Prey capture and processing behaviors vary with prey size and shape in Australian and subantarctic fur seals

When hunting at sea, pinnipeds should adapt their foraging behaviors to suit the prey they are targeting. We performed captive feeding trials with two species of otariid seal, Australian fur seals (Arctocephalus pusillus doriferus) and subantarctic fur seals (Arctocephalus tropicalis). This allowed us to record detailed observations of how their foraging behaviors vary when presented with prey items that cover the full range of body shapes and sizes encountered in the wild. Small prey were captured using suction alone, while larger prey items were caught in the teeth using raptorial biting. Small fish and long skinny prey items could then be swallowed whole or processed by shaking, while all prey items with body depths greater than 7.5 cm were processed by shaking at the water's surface. This matched opportunistic observations of feeding in wild Australian fur seals. Use of “shake feeding” as the main prey processing tactic also matches predictions that this method would be one of the only tactics available to aquatic tetrapods that are unable to secure prey using their forelimbs.     

Prey selection by experienced and naive juvenile Atlantic salmon

Both in foraging groups and in a sequential prey encounter context, learning had a visible effect on the pattern of selection for three live prey types ( Ecdyonurus larvae, Hydropsyche larvae, and Gammarus ) by juvenile Atlantic salmon Salmo salar . Compared to wild-caught fish, naive, hatchery-reared fish that had not been exposed to natural prey ate Hydropsyche larvae in a remarkably low proportion, and consumed a higher proportion of Gammarus. Ecdyonurus experienced a high and rather steady predation rate across the experience gradient, but after a short period of experience with live prey the consumption rate for Hydropsyche increased drastically, and that of Gammarus decreased, matching the selection pattern exhibited by wild fish. Individual fish offered prey in a sequential encounter context increased consumption rates of all the prey types as they gained experience, but the improvement was higher for the prey that were less consumed initially. Fish became more selective as they approached satiation, conforming to the prediction of optimal foraging theory that higher predator's energy requirements, as well as low food availability, result in reduced selectivity. The results also suggest that fish from distinct populations can differ in the degree of diet selectivity according to their energetic requirements for growth. The fast learning response of Atlantic salmon parr towards novel prey probably allows fish to maintain a high foraging efficiency when faced with frequent changes in the availability of different prey types.     

School size affects individual feeding success in three-spined sticklebacks (Gastevosteus aculeatus L.)

The effect of school size on the feeding success of individual three-spined sticklebacks was studied. We found that the proportion of fish feeding on benthic prey increased with school size and that fish in large schools tended to start feeding sooner than fish in small schools. The total number of strikes also increased in larger schools. Despite this evidence for a foraging benefit associated with school membership we propose that improved feeding returns do not alone explain stickleback schooling.     

Size-specific protection against predation by fish in swarming waterfleas,Bosmina longispina

When feeding on non-swarming waterflea prey, sticklebacks, Gasterosteus aculeatus, selected larger items. However, when feeding on swarming prey, the fish fed mainly at the edges of the swarms and smaller items were consumed. This selection of small prey items can be explained by the fact that when attacked by a stickleback the waterfleas escaped towards the centre of the swarm, larger individuals more quickly than smaller ones, thus leaving the stickleback with smaller than average prey at the edge of the swarm.     

Experience affects performance of ten-spined sticklebacks foraging on zooplankton

To study the effects of short-term experience on prey size-selection ten-spined sticklebacks (Pungitius pungitius) were fed 7–13 days with five differing diets of novel prey, Daphnia magna. The diets consisted either of a mixture of two prey size classes (1.7 and 2.2 mm) or of single-sized (1.7 and 2.2 mm) prey. Before and after the diets, the sticklebacks' prey size selection was tested with a 1:1 ratio of 1.7 and 2.2 mm D. magna. Sticklebacks made more attempts to capture large than small prey, but their foraging success was better for small than for large prey. Sticklebacks fed with a diet of both prey sizes chose significantly more large prey on the 13th day than on the 7th day or at the beginning of the experiment. Handling times for both Daphnia size classes decreased slightly with increasing foraging experience. Inexperienced sticklebacks made more unsuccessful strikes on large prey than did experienced fish. Foraging success on large prey improved somewhat with increasing experience in all but one diet group. The results indicate that experience affected ten-spined sticklebacks' prey selection.     

