Mechanisms of selectivity in a nocturnal fish: a lack of active prey choice

Fish that feed on individual zooplankton usually exhibit strong selectivity for large prey. Such selectivity can result from the predator’s active choice of larger prey or from differential encounter rate due to lower detectability of small prey, or both. In diurnal fishes, selectivity is thought to be determined mostly by active choice. In spite of a lack of direct observations, active choice is also considered the prevailing mechanism of prey selectivity in nocturnal fishes. Our objective was to resolve this mechanism in the highly selective, nocturnal zooplanktivorous fish Apogon annularis. Laboratory experiments indicated that the fish’s encounter rate with small prey was lower than that with large prey and that its selectivity became stronger with decreasing light intensity. Feeding efficiency, defined as the ratio between feeding and encounter rates, ranged 41–89% and was positively correlated with prey size. When feeding on a mixture of prey sizes, the fish fed on each size group at a rate similar to that of its feeding on the respective size alone, indicating that selectivity in A. annularis was due to size-dependent encounter rate and differential feeding efficiency. A low visual acuity in A. annularis, as inferred from its inability to detect small prey (<0.9 mm in length), together with the low abundance of large zooplankton in situ, can explain the dominance of differential encounter over active choice in this nocturnal coral-reef fish.     

A field study of prey selection in planktivorous fish larvae

Summary The food selection of larval fish was studied from field samples collected in two areas that differ in productivity. In the area where planktonic primary and secondary production was high the fish larvae showed a tendency to specialise and they selected the largest prey species available as food; in the area of lower production fish were generalists and they fed equally on all size classes.Abundance of prey was found to be one of the decisive factors in the prey selection of planktivorous fish larvae. Large prey species (calanoids) were selected when their absolute abundance was high but when their abundance was low, small-sized cladocerans were preferred. Visibility and stage of life history were also assumed to affect the mode of selection.     

The dynamics of prey choice in fish: the importance of prey size and satiation

Over a number of decades the process of prey choice has been investigated using fishes as model predators. Using fishes for the model has allowed the proximate factors that determine how a mobile predator finds and chooses to eat the prey encountered within a variable 3‐D environment to be estimated. During prey choice a number of constraints exist, in particular most fish predators will eat their prey whole thus their jaws and gut create functional limitations once a prey has been attacked. By considering the relationship between the size of the prey and the predator's feeding apparatus and feeding motivation this study explores the link between mechanistic studies and theoretical, optimal foraging based predictions. How the prediction of prey choices made by the fish following prey encounter can be reconciled with what is likely to be found in the fish's stomach is discussed. This study uses a progression of empirical examples to illustrate how the limits of functional constraints and prey choice at different stages of motivation to feed can be taken into account to improve predictions of predator prey choice.     

Predation on coral spawn by planktivorous fish

The stomach contents of three species of coral reef fish were examined for changes in diet during the annual mass spawning of scleractinian corals on the Great Barrier Reef, Australia. Acanthochromis polyacanthus, Abudefduf bengalensis (Pomacentridae), and Caesio cuning (Caesionidae) were collected before and immediately after the coral spawning to determine whether the composition of the diet changed after the mass coral spawning. The diet of Caesio cuning did not change. The stomach contents of Acanthochromis and Abudefduf showed that these fish: (1) switched from an omnivorous diet to one consisting predominantly of coral spawn, and (2) that they appeared satiated with stomachs >90% full when coral spawn was present in the water column. Fish predation during coral spawning may be an important source of larval coral mortality.     

Local extinction of a coral reef fish explained by inflexible prey choice

While global extinctions of marine species are infrequent, local extinctions are becoming common. However, the role of habitat degradation and resource specialisation in explaining local extinction is unknown. On coral reefs, coral bleaching is an increasingly frequent cause of coral mortality that can result in dramatic changes to coral community composition. Coral-associated fishes are often specialised on a limited suite of coral species and are therefore sensitive to these changes. This study documents the local extinction of a corallivorous reef fish, Oxymonacanthus longirostris, following a mass bleaching event that altered the species composition of associated coral communities. Local extinction only occurred on reefs that also completely lost a key prey species, Acropora millepora, even though coral cover remained high. In an experimental test, fish continued to select bleached A. millepora over the healthy, but less-preferred prey species that resisted bleaching. These results suggest that behavioural inflexibility may limit the ability of specialists to cope with even subtle changes to resource availability.     

