Morphological limitations, prey size selectivity, and growth response of juvenile atlantic salmon, Salmo salar

The head and jaw movements involved in capture, buccal manipulation, ingestion and rejection of prey were investigated using sequential photography of juvenile Atlantic salmon feeding in a simulated stream environment. The results are described and discussed and mouth breadth and gill raker spacing are proposed as morphometric limitations to the range of prey sizes available which remains constant at 0·06 · fish fork length ( PFR ).
A recirculatory flume tank was used to study prey size selectivity behavior. Simplified downstream-drifting prey items elicited a variety of responses depending on their physical size. One hundred percent of offered prey of PFR 0·025 were ingested, while 90 % of prey at PFR 0·051 and 100% of prey at PFR 0·105 were rejected. It is demonstrated that fish show negative selection for prey sizes smaller than PFR 0·025 and that prey of this size elicits maximum growth response.
The validity of the proposed morphometric limitations on the available prey sizes is demonstrated by reference to selectivity behaviour and prey size related differential growth.     

The role of food particle size in the growth of juvenile Atlantic salmon (Salmo salar L.)

The effect of particle size of a commercial pelleted feed on the growth of Atlantic salmon from first feeding alevins to first and second year smolts was investigated using 20-day feeding experiments. Ten experiments were performed over a two year period, each comprising six groups of fish separately selected from stock populations. Each group was presented with one of six sizes of food particle ranging from larger than the respective mean mouth breadth (100% feed size), through 50%, 25%, 12·5%, 6·25% to 3·125%. Experiments were performed in six radial flow/circumferential drain tanks under ambient photoperiod and water temperature. Growth rate was found to be closely related to feed size. Maximum growth in each case was shown only on one size of particle; larger and smaller sizes resulted in reduced growth. The particle size for maximum growth increased in direct proportion to fish length. Fish from 4·2 to 20·3 cm in length showed maximum growth on particle diameters 0·022 to 0·026 × fish fork length (PFR). First feeding alevins were found to show comparable growth rate on particle diameters 0·0115 to 0·090 PFR. Some seasonal variation in growth response was indicated. The results are discussed in relation to developmental and seasonal effects. Possible factors affecting the energetics of prey size related differential growth are discussed.     

Feeding performance of the Hawaiian sleeper,Eleotris sandwicensis (Gobioidei: Eleotridae): correlations between predatory functional modulation and selection pressures on prey

The eleotrid fish Eleotris sandwicensis inhabits lower reaches of streams in the Hawaiian Archipelago, where it feeds on juveniles of native amphidromous gobiid fishes migrating upstream from the ocean. Using high‐speed video and geometric modelling, we evaluated the feeding kinematics and performance of E. sandwicensis on free swimming prey, including two species with juveniles of different characteristic sizes, and compared successful and unsuccessful strikes. With fast jaw movements and a highly expansive buccal cavity, E. sandwicensis achieves high suction performance that enables the capture of elusive prey. Our analyses indicated that the species with larger juveniles (Sicyopterus stimpsoni) could be captured from a distance of up to 18.6% of the predator's body length (BL), but capture of the smaller species (Awaous guamensis) required a closer distance (12.2% BL). Predator–prey distance appears to be the predominant factor determining strike outcome during feeding on juvenile A. guamensis. However, during feeding on juvenile S. stimpsoni, E. sandwicensis shows modulations of strike behaviour that correlate with capture success. Moreover, the ability of E. sandwicensis to capture larger prey fish from longer distances suggests a potential biomechanical basis underlying observations that predation by eleotrids imposes significant selection against large body size in juvenile gobies. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 359–374.     

