Behavioural interactions between the Carangidae (Pisces) and their prey on the fore-reef slope of Aldabra, with notes on other predators

The behavioural interactions of carangids with their prey have been examined on the forereef slope of Aldabra Atoll, Indian Ocean. From underwater observations and from fishing data, Caranx melampygus and C. ignobilis are shown to be predominately piscivores. Slight differences in the sex ratios occurred when compared with earlier work, although length distribution of trolled specimens match very closely. By contrast underwater observations indicate that many more small C. melampygus occur on the reef slope than are caught by trolling and suggest this method of capture is highly selective. The literature ascribes the association of small predatory fish to larger ones with a scavenging role, but present observations suggest that it provides a mechanism whereby small fish can approach and exploit the disorientation of prey during hunting episodes. Other types of predator association are discussed.     

On Group Living and Collaborative Hunting in the Yellow Saddle Goatfish (Parupeneus cyclostomus)1

Intraspecific group hunting has received considerable attention by researchers interested in cooperative behaviour and animal cognition. Differences between species in the complexity of the hunting with respect to communication, coordination and food sharing have typically been interpreted as a reflection of differences in cognitive abilities. Here we describe for the first time collaborative hunting where individuals play different roles in a fish species, the yellow saddle goatfish Parupeneus cyclostomus. Adults in our study area may live either solitarily or in relatively stable groups formed of similar sized and most likely unrelated individuals. The solitary life style was associated with searching for hidden immobile prey on sandy areas while group living was associated with collaborative hunting of mobile prey in corals. Any member of a group could initiate a hunt by rapid acceleration. Partners did not simply follow the attacker but deviated around coral formation to block the prey’s escape routes. Prey that escaped into a coral crevice was typically encircled with maximal inter‐individual distance and pried on by insertion of the barbels into the crevices. As home ranges largely overlapped and no between‐group aggression existed, we propose that it is the hunting of mobile prey in a complexly structured habitat that selects for collaborative hunting and hence for the evolution of group living in yellow saddle goatfish.     

Feeding Habits and Interspecific Feeding Associations of Caranx Latus (Carangidae) in a Subtropical Reef

I studied the feeding behavior and diet of the carangid jack Caranx latus in a subtropical reef at Búzios Island on the southeastern Brazilian coast. Caranx latus foraged alone or in small groups of up to three individuals during daytime employing two main feeding styles: searching for prey while swimming in mid-water and following individuals of other fish species which disturbed the substrate while foraging among the rocks. The labrid wrasse Bodianus rufus was the main fish species followed by the jack. The jack feeds on crustaceans and fishes some of which are benthic rocky bottom dwellers and are caught during following. The behavioral flexibility of C. latus enables this fish to consume both crustaceans from the water column and benthic rocky dweller fishes. There is little dietary overlap between C. latus and the followed B. rufus. The interspecific feeding associations could be regarded as advantageous for C. latus allowing access to a broader range of prey categories and increasing the food intake through the consumption of large benthic fish prey     

Wolf-pack (Canis lupus) hunting strategies emerge from simple rules in computational simulations

We have produced computational simulations of multi-agent systems in which wolf agents chase prey agents. We show that two simple decentralized rules controlling the movement of each wolf are enough to reproduce the main features of the wolf-pack hunting behavior: tracking the prey, carrying out the pursuit, and encircling the prey until it stops moving. The rules are (1) move towards the prey until a minimum safe distance to the prey is reached, and (2) when close enough to the prey, move away from the other wolves that are close to the safe distance to the prey. The hunting agents are autonomous, interchangeable and indistinguishable; the only information each agent needs is the position of the other agents. Our results suggest that wolf-pack hunting is an emergent collective behavior which does not necessarily rely on the presence of effective communication between the individuals participating in the hunt, and that no hierarchy is needed in the group to achieve the task properly.     

