Feeding and aggressive behaviours in juvenile coho salmon (Oncorhynchus kisutch) under chemically-mediated risk of predation

Behavioral responses of a clonal fish to perceived predation risk

Predation threat is a major driver of behavior in many prey species. Animals can recognize their relative risk of predation based on cues in the environment, including visual and/or chemical cues released by a predator or from its prey. When threat of predation is high, prey often respond by altering their behavior to reduce their probability of detection and/or capture. Here, we test how a clonal fish, the Amazon molly (Poecilia formosa), behaviorally responds to predation cues. We measured aggressive and social behaviors both under 'risk', where chemical cues from predatory fish and injured conspecifics were present, and control contexts (no risk cues present). We predicted that mollies would exhibit reduced aggression towards a simulated intruder and increased sociability under risk contexts as aggression might increase their visibility to a predator and shoaling should decrease their chance of capture through the dilution effect. As predicted, we found that Amazon mollies spent more time with a conspecific when risk cues were present, however they did not reduce their aggression. This highlights the general result of the 'safety in numbers' behavioral response that many small shoaling species exhibit, including these clonal fish, which suggests that mollies may view this response as a more effective anti-predator response compared to limiting their detectability by reducing aggressive conspecific interactions.

Metabolic constraints and individual variation shape the trade-off between physiological recovery and anti-predator responses in adult sockeye salmon

Metabolic scope represents the aerobic energy budget available to an organism to perform non-maintenance activities (e.g., escape a predator, recover from a fisheries interaction, compete for a mate). Conflicting energetic requirements can give rise to ecologically relevant metabolic trade-offs when energy budgeting is constrained. The objective of this study was to investigate how aerobic energy is utilized when individual sockeye salmon (Oncorhynchus nerka) are exposed to multiple acute stressors. To indirectly assess metabolic changes in free-swimming individuals, salmon were implanted with heart rate biologgers. The animals were then exercised to exhaustion or briefly handled as a control, and allowed to recover from this stressor for 48 h. During the first 2 h of the recovery period, individual salmon were exposed to 90 ml of conspecific alarm cues or water as a control. Heart rate was recorded throughout the recovery period. Recovery effort and time was higher in exercised fish, relative to control fish, whereas exposure to an alarm cue had no effect on either of these metrics. Individual routine heart rate was negatively correlated with recovery time and effort. Together, these findings suggest that metabolic energy allocation towards exercise recovery (i.e., an acute stressor; handling, chase, etc.) trumps anti-predator responses in salmon, although individual variation may mediate this effect at the population level.

Acoustic and visual adaptations to predation risk: a predator affects communication in vocal female fish

Predation is an important ecological constraint that influences communication in animals. Fish respond to predators by adjusting their visual signaling behavior, but the responses in calling behavior in the presence of a visually detected predator are largely unknown. We hypothesize that fish will reduce visual and acoustic signaling including sound levels and avoid escalating fights in the presence of a predator. To test this we investigated dyadic contests in female croaking gouramis (Trichopsis vittata, Osphronemidae) in the presence and absence of a predator (Astronotus ocellatus, Cichlidae) in an adjoining tank. Agonistic behavior in T. vittata consists of lateral (visual) displays, antiparallel circling, and production of croaking sounds and may escalate to frontal displays. We analyzed the number and duration of lateral display bouts, the number, duration, sound pressure level, and dominant frequency of croaking sounds as well as contest outcomes. The number and duration of lateral displays decreased significantly in predator when compared with no-predator trials. Total number of sounds per contest dropped in parallel but no significant changes were observed in sound characteristics. In the presence of a predator, dyadic contests were decided or terminated during lateral displays and never escalated to frontal displays. The gouramis showed approaching behavior toward the predator between lateral displays. This is the first study supporting the hypothesis that predators reduce visual and acoustic signaling in a vocal fish. Sound properties, in contrast, did not change. Decreased signaling and the lack of escalating contests reduce the fish’s conspicuousness and thus predation threat.