Temporal variation in chum salmon, Oncorhynchus keta, diets in the central Bering Sea in summer and early autumn

Seasonal, ontogenetic, and diel variations in the diets of chum salmon, Oncorhynchus keta, were examined by analyzing the stomach contents of 1398 fish (300–755 mm fork length) collected in the Bering Sea during summer and early autumn of 2002. Whereas mesozooplankton, including euphausiids, hyperiids, and gastropods, constituted the greatest portion of the stomach contents during the summer, forage fishes (Stenobrachius leucopsarus and Atka mackerel, Pleurogrammus monopterygius) were the most important items during early autumn. Although no apparent diel trend was found in feeding intensity, distinct diel differences in prey composition were observed. Chum salmon caught in the morning contained Stenobrachius leucopsarus, whereas those caught in the afternoon had mainly fed on euphausiids. Thus, chum salmon diets change temporally because of changes in prey availability that result from differences in the annual life cycles and diurnal vertical migrations of prey species.     

Variation in the diet of the viperine snake Natvix mama in relation to prey availability

The diet of the viperine snake was compared with food availability in the Ebro Delta, a wetland largely occupied by rice fields, in 1990 and 1991. Snake selection of prey type and size was studied seasonally and by snake group: males, females and immature snakes. Overall, feeding activity (percentage of individuals with prey and number of prey per stomach) increased with food availability. Diet analysis showed that viperine snakes mainly foraged on the green frog Rana perezi (adults and tadpoles) and the carp Cyprinus earpio. Conversely, viperine snakes rejected the mosquito fish Gambusia holbroki which is the most abundant species in autumn, when Natrix maura has a low feeding activity. Statistical comparisons between viperine snake diet and prey availability showed that males selected small carp, immature snakes selected tadpoles and, in spring, females selected frogs. The selection of small carp by males may reflect a sexual divergence of trophic niche related to sexual size dimorphism, as females are larger than males. As tadpoles are presumably easier to catch than fish, tadpole selection by immature individuals may reflect variance in capture abilities. In spring, the selection of frogs by females overlapped with vitellogenesis, suggesting that females compensate for the cost of reproduction by selecting green frogs, which have a greater biomass and higher energy content than fish. Carps eaten in spring were smaller than in summer. Moreover, in summer viperine snakes selected smaller carp than the available mean size. This divergent tendency between carp size selection and carp size availability reveals how seasonal diet shifts in prey size selection may be a response to an increase in prey size.     

The diet of blue whiting, hake, horse mackerel and mackerel off Portugal

This paper deals with the diets of blue whiting Micromesistius poutassou (Risso 1810), hake Merluccius merluccius (L. 1758), horse mackerel Trachurus trachurus (L. 1758), and mackerel Scomber scombrus (L. 1758) off Portugal and explores variations in fish length, water depth, latitude and season. All four species feed on fish; however, hake and mackerel are the first and second most important predators, respectively, blue whiting being the most important fish prey for both species. The diets of blue whiting and horse mackerel are composed mainly of crustaceans. Diet variations according to predator fish size are more important than either latitude or depth. In the diets of blue whiting, hake and horse mackerel, prey importance increases with predator size. For blue whiting and horse mackerel, diet variations with fish length and water depth are correlated: small fish are closely associated with coastal areas where they feed on copepods and decapod larvae. Seasonality in the diet is apparent for blue whiting, hake and mackerel. For blue whiting, the decapod Pasiphaea sivado is the most important prey in summer and autumn, being replaced by the euphausid Meganyctiphanes norvegica in winter. In the diet of hake, seasonality was characterised by the major importance of Macroramphosus scolopax in autumn, whereas the diet of mackerel consisted of zooplankton in summer, fish and decapods in autumn and decapod larvae in winter. Seasonal changes in the diet of horse mackerel correspond to a higher diversity of prey in autumn compared to other seasons (although euphausids are the main prey in all seasons). Seasonality in feeding activity is not as marked for the other species as it is for horse mackerel; the percentage of empty stomachs of horse mackerel is greatest in winter, when spawning takes place at the Portuguese coast.     

The influence of prey size and abundance, and individual phenotype on prey choice by the three-spined stickleback, Gasterosteus aculeatm L.

