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A theoretical model of shoaling behavior based on a consideration of patterns of overlap among the visual fields of individual members
Abstract
We propose a hypothetical visual field overlap (VFO) model for shoaling behavior. While solitary individuals have the disadvantage of a substantial blind zone to their rear, the overlapping visual fields among shoal members allows the shoal to collectively view nearly 360°. A highly polarized shoal (i.e., a school) would be less advantageous than randomly oriented shoals because a substantial area blind to all school members (common blind zone) would occur at some distance behind the school. However, in situations where fishes must orient in one direction, the visual fields of individuals in the school overlap such that the common blind zone is considerably farther behind than the blind zone of any particular fish. A simple geometric relationship between school width, the blind angle, and the distance to the common blind zone predicts that larger schools can detect predators at a greater distance than smaller schools. Additionally, the model provides a novel explanation for the nearly universally observed tendency for fish to school together with like-sized individuals. Finally, the effect of school shape on the visual field overlap pattern would have a strong impact on predator–prey interactions. However, our model does not directly take into account the range of visibility or social interaction affects and applies only to small groups. The VFO model suggests that schooling may have arisen as an adaptation to enhance feeding efficiency by reducing the need for individual vigilant behavior while oriented into the current for feeding. We believe the VFO model promises to provide insight into the behavioral ecology of shoaling fishes and that it is highly amenable to both field and laboratory testing.
Supplementary resource (1)
... body lengths (Barbaro et al., 2009; Katz et al., 2011) based upon empirical estimates solely from minnow schools (Partridge, 1980). Although these assumptions intend to consider some aspects of the sensory system, they do not necessarily reflect the configuration specific to the study species (Rountree & Sedberry, 2009; Romey & Vidal, 2013). This could be a major constraint in our ability to understand and model collective behavior, as recent evidence suggests that relaxing these sensory assumptions can influence model predictions (Lemasson, Anderson & Goodwin, 2009; Harpaz & Schneidman, 2014). ...
... Both species have a wide range of visual coverage limited by a posterior blind area, which may make them vulnerable to predator attacks. However, individuals may be able to increase their " collective " visual coverage by associating in groups where the vigilance can be shared among conspecifics (Rountree & Sedberry, 2009). In other words, an individual's blind area may be compensated for by the visual coverage of its surrounding neighbors, which have the capacity to provide cues to the individual about potential dangers. ...
... 4) is generated in which all of the individual blind areas overlap, and represents a blind zone that is shared between all of members of the group. The size and location of the " collective " blind area could have implications for the group's ability to detect approaching predators (Rountree & Sedberry, 2009). As individuals space themselves out, the distance between the center of the school and the " collective " blind area increases, providing individuals with more time to detect and respond to potential predators approaching from behind (Fig. 4), assuming equal predator attack speeds. ...
Many species of fish rely on their visual systems to interact with conspecifics and these interactions can lead to collective behavior. Individual-based models have been used to predict collective interactions; however, these models generally make simplistic assumptions about the sensory systems that are applied without proper empirical testing to different species. This could limit our ability to predict (and test empirically) collective behavior in species with very different sensory requirements. In this study, we characterized components of the visual system in two species of cyprinid fish known to engage in visually dependent collective interactions (zebrafish Danio rerio and golden shiner Notemigonus crysoleucas) and derived quantitative predictions about the positioning of individuals within schools. We found that both species had relatively narrow binocular and blind fields and wide visual coverage. However, golden shiners had more visual coverage in the vertical plane (binocular field extending behind the head) and higher visual acuity than zebrafish. The centers of acute vision (areae) of both species projected in the fronto-dorsal region of the visual field, but those of the zebrafish projected more dorsally than those of the golden shiner. Based on this visual sensory information, we predicted that: (a) predator detection time could be increased by >1,000% in zebrafish and >100% in golden shiners with an increase in nearest neighbor distance, (b) zebrafish schools would have a higher roughness value (surface area/volume ratio) than those of golden shiners, (c) and that nearest neighbor distance would vary from 8 to 20 cm to visually resolve conspecific striping patterns in both species. Overall, considering between-species differences in the sensory system of species exhibiting collective behavior could change the predictions about the positioning of individuals in the group as well as the shape of the school, which can have implications for group cohesion. We suggest that more effort should be invested in assessing the role of the sensory system in shaping local interactions driving collective behavior.
... Therefore, when interacting with small conspecifics, zebrafish may prioritize maintaining a separation distance at the outer limits of cue resolution in order to improve predator detection and the probability of survival. This is because the farther individuals can space themselves in the group, the faster the group can detect and potentially react to approaching predators (Rountree and Sedberry, 2009). This is important as larger individuals have a higher probability of being targeted by a predator when associating with small conspecifics (Rodgers et al., 2015). ...