Fears from the past? The innate ability of dogs to detect predator scents

Throughout the animal kingdom, antipredator mechanisms are an evolutionary driving force to enable the survival of species classified as prey. Information regarding a predator's location can be determined through chemosensory cues from urine, faeces, visual and/or acoustic signals and anal gland secretions; and in several lab and field-based studies it has been seen that these cues mediate behavioural changes within prey species. These behaviours are often linked to fear and avoidance, which will in turn increase the prey's survival rate. In many studies dogs (Canis lupus familiaris) have been used as a predator species, however, no research has addressed a dog's innate ability to detect predator scents, hence the rationale behind this study. We assessed the innate ability of the untrained domestic dog to detect faecal scents of wild Eurasian brown bear (Ursus arctos arctos) and European lynx (Lynx lynx). The study monitored 82 domestic dogs across the UK and Norway. The dogs were exposed to the two predator faecal scents from Eurasian brown bear and European lynx, a herbivore faecal scent of Eurasian beaver (Castor fiber) and water control. Measurements were taken upon the time spent within a 40 cm radius of each scent and changes in the dog's heart rate when within this 40 cm radius. We found dogs spent a decreased length of time around the predator scents and had an increased heart rate in relation to their basal heart rate. We conclude that dogs can innately sense predator scents of brown bear and lynx and elicit fear towards these odours, as shown through behavioural and physiological changes.

Ecology of sound communication in fishes

Fishes communicate acoustically under ecological constraints which may modify or hinder signal transmission and detection and may also be risky. This makes it important to know if and to what degree fishes can modify acoustic signalling when key ecological factors-predation pressure, noise and ambient temperature-vary. This paper reviews short-time effects of the first two factors; the third has been reviewed recently (Ladich, 2018). Numerous studies have investigated the effects of predators on fish behaviour, but only a few report changes in calling activity when hearing predator calls as demonstrated when fish responded to played-back dolphin sounds. Furthermore, swimming sounds of schooling fish may affect predators. Our knowledge on adaptations to natural changes in ambient noise, for example caused by wind or migration between quiet and noisier habitats, is limited. Hearing abilities decrease when ambient noise levels increase (termed masking), in particular in taxa possessing enhanced hearing abilities. High natural and anthropogenic noise regimes, for example vessel noise, alter calling activity in the field and laboratory. Increases in sound pressure levels (Lombard effect) and altered temporal call patterns were also observed, but no switches to higher sound frequencies. In summary, effects of predator calls and noise on sound communication are described in fishes, yet sparsely in contrast to songbirds or whales. Major gaps in our knowledge on potential negative effects of noise on acoustic communication call for more detailed investigation because fishes are keystone species in many aquatic habitats and constitute a major source of protein for humans.

Chemical encoding of risk perception and predator detection among estuarine invertebrates

An effective strategy for prey to survive in habitats rich in predators is to avoid being noticed. Thus, prey are under selection pressure to recognize predators and adjust their behavior, which can impact numerous community-wide interactions. Many animals in murky and turbulent aquatic environments rely on waterborne chemical cues. Previous research showed that the mud crab, Panopeus herbstii, recognizes the predatory blue crab, Callinectus sapidus, via a cue in blue crab urine. This cue is strongest if blue crabs recently preyed upon mud crabs. Subsequently, mud crabs suppress their foraging activity, reducing predation by blue crabs. Using NMR spectroscopy- and mass spectrometry-based metabolomics, chemical variation in urine from blue crabs fed different diets was related to prey behavior. We identified the urinary metabolites trigonelline and homarine as components of the cue that mud crabs use to detect blue crabs, with concentrations of each metabolite dependent on the blue crab's diet. At concentrations found naturally in blue crab urine, trigonelline and homarine, alone as well as in a mixture, alerted mud crabs to the presence of blue crabs, leading to decreased foraging by mud crabs. Risk perception by waterborne cues has been widely observed by ecologists, but the molecular nature of these cues has not been previously identified. Metabolomics provides an opportunity to study waterborne cues where other approaches have historically failed, advancing our understanding of the chemical nature of a wide range of ecological interactions.