Artificial dietary regime may impair subsequent foraging behaviour of hatchery-reared turbot released into the natural environment

When offered a size range of shrimp Crangon crangon , fewer naïve reared turbot Scophthalmus maximus fed than did wild fish, and those that fed took smaller prey. Analysis of feeding behaviour indicated differences between wild and naïve reared fish in the motivation to feed on novel prey and in prey recognition and capture efficiency. Feeding efficiency and motivation increased with experience and reared fish achieved the feeding rate of wild fish within the 9 days of the experiment. The 'creep' style of approach to shrimp was innate to naïve reared turbot. A comparative feeding experiment confirmed that experienced reared fish ate more prey than naïve reared fish. Naïve reared turbot selected pellets and attacked stones preferentially to shrimp, in contrast to wild and experienced reared fish, which selected shrimp. Stones were attacked due to the memory of pellet-like visual characteristics and this behaviour persisted in some reared fish for at least 6 weeks, illustrating a cost of memory in which changing environmental conditions cause previously learnt information to become misleading. The experimental data also demonstrated that differences may arise in the prey and feeding behaviour of wild and naïve reared fish, but that such differences diminish with experience.     

Experience and social environment influence the ability of young brown trout to forage on live novel prey

Efficient feeding is crucial for the growth, survival and reproductive success of most animals. In artificial-rearing environments, however, animals are deprived of many stimuli normally experienced in the wild, which may alter feeding behaviour, and thus influence their survival and reproductive success upon release in nature. In a laboratory experiment, we investigated the effect of hatchery rearing on the ability of brown trout, Salmo trutta, to capture and consume a novel live prey item. Hatchery-reared and wild-caught trout, originating from the same river, were fed single black crickets, either in isolation or in visual and olfactory contact with another hatchery-reared or wild-caught fish. Total consumption, time to first bite and feeding efficiency were monitored. Wild-caught trout ate more, were quicker to attack, and consumed attacked prey more efficiently than hatchery-reared fish. Food consumption and efficiency increased in both wild and hatchery-reared trout during the experiment. We propose that the differences in feeding ability between wild-caught and hatchery-reared brown trout were mainly due to differences in previous experience of feeding on live prey. Wild-caught trout tended to eat more and sooner when in visual contact with another fish than when in isolation. This trend was not seen for the hatchery-reared fish, which may be due to environmental differences between the hatchery and the natural stream. The initial inability of hatchery-reared fish to forage on live prey may reduce their success when released in the wild, especially when in competition with resident wild fish. Copyright 2001 The Association for the Study of Animal Behaviour.     

Seasonal change in the gastric evacuation rate of rainbow trout feeding on natural prey

Seasonal changes in the gastric evacuation rate ( R ) and gut contents of a wild population of rainbow trout Oncorhynchus mykiss feeding on natural prey at four water temperatures (2, 7, 9 and 12° C) were measured. The R and mass of the gut contents increased with water temperature, and prey items changed seasonally. These results suggest that the R of fish feeding on natural food depends primarily on water temperature, with their consumed prey being a secondary factor.     

Feeding Experience Modifies the Assessment of Ambush Sites by the Timber Rattlesnake, a Sit-and-Wait Predator

To effectively ambush prey, sit‐and‐wait predators must locate sites where profitable prey are likely to return. One means by which predators evaluate potential ambush sites is by recognizing high‐use areas through chemical cues deposited inadvertently by their prey. However, it is unknown whether ambush predators can use chemical cues associated with past prey items in the assessment of potential ambush sites. I examined selection of ambush sites by timber rattlesnakes (Crotalus horridus) exposed to trails made from chemical extracts of the integument of various prey species. I evaluated the role of feeding experience in ambush site selection by comparing the behavior of timber rattlesnakes before and after feeding experience with different sized prey items. Timber rattlesnakes are more likely to select ambush sites adjacent to chemical trails from prey with which they have had feeding experience, but only those fed relatively large prey showed an increase in responsiveness. Increased responsiveness after feeding experience was exhibited in experiments using integumentary extracts of mammals (the natural prey of timber rattlesnakes), but not in those using extracts of fish. These results indicate that ambush predators may learn to recognize chemicals on the integument of profitable food items, and use that experience when subsequently selecting ambush sites. Additionally, these findings provide evidence that size‐dependent predation by snakes may be, in some species, a result of active prey selection.     