... For example, in a low-risk environment with an easily visible horizon, individuals may maintain long separation distances, which would lead to low density groups with low intraspecific competition (Krause and Ruxton, 2002). Low density fish schools could then detect predators more quickly because of greater group visibility (Rountree and Sedberry, 2009;Pita et al., 2015). Additionally, in these low-density groups, the presence of a threat could be quickly transmitted across the group via social information, which could lead to rapid changes in the spatial configuration of the school ending in spatially tighter (e.g., higher density) schools. ...
Many fish form schools and maintain visual contact with their neighbors in a three-dimensional environment. In this study, we assessed whether zebrafish modified their spacing and interaction time in an additive or multiplicative way relative to multiple sources of social information using computer animations. We simultaneously manipulated: (a) the size of the virtual conspecific (as a proxy of social cue magnitude), (b) the position of the virtual conspecific in the water column (as a proxy of the level of perceived risk), and (c) the absence/presence of the visual horizon (as a proxy of depth perception). We found that the size of the virtual conspecific independently affected spacing behavior (zebrafish increased their separation distance as conspecific size increased). However, some of these factors interacted significantly, such that their effects on social behavior depended on each other. For instance, zebrafish increased their separation distance under high risk conditions when the virtual conspecific was larger, but this risk effect disappeared when the conspecific was the same size or smaller, likely to avoid aggression. Also, zebrafish increased their separation distance when depth perception was enhanced under low risk conditions, but the effect of depth perception disappeared under high risk conditions. Overall, we found that certain dimensions of the visual social environment affected zebrafish spacing behavior in different ways, but they did not affect social interaction time. We discuss the implications of these findings for the spatial organization of fish schools.
... Communal movement and visual fields have often been modelled: mathematically (Rountree & Sedberry, 2009), in particle models (Newman & Sayama, 2008), in neurobiological models (Lemasson et al., 2009) or in individual-based models (Hemelrijk & Hildenbrandt 2012, Romey & Vidal, 2013. However, these studies do not include agent-based models in connection to cuemotivated predator avoidance, but focus on continuously incoming information on others' movements, vigilance or feeding. ...
Group living is of benefit to foraging individuals by improving their survival, through passive risk dilution by sheer numbers and through increasingly more active processes, ranging from cue transmission to alarm calling. Cue transmission of information within a group cannot easily be tracked in the field, but can be studied by modelling. An unintentional visual cue can be given by a fleeing action, and when it occurs in the visual field of an individual, can by contagion incite it to flee as well, making such a cue functional in anti-predator warning. The visual field is limited not only by morphology, causing a blind angle at the back, but also by behaviour. For instance, foraging with the head down can cause an extra “blind” angle in front for cues from other individuals, changing an unobstructed frontal visual field to a split lateral shape.
The questions of the present study are: how do visual fields, in terms of their size and blind angles, influence survival of individuals in a group through their effect on non-attentional reception of cues to danger among group members after attentional detection of a predator, and how can we quantify this?
We use an agent-based spatially explicit model to investigate the effect of contagious fleeing after detection of predators on survival rate. This model is a bottom-up model of foraging agents in a simple environment, where only assumptions about basic competences are made. We vary the size and the shape of the visual field (lateral, with the additional frontal “blind” angle, versus a frontal continuous view), the group size, the movement probability, and the style of movement (regular movement or start-stop movement) in residential groups. We devise a measure for the transmission rate and we measure the length of the transmission chains.
We find that, as expected, in a residential group, a larger visual field enhances survival rate. Moreover, a lateral field is more effective than a frontal field of the same total size because it increases the field of vision and therefore the non-attentional reception of visual cues about danger during, for instance, foraging, for all but the largest visual fields. This is demonstrated by the higher transmission rates and longer chains of transmission for lateral fields. Better transmission for lateral visual fields results in more synchronized fleeing behaviour. As long as the visual field is large enough, having a blind angle in front does not detract from sufficiently effective transmission. These findings should be taken into account in empirical studies of vigilance in groups of foraging animals.
... Improvements to this basic model have included changing the rules to be based on cohesion, separation, and alignment zones instead of set limits (Tien et al., 2004;Hemelrijk and Hildenbrandt, 2008), and incorporating a blind zone behind each "fish" to represent the sensory capabilities of vision and the lateral line (Hemelrijk and Hildenbrandt, 2008;Rountree and Sedberry, 2009). These models can be expanded to include different behavioral states, such as food-seeking or safety-seeking (Pitcher et al., 1985;Nonacs et al., 1998;Krause et al., 2000a,b;Reebs, 2000), presence of a threat (Tien et al., 2004), leadership (Huth and Wissel, 1992;Krause et al., 2000b), methods for information transfer through the shoal Rieucau et al., 2016), and environmental conditions such as thermoclines, pycnoclines, and light levels (Fu, 2016;Rieucau et al., 2016). ...