Effects of diet quality and stress on interference behaviour of larval ringed salamanders

Interference behaviour (aggression and cannibalism) can be influenced by both intrinsic factors, such as animal physiology, size, or motivation, and extrinsic factors, such as presence of competitors, predators, or prey. Our experiment examined the effects of differences in diet quality of focal salamanders and their opponents, and levels of handling on biting by pairs of larval ringed salamanders,Ambystoma annulatum, before and after presentation of prey. Diet quality and handling affect the intrinsic qualities of body size/condition and stress, respectively. Presence of prey and diet quality of opponents are extrinsic factors. Unstressed larvae on high-quality diets bit their high-condition opponents more frequently than those that had been on low-quality diets in both the presence and absence of food. Stressed larvae (all on high quality diets) showed low levels of biting when food was absent. However, when food was present, the level of biting depended on the diet quality of their opponents: stressed larvae bit opponents on low-quality diets more than opponents on high-quality diets. Overall, both intrinsic and extrinsic factors influenced interference behaviour, with larvae on high-quality diets exhibiting relatively high levels of biting unless they had experienced handling stress, and the effect of handling stress depending on the condition of the opponent in paired interactions.

Predator-induced neophobia in juvenile cichlids

Predation is an important but often fluctuating selection factor for prey animals. Accordingly, individuals plastically adopt antipredator strategies in response to current predation risk. Recently, it was proposed that predation risk also plastically induces neophobia (an antipredator response towards novel cues). Previous studies, however, do not allow a differentiation between general neophobia and sensory channel-specific neophobic responses. Therefore, we tested the neophobia hypothesis focusing on adjustment in shoaling behavior in response to a novel cue addressing a different sensory channel than the one from which predation risk was initially perceived. From hatching onwards, juveniles of the cichlid Pelvicachromis taeniatus were exposed to different chemical cues in a split-clutch design: conspecific alarm cues which signal predation risk and heterospecific alarm cues or distilled water as controls. At 2 months of age, their shoaling behavior was examined prior and subsequent to a tactical disturbance cue. We found that fish previously exposed to predation risk formed more compact shoals relative to the control groups in response to the novel disturbance cue. Moreover, the relationship between shoal density and shoal homogeneity was also affected by experienced predation risk. Our findings indicate predator-induced, increased cross-sensory sensitivity towards novel cues making neophobia an effective antipredator mechanism.

Sight or smell? Behavioural and heart rate responses in subordinate rainbow trout exposed to cues from dominant fish

Many animals, including fish, can utilize both vision and the chemical senses in intra-specific communication. However, the relative influence of these sensory modalities on behavioral and physiological responses in social interactions is not well understood. The aim of this study was therefore to investigate the relative effects of visual and chemical stimuli from dominant individuals on the behavioral and physiological responses of subordinate rainbow trout (Oncorhynchus mykiss). External electrodes were used to detect ECG signals from free-swimming fish. This method allowed the simultaneous recording of behavioral and physiological responses, and possible sex differences in these responses were also investigated. The results suggest that, in this context, visual cues are more important than chemical cues in settling the social hierarchy in rainbow trout because a combination of chemical and visual exposure generally yielded a response in focal fish that was similar to the response elicited by visual exposure alone. Both activity and physiological responses were most pronounced during the first ten seconds after exposure, with subordinate fish moving closer to the dominant, accompanied by a strong bradycardic response. Furthermore, females acted more boldly and moved closer to the dominant fish than males, but here the effect of the modes was additive, with a stronger effect of the combined visual and chemical exposure. Overall, the extra information furnished to the fish in the form of chemical cues did not change either the behavioral or the physiological response. This result suggests that visual cues are more important than chemically mediated ones for social communication and individual recognition in rainbow trout.

Understanding the importance of episodic acidification on fish predator-prey interactions: does weak acidification impair predator recognition?

