chimpanzee subject, named Pan, had had extensive previous experience in auditory-visual intermodal matching tasks. We found clear evidence of a species-specific effect: the chimpanzee and human subjects both performed better at recognizing conspecifics than non-conspecifics. Our results suggest that Pan's early exposure to human caretakers did not seem to favor a perceptual advantage in better discriminating familiar humans compared to familiar conspecifics. The results also showed that Pan's recognition of non-conspecifics did not significantly improve over the course of the experiment. In contrast, human subjects learned to better discriminate non-conspecific stimuli, suggesting that the processing of recognition might differ across species. Nevertheless, this comparative study demonstrates that species-specificity significantly affects intermodal individual recognition of highly familiar individuals in both chimpanzee and human subjects.     

Back to school: can antipredator behaviour in guppies be enhanced through social learning?

The ability to recognize and respond to predators often has a learned component, but few studies have examined the role of social learning in the development of antipredator behaviour. We investigated whether wild-caught juvenile guppies, Poecilia reticulata, from a low-predation river in Trinidad increase their response towards a novel predator through association with conspecifics from a high-predation river. We assigned fish to one of three treatment groups: (1) repeated exposure to a model accompanied by high-predation conspecifics; (2) repeated exposure to a model with low-predation conspecifics; (3) a control group in which focal fish interacted with high-predation fish in the absence of the model. Guppies trained with high-predation, but not low-predation, ‘demonstrators’ significantly improved their antipredator behaviour (spent more time schooling and inspected the model from further away). The guppies assigned to the control group showed no significant improvement in antipredator behaviour after the training period, suggesting that association with experienced conspecifics in the absence of the model is not sufficient to enhance the antipredator behaviour of na?ve fish. We conclude that guppies can improve their antipredator behaviour through association with more experienced conspecifics in the presence of visual cues simulating high predation risk. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.        

Response to Video and Computer-animated Images by the Tiger Barb, Puntius tetrazona

Video playback has become an important tool for testing certain questions about animal visual perception. While a few studies have compared the response of test subjects to live and video stimuli, it is generally assumed a priori that the animals will respond to the video image as if it were real. Since video devices are tuned to the spectral sensitivities and flicker fusion frequencies of the human eye and brain, it is important to conduct preliminary tests to compare the response of test subjects between live and video images. Here we compare schooling behavior of tiger barbs, Puntius tetrazona, when presented with a school of live fish, an analog video school of fish and a video of a computer-animated school of fish. Test subjects did not show a significantly different schooling behavior response to any of these stimuli. Additionally, when given the choice of schooling with a conspecific versus a heterospecific, tiger barbs schooled significantly more often with the conspecifics for all three stimulus formats. This study provides evidence that tiger barbs do not choose to discriminate between live and video models of fish and that this technique can be used to test future questions concerning visual perception for this species.     

Digital video playback and visual communication in lizards

Experimental analyses of dynamic visual signals have to overcome the technical obstacle of reproducing complex motor patterns such as those found in courtship and threat displays. Video playback offers a potential solution to this problem, but it has recently been criticized because of sensory differences between humans and nonhuman animals, which suggest that video stimuli might be perceived as deficient relative to live conspecifics. Quantitative comparisons are therefore necessary to determine whether video sequences reliably evoke natural responses. Male Jacky dragons, Amphibolurus muricatus, compete for territories using complex displays delivered in a rapid stereotyped sequence. We evaluated video playback as a technique for studying this visual signal. Digital video sequences depicting a life-sized displaying male were indistinguishable from live male conspecifics in the rate and structure of aggressive displays evoked. Other measures of social behaviour suggested that video stimuli were more effective in this context. Lizards produced significantly more appeasement displays and had higher rates of substrate licking and locomotor activity in response to video playback than to confined male opponents, which failed to produce aggressive displays. Lizards tracked temporal changes in the display rate of video stimuli and were also sensitive to individual differences in morphology and behaviour between video exemplars. These results show that video stimuli are appropriate for the experimental analysis of Jacky dragon aggressive displays. We compare the potential shortcomings of video playback with those of other techniques and conclude that no approach offers a panacea, but that several have complementary characteristics. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Recent social environment affects colour-assortative shoaling in juvenile angelfish (Pterophyllum scalare)

