Trophic ecology of striped bass, Morone saxatilis, in a freshwater reservoir (Lake Texoma, U.S.A.)

Many vertebrate predators consume a wide variety of prey types, depending upon availability and vulnerability. In contrast, striped bass, Morone saxatilis , that have been introduced to Lake Texoma (Oklahoma-Texas, U.S.A.) use a very limited array of fish (mostly clupeids of the genus Dorosoma ) as prey. As a large, mobile predator, M. saxatilis should be capable of capturing and consuming numerous other species of fish that are available in the reservoir. However, examination of 1845 stomachs year-around over 5 years showed that the only marked 'switching' among prey was from Dorosoma to a diet including a high percentage of insects during spring–early summer, ignoring most other fish taxa that could have served as food. Even under essentially starvation conditions in late summer of years with scarce Dorosoma, M. saxatilis in Lake Texoma did not switch to other available fish as prey. Patterns of predation by M. saxatilis are trenchantly different from place to place: very narrow prey selectivity even under starvation conditions has been reported once previously for the species in a freshwater reservoir, but in its native marine and estuarine environment and in some other reservoirs the species is more catholic in its use of prey. Why this large predator shows fidelity in some environments to particular prey, even to the extent of starvation, remains an enigma.     

Estimating piscine prey size from partial remains: testing for shifts in foraging mode by juvenile bluefish

Knowledge of prey sizes consumed by a predator aids in the estimation of predation impact. Young-of-the-year bluefish, Pomatomus saltatrix, attack their prey tail-first and often bite their prey in half; this poses a unique problem in determining prey sizes from stomach content analysis. We developed a series of linear regressions to estimate original prey lengths from measurements of eye diameter and caudal peduncle depth for striped bass, Morone saxatilis, bay anchovy, Anchoa mitchilli, American shad, Alosa sapidissima, blueback herring, Alosa aestivalis, Atlantic silverside, Menidia menidia, and white perch, Morone americana. We then used these regressions to estimate original prey sizes from pieces of prey found in stomachs of bluefish collected in the Hudson River estuary from 1990–1993. Lengths of prey that were swallowed whole were compared to estimated lengths of prey that were consumed in pieces. Lengths of prey that were consumed in pieces were larger than prey that were consumed whole. We determined the prey length/predator length ratio at which bluefish began shifting from swallowing their prey whole to partial consumption. Shifting occurred at a ratio of approximately 0.35 irrespective of prey species, suggesting that prey length plays an important role in predator foraging decisions and may contribute to gape limitations. Shifts in foraging mode effectively reduce gape limitation and allow bluefish to consume larger prey sizes which may increase their effect on prey populations.     

Effects of season and body-size on the distribution and diet of the Iberian chub Leuciscus pyrenaicus in a lowland catchment

Seasonal and ontogenetic variations in space and food use by Leuciscus pyrenaicus were analysed in a Portuguese lowland catchment. Large fish occurred mostly in deep permanent-flowing sites whereas small fish occurred mostly in the shallowest intermittent-flowing site. No seasonal or size-related changes in feeding intensity were found, but the diet changed both across seasons and throughout ontogeny. The diet was dominated by aquatic prey over all seasons, but during winter and summer more plant material and terrestrial prey, respectively, were eaten. Throughout ontogeny fish shifted from soft-bodied to hard-shelled prey and decreased animal prey breadth. Mean prey size increased with fish size but the prey size spectrum was more variable for medium-sized fish than for either small or large fish. It is suggested that: (i) large fish avoid shallow drying areas owing to the risk of mortality, either by thermal and respiratory stresses or increased predation by mammals and birds; (ii) seasonal changes in diet are a response to differences in prey availability; and (iii) morphological constraints, prey handling costs and habitat partitioning are responsible for size-related changes in diet.     