The ability of prey to recognize predators is a fundamental prerequisite to avoid being eaten. Indeed, many prey animals learn to distinguish species that pose a threat from those that do not. Once the prey has learned the identity of one predator, it may generalize this recognition to similar predators with which the prey has no experience. The ability to generalize reduces the costs associated with learning and further enhances the ability of the prey to avoid relevant threats. For many aquatic organisms, recognition of predators is based on odor signatures, consequently any anthropogenic alteration in water chemistry has the potential to impair recognition and learning of predators. Here we explored whether episodic acidification could influence the ability of juvenile rainbow trout to learn to recognize an unknown predator and then generalize this recognition to a closely related predator. Trout were conditioned to recognize the odor of pumpkinseed sunfish under circumneutral (~pH 7) conditions, and then tested for recognition of pumpkinseed or longear sunfish under both neutral or weakly acidic (~pH 6) conditions. When tested for a response to pumpkinseed odor, we found no significant effect of predator odor pH: trout responded similarly regardless of pH. Moreover, under neutral conditions, trout were able to generalize their recognition to the odor of longear sunfish. However, the trout could not generalize their recognition of the longear sunfish under acidic conditions. Given the widespread occurrence of anthropogenic acidification, acid-mediated impairment of predator recognition and generalization may be a pervasive problem for freshwater salmonid populations and other aquatic organisms.

Social context, competitive interactions and the dynamic nature of antipredator responses of juvenile rainbow trout Oncorhynchus mykiss

The effects of food ration and social context, as well as possible interactions, on the antipredator behaviour of juvenile rainbow trout Onchorhynchus mykiss were investigated in a pair of laboratory studies. In experiment 1, pairs of dominant and subordinate O. mykiss were exposed to conspecific alarm cues when maintained under high or low food rations. Under high food rations, dominant individuals responded to predation risk, whereas subordinates used the opportunity to feed. Under low food ration, however, the opposite pattern was observed, where subordinates responded to predation cues and dominants did not. Experiment 2 consisted of performing the same experiment, however separating the dominant and subordinate O. mykiss 3 h before testing. When tested separately, dominant and subordinate individuals did not differ in their responses to alarm cues, regardless of food ration. These results demonstrate that there is a complex interaction between current energy status and social context on decision making by prey animals.

Disturbance chemical cues determine changes in spatial occupation by the convict cichlid Archocentrus nigrofasciatus

I studied the effect of disturbance chemical cues on fish that make trade-offs between foraging in an open area and remaining in a safe refuge. I used convict cichlids Archocentrus nigrofasciatus that were either visually exposed to a predator (n = 8) or exposed to water conditioned by chemical cues from disturbed conspecifics (n = 8). Fish visually exposed to a predator decreased their ingestion rate and spent more time in the refuge than in the foraging area, while fish receiving water from frightened conspecifics did not alter their ingestion rate or time spent in the refuge and foraging site, but increased their spatial occupation (i.e., motion). These results suggest that convict cichlids recognized the predator by visual cues. Moreover, disturbance cues are a form of threatening public information that may increase fish spatial occupation due to increased exploring behaviour; but is not sufficiently alarming to alter feeding or increase refuge use.

Metabolic fright responses of different-sized largemouth bass (Micropterus salmoides) to two avian predators show variations in nonlethal energetic costs

Recently, researchers have identified that nonlethal costs of predation may arise not only from lost energy intake but also potentially from increased energetic expenditure. During periods of heightened stress following unsuccessful predation attempts, organisms may remain in an altered physiological state with elevated metabolism for some time. Few studies have quantified these nonlethal energetic costs of predation. We monitored the cardiac response (cardiac output (Q), heart rate (fH), and stroke volume (SVH)) of largemouth bass, Micropterus salmoides, ranging in size from 200 to 425 mm, to simulated avian predation attempts by great blue heron, Ardea herodias, and osprey, Pandion haliaetus. Fear bradycardia during a 30-s predation attempt varied depending upon the size of the fish and the type of predator. The magnitude of the bradycardia decreased with increasing size of the fish; however, the disturbances were more extreme in response to osprey than to blue heron models. Maximal cardiac disturbance following simulated predation attempts by osprey were consistent among size classes of bass. However, the magnitude of the disturbance following heron predation attempts reduced as the size of the fish increased. Size-specific trends were even more extreme for cardiac-recovery durations. Largemouth bass of all sizes exposed to osprey predation attempts required ~40 min for Q and fH and ~30 min for SVH to return to predisturbance levels. Although small bass exposed to heron predation attempts required recovery times similar to fish exposed to osprey predation attempts, as the size of largemouth bass exposed to the heron model increased above ~300 mm, the recovery time decreased significantly. We conclude that the size-specific response of largemouth bass to different predators is reflective of their ability to assess the risk posed by different predators. In addition, the nonlethal costs of predation can be substantial and should be considered in future bioenergetics models.