Theory predicts that fish should show colour-assortative shoaling in order to avoid the oddity effect whereby individuals that differ in some feature from the group majority appear to incur increased risk of predation. It has also been shown that early experience plays an important role in affecting social preferences in some fish species. In this study, the importance of colour phenotype in promoting assortative shoaling and the role played by the recent social environment on its expression were investigated in juvenile angelfish, Pterophyllum scalare. Individuals of the uniformly black and golden colour morphs were housed in a group with conspecifics of like and unlike body colour to themselves, as well as in mixed-colour groups for 4 weeks. Subsequently, they were subjected to a binary choice to shoal with a group of conspecifics composed of unfamiliar fish of either a like or unlike colour phenotype to themselves. The response of the individuals to the colour attributes of the shoals was related to their recent social environment. Fish in like- and mixed-colour previous treatments showed a preferential association with like colour conspecifics. In contrast, the shoaling behaviour exhibited by fish previously maintained with a group of unlike-coloured conspecifics (cross-housed treatment) indicated no significant preference for any of the shoals. The results suggest that angelfish use body colouration as an intraspecific shoaling cue and that learning, in the form of recent familiarisation with a specific colour phenotype of conspecifics, can affect colour-assortative shoaling preferences in this species. This learning component of the choice need not be restricted to early developmental stages.     

Can Paradise Fish (Macropodus opercularis,Anabantidae) Recognize a Natural Predator? An Ethological Analysis

Predator recognition and avoidance by paradise fish have been studied with allopatric species and model experiments. The effect of sympatric predators has not been investigated. Here I report that reactions of paradise fish towards a sympatric predator (Channa micropeltes) are quantitatively different from those shown towards an allopatric predator or different harmless species of fishes. I investigate the possible cues eliciting this differential response and show that visual as well as olfactory stimuli play roles. Olfactory stimuli from the sympatric predator alone elicit an elevated level of activity from paradise fish; the appearance of the sympatric predator (with or without olfactory stimuli) results in an exploratory and display reaction. I speculate what visual stimuli may play roles in predator recognition in paradise fish and I suggest that previously asserted key stimuli such as the eyes of the encountered heterospecific fish may not differentiate the harmful species from innocuous. I conclude that the antipredatory behavior of paradise fish may be affected by both genetic factors and learning and that the relative importance of the former or latter factor may vary depending on the situation.     

The effects of stimulus type on consistency of responses to conflicting stimuli in Siamese fighting fish

Individuals must often decide among a range of behavioral responses in a given situation and individuals will likely vary from one another in the choices they make. A rarely explored example of this is when both a male and a female are present simultaneously and a male must decide to fight or court. Consistent individual differences have been found in decision-making in Siamese fighting fish; however, dummies were used to explore this question and it is possible that individuals respond differently in this situation when faced with live stimuli as a result of feedback. Therefore, this study investigated whether males behave similarly to dummy and live stimuli by exposing them to male and female conspecifics both simultaneously and separately. Subjects were tested first with dummies and then in the same situations using live conspecifics. It was hypothesized that males would respond in the same manner to both the dummy and live fish. The results indicated that individuals behaved similarly to the dummy fish and the live fish in corresponding conditions. In addition, repeatability levels suggest that individuals behaved similarly across trials of a given treatment type while differing from one another in their responses. This study demonstrates that males are consistent in their responses even when receiving feedback from conspecifics and supports the continued use of dummy stimuli as an effective tool.     