Prey selection behaviour of three-spined sticklebacks, Gasterosteus aculeatus L., was studied in two experiments. Where possible, the experimental apparatus satisfied the assumptions of the simplest optimal diet model (the basic prey model); prey were presented sequentially, the fish could not search for and handle prey at the same time, and net energy gain, handling time and encounter rate were fixed. Experiment 1 presented fish with a range of Asellus sizes so that pursuit ( p ) and handling ( h ) time could be related to prey size. Published energy values of Asellus together with pursuit and handling times were used to calculate E /( p+h ) for Asellus measuring 3,4,5,6,7 and 9 mm. Pursuit times did not differ with prey size but handling times did. E /( p+h ) was very variable particularly at the larger prey sizes. Experiment 2 presented fish with two sequences of prey differing in the encounter rate with the most profitable prey sizes. Fish did not select the diet predicted by the basic prey model tending to always ignore the largest prey even when net energy gain would have been maximized by including them in the diet. Further analysis showed that the probability of a prey size being taken was a function of prey size, fish stomach fullness and encounter rate. It is concluded that the basic prey model is too simple to capture the behaviour of the fish. One of its main faults is that the changing state of the fish through the feeding bout is ignored.     

Ontogeny of Acoustic and Feeding Behaviour in the Grey Gurnard, Eutrigla gurnardus

Although sound production in teleost fish is often associated with territorial behaviour, little is known of fish acoustic behaviour in other agonistic contexts such as competitive feeding and how it changes during ontogeny. The grey gurnard, Eutrigla gurnardus, frequently emits knock and grunt sounds during competitive feeding and seems to adopt both contest and scramble tactics under defensible resource conditions. Here we examine, for the first time, the effect of fish size on sound production and agonistic behaviour during competitive feeding. We have made sound (alone) and video (synchronized image and sound) recordings of grey gurnards during competitive feeding interactions. Experimental fish ranged from small juveniles to large adults and were grouped in four size classes: 10–15, 15–20, 25–30 and 30–40 cm in total length. We show that, in this species, both sound production and feeding behaviour change with fish size. Sound production rate decreased in larger fish. Sound duration, pulse duration and the number of pulses increased whereas the peak frequency decreased with fish size, in both sound types (knocks and grunts). Interaction rate and the frequency of agonistic behaviour decreased with increasing fish size during competitive feeding sessions. The proportion of feeding interactions accompanied by sound production was similar in all size classes. However, the proportion of interactions accompanied by knocks (less aggressive sounds) and by grunts (more aggressive) increased and decreased with fish size, respectively. Taken together, these results suggest that smaller grey gurnards compete for food by contest tactics whereas larger specimens predominantly scramble for food, probably because body size gives an advantage in locating, capturing and handling prey. We further suggest that sounds emitted during feeding may potentially give information on the motivation and ability of the individual to compete for food resources.     

Size-scaling of zooplankton foraging in Arctic charr

Prey capture rate (number of prey s⁻¹) and the mode of feeding of Arctic charr Salvelinus alpinus were studied by performing foraging experiments with two sizes (1·1 and 1·8 mm) of Daphnia longispina prey. Arctic charr were particulate feeders at all densities tested. Adjusted for the effect of prey density, the capture rate showed a hump-shaped relationship with Arctic charr size for both sizes of D. longispina . Estimated attack rates ( a ) also tended to show a hump-shaped relationship with fish size. The estimated size-scaling exponent of the attack rate function, however, was relatively small, implying small changes in attack rate over fish sizes. Simultaneous estimations of a and handling time were used in combination with published data on fish metabolism and dry mass rations of prey to estimate maintenance resource density of prey as a function of Arctic charr mass. Maintenance resource densities increased monotonically with Arctic charr size, and rapidly as optimum fish size relative to attack rate on prey was passed.     