With the increased uncertainty introduced through climate change and fishing pressure, having accurate estimates of fish biomass is essential for global ecosystem and economic health. Acoustic surveys are an efficient way to determine population size for pelagic species in the Northeast Atlantic (NEA), but acoustic population estimates still contain uncertainty and are difficult for some species. For example, Atlantic mackerel (Scomber scombrus) is one of the most valuable fisheries in the NEA and is not monitored acoustically, as mackerel lack the swim bladder that provides the strongest acoustic echo (target strength) at common assessment frequencies. For all pelagic species, and especially for mackerel, behavior is a source of variation in acoustic measurements and therefore in population estimates. Behavior is mediated by both extrinsic and intrinsic factors, such as the environment and the life history of the fish. In turn, behavior affects the density of the shoal and the tilt angle of the fish relative to the survey vessel, affecting their target strength, which affects the biomass estimate. Some fish may also undergo an anti-predator response to survey vessels, changing their behavior in response to the survey. Understanding these behaviors and incorporating them into acoustic stock assessment methods can improve the accuracy of population estimates. Individual-based models (IBM) of fish shoals provide a pathway for incorporating behavior into acoustic methods. IBMs have been used extensively to build theoretical models of fish shoals, but few have been successfully tested in lab or field conditions. As computational power and monitoring technology improve, modeling the collective behavior of pelagic fishes will be possible. Novel, interdisciplinary approaches to data collection and analysis will help translate theoretical IBMs to the fisheries science domain. Beyond acoustic stock assessments, this approach can be used to investigate knowledge gaps in the effects of fisheries-induced evolution and the potential for range shifts under climate change. Further work to synthesize existing Frontiers in Marine Science | www.frontiersin.org 1 May 2020 | Volume 7 | Article 357 Wassermann and Johnson Improving Fisheries Sustainability With Behavior models and incorporate field data will help determine how environmental, ecological, physiological, and anthropogenic factors, often affecting both behavior and acoustic surveying, are interconnected. Moving from theoretical models to practical applications will be a valuable tool in tackling the uncertainty that accompanies further fisheries exploitation and warming oceans.
... Most evolutionary optimization models of animal growth and survival focus on behaviour, size or other phenotypic traits while the internal regulatory processes are often ignored (Fawcett et al., 2014;Grafen, 1984). For fish, this includes social behaviour (Rountree and Sedberry, 2009;van der Post and Semmann, 2011), diel vertical migration (Burrows, 1994) and habitat choice (Fiksen et al., 1995;Kirby et al., 2000), but see Salzman et al. (2018). Here we take the opposite perspective, and study optimal internal regulation by hormone systems for animals that cannot choose their external environment. ...
Growth is an important theme in biology. Physiologists often relate growth rates to hormonal control of essential processes. Ecologists often study growth as a function of gradients or combinations of environmental factors. Fewer studies have investigated the combined effects of environmental and hormonal control on growth. Here, we present an evolutionary optimization model of fish growth that combines internal regulation of growth by hormone levels with the external influence of food availability and predation risk. The model finds a dynamic hormone profile that optimizes fish growth and survival up to 30 cm, and we use the probability of reaching this milestone as a proxy for fitness. The complex web of interrelated hormones and other signalling molecules is simplified to three functions represented by growth hormone, thyroid hormone and orexin. By studying a range from poor to rich environments, we find that the level of food availability in the environment results in different evolutionarily optimal strategies of hormone levels. With more food available, higher levels of hormones are optimal, resulting in higher food intake, standard metabolism and growth. By using this fitness-based approach we also find a consequence of evolutionary optimization of survival on optimal hormone use. Where foraging is risky, the thyroid hormone can be used strategically to increase metabolic potential and the chance of escaping from predators. By comparing model results to empirical observations, many mechanisms can be recognized, for instance a change in pace-of-life due to resource availability, and reduced emphasis on reserves in more stable environments.
This article has an associated First Person interview with the first author of the paper.
... http://dx.doi.org/10.1101/511972 doi: bioRxiv preprint first posted online Jan. 11, 2019; could detect predators more quickly because of greater group visibility (Rountree and Sedberry 2009;Pita et al. 2015). In these low-density groups, individuals may orient themselves utilizing regions of the visual field with low acuity to monitor changes in conspecific behavior while the centers of acute vision may be focused on the environment for threats (Butler and Fernández-Juricic 2018). ...