The Role of Agonistic Sounds in Male Nest Defence in the Painted Goby Pomatoschistus pictus

Animals often vocalize during territorial challenges as acoustic signals may indicate motivation and fighting ability and contribute to reduce aggressive escalation. Here, we tested the function of agonistic sounds in territorial defence in the painted goby. Pomatoschistus pictus, a small vocal marine fish that defends nests during the breeding season. We first measured the number of times a male approached, avoided, explored, entered and exited two unattended nests associated with either conspecific agonistic sounds or a control: silence or white noise. Acoustic stimuli were played back when the male approached a nest. In a second experimental set, we added visual stimuli, consisting of a conspecific male in a small confinement aquarium near each nest. Even though we found no effect of the visual stimuli, the sound playbacks induced similar effects in both experimental conditions. In the sound vs. silence treatment, we found that when males approached a nest, the playback of conspecific sounds usually triggered avoidance. However, this behaviour did not last as in longer periods males visited nests associated with agonistic sounds more often than silent ones. When the control was white noise, we found no significant effect of the playback treatment in male behaviour. Although we cannot exclude the possibility that other sounds may dissuade nest occupation, our results suggest that agonistic sounds act as territorial intrusion deterrents but are insufficient to prevent nest intrusion on their own. Further studies are needed to test the significance of sound production rate, spectral content and temporal patterns to deter territorial intrusion in fish.     

Chemical modulation on heterogeneous growth in Prochilodus lineatus (Valenciennes, 1847) (Pisces; Characiformes).

In the present study, the effect of chemical factors released by conspecifics on growth variability (heterogeneous growth - HetG) in a gregarious fish species (Prochilodus lineatus) was tested. HetG was assessed by the weight variation coefficient in two consecutive 21-day periods. The fish were grouped in tanks (4 fish in each) that received running water with constant draining. The tanks contained either conspecifics (C) or non-conspecifics (N). Four conditions were established in accordance with the tank water supply: a) water with previous contact with conspecifics throughout the experiment (CC); b) water without previous contact with conspecifics throughout the experiment (NN); c) water with previous contact with conspecifics in just the first period, 0 to 21 days (CN); and d) just in the period from 21 to 42 days (NC). At the end of the experiments, the occurrence of chemical modulation on the growth variability in P. lineatus was verified: the fish that received water with previous contact with a conspecific (C) presented exacerbation of HetG. This corroborates the notion that the predominant mechanism in the determination of intraspecific growth variation in the gregarious species is associated to chemical factors released by conspecifics.     

Role of Olfactory and Visual Cues in the Attraction/Repulsion Responses to Conspecifics by Gregarious and Solitarious Desert Locusts

Desert locusts [Schistocerca gregaria Forskål (Orthoptera, Acrididae)] change phase in response to population density: solitarious insects avoid one another, but when crowded they change to the gregarious phase and aggregate. The attraction/repulsion responses of gregarious and solitarious locusts maintain phase differences in locust populations. Despite considerable research, the cues for aggregation are poorly understood; moreover, the repulsion response of solitarious locusts has not previously been investigated. This study analyzes the role of visual and olfactory stimuli in triggering these different responses to conspecifics. Isolation-reared insects were repelled by both olfactory and visual stimuli from other locusts. Crowd-reared insects were attracted by the combination of olfactory and visual cues. In addition, olfactory stimuli affected other behaviors in both phases, and behavioral differences between isolation- and crowd-reared locusts were clear even in the absence of conspecifics. The sensory and neurological mechanisms underlying these responses are not well understood and will form the basis for neurobiological investigations of locust phase.     

Predation risk assessment by olfactory and visual cues in a coral reef fish

Assessment of predation risk is vital for the success of an individual. Primary cues for the assessment include visual and olfactory stimuli, but the relative importance of these sources of information for risk assessment has seldom been assessed for marine fishes. This study examined the importance of visual and chemical cues in assessing risk for the star goby, Asterropteryx semipunctatus. Visual and chemical cue intensities were used that were indicative of a high threat situation. The behavioural response elicited by both the visual cues of a predator (the rock cod, Cephalopholis boenak) and the chemical alarm cues from conspecifics were similar in magnitude, with responses including a decrease in feeding strikes and moves. A bobbing behaviour was exhibited when the predator was visible and not when only exposed to the chemical alarm cue. When visual and chemical cues were presented together they yielded a stronger antipredator response than when gobies were exposed solely to conspecific alarm cues. This suggests additivity of risk assessment information at the levels of threat used, however, the goby’s response is also likely to depend on the environmental and social context of the predator–prey encounter. This study highlights the importance of chemical cues in the assessment of predation risk for a coral reef fish.     