Feeding and Refuge Use by Small Fish in the Presence of Cyanobacteria Blooms

We studied the effect of cyanobacteria on foraging and refuge use in small fish. We measured pike larval feeding in the presence of cyanobacteria by counting leftover prey. Our results showed that feeding by pike larvae on zooplankton prey decreased significantly in the presence of non-toxic cyanobacteria. The behaviour can be due to lowered vision caused by turbidity or clogging of the gills. Further, we tested whether the three-spined stickleback use toxic cyanobacteria as a refuge against predators in a choice experiment. The choice experiment was performed in a Y-maze fluviarum, where the fish could select between two different environments. Our results support the refuge use hypothesis because the three-spined stickleback clearly preferred toxic cyanobacteria to the chemical predator signal. To conclude, cyanobacteria decrease feeding rates in fish larvae, but may function as important refuge for e.g. sticklebacks, during predation pressure in pelagic algal blooms.     

Mechanisms of drift-feeding behavior in juvenile Chinook salmon and the role of inedible debris in a clear-water Alaskan stream

Drift-feeding fish are challenged to discriminate between prey and similar-sized particles of debris, which are ubiquitous even in clear-water streams. Spending time and energy pursuing debris mistaken as prey could affect fish growth and the fitness potential of different foraging strategies. Our goal was to determine the extent to which debris influences drift-feeding fish in clear water under low-flow conditions when the distracting effect of debris should be at a minimum. We used high-definition video to measure the reactions of drift-feeding juvenile Chinook salmon (Oncorhynchus tshawytscha) to natural debris and prey in situ in the Chena River, Alaska. Among all potential food items fish pursued, 52 % were captured and quickly expelled from the mouth, 39 % were visually inspected but not captured, and only 9 % were ingested. Foraging attempt rate was only moderately correlated with ingestion rate (Kendall’s τ?=?0.55), raising concerns about the common use of foraging attempts as a presumed index of foraging success. The total time fish spent handling debris increased linearly with foraging attempt rate and ranged between 4 and 25 % of total foraging time among observed groups. Our results help motivate a revised theoretical view of drift feeding that emphasizes prey detection and discrimination, incorporating ideas from signal detection theory and the study of visual attention in cognitive ecology. We discuss how these ideas could lead to better explanations and predictions of the spatial behavior, prey selection, and energy intake of drift-feeding fish.     

The costs of becoming nocturnal: feeding efficiency in relation to light intensity in juvenile Atlantic Salmon

1. Most animals are active by day or by night, but not both; juvenile salmonids are unusual in that they switch from being predominantly diurnal for most of the year to being nocturnal in winter. They are visual foragers, and adaptations for high visual acuity at daytime light intensities are generally incompatible with sensitive night vision. Here we test whether juvenile Atlantic Salmon Salmo salar are able to maintain their efficiency of prey capture when switching between diurnal and nocturnal foraging.
2. By testing the ability of the fish to acquire drifting food items under a range of manipulated light intensities, we show that the foraging efficiency of juvenile salmon is high at light intensities down to those equivalent to dawn or dusk, but drops markedly at lower levels of illumination: even under the best night condition (full moon and clear sky), the feeding efficiency is only 35% of their diurnal efficiency, and fish will usually be feeding at less than 10% (whenever the moon is not full, skies are overcast or when in the shade of bankside trees). Fish were unable to feed on drifting prey when in complete darkness.
3. The ability of juvenile salmon to detect prey under different light intensities is similar to that of other planktivorous or drift-feeding species of fish; they thus appear to have no special adaptations for nocturnal foraging.
4. While winter drift abundance is slightly higher by night than by day, the difference is not enough to compensate for the loss in foraging efficiency. We suggest that juvenile salmon can nonetheless switch to nocturnal foraging in winter because their food requirements are low, many individuals adopting a strategy in which intake is suppressed to the minimum that ensures survival.     