Many fish form schools and visually track the position of their neighbors in a 3D environment. In this study, we assessed whether zebrafish modified their spacing behavior and interaction time in an additive or multiplicative way relative to multiple sources of visual social information using video playbacks. We simultaneously manipulated: (a) the magnitude of the social cues (by varying the size of the virtual fish), (b) the level of social risk (low, high based on the position of the virtual fish in the water column), and (c) the perceived depth of the social cues (visual horizon absent or present). Each of these factors independently affected spacing behavior (zebrafish increased the separation distance with larger virtual fish, under lower visual social risk, and when depth perception was enhanced), but they did not affect interaction time. However, some of these factors interacted significantly, such that their effects on social behavior depended on each other. For instance, zebrafish decreased their separation distance under high vs. low risk conditions when the virtual fish was the same or smaller size, but this risk effect disappeared with larger virtual fish likely to avoid aggression. Also, zebrafish increased their separation distance when perceived depth was enhanced under low risk, but the perceived depth effect became less pronounced under high risk probably due to dilution effects. Overall, the effects of certain visual social parameters depend on the intensity of other visual social parameters, ultimately tuning up or down different social behavioral responses. We discuss the implications for the spatial organization of fish schools.
Significance Statement
Many fish form schools and visually track the position of their neighbors in a 3D environment. We found that zebrafish consider multiple visual social sources of information simultaneously to modify their neighbor distance. Thus, their spacing behavior appears to follow multiplicative rules, whereby the spacing response to a visual social parameter depend on the intensity of a different visual social parameter.
... Kowalko et al., 2013;Pita et al., 2015) gives great potential for testing how the information transfer driving swarm intelligence is in turn determined by sensory systems. This is especially true with new models of collective movement and collective detection of predators that make more realistic assumptions about the sensory properties of animals (Lemasson et al., 2013;Rountree and Sedberry, 2009), an approach which is supported by older experimental work in fish (Hunter, 1969). The study of Pita et al. (2015), for example, measured the field-of-view and visual acuity of two species commonly used to study collective behaviour, zebrafish (Danio rerio) and the golden shiner (Notemigonus crysoleucas). ...
Larger groups often have a greater ability to solve cognitive tasks compared to smaller ones or lone individuals. This is well established in social insects, navigating flocks of birds, and in groups of prey collectively vigilant for predators. Research in social insects has convincingly shown that improved cognitive performance can arise from self-organised local interactions between individuals that integrates their contributions, often referred to as swarm intelligence. This emergent collective intelligence has gained in popularity and been directly applied to groups of other animals, including fish. Despite being a likely mechanism at least partially explaining group performance in vertebrates, I argue here that other possible explanations are rarely ruled out in empirical studies. Hence, evidence for self-organised collective (or ‘swarm’) intelligence in fish is not as strong as it would first appear. These other explanations, the ‘pool-of-competence’ and the greater cognitive ability of individuals when in larger groups, are also reviewed. Also discussed is why improved group performance in general may be less often observed in animals such as shoaling fish compared to social insects. This review intends to highlight the difficulties in exploring collective intelligence in animal groups, ideally leading to further empirical work to illuminate these issues.
... The interaction radius together with the blind angle define an interaction zone whose size is equal for all the simulated fish and deduced from empirical data. We set the interaction radius to around 16 cm, which is also compatible with the size of the decision zone and the blind angle is fixed at 60 degrees from experimental considerations on the visual system of fish [17]. ...
Recent experiments by Ward et al. have shown that fish in a moving fish group detects hidden predators faster and more accurately than isolated individuals. The increase in speed, in particular, seems to be a consequence of the movement-mediated nature of the interactions used by fish to share information. The present work aims at investigating the link between movement and information transfer underlying collective decisions in fish. We define an individual-based self-propelled particle (SPP) model of the decision-making process analyzed by Ward et al. We fit it to data in order to deduce the smallest set of interaction rules consistent with the experimentally observed behaviour. We infer the relative weight of different social forces on fish movement during the decision-making process. We find that, in order to reproduce the observed experimental trends, both the social forces of alignment and attraction have to be introduced in the model, alignment playing a more important role than attraction. We finally apply this model to make theoretical predictions about fish ability to detect and avoid a moving predator in a natural environment such as open water.
... Although isolated prey have been reported to be more vulnerable to capture than those forming shoals (Rountree and Sedberry, 2009), we observed that the kingfishers focused their attacks more often on shoals than on isolated fish, although the capture success did not differ between these two types of prey. This tendency to attack groups of fish was common to all experiments and is consistent with previous studies in which the prey density was an important determinant of fishing site selection in kingfishers (Reyer et al., 1988). ...