The Feeding Behaviour of a Sit and Wait Predator Ranatra dispar (Heteroptera: Nepidae): The Effect of Prey Density and Age Structure on the Number of Prey Eaten

Functional response experiments were performed in the laboratory to examine the effect of prey density (as observed in the field) on feeding behaviour, and to measure handling-times and attack-rates for each instar and adult of Ranatra dispar Montandon (Heteroptera: Nepidae) feeding on five size-classes of its common prey, Anisops deanei Brooks (Heteroptera: Notonectidae). The most generally applicable response was the Type 2, although for both the predator fifth instar and adult female and male feeding on the two smallest prey sizes, the asymptote or plateau was not observed even at the highest prey density given. Generally, the handling-time increased as prey-size increased, and decreased as the predator size increased. The attack-rate surface was far more complex. For the first two predator instars (I and II), the maximum attack-rate occurred on the smallest prey sizes (1 and 2). The maximum attack-rate for predator instar III was almost the same for prey sizes 1 and 2, that of predator instar IV was greater for prey size 2, while in the three largest predator sizes (V, female and male), the maximum attack-rate was found for prey size 3. Predator instar V had the largest attack-rate values over all prey sizes, and both the predator adult female and male had lower attack-rates for various prey sizes than instars V, IV and, to some degree, III. The results support the suggestion that small predator instars will usually compete with large instars for prey, unless they are spatially or temporally separated. Observations in the field indicate that a distinct age-specific spatial distribution exists in R. dispar and the prey, A. deanei, with the smallest individuals being found predominantly in the shallow (littoral zone) water, while the larger individuals are found in the deeper water.     

Foraging behaviour and feeding success of the black-necked stork (Ephippiorhynchus asiaticus) in Dudwa National Park, Uttar Pradesh, India

Foraging behaviour of the black-necked stork (BNS) Ephippiorhynchus asiaticus was studied in 1996 and 1997 in Dudwa National Park (DNP), Uttar Pradesh, India. The storks were observed using tactile and visual techniques to catch fish. Of the 929 fish seen caught in 2 years, 894 (96%) were caught by a tactile mode of feeding and the remaining 35 (4%) by a visual mode of feeding. The rate of foraging attempts by BNS fluctuates with that of season in DNP and coincides with prey abundance. Immediately after the monsoon when the water level was high, BNS had to search for prey more often, as the prey became widespread. Whereas in summer when the water level decreased, the concentration of the fish was higher, which helps BNS to catch fish in quick succession. Prey behaviour and the condition of the wetland determined the selection of the tactile foraging technique among the BNS in DNP. The black-necked storks were more successful in the early hours of the day (06:00–10:00) and they were generally more successful or preferred to feed on medium-sized fish (i.e. 5–10 cm) in DNP. Prey profitability was highest for larger size fish and decreased as the prey size decreased. Principal component analysis showed that prey size, handling time of each prey, the month and water depth determined foraging success in 1996 and 1997.     

Prey orientation in piscivorous brown trout

Piscivorous brown trout Salmo trutta change their feeding behaviour depending on prey species, prey size and number of prey eaten. In trout which had eaten fish recently, most had one fish in their stomach, but up to 16 prey fish were found. Individuals of the small-sized minnow Phoxinus phoxinus were swallowed chiefly tail first, whereas individuals of the larger Arctic charr Salvelinus alpinus were taken both head and tail first. The largest charr were swallowed head first. In stomachs containing more than one fish prey, prey orientation was likely to be mixed. For all three types of prey orientation (i.e. tail first, head first and mixed), significant and positive correlations existed between prey length and predator length. The maximum prey size eaten tail first or in mixed orientation was about 70–85% of the size of prey eaten head first, indicating morphological advantages in eating the prey head first.     

