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.     

Temporal and spatial variation in potential and realized growth rates of age‐0 year northern rock sole

The possibility of prey limitations on the growth performance of age‐0 year northern rock sole Lepidopsetta polyxystra was evaluated at three sites along the north‐east coast of Kodiak Island, Alaska, U.S.A., by comparison of observed to potential growth rates. Growth potential was measured in the laboratory across the range of temperatures encountered by this species during the first summer of life. Growth potential ( g _L, mm day⁻¹) increased with water temperature (T) between 2 and 13° C, according to: g _L = 0·0151 + 0·3673·log₁₀(T). There were significant differences in growth rate between the three field sites such that Holiday Beach fish were 7·1 mm longer than Shakmanof Beach fish by mid‐September, with Pillar Creek Cove fish of intermediate size. Temperature differences between sites accounted for less than half of this variation. The remainder may have been related to differences in prey availability among the sites in association with observed differences in sediment characteristics. In addition to the spatial variability, there was significant monthly variation in growth performance. Realized growth rates between July and August were in excess of 85% of potential. Between August and September, however, realized growth fell to 43–71% of potential indicating a decline in conditions for growth. The spatial variation in growth rates was not density‐dependent as the site with the highest fish densities (Holiday Beach) also supported the highest growth rates. The available data indicates that for this subtidal species, interannual variation in growth may be more important than site variation.     

Gape-limitation,foraging tactics and prey size selectivity of two microcarnivorous species of fish

Summary Patterns of prey size selectivity were quantified in the field for two species of marine microcarnivorous fish, Embiotoca jacksoni and Embiotoca lateralis (Embiotocidae) to test Scott and Murdoch's (1983) size spectrum hypothesis. Two mechanisms accounted for observed selectivity: the relative size of a fish in relation to its prey, and the type of foraging behavior used. Juvenile E. jacksoni were gape limited and newborn individuals achieved highest selectivity for the smallest prey size by using a visual picking foraging strategy. As young E. jacksoni grew, highest preference shifted to the next larger prey sizes. When E. jacksoni reached adulthood, the principal mode of foraging changed from visual picking to relatively indiscriminant winnowing behavior. The shift in foraging behavior by adults was accompanied by a decline in overall preference for prey size; sizes were taken nearly in proportion to their relative abundance. Adult E. lateralis retained a visual picking strategy and achieved highest selectivity for the largest class of prey. These differences in selectivity patterns by adult fish were not explained by gape-limination since adults of both species could ingest the largest prey items available to them. These results support Scott and Murdoch's (1983) hypothesis that the qualitative pattern of size selectivity depends largely on the range of available prey sizes relative to that a predator can effectively harvest.     

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.     

Spring cannibalism on 1 year walleye pollock in the Doto area,northern Japan: is it density dependent?

Cannibalism in walleye pollock off the eastern coast of the Hokkaido Island, Japan was important only during spring (April to June), and its importance increased from 0% in dry mass for <200 mm L _S fish to 48·9% for >400 mm L _S fish. Most of the prey was represented by age 1 year fish, showing a unimodal body size distribution with a mode at 121–130 mm. Although cannibal body size was larger in deeper (>150 m) water, there was no difference in prey size by depth, suggesting impingement of the predators inhabiting deeper water into the shallow areas to cannibalize 1 year fish. The minimum ratio cannibal: prey size was 1·74. There was a positive but non-significant correlation between the contribution of cannibalism to a potential predator's (>300 mm) diet and an estimate of the previous year's recruitment. This was due to an extremely high contribution of cannibalism during 1992, when a distinctly larger size of predators seemed to bias the contribution. When the 1992 data were removed from the analysis, a significant correlation was obtained ( r ²=0·77, P <0·01), showing that Pollock cannibalism is rather density dependent. Based on the results, it is hypothesized that the'overflow' of 1 year fish from the shelf waters due to their high abundance and the weak stratification in the spring water column results in increased co-occurrence with adult fish and consequent cannibalism.     

The effects of predator size, temperature, and prey characteristics on gastric evacuation in whiting

No effect of prey size on gastric evacuation rate was found in whiting Merlangius merlangus .Prey energy density explained most of the data variation among fish prey, and evacuation time increased by a factor 1·6–1·7 with prey energy density increasing from 3·4 to 5·6 kJ g⁻¹. The power model expanded to include predator size, temperature, and prey energy density could describe gastric evacuation in whiting fed fish prey. Krill Meganyctiphanes norvegica was evacuated at a rate similar to fish prey of equal energy density, while the evacuation of brown shrimp Crangon crangon took almost twice as long.     