Impact of olfactory non-host predator cues on aggregation behaviour and activity in <Emphasis Type="Italic">Polymorphus minutus</Emphasis> infected <Emphasis Type="Italic">Gammarus pulex</Emphasis>

Parasites with a complex life cycle are supposed to influence the behaviour of their intermediate host in such a way that the transmission to the final host is enhanced, but reduced to non-hosts. Here, we examined whether the trophically transmitted bird parasite Polymorphus minutus increases the antipredator response of its intermediate host, the freshwater amphipod Gammarus pulex to fish cues, i.e. non-host cues (‘increased host abilities hypothesis’). Aggregation behaviour and reduced activity are assumed to decrease the predation risk of gammarids by fishes. Uninfected G. pulex are known to aggregate in the presence of a fish predator. In the present study, gammarids were allowed to choose either to join a group of conspecifics or to stay solitary (experiment 1) or between two groups differing in infection status (experiment 2), both in the presence or absence of fish odour. The perception of the groups was limited to mainly olfactory cues. Contrary to the ‘increased host abilities hypothesis’, in infected gammarids of experiment 1, fish cues induced similar aggregation behaviour as in their uninfected conspecifics. In experiment 2, uninfected as well as infected gammarids did not significantly discriminate between infected and uninfected groups. Although only uninfected gammarids reduced their activity in the presence of predator cues, infected G. pulex were generally less active than uninfected conspecifics. This might suggest that P. minutus manipulates rather the general anti-predator behaviour than the plastic response to predation risk.     

Responses of larval dragonflies to conspecific and heterospecific predator cues

Abstract.  1. In cannibalistic populations, smaller individuals are subject to predation by larger conspecifics, and small individuals commonly alter their behaviour in response to cannibals. Little is known, however, about the underlying cues that trigger such responses and how the behavioural responses to conspecific cannibals differ from heterospecific predators.
2. This study tests which cues are used for the detection of conspecific predators in the larva of the dragonfly Plathemis lydia and how the behavioural response to cannibals differed from the response to heterospecific predators.
3. Individuals were exposed to chemical cues, visual cues, and a combination of both cues from conspecifics as well as no predator and heterospecific predator controls during which their activity and feeding rates were observed.
4. Individuals increased their activity, spatial movement and feeding behaviour in response to either visual or chemical cues from conspecific predators, which was opposite to responses displayed with cues from heterospecific predators. Interestingly, the responses to visual and chemical cues from conspecifics combined were weaker than to either cue in isolation and similar to the no cue control.
5. The results clearly indicate that individuals are able to use chemical and visual cues to detect even very subtle differences in phenotype of conspecific predators.
6. The opposite response in behaviour when exposed to conspecific cannibals vs. heterospecific predators suggests that the presence of cannibals will increase the mortality risk of small individuals due to heterospecific predation. This risk-enhancement is likely to have important consequences for the dynamics of predator–prey interactions.     

The response of female guppies, Poecilia reticulata, to chemical stimuli from injured conspecifics

Predation is an important component of the life history and behavior of many fish. In some species of teleost fishes, conspecifics respond to chemical stimuli from injured conspecifics by displaying a fright response, such as increased cohesion of schooling. The purpose of this experiment was to investigate whether female guppies, Poecilia reticulata, display a fright response when exposed to a filtered extract made from wounded conspecifics. The schooling behavior of female guppies was observed for ten minute periods for each of three treatments: baseline treatment (no substances added to the aquarium water), sham treatment (distilled water added to the aquarium water), and alarm treatment (fish extract added to the aquarium water). Female guppies responded with a fright reaction when exposed to a filtered extract prepared from wounded conspecifics; female swam in a more cohesive school during the alarm treatment than they did in either the baseline or sham treatments. The guppy fright reaction response indicates that the fish extract is recognized by conspecifics. This revised version was published online in August 2006 with corrections to the Cover Date.     