Shape of Single and Multiple Central‐Place Territories in a Stream‐Dwelling Fish

Territory shape yields important insights into how animals exploit local resources. Territories of stream‐dwelling salmonids are typically (1) mapped around a single central‐place, (2) described as circular, elliptical or teardrop‐shaped, and (3) believed to reflect their exploitation of drifting invertebrate prey. In this study, we tested the current view of territory shape by mapping multiple central‐place territories for 50 young‐of‐the‐year Atlantic salmon. Multiple central‐place areas were more elongated (eccentricity: median = 1.301, range = 1.043–2.784) than the foraging patterns around each central place (eccentricity: median = 1.135, range = 1.014–1.385). In addition, multiple central‐place areas were elongated along the stream length (33 of 50 fish), whereas the foraging areas around each station tended to be elongated along the stream width (32 of 50 fish). These findings may be explained by the way that stream salmonids interact with drifting prey. At each central place, a wider foraging area should provide an increased access to prey drifting downstream. Similarly, by regularly patrolling a large multiple central‐place area along the stream axis, a territorial fish may increase its access to drifting prey by excluding competitors from upstream areas. Further studies are needed on the ecological factors that determine territory shape in stream fish and multiple central‐place foragers.     

Experiments on whether schooling by their prey affects the hunting behaviour of cephalopods and fish predators

The experiments investigated the effects of a school of live prey fish on the hunting behaviour of squid, cuttlefish and pike (ambush predators) and perch (a chasing predator). The hunting behaviour of all species is described, including some previously unrecorded actions.
For all species the increasing shoal size from one fish to six, to 20, decreased the success of the predators' attacks per encounter with a prey. This was partly because attacks on larger shoals lasted longer, and fish became increasingly difficult to catch as a hunt went on. However, for some of the species there was a clear effect of the shoals at the start of the hunt. For other species it was less conclusive.
The disruptive effect of the shoals was shown to act for the ambush predators by interfering with the "optimal" sequence of an attack and causing avoidance actions together with so-called irrelevant behaviour. For the perch, shoals seemed to disrupt by forcing the predator continually to switch targets during his pursuit.
The artificiality of the experiments is discussed in relation to field observations. The relatively high incidence of avoidance and irrelevant behaviour seems to be an artefact, but the main results are quite compatible with field studies. In nature fish predators may be able to get sufficient food by restricting their attacks to individuals which are conspicuous either by an abnormal appearance or by becoming separated from the school. An individual prey fish no doubt gains a considerable advantage from the school by burying itself in the crowd. Our experiments show that a shoal can provide further protection in hampering the attack of a predator.     

Predation on adult Atlantic salmon, Salmo salar L., by otters, Lutra lutra (L.), within the River Dee system, Aberdeenshire, Scotland

Predation on adult salmon, Salmo salur L., by otters, Lurra lutra (L.), varied seasonally on the R. Dee, Aberdeenshire, Scotland, being highest during the spawning season in winter. Predation is described for some tributaries of the river. Male fish were caught by otters more often than females, and it is suggested that they were most vulnerable during their extensive excursions up and down stream, particularly as they negotiated shallow riffles. Otters appeared to prey upon healthy fish rather than those infested with Saprolegnia sp. but there was no difference in the size, freshwater- or sea-age offish killed by otterscompared with 'kelts' which had died non-violently. At least some of the otters obtained most of their daily food requirements by catching a single salmon per night.
Considerable numbers of adult salmon may be killed by otters during the spawning period but it is suggested that, because most are males, this is unlikely to affect the breeding success of the salmon population. Most predation occurs outside the fishing season and so is unlikely to reduce the numbers of salmon caught by anglers.     

Prey selection by young trout fry (Salmo trutta)

Factors influencing the choice of invertebrate food by early feeding trout are sought, special attention being given to the dimensions of the prey. The stomach contents of fish of similar age feeding on lake plankton and drifting stream fauna are compared. Little consistent preference was displayed for the smaller planktonic organisms but prey size was of importance in the distribution of stream benthos among predators of different lengths. The width of the prey was of greater significance to selection than the mean volume or the area of the largest surface of the food items. When the fish were in obvious competition for food the correlation between predator length and all prey dimensions improved.     