Diet of farm‐escaped sturgeon in the Yangtze River

The diet of farm‐escaped sturgeon accidentally introduced into the Yangtze River ecosystem was investigated from samples collected over 10 days. The fish had consumed adequate quantities of natural food, with prey type related to fish body size. The fish were categorized according to their primary dietary component: demersal fish were most common prey of kaluga Huso dauricus and H. dauricus × Acipenser schrenckii hybrids of total length 120.3–188.7 cm. Shrimp Macrobrachium nipponensis made up the major portion of the diet of H. dauricus × A. schrenckii and Acipenser baerii × A. schrenckii of total length 91.1–106.8 cm. Gammarid amphipods were consumed by A. baerii × A. schrenckii of total length 35.0–81.2 cm. The body length of prey was proportional to the fork length of the sturgeon, Y = 0.19X‐9.46, R² = 0.997. Escaped sturgeon had travelled at least 898 km downstream from the point of escape in the Yangtze River and were feeding on native macrobenthos and demersal fish.     

Feeding habits of Schizothorax oconnori Lloyd, 1908 in the Yarlung Zangbo River,Tibet

Seasonal, ontogenetic and sexual feeding habits of Schizothorax oconnori in the Yarlung Zangbo River were studied using percentages by number, weight and index of relative importance (IRI). The Schoener overlap index was applied to compare diets related to season, fish size and sex. The feeding intensity exhibited seasonal trends, with a minimum food intake in summer. Overall, 103 prey taxa belonging to seven prey categories (diatoms, green algae, cyanophytes, other algae, small invertebrates, macroinvertebrates and remains) were identified in 136 guts. Highest feeding diversity and evenness were recorded in summer, while lowest values of both indices occurred in autumn. S. oconnori fed almost exclusively on periphyton. Diatoms were the most important prey in terms of number, weight and IRI, followed by macroinvertebrates; other prey were also occasionally ingested. S. oconnori is a generalized and opportunistic feeder relying upon a wide trophic spectrum, but mainly feeds on diatoms as they are the most prevalent periphyton. There were high diet overlaps among seasons, fish sizes and between sexes. S. oconnori showed no dietary preference for any of the other prey available in the periphyton community.     

Prey selection by age-0 walleye pollock, Theragra chalcogramma, in nearshore waters of the Gulf of Alaska

Juvenile walleye pollock, Theragra chalcogramma, is the dominant forage fish on the continental shelf of the Gulf of Alaska, yet little is known about the feeding habits of this important interval of pollock life history. The taxonomic composition and size of prey found in the stomachs of age-0 juveniles collected at three nearshore locations in the Gulf of Alaska in September 1990 were compared to the composition and size of zooplankton collected in concurrent plankton tows. The maximum length of prey consumed increased dramatically over the length range of pollock examined (58–110 mm) from approximately 7 mm to 30 mm, due mainly to the consumption of large euphausiids and chaetognaths by the bigger individuals. The maximum width of prey changed little over this size range although there was a general increase in prey width with increasing predator size. The minimum prey length and width did not change with increasing fish size. Juvenile pollock generally selected the larger prey sizes relative to what was available. Juvenile pollock showed a marked preference for adult euphausiids and decapod larvae and an avoidance of copepods and chaetognaths relative to the numbers collected in net tows. These results are discussed relative to the feeding ecology of these juvenile fishes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of Age and Size on the Response to Novel Prey by Fry of the Cichlid Fish Cichlasoma managuense (Pisces: Cichlidae)

The response to a novel prey item was investigated during the first month of feeding of the cichlid fish Cichlasoma managuense. The relative contribution of size and age to improvement in predatory behavior was addressed. Group I (control) was fed nauplii of Artemia salina and group II a manufactured flakefood diet. Group II fish were tested for their ability to prey on a novel diet, the nauplii of Artemia salina. Latency to respond to the presence of novel prey decreased and the number of capture attempts increased with increasing experience with the artificial diet and with age. As size increased so did the number of capture attempts, but the latency did not change. Size and experiential and maturational factors may affect parts of the predatory behavior differentially. During the first month of feeding, age may be more important than size for the decrease in latency and the increase in the number of capture attempts. The number of capture attempts during the first 30 s of the observation period and the capture success increased faster than the latency decreased. Latency to respond to novel prey may mature at a slower rate than the number of capture attempts.     