Species recognition in the blackbordered damselfish Dascyllus marginatus (Rüppell): An evaluation of computer-animated playback techniques

Blackbordered damselfish, Dascyllus marginatus, were presented with computer animations of varying backgrounds, with and without moving stimuli of conspecifics and sympatric heterospecifics. When subjects were released into the test tank their initial escape response was nearly always towards a fish stimulus, independent of species identity, rather than towards a background stimulus, even when the former where presented in the open and the latter displayed a coral shelter. Subjects did, however, show distinct preferences according to species with regards to the total time spent with each animation. D. marginatus significantly preferred conspecifics over Dascyllus trimaculatus, but did not prefer conspecifics over Dascyllus aruanus, a species with which they frequently shoal in nature. These results demonstrate that computer animations can be used in ethological studies of coral reef fishes and to isolate critical visual signals. Results also show that reef fishes can use visual signals to make sophisticated discriminations among species that can be modulated according to the task at hand.     

Separating natural responses from experimental artefacts: habitat selection by a diadromous fish species using odours from conspecifics and natural stream water

Animals use sensory stimuli to assess and select habitats, mates and food as well as to communicate with other individuals. One way they do this is to use olfaction, whereby they identify and respond to chemical cues. All organisms release odours, which mix with other chemical substances and ambient environmental conditions. The result is that animals are frequently immersed in a complex, highly dynamic sensory environment where they must identify and respond to only some of the potential stimuli they encounter in the face of significant levels of background noise. Understanding how organisms respond to different chemical cues is therefore dependent on knowing how these responses might be influenced by potential interactions with other stimuli. To test this, we examined whether the diadromous fish Galaxias maculatus was attracted to conspecific odours and whether this response differed when cues were offered in an artificial environment lacking other potential chemical stimuli (tap water) or a more natural background environment (stream water). We found that (1) fish responded to both natural stream water odours and those from conspecifics but the response to the latter was stronger; (2) the attraction to conspecific odours was stronger in tap water than in stream water, which indicates the importance of these odours may be overestimated when they are offered in artificial media. We also conducted a brief literature review, which confirmed that artificial media are commonly used in experiments and that the background environment is often not considered. Our results show that future research testing the responses of organisms to auditory, olfactory and visual cues should carefully consider the context in which cues are presented. Without doing so, such studies may inaccurately assess the importance of sensory cues in natural situations in the wild. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Social spacing in the mormyrid fish Gnathonemus petersii (Pisces): A multisensory approach

The sensory basis of group cohesion in the weak-electric fish Gnathonemus petersii was investigated in a circular tank with groups of four fish each, interacting through a wide-meshed plastic screen with intact or operated conspecifics, or with other stimulus objects. We confined these stimuli to one or two peripheral holding compartments. The response measures were obtained from the free swimming fish and included (1) the time the fish spent together as a group, (2) the time they spent in front of the holding compartments, (3) the circular distribution of the fish's positions, and (4) the mean nearest neighbour distances. Under empty compartment conditions, four different groups were tested, consisting of either (1) intact, electrically active fish, or (2) electrically ‘silent’ fish (with their electric organ surgically rendered inoperative), or (3) blind, or (4) ‘silent’ and blind animals. The loss of either sensory modality, vision or feedback from electric organ discharge, led to changes of comparable size, decreasing the time spent as a group and increasing the mean nearest neighbour distance. In fish lacking both modalities, group cohesion was further impaired. With stimuli present in one or both holding compartments, the strength of social attraction depended on the nature of the stimulus: the more intact stimulus conspecifics were present, the more densely did the fish group in front of the stimulus compartment. ‘Wired-in’ electric organ discharges (simulating waveform and intensity) and electrically ‘silent’ fish were equally attractive, but only half as attractive as intact fish. Blind free swimming fish aggregated with intact and also with ‘silent’ conspecifics. Under dim light conditions, group cohesion was predominantly, though not exclusively, affected by electrosensory feedback from the electric organ discharge and visual input. Mechanical and olfactory cues may also be involved.     