Red-tailed monkeys (Cercopithecus ascanius) hunt green pigeons (Treron calva) in the Kalinzu Forest in Uganda

Red-tailed monkeys (Cercopithecus ascanius) were observed hunting green pigeons (Treron calva) in the Kalinzu Forest in Uganda. During 2 h 39 min, I observed two cases of successful hunting and one case of unsuccessful hunting in a Ficus saussureana tree. Red-tailed monkeys stalked the pigeons until they were within 2–3 m, and then jumped and caught them. In both successful cases, blue monkeys (C. mitis) ran to the hunting site from adjacent trees in order to poach the prey, and the red-tailed monkeys fled. One of these red-tailed monkeys dropped the pigeon while fleeing, and the blue monkey climbed down from the tree to search for it. This is the first record of cercopithecoid monkeys hunting birds that are outside of the nest and moving freely, and also the first record of red-tailed monkeys hunting vertebrates. However rare it may be, the repeated hunting attempts using similar techniques and the immediate attempt of blue monkeys to poach the prey suggest that this type of hunting was not a one-time event that happened by chance. Blue monkeys and an adult chimpanzee (Pan troglodytes schweinfurthii) in and around the fig tree did not attempt to hunt. The hunting of volant birds may be enabled by the small body size and the quick movements of red-tailed monkeys.     

Is there always an influence of shoal size on predator hunting success?

Theoretical and empirical studies predict that there should be a decrease in hunting success of predators with increasing prey group size. Most of these studies investigated situations in which predator and prey were in full view of each other before, during and after an attack. In this study, single rock bass Ambloplites rupestris were given an opportunity to launch surprise attacks at shoals of creek chub Semotilus atromaculatus that ranged in size from two to 13 fish. There was no significant influence of either shoal size or attack distance on predator success rate and no significant relationship between attack distance and shoal size. Furthermore, it was found that the leading fish of a shoal was attacked significantly more often than fish in other shoal positions, indicating that predation risk was not shared equally among shoal members. Also, leading fish in larger shoals (eight to 13 fish) were not more likely to survive a predator attack than ones in small shoals (two to seven fish).The consequences of these results are discussed in the general context of antipredator benefits of grouping.     

Use of terrestrial arthropod prey by a stream-dwelling cyprinid fish

Synopsis The spatio-temporal availability to, and use of terrestrial (allochthonous) prey by, a subset of the fish assemblage of a Virginia piedmont stream were investigated during spring and summer. Terrestrial invertebrates were most abundant in drift nets, but least common in fish stomachs, during April. In contrast, during August, terrestrial prey dominated the diet of the major cyprinid fish,Notropis ardens, even though terrestrial prey availability in the stream declined by an order of magnitude. Several other co-occurring fishes only rarely consumed prey of terrestrial origin. Drifting terrestrial invertebrates, were concentrated at the surface near the thalweg, compared to stream margin or substrate locations. This study suggests that morphologial and behavioral constraints of individual fish taxa, including predator avoidance responses, regulate the use of terrestrial prey within a given fish assemblage.     

Intraguild interactions among generalist predator functional groups drive impact on herbivore and decomposer prey

Different functional groups of generalist predators may complement each other in controlling prey populations; but intraguild interactions, common among generalist predators, may also reduce the strength of top–down control. In natural communities greater alterations to ecosystem function are expected if a whole functional group declines in abundance or is lost. Therefore studying functional group diversity is important for predicting effects of predator loss. We studied the top–down impact of web‐building spiders, hunting spiders and ants, which are highly abundant generalist predators in most terrestrial ecosystems, on prey from the herbivore and decomposer system of a grassland food web. The density of the three predator groups was manipulated by continuous removal in a three‐factorial designed field experiment, which was carried out for two years. We found no positive effect of increasing predator functional group richness on prey control. However there was evidence for strong composition effects between the functional groups. The presence of ants in predator assemblages reduced the prey suppression through mostly trait‐mediated intraguild interactions, while hunting and web‐building spiders contributed additively to prey suppression and reduced the density of herbivore and decomposer prey by 50–60%. A trophic cascade on plant biomass triggered by web‐builders and hunting spiders was diminished at levels of higher predator group diversity. In conclusion, our experiments showed that intraguild interactions strongly influence the strength of top–down control by generalist predators. Among spiders there was evidence for a positive relation between functional group richness and prey suppression but the overall outcome strongly depended on the occurrence of interference, driven by trait‐mediated indirect interactions.     