Simulation models regarding groups of fish and birds based on individual movement decision rules have become increasingly sophisticated. Recent studies have started to tie together how the rules of homogeneous independent-acting individuals lead to emergent group behaviors. However, there is less research on the role that heterogeneity within a group has on these emergent properties. Heterogeneity in real animal groups due to hunger, sex, body size, species, and age can influence speed, nearest neighbor distance, and viewing angle. In our study we examine how differences in viewing angle (or its complement: blind zone) within a group influence emergent properties such as group size, polarization, group shape, and segregation. Simulated groups were assembled with different mixes of blind zones (e.g. half the members with a blind zone of 60 degrees and half with a blind zone of 120 degrees). Significant differences in many of the measured emergent properties were found and were related to the level of heterogeneity as well as the absolute value of the blind zone. In homogeneous groups, increased values for the blind zone led to groups that were: smaller, more elongated, and denser. In heterogeneous groups the sum of blind zones predicted emergent group behaviors. Specifically, as the sum of the blind zones increased: group size and density decreased and the shape of the group became rounder. However, several mixes produced emergent properties that were very different than the predicted regressions. Our findings suggest that it will be important for researchers to look at how individual differences in blind zones within real groups such as fish schools and bird flocks influence emergent behaviors. Our findings also have applications to designing sensor systems for car navigation systems and robotic arrays.
Meristic characteristics of sand lance taken from Stellwagen Bank indicated the species to be the American sand lance,Ammodytes americanus.Bottom trawl data, ichthyoplankton surveys, and diver and submersible observations demonstrated a significant increase in relative abundance of sand lance since about 1975 on Stellwagen Bank; this trend was typical of the Northwest Atlantic from Cape Hatteras, N.C., to the Gulf of Maine. School shapes were constant in appearance, vertically compres sed, tightly compacted, and bluntly linear from a dorsal and ventral view. School strengths varied from about 100 to tens ofthousands ofindividuals with the nearest-neighbor distance ranging from '4 to 1'h body lengths. The swimmingmotion issinusoidal in form and eellike in appearance. Swimmingspeeds varied from 15 to over 120 cm/s. Copepods were the most important food source, constituting 41% ofthe total weight offood consumed; sand lance feed in school fonnation between midwater and the surface. Sand lance bury themselves totally or partially in clean sandy substrates when not schooling.
Assumes that a fish chooses the position that maximizes its net energy intake rate. The fish's habitat is represented as a series of stream cross-profiles, each divided into vertical strips characterized by water depth and velocity. The fish may select a focal point in any of these strips, and include several neighbouring strips in its foraging area. Number of prey the fish encounters depends on its reaction distance to prey, water depth, and water velocity; proportion of detected prey the fish is able to capture declines with water velocity. The fish's net energy intake rate is its gross energy intake rate from feeding minus the swimming cost calculated by using water velocity at the fish's focal point. -from Authors
We studied visual optics using ophthalmoscopy in six species of coraciform birds, five species from the family Alcedinidae (kingfishers) and one from the family Meropidae (bee-eaters). All six species had large angular separations between the two foveae of one eye (angle α); angle α was greater than 40° in all cases, the largest separation so far reported for any group of vertebrates. In all kingfishers, but not in the bee-eater, the plane containing the projections of both foveae in one eye (angle ϑ) was rotated from the horizontal plane so that the projection of the monocular fovea was lower than the projection of the binocular fovea.
Retinal ganglion cell isodensity maps were obtained for the sacred kingfisher (Halcyon sancta) and laughing kookaburra (Dacelo gigas). These maps were constructed in the usual way for the peripheral regions of the retina. For the high-density, multi-layered, central region of the retinal ganglion cell layer, we used a combined retinal wholemount-cross-sectional technique. It was discovered that the ‘horizontal streak’, formed by the elliptically shaped contours of retinal ganglion cell isodensity, deviated inferiorly at its nasal extent from the line of the foveae. This deviation had the same sign, and slightly greater magnitude, as the rotation (angle ϑ) observed ophthalmoscopically when the eyes were in their primary position.
Our new observations provide new insights into the functional significance of the bifoveate visual organization. In particular, the relationship between angle α and angle ϑ suggests one strategy which could be used by kingfishers to maximize their visual capabilities when hunting from a perch above the substrate.
Teleost fish are frequently in danger of being preyed upon. Predators of many taxa are specialised piscivores and there are more of them that attack small fish. Therefore, young fish normally live under a high risk of predation, which decreases as they grow older and bigger. Growing fast is not only a good strategy for escaping the prey spectrum of many predators, but also for increasing reproductive success; bigger teleosts are generally able to produce more numerous offspring than smaller ones because bigger females produce more eggs, and bigger males can defend breeding sites better and have a higher social rank; for example, dominant male threespined sticklebacks (Gasterosteus aculeatus) maintain larger territories and have priority of access to females (Li and Owings 1978). Thus, there is a high selection pressure to feed most efficiently in order to grow quickly.
Throughout history, human hunters for fish have made use of their knowledge of fish behaviour in order to make catches. There are more than 22000 different species of teleost fishes, each with its own characteristic world of reaction and behaviour, so that numerous appropriate fish capture systems have been invented. Outlines of our knowledge of the sensory ability and behaviour of fishes have been summarised in the earlier chapters of this book. Fish behaviour is involved in catching fish, both on the oceanic scale, where the annual cycles of maturity cause migrations so that fish are found in different locations that become known to the fisherman by observation of their availability, and on a smaller scale, where the reactions of a fish to each part of an approaching trawl can cause the fish to swim into the codend. In order to be successful, the fisherman must have local knowledge of the day-to-day movements of the fish and of their likely distribution. In all fisheries, one of the most important of the fisherman’s skills is to use the appropriate gear at the right time in the right place.