Computer-manipulated stimuli as a research tool in Mozambique tilapia <Emphasis Type="Italic">Oreochromis mossambicus</Emphasis>

Multimodal communication is essential in social interactions in cichlid fish, including conspecifics’ recognition, agonistic interactions and courtship behaviour. Computer-manipulated image stimuli and sound playback offer powerful tools to assess the relative relevance of visual and acoustic stimuli in fish behavioural studies, but these techniques require validation for each taxon. The aim of the present study was to investigate whether Mozambique tilapia Oreochromis mossambicus responds to computer-manipulated visual stimuli and acoustic playback. Six experiments were conducted: computer animation playback, video playback, interaction with a mirror, presentation of a live male in a jar alone and combined with courting sound playback or with white noise playback. Individual agonistic interactions (lateral displays, up and down swimming, butting) and courting behaviours (tilting leading, digging) were tallied for each experiment. Our results suggest that non-interactive computer-manipulated visual stimuli is not a suitable tool in behavioural research with Mozambique tilapia. In contrast, interaction with a live male in a jar seems to remain the best visual research instrument inducing significant strong behavioural responses. Although none or only a few agonistic interactions were observed towards video playbacks or computer animations, such interactions significantly increased towards a male in jar and were modulated by courtship sound playback, suggesting the additional relevance of sound playback as a tool in behavioural research with Mozambique tilapia, including the study of multimodal signalling.     