Optimal foraging of sticklebacks on swarming prey

Hungry sticklebacks, Gasterosteus aculeatus, preferentially attacked the densest region of a swarm of water fleas, but with decreasing attack readiness they increasingly preferred less dense regions. Such a hunger dependent change in feeding preference has not yet been dealt with by optimal foraging theory. A model, which assumes that high swarm densities provide high feeding rates (because of small inter-prey distances) but also high costs of confusion, predicts that a predator should always choose the lowest prey density in which it can achieve a feeding rate sufficient to satisfy its hunger. Some predictions of the model were experimentally verified. Hungry fish have a higher feeding rate in a high prey density than in a lower density and less hungry fish have a higher rate in a low density than in a high density.     

Schistocephalus infestation improves prey-size selection by three-spined sticklebacks, Gasterosteus aculeatus

Schistocephalus -infected sticklebacks ate slightly less but fed more selectively on larger prey than non-infested fish. It follows that, in the short-term, the net energy gain in foraging is larger for infested fish. This matches the expectation that infested sticklebacks compensate energy losses due to Schistocephalus.     

Feeding of wild and hatchery reared Atlantic salmon (<Emphasis Type="Italic">Salmo salar</Emphasis> L.) smolts during downstream migration

In general, hatchery salmonid smolts experience higher mortality during migration than wild smolts, which is suggested to be due to domestication effects and that hatchery fish lack experience of the natural environment. However, possible differences in feeding during smolt migration between hatchery and wild smolts have rarely been addressed. We compared the number of feeding smolts and stomach fullness among wild Atlantic salmon smolts, hatchery-reared smolts released as 1-year-old parr, and hatchery-reared smolts released as 2-year-old smolts during their descent to sea in River Tornionjoki. In addition, estimations of prey selection among the smolt groups were conducted. A high proportion of wild smolts and smolts stocked as parr actively fed during the smolt migration. A lower proportion of smolts stocked as smolts was feeding and their stomach fullness were much reduced in comparison with the two other groups. The study also indicated that the feeding of migrating smolts is selective rather than opportunistic. In conclusion, this study suggests that stocked 2-year-old smolts may enter sea with an inferior foraging behaviour and it is a possibility that this may contribute to the observed low post-smolt survival in the Baltic Sea.     

Behavioral context of echolocation and prey-handling sounds produced by killer whales (Orcinus orca) during pursuit and capture of Pacific salmon (Oncorhynchus spp.)

Availability of preferred salmonid prey and a sufficiently quiet acoustic environment in which to forage are critical to the survival of resident killer whales (Orcinus orca) in the northeastern Pacific. Although piscivorous killer whales rely on echolocation to locate and track prey, the relationship between echolocation, movement, and prey capture during foraging by wild individuals is poorly understood. We used acoustic biologging tags to relate echolocation behavior to prey pursuit and capture during successful feeding dives by fish-eating killer whales in coastal British Columbia, Canada. The significantly higher incidence and rate of echolocation prior to fish captures compared to afterward confirms its importance in prey detection and tracking. Extremely rapid click sequences (buzzes) were produced before or concurrent with captures of salmon at depths typically exceeding 50 m, and were likely used by killer whales for close-range prey targeting, as in other odontocetes. Distinctive crunching and tearing sounds indicative of prey-handling behavior occurred at relatively shallow depths following fish captures, matching concurrent observations that whales surfaced with fish prior to consumption and often shared prey. Buzzes and prey-handling sounds are potentially useful acoustic signals for estimating foraging efficiency and determining if resident killer whales are meeting their energetic requirements.     

The influence of turbidity on three-spined stickleback foraging

Three-spined sticklebacks Gasterosteus aculeatus were used as a model to test for the effects of turbidity on visual-predator foraging. While the number of feeding lunges and number of prey consumed did not change with increasing turbidity, reactive distance to prey decreased.     