The three-dimensional structure of pelagic fish schools was investigated at sea during the night by means of underwater stereo-photography. Jack mackerel Trachurus japonicus and mackerel Scomber sp. were photographed. At night, compact and polarized schools were main-tained and, on the other hand, some fish were dispersed or aggregated in very small schools. Es-timated body lengths ranged from 13 to 31cm with an average of 20cm for a school of jack mackerel and 14 to 23cm with an average of l6cm for a school of mackerel. Within the school, jack mackerel of similar size tended to swim next to each other. The mean distance to nearest neighbour was 1.43 body lengths for the jack mackerel school and 1.51 body lengths for the mackerel school. In the vertical plane, jack mackerel seem to swim at the same level as their nearest neighbours at one moment and take up diagonal positions to them at another. In the horizontal plane, jack mackerel less frequently have nearest neighbours at the side. The density estimated for the jack mackerel school ranged from 6.6 to 19.5 fish/m3.
The visibility of herring Clupea harengus and mysids Praunus flexuosus was studied in the shallow water of the Kiel Fjord (Baltic Sea). The apparent brightness contrast of the predator (herring) and its prey (mysid) were measured from various visual angles with an underwater video system. The camera looked at the prey from the predator's point of view and vice versa. During a head-on encounter, the contrast of the prey against the ocean background was very poor. With increasing visual angles (relative to a horizontal plane), the visibility of the prey improved. The predator was most visible from below, but was excellently camouflaged when observed from above or from a horizontal Line of vision. When the herring was located 30 to 90 degrees below the mysid, the prey was fairly visible to the predator but the predator was nearly invisible to the prey. By incorporating these ocean optics and taking into account the limits of the maneuvering potential of cruising herring, an optimal attack angle can be postulated: at oblique predator-prey configurations between 30 and 60 degrees, herring are still capable of modulating their normal cruising to an upward directed position while the preconditions of a successful attack - to see and not to be seen - are fulfilled. In Kiel Aquarium, adult herring attacked mysids from an average angle of 48 degrees from below. In situ measurements from inside a feeding school of juvenile herring showed a mean attack angle of 43 degrees.
We videotaped Arctic grayling Thymallus arcticus feeding on large Daphnia middendorffiana drifting at different water velocities in an experimental stream with and without stream debris. The angle and distance at which fish first located each prey was determined from the videotapes. Both measures were affected by stream velocity and added debris. Location distance was unchanged at the lower velocities (11.6 and 32.3 cm/s) but declined at higher velocities. However, prey encounter rate increased up to water velocities of 45.8 cm/s, and thus water velocity compensated for reduced search area. Added debris always shortened location distance and decreased location angle. These findings have implications for position choice in streams and search strategies.
An evolutionarily-stable-strategy model for the antipredator advantages of group living in animals is described and termed here the attack-abatement effect. The model considers the possible benefits to the individual of joining or not joining a group and is based on the relative fitnesses of these competing strategies in the same population. It is shown that neither avoidance of contact with the predator nor dilution of the effects of its attack once encountered is advantageous on its own, but in combination they reduce the risk to grouped prey. This is so even in the absence of predation at the margins of the group (as is assumed in Hamilton's selfish-herd model) and without the need for active group defense, evasion measures, or confusion of the predator. It is suggested that avoidance and dilution effects of group living cannot be considered separately, but should be regarded as effective only when in combination (the abatement effect).
Predators and food are the keys to understanding fish shoals; synchronised co-operation defeats predators, and optimal food gathering in shoals reflects a shifting balance between joining, competing in, or leaving the group. In the wild, predators may arrive while shoaling fish are feeding, and so vigilance is a crucial behaviour. Once detected, predator defence takes precedence over feeding, since an animal’s life is worth more than today’s dinner.