RHEOTROPISM IN FISHES

(1) The fluid properties of air and water enable animals to orientate to flow and this behaviour in water is termed rheotaxis. Fish, however, have a wide range of responses to currents, extending beyond simple orientation, and the term rheotropism is therefore used as a ‘portmanteau’ word to describe all such reactions. (2) Fish detect currents directly by flow over the body surface or indirectly by other stimuli. Indirect responses are more common and occur in response to visual, tactile and inertial stimuli resulting from displacement of the fish by the current. Reactions to displacement of visual images are called optomotor reactions. The lateral line is not involved except in the detection of small localized jets of water. It has not been demonstrated that any fish can detect the current by electrical stimuli, although it is theoretically possible for some to do so. (3) In the basic form of rhotaxis the fish heads upstream and maintains station by stemming the current. Current detection thresholds fall within the range 0.4 to 10 cm/s for tactile stimuli but may be as low as 0.03 cm/s for visual stimuli. (4) Visual responses have been studied by simulating displacement by the current in optomotor apparatus. Fish respond to a rotating black-and-white-striped background by compensatory movements of the head and eyes - optokinetic nystagmus - or by the optomotor reaction, in which the fish swims with the background. (5) Fish show an orthokinesis in optomotor apparatus, their mean swimming speed increasing with the speed of rotation of the background. The precise form of the relationship varies between species and there is also considerable individual variation in performance. Fish accelerate and decelerate relative to the background, fixating on a particular stripe for short periods. (6) Factors limiting the appearance of the optomotor response are contrast, illuminance, acuity, critical flicker fusion frequency and spectral sensitivity. (7) Fish tolerate retinal image movements equivalent to those received when they are carried forwards by the current but not to those received when they are carried backwards. There are ganglion cells in the optic tectum which are sensitive to the direction of movement of targets across the visual field. In the goldfish there are significantly more units sensitive to movements in the temporo-nasal than in the opposite direction. (8) There are close parallels between the behaviour of fish in schools and in an optomotor apparatus. The optomotor response is apparently innate, occurring in newly hatched fry. (9) Physical and chemical factors can modify rheotaxis. Temperature and olfactory stimuli affect both the sign of the taxis and the kinetic component of the behaviour. (10) Thyroid hormones which are involved in the control of migration have been shown to affect the kinetic component of rheotaxis. (11) Fish show a number of hydrodynamic adaptations to life in currents. Morphological modifications are greatest in fish from torrential streams, which show extreme dorsoventral flattening and have specialized adhesive organs. Other fish select areas of low velocity or decrease their buoyancy with increasing current speed. (12) Rheotropic behaviour plays an important role in the distribution of fish within stream systems, in the maintenance of territory and station and in feeding behaviour. Territory, station and spawning sites in salmonids are all selected in relation to water velocity. (13) Water currents are thought to provide either a transport system or directional clues for fish on migration. The fish either does not respond to the current and is carried passively downstream, or it makes an orientated movement, swimming up- or downstream. (14) Eggs and larvae are known to drift passively downstream from their spawning grounds and some adult fish may also drift passively. In the sea both adult and juvenile fish use a form of modulated drift associated with vertical migration. Fish move up into midwater either by direct tidal selection or in relation to the diel cycle of illuminance. In fresh water the downstream migrations of salmonid fry, and smolts under some conditions, occur by modulated drift. (15) There is no evidence that fish migrating in the sea orientate to the current, but in fresh water the upstream migrations of diadromous fish are clearly orientated movements. (16) Water velocity is a major factor for salmonids migrating upstream. For fry it limits the occurrence of upstream migrations and for adults it can also prevent upstream movement. But migrations are often initiated by freshets, and changing water velocity is thought to be the most important factor associated with a freshet. (17) Both environmental and genetic factors affect the direction of migration in relation to the current. In some sockeye salmon fry direction is determined by temperature, but in others the overall direction of movement is genetically determined and environmental factors only modify the behaviour. (18) Rheotropic behaviour has a number of important practical applications in the capture of fish and in guiding them past dams and power stations. (19) The optomotor response plays a basic role in the capture of roundfish by trawls under conditions when the fish can see the gear. Many fish are caught because they become fatigued after a prolonged period of swimming at the same speed as the trawl. (20) Most success in guiding fish away from hazardous areas and bypassing them round dams has been achieved with mechanical barriers which depend on rheotropic reactions of the fish. (21) Louvre screens are very successful in deflecting juvenile salmonids migrating downstream past small dams but are impracticable at large dams. Instead, the turbine intakes are commonly sited at a considerable depth and fish are bypassed by mechanical screens either at the surface of the forebay or into the gatewells immediately upstream of the turbine intakes. (22) With upstream migrants the basic problem is to attract fish to the lower end of the fishways. An adequate ‘attraction velocity’ is an important feature of fishways, which must be sited so that the fish avoid the high velocity discharges from spillways and turbines.     

Effects of nutritional state on the shoaling tendency of banded killifish, Fundulus diaphanus, in the field

Although group living can confer benefits to individuals in terms of reduced predation risk and enhanced foraging success, it may also be associated with costs, such as increased competition for food. The nutritional state of an individual could therefore affect its readiness to join or remain in groups. We investigated the shoaling behaviour of banded killifish by following marked individuals, whose nutritional state had been experimentally manipulated, and recording their shoaling behaviour in a field enclosure containing unmarked conspecifics. Overall, food-deprived fish spent more time alone, and therefore less time shoaling, than well-fed individuals. When shoaling, however, food-deprived fish were not found in smaller shoals than well-fed conspecifics. Furthermore, they did not show a greater latency to shoal initially. Having joined a shoal, however, food-deprived fish left shoals more frequently to be alone than fed fish. Rates of change in the membership size of shoals occupied by either well-fed or food-deprived fish did not differ. We conclude that nutritional state seems to affect an individual's decision to continue shoaling once an association has been made. This study is the first to investigate experimentally state-dependent changes in the size of social groups in fish under field conditions. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.