Prey selection and functional morphology through ontogeny of Amphiprion clarkii with a congeneric comparison

The relationship between the complexity of the feeding apparatus and prey selection through ontogeny was examined in Amphiprion clarkii larvae. Larvae were reared from 1 to 10 days post-hatch (dph) on a diet of rotifers, wild-caught plankton and newly hatched Artemia sp. nauplii. Results were compared with available data on the relationship between functional morphology and prey selection of Amphiprion frenatus to establish patterns of functional morphology and prey selection between the larvae of two species of coral-reef fishes. Larvae of both species exhibited an increase in selection of larger prey through ontogeny coincident with an increase in the complexity of the feeding apparatus. The first elements to ossify in larvae of both species were the pharyngeal teeth ( A. clarkii : 5 dph, near ± s.d. Standard length, L _S,4· 3 ± 0· 2 mm; A. frenatus : 5 dph, L _S5· 0 ± 0· 4 mm) which, in combination with the development of a more functional feeding apparatus, may have permitted larvae to better process new types and sizes of prey. Prey items, however, were selected differentially between the two fish species, which could not be fully explained by the functional state of the feeding apparatus. While prey selection is influenced by the functional state of the feeding apparatus, all aspects of larval fish biology (morphology, behaviour and physiology) should be considered.     

Behavioural aspects of predation by juvenile Atlantic salmon (Salmo salar L.) on particulate,drifting prey

A recirculatory flume tank simulating a simplified stream environment was used to study the feeding behaviour of juvenile Atlantic salmon (Salmo salar L.), 5.1 to 9.4 cm in fork length (from tip of snout to fork of tail), on artificial particulate prey passively drifting in the water current. Changes in feeding behaviour at two different times of the year and when fish were presented with prey of different sizes are described and quantified. Responsiveness to food was greatly reduced in autumn as compared to summer. The maximum distances at which prey elicited a response decreased in autumn to 40% of the summer value, and the maximum distances which fish traversed in order to capture prey decreased by 80% over the same period. During the peak growing season, the response to a range of prey sizes from 0.013 to 0.102 × fish fork length was directly related to prey size and could be accounted for on the basis of visual theory alone. Capture distances were closely related to fixation distances. Maximum capture distance increased to a peak value for prey of between 0.025 and 0.069 × fork length, while larger prey were never captured and the smallest prey rarely evoked a response. Prey size selectivity also operated after capture, through rejection versus retention of the prey.     

Light intensity, prey detection and foraging mechanisms of age 0 year yellow perch

The ability of age‐0 year yellow perch Perca flavescans to detect prey using visual and mechano‐sensory input was examined during laboratory feeding trials at varying light intensities. Perch were highly effective predators and captured Daphnia pulicaria with 94% overall foraging success at light levels ranging from 0 to 3400 lx. Maximum average reaction distances (5·0 ± 0·8 cm, mean ±  s . e .) occurred in front of the fish at 3000 lx and significantly decreased as light intensities fell to <2 lx, with minimum reaction distances (2·8 ± 0·1 cm) observed in the dark. Following chemical ablation of the lateral line, yellow perch showed a significant reduction in reaction distance when compared to the untreated fish at 3000 lx, suggesting that the lateral line may augment visual prey detection at high light levels. A model was created to predict reaction distances for fish feeding with multiple sensory systems that can be applied to a variety of photic environments. This study provides a better understanding of the contribution of vision and the lateral line to prey detection, and relates the reaction distance of age‐0 year yellow perch to light intensities similar to those experienced in nature.     

An experimental study of body size and food size relations in crucian carp,Carassius carassius

Synopsis In dense, single-species assemblages of crucian carp, competition is intense and results in populations of stunted fish. To explore mechanisms underlying this competition, we measured handling times, return rates, and prey choice for five sizes of crucian carp feeding on six sizes of a standardized food. Handling times increased with prey size and decreased with fish size. Return rates (dry mass ingested per unit handling time) increased dramatically with fish size, and generally decreased with increasing prey size, especially for small fish. Patterns of return rates among size-classes suggested that one or more size-related shifts in feeding efficiency exist for crucian carp; combined with physiological stresses related to winter anoxia, the inability of fish to make these shifts may contribute to size structures observed in high-density populations. Comparisons of relations among fish size, prey size, and return rates for crucian carp and bluegill, Lepomis macrochirus, suggest that similar intraspecific competitive relations exist between generalist species with size-structured populations. Despite differences in return rates among prey sizes, the extent of food selectivity based on prey size exhibited by crucian carp in two types of choice trials was less than predicted. Crucian carp commonly take in several items before mechanically processing food with their pharyngeal apparatus; this multiple prey processing may contribute to disparities between observed choice patterns and those predicted based on return rates for single prey.     