Species-specific responses of planktivorous fish to the introduction of a new piscivore: implications for prey fitness

1. Antipredator behaviour by the facultative planktivorous fish species roach (Rutilus rutilus), perch (Perca fluviatilis) and rudd (Scardinius erythrophthalmus) was studied in a multi‐year whole‐lake experiment to evaluate species‐specific behavioural and numerical responses to the stocking of pikeperch (Sander lucioperca), a predator with different foraging behaviour than the resident predators large perch (P. fluviatilis) and pike (Esox lucius). 2. Behavioural responses to pikeperch varied greatly during the night, ranging from reduced activity (roach and small perch) and a shift in habitat (roach), to no change in the habitat use and activity of rudd. The differing responses of the different planktivorous prey species highlight the potential variation in behavioural response to predation risk from species of similar vulnerability. 3. These differences had profound effects on fitness; the density of species that exhibited an antipredator response declined only slightly (roach) or even increased (small perch), whereas the density of the species that did not exhibit an antipredator response (rudd) decreased dramatically (by more than 80%). 4. The maladaptive behaviour of rudd can be explained by a ‘behavioural syndrome’, i.e. the interdependence of behaviours expressed in different contexts (feeding activity, antipredator) across different situations (different densities of predators). 5. Our study extends previous studies, that have typically been limited to more controlled situations, by illustrating the variability in intensity of phenotypic responses to predators, and the consequences for population density, in a large whole‐lake setting.     

Food supply and prey selection in planktivorous cyprinidae

Summary In small Bavarian lakes, the gut contents of the Cyprinid fish roach (Rutilus rutilus), rudd (Scardinius erythrophtalamus), bream (Abramis brama) and bleak (Alburnus alburnus), and the actural food supply during the fish's feeding period were examined in relation to the species composition of zooplankton. Accompanied by feeding experiments in the laboratory, the selective effect of fish predation could be attributed to the distribution patterns of prey and predator in time and space, to the prey's specific visibility and escape ability and, to some extent, to the fish's active choice. The possibility that the species composition of zooplankton is regulated was indicated only in the fish's positively abundance-dependent preference for the prey types and restricted to only a few plankter species.     

Interference competition in a planktivorous fish (Rutilus rutilus) at different prey densities and temperatures

The effect of interference competition can be assessed by comparing the capture rate of a predator foraging alone with that of the predator within a group. Since such an effect could be prey density dependent, a constant density of prey must be maintained while assessing this effect, irrespective of the elimination of prey by predation. However, when studying a predator-harvester, such as a planktivorous fish, which collects zooplankton at a rate of up to 1 prey s^?1, instantaneous replacement of each consumed prey item is not feasible. This problem was solved in short-lasting mesocosm experiments by minute-by-minute supplementation to replace eliminated Daphnia and maintain a constant average prey density. Such experiments were performed with different numbers of foraging roach (Rutilus rutilus) at three prey densities and in two ranges of ambient temperature. The number of Daphnia required at the start of each experiment to establish the initial prey density and the number that it was necessary to add per minute were determined in experiments conducted without prey supplementation and in preliminary experiments with prey supplementation. The results of this study revealed that fish foraging in a group eat less, due to both exploitation and non-aggressive competition for space. Moreover, the effect of interference competition was stronger at higher temperatures, irrespective of the prey density, indicating that natural populations of roach foraging in shoals may suffer more from competitive interactions in warmer waters.     

Mate choice when males are in patches: optimal strategies and good rules of thumb

In standard mate-choice models, females encounter males sequentially and decide whether to inspect the quality of another male or to accept a male already inspected. What changes when males are clumped in patches and there is a significant cost to travel between patches? We use stochastic dynamic programming to derive optimum strategies under various assumptions. With zero costs to returning to a male in the current patch, the optimal strategy accepts males above a quality threshold which is constant whenever one or more males in the patch remain uninspected; this threshold drops when inspecting the last male in the patch, so returns may occur only then and are never to a male in a previously inspected patch. With non-zero within-patch return costs, such a two-threshold rule still performs extremely well, but a more gradual decline in acceptance threshold is optimal. Inability to return at all need not decrease performance by much. The acceptance threshold should also decline if it gets harder to discover the last males in a patch. Optimal strategies become more complex when mean male quality varies systematically between patches or years, and females estimate this in a Bayesian manner through inspecting male qualities. It can then be optimal to switch patch before inspecting all males on a patch, or, exceptionally, to return to an earlier patch. We compare performance of various rules of thumb in these environments and in ones without a patch structure. A two-threshold rule performs excellently, as do various simplifications of it. The best-of-N rule outperforms threshold rules only in non-patchy environments with between-year quality variation. The cutoff rule performs poorly.     