Hunted Woolly Monkeys (Lagothrix poeppigii) Show Threat-Sensitive Responses to Human Presence

Responding only to individuals of a predator species which display threatening behaviour allows prey species to minimise energy expenditure and other costs of predator avoidance, such as disruption of feeding. The threat sensitivity hypothesis predicts such behaviour in prey species. If hunted animals are unable to distinguish dangerous humans from non-dangerous humans, human hunting is likely to have a greater effect on prey populations as all human encounters should lead to predator avoidance, increasing stress and creating opportunity costs for exploited populations. We test the threat sensitivity hypothesis in wild Poeppigi''s woolly monkeys (Lagothrix poeppigii) in Yasuní National Park, Ecuador, by presenting human models engaging in one of three behaviours “hunting”, “gathering” or “researching”. These experiments were conducted at two sites with differing hunting pressures. Visibility, movement and vocalisations were recorded and results from two sites showed that groups changed their behaviours after being exposed to humans, and did so in different ways depending on the behaviour of the human model. Results at the site with higher hunting pressure were consistent with predictions based on the threat sensitivity hypothesis. Although results at the site with lower hunting pressure were not consistent with the results at the site with higher hunting pressure, groups at this site also showed differential responses to different human behaviours. These results provide evidence of threat-sensitive predator avoidance in hunted primates, which may allow them to conserve both time and energy when encountering humans which pose no threat.     

A sonar fish counter

The basic principles of a new technique for counting fish moving up or down stream are given. The counter is based on the ability to detect the presence of fish in an acoustic beam directed across the river. The counter is capable of counting several fish moving up or down stream simultaneously and is designed for use in wide open channels. The results of field trials with the equipment at two different sites are discussed together with the problems encountered. The results obtained are in good agreement with other methods of counting fish but the totals obtained were insufficient to put confidence limits on the counter.     

Prey preferences of the leopard (Panthera pardus)

Leopards Panthera pardus have a catholic diet and are generally thought to prey on medium-sized ungulates; however, knowledge on which species are actually preferred and avoided is lacking, along with an understanding of why such preferences arise. Twenty-nine published and four unpublished studies of leopard diet that had relative prey abundance estimates associated with them were analysed from 13 countries in 41 different spatial locations or temporal periods throughout the distribution of the leopard. A Jacobs' index value was calculated for each prey species in each study and the mean of these was then tested against a mean of 0 using t or sign tests for preference or avoidance. Leopards preferentially prey upon species within a weight range of 10–40 kg. Regression plots suggest that the most preferred mass of leopard prey is 25 kg, whereas the mean body mass of significantly preferred prey is 23 kg. Leopards prefer prey within this body mass range, which occur in small herds, in dense habitat and afford the hunter minimal risk of injury during capture. Consequently, impala, bushbuck and common duiker are significantly preferred, with chital likely to also be preferred with a larger sample size from Asian sites. Species outside the preferred weight range are generally avoided, as are species that are restricted to open vegetation or that have sufficient anti-predator strategies. The ratio of mean leopard body mass with that of their preferred prey is less than 1 and may be a reflection of their solitary hunting strategy. This model will allow us to predict the diet of leopards in areas where dietary information is lacking, also providing information to assist wildlife managers and conservation bodies on predator carrying capacity and predator–prey interactions.     

Ecological characteristics of territories of male cheetahs (Acinonyx jubatus)

Ecological characteristics of nine male cheetahs territories in the Serengeti National Park are described. Territories were centred on areas that provided cover but did not have particularly high rainfall or prey abundance. Resident male cheetahs did not occupy their territories continuously. Some territories were vacated in the dry season, others in the wet season, but in both cases this occurred when numbers of Thomson's gazelles (females' main prey) and female cheetahs seen there were low. Although changes in territory ownership appeared to be related to demographic rather than ecological factors, territories that remained vacant seemed to contain rather little prey. Over two years, evidence suggested that territories occupied by males of larger group size contained greater numbers of Thomson's gazelles.     