The probability that bluegills (sunfish) (Lepomis macrochirus) will locate prey varies with distance and position of the prey in three-dimensional space. Location ability is greatest in the hemisphere directly ahead of the fish. The probability that bluegills will locate prey declines steadily when prey are placed to the side, behind, or directly above or below the fish. Bluegills could locate prey farthest when placed slightly to the side of the area directly in front of the fish. Under all experimental conditions, fish could locate prey, at least some of the time, at distances considerably farther than suggested by reported reactive distance measurements, indicating that the water volume searched by bluegills has generally been underestimated.Key words: planktivorous fish, zooplankton, reactive distance, location volume, predator search
Reviews the book
The Vertebrate Eye and its Adaptive Radiation by G.L. Walls (see record
1943-00068-000). This book provides an account of the eye and its capacities from the standpoint of general biological utility, rather than from the more conventional standpoints of anatomy, physiology, ophthalmology, psychophysics, psychophysiology, histology, or pathology. The book is divided into three principal parts, each containing six chapters. Part I is "basic" (139 pages); Part II, "ecologic" (410 pages); and Part III, "synoptic" (136 pages). There is an extended bibliography (24 pages), and a detailed index and glossary (64 pages), The book contains 197 illustrations. With regard to the author's professed ambition to produce a "progressive" treatment of the vertebrate eye and its performances, the present reviewer feels bound in all honesty to submit the pronouncement that he has not succeeded. The three parts of the book do not truly represent a progressive development, but are concerned with three essentially diverse aspects of the eye and vision. Dr. Walls has laid a broad groundwork for the study of the eye and its functions. The use of the comparative method gives breadth of scope, and the utilization of the genetic method provides perspective. However, students of vision will find many places where the plan is only sketchily outlined. (PsycINFO Database Record (c) 2012 APA, all rights reserved)
This treatise on comparative ophthalmology is written both for the layman and the specialist. Part 1 outlines the essentials of the vertebrate (human) eye, the histology and physiology of the vertebrate retina, and discusses scotopic and photopic vision. To this is added an account of the embryological and evolutionary genesis of the eye. Part 2 discusses the following topics: adaptations to arhythmic activity as seen in photomechanical retinal changes and in pupil mobility; adaptations to diurnal activity; adaptations to nocturnal activity; adaptations to space and motion; adaptations to media and substrates including aquatic and aerial vision; and adaptations to photic quality including color vision in animals, dermal color-changes, and coloration of the eye. Part 3 traces the history of the eye from the lowest to the highest living vertebrates. There is a 24-page bibliography and an index and glossary. (PsycINFO Database Record (c) 2012 APA, all rights reserved)
and Summary
The aim of this work was to investigate the relationship between shoal size and vigilance. The behaviour of minnows ( Phoxinus phoxinus ) foraging on an artificial food patch during the simulated stalking approach of a model predator (pike: Esox lucius ) was recorded for shoals of 20, 12, 6 and 3 fish. Minnows in large shoals reduced their foraging sooner but remained feeding on the patch longer than in small shoals. The relatively late reaction of small shoals to the model and the rapid cessation of feeding once the predator was detected, indicates that small shoals were less vigilant than large shoals. The gradual reduction of foraging in large shoals was accompanied by an increasing number of investigative approaches in which individuals monitored the model's approach. This enabled minnows in larger shoals to balance more efficiently the conflicting demands of feeding and watching for predators.
A model to explain the behavioural mechanisms underlying the fountain manoeuvre, a predator-evasion response shown by fish shoals is tested. It is proposed that the responses of individual fish are constrained by requirements to (1) visually monitor the predator's behaviour, (2) minimise the energetic cost of escape, and (3) maximise the rate of passage around the predator. The model predicts that individuals will swim away from the threat at a constant angle determined by the rear limit of the visual field and that the range of reaction will be constrained by water visibility. The model's predictions were upheld in tests conducted in 1984 using a shoal of juvenile whiting, Merlangius merlangus (L.). It is concluded that the principal determinant of the fountain manoeuvre is the visual field of the fish.
The three-dimensional structure of schools of saithe (Pollachius virens) and the interactions between individuals over time were analyzed in 12,240 frames of videotape sampled at 2.7 Hz. Time series analyses of the interactions between identified individuals allowed testing of assumptions of anonymity vs. leadership in schools and investigation of the transfer of information between individuals by which collective decisions are made. Results include the following:1. The three-dimensional structure of schools of saithe (Pollachius virens) and the interactions between individuals over time were analyzed in 12,240 frames of videotape sampled at 2.7 Hz. Time series analyses of the interactions between identified individuals allowed testing of assumptions of anonymity vs. leadership in schools and investigation of the transfer of information between individuals by which collective decisions are made. Results include the following:1.
Saithe match changes in both swimming direction and speed of their neighbors but correlations are greater for swimming speed. Average speed of the school does not greatly affect correlations between neighboring fish although the reaction latencies may be somewhat increased. As shown previously (Partridge et al. 1980) nearest neighbor distance (NND) decreases with increasing school velocity.Saithe match changes in both swimming direction and speed of their neighbors but correlations are greater for swimming speed. Average speed of the school does not greatly affect correlations between neighboring fish although the reaction latencies may be somewhat increased. As shown previously (Partridge et al. 1980) nearest neighbor distance (NND) decreases with increasing school velocity.
2.2.