Effects of habitat enrichment and food availability on the foraging behaviour of juvenile Atlantic Cod (<Emphasis Type="Italic">Gadus morhua L</Emphasis>)

The environment can play an important role in shaping how an animal behaves, and how well the animal performs in a particular environment can be influenced by early experiences. The tradition of releasing captive-reared juveniles into the wild in an effort to strengthen wild fish populations has often had little success owing to high post-release mortality. Fish reared under standard hatchery conditions are provided with fewer stimuli and they receive excess quantities of pellet food that are easy to handle and consume. Captive reared fish therefore appear to be under-stimulated and overfed. Several studies have demonstrated that simple structural enrichment in the rearing facilities promotes flexible behaviour compared to fish reared in plain, standard hatchery tanks. Less attention has been given to the effects of the diet. Here we use a cross-factored design to test the relative role of food ration and spatial enrichment on foraging behaviour. Our results show that fish from enriched environments, regardless of previous food-ration size, were more reluctant to start feeding on the first day in a novel arena. On day two and three, however, fish with prior experience of a low food ration showed greater foraging activity and efficiency than fish fed on full rations. On the second and third day, prior experience with enrichment was less important. We discuss how early feeding experience in combination with structural enrichment may contribute in producing fish that are better suited for release into the wild.     

Prey capture and selection throughout the breeding season in a deep‐diving generalist seabird,the thick‐billed murre

Generalist seabirds forage on a variety of prey items providing the opportunity to monitor diverse aquatic fauna simultaneously. For example, the coupling of prey consumption rates and movement patterns of generalist seabirds might be used to create three‐dimensional prey distribution maps (‘preyscapes’) for multiple prey species in the same region. However, the complex interaction between generalist seabird foraging behaviour and the various prey types clouds the interpretation of such preyscapes, and the mechanisms underlying prey selection need to be understood before such an application can be realized. Central place foraging theory provides a theoretical model for understanding such selectivity by predicting that larger prey items should be 1) selected farther from the colony and 2) for chick‐feeding compared with self‐feeding, but these predictions remain untested on most seabird species. Furthermore, rarely do we know how foraging features such as handling time, capture methods or choice of foraging location varies among prey types. We used three types of animal‐borne biologgers (camera loggers, GPS and depth‐loggers) to examine how a generalist Arctic seabird, the thick‐billed murre Uria lomvia, selects and captures their prey throughout the breeding season. Murres captured small prey at all phases of a dive, including while descending and ascending, but captured large fish mostly while ascending, with considerably longer handling times. Birds captured larger prey and dove deeper during chick‐rearing. As central place foraging theory predicted, birds travelling further also brought bigger prey items for their chick. The location of a dive (distance from colony and distance to shore) best explained which prey type was the most likely to get caught in a dive, and we created a preyscape surrounding our study colony. We discuss how these findings might aid the use of generalist seabirds as bioindicators.     

Changing handling times during feeding and consequences for prey size selection of 0+ zooplanktivorous fish

The interrelationship of fish size, prey size and handling time within a 15-min feeding period was studied in three size groups of 0 + roach, Rutilus rutilus, and bleak, Alburnus alburnus. Four size classes of cladoceran prey were used to measure changes in feeding rate and handling time from initial rapid feeding to sustained feeding. Observed differences in increase of handling time between prey size classes led to a change in the prey profitability ranking of those size classes within the first 2 min of the experiments. A 1-min feeding period is interpreted as reflecting an intermediate motivational status between extreme hunger and satiation. The use of average handling times for this period revealed a substantial change in prey profitability estimates compared to previous studies which used handling times based on short-term (a few seconds up to 1 min) feeding. It is not the largest prey items a fish can handle and swallow that are most profitable, but prey of intermediate size. By this approach a closer fit between expectations derived from optimal foraging theory and empirical data on prey size selection of 0 + zooplanktivorous fish is qualitatively achieved. Optimal prey size was found to be close the mouth gape width in small fish of 15 mm standard length, decreasing to 50% of mouth gape width in fish of 40 mm standard length.