Monopolization of food by zebrafish (Danio rerio) increases in risky habitats

Dominant zebrafish (Danio rerio) previously have been shown to reduce their monopolization of food when foraging in structurally complex habitats compared with open habitats. Complex habitats may be more difficult to defend but may also be safer. To decouple these effects, we compared aggression and monopolization of food in groups of zebrafish foraging in an open habitat and one with overhead cover, as well as in an open habitat and a complex (vegetated) habitat. Covered and open habitats should have been equally defendable. In our experiments, fish used covered habitats more than open ones, suggesting that the perceived risk of predation was lower in covered habitats. There was no difference in use of vegetated and open habitats, suggesting that these habitats, which should differ in defendability, did not differ in safety. We found that the degree of food monopolization (expressed in the coefficient of variation within groups) at risky feeders was significantly greater in open habitats than in covered, but not vegetated, habitats. We did not find a difference in aggression between habitats. These results indicate that resource monopolization in groups of zebrafish is greater in risky habitats and support the hypothesis that the lower monopolization of food in complex habitats could result from greater safety in those habitats rather than, or in addition to, the reduction in defendability.

Fighting behaviour, vigilance and predation risk in the cichlid fish Nannacara anomala

I studied the fighting behaviour of a small South American cichlid fish, Nannacara anomala, in relation to predation risk. In this species, which reduces vigilance during escalated fighting, differences in fighting ability are most accurately assessed during mouth wrestling. Thus, there is a potential trade-off between agonistic behaviours that are effective for assessing differences in fighting ability between contestants and agonistic behaviours that minimize the risk of being killed by predators. I investigated whether breaks between bouts of mouth wrestling improved the contestants' ability to flee from an approaching model fish predator. During the breaks between mouth wrestling the contestants had a significantly longer escape distance than males that were actively mouth wrestling. Fighting males also changed their fighting behaviour after the introduction of the model predator compared with males that were not exposed to it. Low-intensity behaviours such as lateral display and tail beating were preferred to mouth wrestling. In addition, both the number and duration of bouts of active mouth wrestling decreased significantly after the introduction of the model predator while the duration of the breaks between bouts of active mouth wrestling increased significantly compared with the control. Including predation risk in the costs of intense fighting may increase our understanding of both the temporal structuring of fighting behaviour and the evolution of fighting behaviour in general.1998 The Association for the Study of Animal Behaviour

Size-dependent foraging behaviour and use of cover in juvenile coho salmon under predation risk

Foraging and use of cover by juvenile coho salmon (Oncorhynchus kisutch) were affected by predation threat in both seminatural channels and laboratory streams. In the field, coho salmon preferred stream sections with brushy cover only when under threat from hunting common mergansers. The mergansers had their highest capture success in pools without cover. Predation threat also caused coho salmon to use cover more as foraging habitat and to aggregate more in favourable positions at the head of the pool. In the laboratory, under simulated predation threat, fish using a refuge were significantly larger than those in the risky habitat. This pattern persisted for 2 days after the predation threat was discontinued. The average growth of coho salmon under predation threat was depressed and the difference in growth between large and small individuals was less than in control groups. We argue that larger fish were more averse to predation risk than smaller fish and that the smaller fish took advantage of feeding opportunities indirectly provided as a result of the predation risk. We speculate that in natural environments, predation may depress growth rates because of risk-avoidance behaviour but may also serve to reduce growth-rate differences among size classes within a cohort.