Apparent size choice of zooplankton by planktivorous sunfish: exceptions to the rule

Synopsis Choice of the apparently largest prey has been implicated as an important component of the size choice behavior of several planktivorous fish species. In this study we describe the effect of several aspects of prey placement, apparent and absolute size, and motion on the choice behavior of bluegill or white crappie. In binary choice experiments, bluegill usually choose Daphnia prey on the basis of apparent size. However, when both prey were close to the fish and the absolutely larger prey was apparently smaller, the fish commonly chose the absolutely larger. The horizontal placement of two prey also altered choice such that the more forward directed prey was chosen even when apparently smaller. White crappie, when offered a choice between a diaptomid copepod or daphnid prey, chose the daphnid most of the time. Bluegill sunfish offered moving versus non-moving heat-killed daphnids commonly chose the one in motion. Apparent size choice is still a good overall describer of bluegill and white crappie prey choice, but it is not the only mechanism involved in prey choice behavior of these fish.     

Behavior of fish predators and their prey: habitat choice between open water and dense vegetation

Synopsis Behavior of largemouth bass, Micropterus salmoides, and northern pike, Esox lucius, foraging on fathead minnows, Pimephales promelas, or bluegills, Lepomis macrochirus, was quantified in pools with 50% cover (half the pool had artificial stems at a density of 1000 stems m⁻²). Both predators spent most of their time in the vegetation. Largemouth bass searched for bluegills and ambushed minnows, whereas the relatively immobile northern pike ambushed all prey. Minnows were closer to predators and were captured more frequently than bluegills. Even when minnows dispersed, they moved continually and eventually wandered within striking distance of a predator. Bluegills dispersed in the cover with predators. Bass captured the few bluegills that strayed into the open and pike captured those that approached too closely in the cover. The ability of predators to capture prey while residing in habitats containing patches of dense cover may explain their residence in areas often considered to be poor ones for foraging. The unit is sponsored jointly by the United States Fish and Wildlife Service, Ohio Department of NaturalResources, The Ohio State University, and the Wildlife Management Institute     

Assessing predation risk: optimal behaviour and rules of thumb

We look at a simple model in which an animal makes behavioural decisions over time in an environment in which all parameters are known to the animal except predation risk. In the model there is a trade-off between gaining information about predation risk and anti-predator behaviour. All predator attacks lead to death for the prey, so that the prey learns about predation risk by virtue of the fact that it is still alive. We show that it is not usually optimal to behave as if the current unbiased estimate of the predation risk is its true value. We consider two different ways to model reproduction; in the first scenario the animal reproduces throughout its life until it dies, and in the second scenario expected reproductive success depends on the level of energy reserves the animal has gained by some point in time. For both of these scenarios we find results on the form of the optimal strategy and give numerical examples which compare optimal behaviour with behaviour under simple rules of thumb. The numerical examples suggest that the value of the optimal strategy over the rules of thumb is greatest when there is little current information about predation risk, learning is not too costly in terms of predation, and it is energetically advantageous to learn about predation. We find that for the model and parameters investigated, a very simple rule of thumb such as 'use the best constant control' performs well.     

Mechanisms of prey selection by predaceous stoneflies: roles of prey morphology,behavior and predator hunger

Summary Laboratory feeding experiments using Hesperoperla pacifica (Banks), Perlidae, and Megarcys signata (Hagen), Perlodidae, as predators and Baetis tricaudatus Dodds and Ephemerella altana Allen as prey indicate a strong effect of prey morphology and mobility and predator hunger on prey selection by stoneflies. Knowledge of both dietary composition and feeding behavior was necessary to fully understand prey selection by these stoneflies.Fasted stoneflies presented with live prey ate more E. altana while satiated stoneflies ate approximately equal numbers of the two mayfly species. This pattern of dietary composition was the result of a reduction of attack frequency on the slower swimming E. altana with predator satiation and a continued high attack rate on B. tricaudatus regardless of recent feeding history. In contrast, fasted H. pacifica fed fresh frozen mayflies ate more B. tricaudatus indicating the importance of differences in prey mobility in controlling dietary composition.The high degree of similarity in patterns of feeding and mechanisms underlying those patterns for H. pacifica and M. signata suggest that they may be using similar rules for choosing mayfly prey and we suggest that mayfly prey are ranked by stoneflies on the basis of handling times. A general mechanistic model for stoneflies feeding on mayflies is presented and discussed.     