Feeding ecology and growth of O-group flatfish (sole, dab and plaice) on a nursery ground (Southern Bight of the North Sea)

The food composition of O-group sole Solea solea , dab Limanda limanda and plaice Pleuronectes platessa on a nursery ground at Gravelines, France, included 17–25 taxa. Sole (new settlers) fed mainly on harpacticoid copepods and when 50 mm in size, on polychaetes (Terebellidae). Dab (<40 mm) consumed mainly polychaetes (Magelonidae and Spionidae), and later amphipods, polychaetes (Spionidae) and Hydrozoa. O-group plaice diet was dominated by polychaetes (Terebellidae), crustaceans and molluscs at all sizes. O-group sole, dab and plaice did not compete for food resources, each species being specialized in different prey items. Growth rates during May-July 1998 varied between 0·5 and 0·67 mm day⁻¹ for sole, 0·12 and 0·24 mm day⁻¹ for dab and 0·55 and 0·81 mm day⁻¹ for plaice. For sole and plaice, these estimates were similar to those recorded in other nurseries and also close to the maximal growth predicted by experimental models. This suggests that their growth was not limited by food during the first summer of life.     

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.     

Growth studies with fish—overcoming the problems of size variation

The relationship between growth rate and fish size is described by the equation log_e G _w= a —0·4 log_e W, where G_w is the specific growth rate and W is fish weight. Since the intercept (a) represents the log_e G_w of a fish unit size, the relationship presents a method allowing comparison of data from experiments involving fish of different sizes. The application of the method is demonstrated by examining the effects of environmental temperature on growth rates of cod, Gadus morhua , and it is suggested that the optimum temperature for growth of cod is 13–15° C.     

Differential effects of low pH and aluminium on the caudal neurosecretory system of the brook trout, Salvelinus fontinalis

Brook trout were subjected to soft water at pH 6·5, 5·5 or 5·0 without aluminium added, or to water at pH 5·5 with 200,300 or 500 μg Al I^-1 added. The response of the caudal neurosecretory system to low pH or aluminium was evaluated after one week by measuring the urotensin I and urotensin II concentrations in the urophysis by radioimmunoassay, and by morphometric analysis of the caudal neurosecretory cells. A positive correlation was found between urotensin I concentrations and acidity, and a negative correlation was found between urotensin II concentrations and total aluminium in the water. Morphometric indices (cell size and proportion of lobed nuclei in the caudal neurosecretory cells) suggested increased synthetic activity in the caudal neurosecretory cells of fish at pH 5·5 compared to pH 6·5.     

Effects of environmental factors on survival, growth, and production of American shad larvae

Episodic increases in temperature of 5°C above 20° C, over 48 h or declines in pH of 1·0 unit from pH 7·0 reduced survival of yolk-sac and feeding-stage larvae of American shad Alosa sapidissima . Over 16 days all measures of survival, growth, and production were more favourable at each higher temperature in the 15–25° C range. More favourable responses were also obtained at the higher prey level (500 v . 50 Artemia nauplii l^-1) and at the higher pH (7·5 v . 6·5). Combinations of high temperature and high prey levels, at pH 7·5, led to highest larval production. Little growth or production occurred at 15° C, regardless of pH or prey level. The effect of pH was strong with respect to survival, but weak with respect to growth. In attempts to restore American shad populations by larval stocking, release times and sites can be critical to optimize survival and eventual returns. Releases of larvae potentially will be most effective when made at temperatures >20° C, pH>7·0, and prey levels >50 1^-1. These conditions are most likely to occur in Maryland tributaries of Chesapeake Bay between mid-May and early June.     