Implications of African wild dog (Lycaon pictus) denning on the density and distribution of a key prey species: addressing myths and misperceptions

African wild dogs (Lycaon pictus) are often the least popular large carnivore among game ranchers because of their perceived impact on prey populations. Landowner perceptions include that wild dogs greatly deplete prey during their three-month denning period, take prey that could otherwise be sold for hunting and cause prey to move away from the vicinity of their den sites. Landowners’ tolerance towards African wild dogs could thus be improved with a more rigorous understanding of the actual impact of wild dogs on prey populations during the denning period. Using impala density data and wild dog denning records from Sango Ranch in the Savé Valley Conservancy, Zimbabwe, we compared impala densities between pre-denning, denning and post-denning season and between inside and outside the denning home range. Our results indicate that wild dog denning does not cause a significant local reduction in prey around the den and does not cause prey to move away from denning areas. However prey species did occur in lower density inside the denning home ranges than outside, in all seasons. This result indicates that wild dogs select dens in areas of lower prey density, perhaps as an avoidance mechanism for lions. Accordingly, contrary to what landowners believe, wild dogs do not have a significant impact on prey populations during their denning season.     

Growth dynamics of myotomal muscle fibres in a carangid, Caranx malabaricus (Cuv. et Val.)

The growth pattern of myotomal red, pink and white muscle and its relation to somatic growth in Caranx malabaricus are described. The growth pattern of red muscle was by an increase in fibre number in early size classes (< 22 cm f.l.) and thereafter mainly by increase in fibre diameter and partly by increase in fibre number. The growth of pink muscle was mainly by an increase in fibre diameter, but in smaller fish an increase in fibre number was also evident. White muscle growth was mainly by an increase in fibre diameter and partly by increase in fibre numbers in fish < 22 cm f.l., but only by an increase in fibre diameter from 22 cm f.l. onwards. Caranx malabaricus is a slow-to-moderate growing species and its fibre growth pattern matches with such somatic growth.     

First feeding success with two types of prey by the Central American cichlid fish,Cichlasoma managuense (Pisces,Cichlidae): morphology versus behavior

Synopsis The behavioral and morphological correlates of differential capture success at first feeding of the cichlid fish Cichlasoma managuense were investigated. Capture efficiency with nauplii of Artemia salina was 69% compared with only 6% with Daphnia. These drastic differences are attributed largely to the different morphology and behavior of the prey that may have produced behavioral changes in the fish. Daphnia is more evasive and harder to handle than Artemia as reflected in the significantly higher number of missed capture attempts and in spit-outs, a measure of handling effort. Fish hunting Artemia had a high capture rate when they made many attempts. By contrast, fish hunting Daphnia were successful when they handled the prey persistently, made many spit outs and many attempts. Large behavioral variation was observed and may be the substrate for later food specialization. In contrast to previous studies, morphological variation in the predators and prey was kept low; no measured morphological trait of the predators explained a significant portion of the variation in capture rate. The behavioral differences among the predators in each group alone significantly explain the variation in capture rate. The developmental stage of the fish and the behavior of the prey need to be considered when comparing the capture efficiency between species.     

Factors affecting the predation of otter (Lutra lutra) on European pond turtle (Emys orbicularis)

In this case study, the ecological background of an unusual hunting behaviour was investigated, when otters Lutra lutra preyed upon European pond turtles Emys orbicularis in a Hungarian fish pond system during an 18-month period. Predation on turtle was found only during cold periods (established by spraint analysis and also by the collection of 182 turtle carcasses in 2003). The relationship was not close between fish availability and turtle consumption ( r _P =−0.325, P =0.19). The crude protein content of the turtle head and leg was higher than that of fish, frog and turtle body, whereas the energy content of the samples was similar. The mean body weight of the killed turtles (460 g) fell within the range of the optimal prey size of the otter. Turtles were used as cache foods by otters during extreme environmental conditions (as in the long winter), but occurred only rarely as buffer foods during moderate winter. In fish ponds, the conservation of the coexistent otter and turtle depends on pond management. The maintenance of a higher fish availability in ponds during winter makes it possible to avoid the need to acquire a proper hunting technique on turtle, indicated by the scarcity of primary fish food.