Relative significance of direct and indirect effects of predation by planktivorous fish on zooplankton

One of the most obvious features of tropical lakes and reservoirs is the small body size of their zooplankton taxa. It is believed that this is the result of high and persistent predation by abundant planktivorous fish, which select large-bodied zooplankton prey thus making them more vulnerable to extinction in tropical as compared to temperate habitats. Do these extinctions result directly from fish predation? Could the high predation-induced mortality alone be responsible for an extermination of the population from a habitat? Or could indirect effects of predation be responsible? Some important indirect effects can be seen at the demographic level; these include reduced reproduction in the population resulting from higher vulnerability of ovigerous females to predation by visually oriented planktivores. Other important indirect effects can be observed at the individual level; these include shifts in behavior (from foraging to predator avoidance) and adjustments in physiology (from high to low feeding rate) in those planktonic animals which detect danger from their predators by sensing either the ‘predator odor’ or an ‘alarm substance’ originating from injured conspecific prey. Although a zooplankton species density may mostly result from the brutal force of direct predator impact on the population (mortality), it is more likely that its distribution in time and space could be attributed to a combination of indirect effects of predation on individual behavior and physiology. An example of periodicity in density and depth distribution patterns of Cahora Bassa zooplankton species and their periodic exterminations seems to confirm the role of indirect effects of predation by planktivorous fish.     

Diel pattern of pelagic distribution and feeding in planktivorous fish

Summary The spatial distribution of juvenile roach (Rutilus rutilus), rudd (Scardinius erythrophtalamus), bream (Abramis brama) and bleak (Alburnus alburnus) was registered by echosounding during two years in small Bavarian lakes. The gut contents of the fish were analysed in order to reconstruct their rhythmicity of food intake. Fish were found in the pelagic zone only during the night, feeding before midnight with maximal rate. The vertical position of the fish was dependent on the water transparency following a light intensity in the order of 10^-3 Lux. The diel migration between littoral and pelagic zones was a mainly horizontal one with a vertical component. Both the speed of increase of fish density in the open water and of the vertical movement was correlated to the speed of change of light intensity. Shoals broke up into single fish during the offshore migration. The span of presence in the pelagic zone was identical with the duration of the dark period. During winter no diel migration nor periodical feeding took place.     

The effects of predation by planktivorous juvenile fish on the microbial food web

The feeding impact of planktivorous fish on microbial organisms is still poorly understood. We followed the seasonal dynamics of the food web in two natural fishponds for two years: one was stocked with planktivorous whitefish while the other had no planktivorous fish. The aim of the study was the simultaneous assessment of the feeding behaviours of planktivorous fish and of bacterivorous meta-/protozooplankters. We hypothesized that in the presence of planktivorous fish there would be fewer metazooplankton, more protozoans and decreased numbers of bacteria. Our results showed that the amount of metazooplankton eaten by the fish was indeed negatively correlated with metazooplankton biomass. The feeding impact of planktivorous fish in shaping the microbial loop was remarkable. The main grazers of bacteria in the fishpond were ciliates, whereas in the pond without fish these were heterotrophic nanoflagellates. In the fishless pond the role of the top predator shifted to the predaceous metazooplankter Leptodora kindtii which controlled the abundance of herbivorous metazooplankters. We found a negative relationship between the number of bacteria and flagellates in the fishless pond, while the number of bacterivorous ciliates was suppressed by predaceous ciliates. Therefore the bacteria-grazing activity was higher in the absence of planktivorous fish.     

Kleptoparasitic prey competition in shoaling fish: effects of familiarity and prey distribution

Familiarity is thought to stabilize dominance hierarchies andreduce aggressive interactions within groups of socially livinganimals. Though familiarity has been widely studied in shoalingfish, few studies have investigated changes in prey competitionas a function of time spent together within groups of initiallyunfamiliar individuals. In this study, we created shoals ofthree-spined stickleback (Gasterosteus aculeatus) and monitoredchanges in foraging rates and related competitive behaviorswithin shoals over a 4-week period in experimental series whereprey was spatially and temporally concentrated or dispersed.Prey share was unequal under both prey distribution modes, anddisparity in prey share was not seen to change as trials progressed.Interestingly, the contest rate for prey items fell over timewhen individuals were competing for dispersed prey but not whenprey were concentrated. We found no evidence that fish showedassociation preferences for either group members that had consumeda greater or lesser proportion of prey during trials. Thoughthe intensity of competition may be reduced by increased groupstability in nature, this is likely to be strongly dependenton the way prey resources are distributed through space andtime.