Some aspects of the biology of a cyprinid, Aspius vorax Heckel

Scales were used for the determination of age with back-calculation of length. The oldest fish was VII + years old. Back-calculation did not exhibit Lee's phenomenon. The most rapid growth occurred in summer at water temperatures over 25°C. The growth in weight was c . 331 g year^-1 after IV vears of life. Growth was well described by von Bertalanffy equation : l_l - 91·0 [ l= ^{0·122(l0·25)}] The length-weight relationship followed the cube law (b = 3·0601) K_n ranged from 0·74 to 1·18 with a mean value of 1·0. Spawning occurred in January, fecundity was 74 509 with a mean of 1157 eggs -¹ body weight. Mean diameter of eggs was 1071 (pm). A developed ovary had ova of two sizes, immature oocytes and mature ova. The fish is a carnivorous feeder.     

Sibling cannibalism among juvenile vundu under controlled conditions. I. Cannibalistic behaviour, prey selection and prey size selectivity

Sibling cannibalism among vundu Heterobranchus longifilis larvae started at the age of 4 days, with the prey caught tail-first then swallowed up to the head, which was eventually discarded (type I cannibalism). At 17 days old, this type of cannibalism vanished and was replaced by the ingestion of the whole prey (type II cannibalism), which could only be exerted by predators six times as heavy as their prey. Type II cannibalism consisted of a seemingly opportunistic ambush attack by a formerly passive predator towards a disorientated prey. It required no preliminary aggression or chase, or even contact with the prey, suggesting that the attack was not mediated by the tactile sense, and that cannibalism was independent of aggressive behaviour. When alternative food resources (formulated feed, live tilapia prey) were available, the intensity of cannibalism decreased but pellet-eaters or tilapia predators always achieved lower growth rates than those feeding on conspecifics, suggesting that cannibalism was the most advantageous foraging tactic. Losses to cannibalism among populations of 30-day old juvenile vundu with an initial ratio of 4% of cannibals were as high as 75·5–79·9% over 15 days. Predation peaked during the first days (up to 2·8 prey C⁻¹ day⁻¹), then vanished progressively as surviving prey grew faster than cannibals and escaped their predation. Cannibals preferred consuming the largest prey available with respect to the logistics of cannibalism (body weight ratio of 6·0). This preference for large prey was interpreted both as a foraging tactic aiming to maximize the energetic return, and as foraging strategy enabling the cannibals to exploit their prey as long as possible. Based on these data, comprehensive models of the impact of cannibalism on vundu populations were developed.     

The effects of food consumption rate, body size and temperature on net food conversion efficiency in saithe and whiting

Net food conversion efficiency κ was estimated from growth experiments on saithe Pollachius virens and whiting Merlangius merlangus that were fed natural prey at a range of ration levels including satiation rations. The conversion efficiency, which specifies the net energy fraction of ingested energy C , was described appropriately by a simple power function of food consumption rate as κ  = 0·272 C ^0·18 and κ  = 0·426 C ^0·11 for saithe and whiting. This functional relationship was supported by the patterns of accretion of lipids and proteins in saithe. No significant effects of temperature and body size on κ was, however, demonstrated in this study except for the indirect influence using feeding levels (rations expressed relatively to satiation rations) in bioenergetics models.     

Size-biased predation by the gila topminnow poeciliopsis occidentalis (Baird and Girard)

The objective of this study is to document under both field and laboratory conditions the phenomenon of size-biased feeding by the Gila topminnow, Poeciliopsis occidentalis, and to shed light on the mechanisms responsible for this process. Adult female Gila topminnows feed in Monkey Spring, Santa Cruz County, Arizona, on a variety of food items, but Hyalella azteca is an important element of its invertebrate diet. The size of the amphipods ingested by female topminnows is a function of the size of the fish, with larger fish taking larger prey. There is also a strong preference for topminnows to select large amphipods from within the size limits imposed by the width of the mouth.The correlation between fish size and prey size was also seen in the laboratory, utilizing Daphnia similis and food pellets as prey. The tendency to select large prey from a range of available sizes was prominent with both Daphnia and pellets. The feeding period in the laboratory is characterized by two distinct phases. During the first phase, feeding is rapid and nonselective. As the feeding progresses, the rate of feeding drops dramatically and selectivity increases.More complex laboratory feeding experiments gave equivocal results. When fish were presented pellets of two sizes at different densities and at different ratios of small to large, size biased feeding was not clearly evident. Both predator and prey densities, agonistic behavior, and reaction distance should be explored more thoroughly as next steps in attempts to mimic feeding behavior in nature.Extracted from a Ph. D. thesis written by the junior author