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.     

Modulation in Feeding Kinematics and Motor Pattern of the Nurse Shark <Emphasis Type="Italic">Ginglymostoma cirratum</Emphasis>

Synopsis Studies of feeding in bony fishes have almost universally demonstrated the ability of individuals to modulate their method of capture in response to differing stimuli. Preliminary evidence indicates that morphologically specialized inertial suction feeding sharks are the most likely fishes to lack inherent modulatory ability. We examined the ability of the nurse shark, Ginglymostoma cirratum, to modulate its feeding behavior based on different food types and sizes. G. cirratum is an inertial suction feeding fish that is apparently stereotyped in its food capture behavior. Electromyography showed no statistical difference between feeding motor patterns based on food type (squid or fish) or size (gape width or twice gape width), although there were slight inter-individual differences in the onset of muscle firing for some muscles. Kinematic analysis showed a statistical difference in variables associated with durations for different food types, with the durations for all variables being faster for squid bites than fish bites, but no difference based on the size of the food item. This apparent lack of modulation may be associated with specialization of the morphology and behavior of G. cirratum for obligate suction prey capture. This functional specialization constrains the method in which G. cirratum captures prey but does not appear to result in dietary specialization. An unusual post capture spit-suck manipulation allows this shark to handle and ingest large prey.     

The effect of prey morphology on the feeding behaviour and population growth of the predatory rotifer Asplanchna sieboldi: a case study using five species of Brachionus (Rotifera)

1. We investigated the numerical response, functional response and prey preference of Asplanchna sieboldi to five different prey brachionids. We also analysed the feeding behaviour of the predator in terms of encounters, attacks, capture and prey ingested per unit time. 2. The five prey species (Brachionus havanaensis, B. rubens, B. patulus, B. macracanthus and B. calyciflorus) differed in their body size and spine length. 3. The population growth rates of A. sieboldi ranged from 0.074 ± 0.03 to 0.431 ± 0.02 depending on prey type and density. There was a significant impact of the spine length rather than body size per se on the population growth rates of the predator. 4. The maximum number of prey consumed depended on both body size and spine length. In the functional response analyses, the plateau was reached at a prey density of 4–8 ind. mL^?1. 5. There was a significant impact of prey density on the prey preference of the predator.     

Modelling Size-Dependent Cannibalism in Barramundi Lates calcarifer: Cannibalistic Polyphenism and Its Implication to Aquaculture

This study quantified size-dependent cannibalism in barramundi Lates calcarifer through coupling a range of prey-predator pairs in a different range of fish sizes. Predictive models were developed using morphological traits with the alterative assumption of cannibalistic polyphenism. Predictive models were validated with the data from trials where cannibals were challenged with progressing increments of prey sizes. The experimental observations showed that cannibals of 25–131 mm total length could ingest the conspecific prey of 78–72% cannibal length. In the validation test, all predictive models underestimate the maximum ingestible prey size for cannibals of a similar size range. However, the model based on the maximal mouth width at opening closely matched the empirical observations, suggesting a certain degree of phenotypic plasticity of mouth size among cannibalistic individuals. Mouth size showed allometric growth comparing with body depth, resulting in a decreasing trend on the maximum size of ingestible prey as cannibals grow larger, which in parts explains why cannibalism in barramundi is frequently observed in the early developmental stage. Any barramundi has the potential to become a cannibal when the initial prey size was <50% of the cannibal body length, but fish could never become a cannibal when prey were >58% of their size, suggesting that 50% of size difference can be the threshold to initiate intracohort cannibalism in a barramundi population. Cannibalistic polyphenism was likely to occur in barramundi that had a cannibalistic history. An experienced cannibal would have a greater ability to stretch its mouth size to capture a much larger prey than the models predict. The awareness of cannibalistic polyphenism has important application in fish farming management to reduce cannibalism.