Saithe simultaneously match the headings and swimming speeds of at least their first two nearest neighbors within the school (NN1 and NN2). Partialling out the correlation between a fish's neighbors demonstrates that a fish's correlation to his second nearest neighbor (NN2) is not simply a transitive function of mutual correlation between the NN1 and NN2.Saithe simultaneously match the headings and swimming speeds of at least their first two nearest neighbors within the school (NN1 and NN2). Partialling out the correlation between a fish's neighbors demonstrates that a fish's correlation to his second nearest neighbor (NN2) is not simply a transitive function of mutual correlation between the NN1 and NN2.
3.3.
Several sources of individual variation in schooling performance were examined. In all respects except one, that of preferred positions within the school, saithe showed no individual differences, i.e., some were not better schoolers than others. Although fish in the school differed in length by up to a factor of 2.5, no size related effects in NND or nearest neighbor positioning were found.Several sources of individual variation in schooling performance were examined. In all respects except one, that of preferred positions within the school, saithe showed no individual differences, i.e., some were not better schoolers than others. Although fish in the school differed in length by up to a factor of 2.5, no size related effects in NND or nearest neighbor positioning were found.
4.4.
Single Linkage Cluster Analysis (SLCA) of the cross-correlations of fishs' swimming speeds and directions demonstrated quantitatively the existence of subgroups within schools if they contain more than 10–11 members. Subgroups acting more-or-less independently in terms of short term variations in speed and direction nonetheless remained within the school as a whole and were not often apparent to observers since members of one group interdigitated with those of another. How individuals know to which subgroup they belong remains unanswered.Single Linkage Cluster Analysis (SLCA) of the cross-correlations of fishs' swimming speeds and directions demonstrated quantitatively the existence of subgroups within schools if they contain more than 10–11 members. Subgroups acting more-or-less independently in terms of short term variations in speed and direction nonetheless remained within the school as a whole and were not often apparent to observers since members of one group interdigitated with those of another. How individuals know to which subgroup they belong remains unanswered.
A photographic technique is described for determining the three-dimensional position of fish schooling in front of a mirror in a flow tank. School structure is discussed in terms of the distance, horizontal bearing, and elevation of nearest neighbours. Nearest neighbour distances were measured snout-to-snout. A technique of analysis is described which considers the probability distribution of nearest neighbours in space. At speeds of flow 0 to 0·125 metres per second it was possible to show that Phoxinus, a facultative schooler, tended to maintain a school structure as previously reported for obligate schooling species. The structure was present only in a dynamic statistical sense and not in the sense of a rigid crystal lattice. Minnows maintained themselves at approximately 0·9 of their body length from each other under normal conditions and the bearings of neighbour fish suggested an attempt to maintain an optimum packing at this distance. At high speeds of flow the school structure tended to break up as fish sought areas of refuge from the current. All minnow schools were ellipsoidal in shape. The strategic and tactical methods by which schooling fish derive anti-predator advantage are discussed in relation to the school structure.
Fish do not need a leader or external stimuli for their school organization. The model presented shows that the group movement of a school can be maintained by interactions in which each individual controls its movement in relation to its neighbours. Our three-dimensional simulations reproduce the typical characteristics of real schools, if the behaviour of the single fish is based on four patterns: attraction, repulsion, parallel orientation and averaging the influences of at least four neighbours. The results of our simulations agree with experimental data in many points, as is demonstrated here for the polarization, nearest neighbour distance and internal dynamics.
The model proposed by Hamilton, which shows how flocking can arise as a result of “selfish” behaviour by individual prey animals attempting to avoid being in the position of greatest risk in relation to a predator, applies strictly only to cases where the predator emerges in the midst of an aggregation of potential prey. Another model is required to account for cases where the predator approaches a loose aggregation from outside its immediate habitat. Such a model is proposed here, although it is limited to cases in which the predator searches by visual means. It indicates, as in Hamilton's case, that the most probable dispersion is a tight circular flock of all the prey animals in the vicinity. Normally, the safety of the group as a whole is likely to be best assured by behaviour which provides the greatest individual safety. Problems in applying this or other models which depend on visual search to three-dimensional predation situations are considered briefly, and aspects of the testability and empirical utility of models proposing selective advantages are discussed.
Two methods are described for measuring the three-dimensional co-ordinates of the individual fish in a school photographed from above. The Stereo Method uses a double-prism device which produces two images on the film taken from positions slightly apart, and the distance of the fish from the camera can be determined by its parallax against the background. The Shadow Method depends on the fact that the shadow of a an object in the sun is farther to the side the higher the object above the background. The coordinates in the horizontal plane are easily determined once the distance of the fish from the background is known.The advantages and disadvantages of the two methods are considered, and a sample set of photographs of a school of 10 pilchards, made with the Shadow Method, is analysed. These results are then used to calculate the distance and bearing in the horizontal and vertical planes between nearest neighbours in the school, and to give information about the regularities in geometrical spacing of the fish. The photographs were also measured to show to what extent the fish were swimming parallel.
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