First Year Ontogenetic Diet Patterns in Two Coexisting <Emphasis Type="Italic">Sander</Emphasis> Species, <Emphasis Type="Italic">S. lucioperca</Emphasis> and <Emphasis Type="Italic">S. volgensis</Emphasis> in Lake Balaton

First year ontogenetic diet patterns and size-dependent diet partitioning of two coexisting piscivorous percids, pikeperch and Volga pikeperch were analysed in Lake Balaton. Diet patterns of the two pikeperches followed a common trend, catching larger preys at larger sizes, but in details they were different. Until 20–30 mm size age-0 pikeperch fed on planktonic microcrustaceans and were found only in the pelagic zone. After this size, from the end of May, a part of the stock shifted habitat and moved to the littoral zone to feed on Limnomysis benedeni. Majority of the stock stayed in the pelagic zone and shifted to Leptodora kindtii. In the pelagic group early piscivory, mainly cannibalism, occurred only sporadically. However, the few individuals that reached 100 mm size during the first year were typically piscivorous. Volga pikeperch stayed in the pelagic zone during the first year and shifted from planktonic microcrustaceans to benthic macroinvertebrates. General patterns of the first year diet ontogeny were similar in the different areas of Lake Balaton in both pikeperches with only some differences in the secondary food resources, especially in the share of chironomids. Chironomids were eaten at highest ratio by the two fish species in the Keszthely basin in accordance with their abundance pattern. Maximum prey sizes increased significantly with predator size in both pikeperch and Volga pikeperch. However, pikeperch ate much larger prey than did Volga pikeperch at the same size although they have a similar mouth gape to length ratio. These two, closely related pikeperches, although occur in the same habitat and have similar patterns of diet ontogeny, effectively partition food resources in Lake Balaton during the first growing season, except in the very beginning of their ontogeny at L = 5–10 mm size. It appears that age-0 Volga pikeperch is not a significant competitor for the economically highly rated pikeperch in Lake Balaton. On the other hand, it is age-0 pikeperch that might affect Volga pikeperch during its early ontogeny, in the planktivorous stage.     

Ontogenetically stable dimorphism in a lacertid lizard (Acanthodactylus boskianus) with tests of methodology and comments on life-history

Recent arguments in the literature prompted us to compare methods for assessing sexual dimorphism in body proportions of lacertid lizards, using Acanthodactylus boskianus . Although expressing body-part measurements as proportional to head length was the most effective method, we recommend using trunk length for the baseline as a general method for lizards. We also argue that, when aiming to assess sexual dimorphism in body proportions of lizards, if the context is ecological, all available adults should be included. However, for morphology and taxonomy, small sub-samples of the largest individuals that maximally express their genetic morphological potential should be used. In A. boskianus , the sexual dimorphism of mensural characters in adults was typical: males were larger, with relatively larger head and appendages. However, the ontogeny of this dimorphism was unusual in that the differences existed already in youth and thereafter persisted isometrically. The sexual dimorphism of meristic characters was male-biased in numbers of femoral pores and of caudal vertebrae, and female-biased in numbers of ventral plates along the trunk and of precaudal vertebrae. Size dimorphism may conceivably play a role in sex recognition because two potential visual cues (i.e. size dimorphism and dichromatism) appear to complement each other.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society 2009, 97 , 275–288.     

Diel feeding periodicity, daily ration and prey selection of a riverine population of juvenile chinook salmon, Oncorhynchus tshawytscha (Walbaum)

The diel feeding periodicity, daily ration and prey selection of juvenile chinook salmon, Oncorhynchus tshawytscha , were studied in relation to the available prey. Maximum dry weight of food intake occurred about dawn, when mayflies were the major prey, but the greatest number of freshly eaten prey occurred during the afternoon, when chironomids and terrestrial dipterans predominated. Feeding activity at night was low, with smaller mayflies comprising up to 50% of the prey. During the day the young salmon fed selectively on chironomids and the larger mayflies, while trichopterans and terrestrial taxa were under-represented in the diet. Food consumption over the 24-h period averaged 8.3% of the fish dry body weight. Prey abundance in the drift explained about 50% of the composition of the diet. Although the fish selected larger mayflies, size apparently was not a main criterion for selection because chironomids, although smaller than mayflies, were also frequently eaten. Previous dietary experience of the fish and the diel pattern of prey abundance appear to best explain the selective feeding of juvenile chinook salmon.     

Correspondence between Ontogenetic Shifts in Morphology and Habitat Use in Minnow Phoxinus Phoxinus

The morphology of minnows Phoxinus phoxinus from two rivers at the south-east of England was analysed on mensural characters and qualitative traits (occurrence of appendages, pattern of pigmentation and scalation). Eight developmental intervals were identified from the qualitative traits, and bivariate and multivariate analysis revealed that allometric growth occurs occasionally during ontogeny, mainly in earlier developmental periods. Body shape is under the influence of rapid increase during development, mainly due to the quick growth in early developmental intervals. The exclusion of general size, remarkable in early developmental intervals, revealed several shape characters that mainly determine minnow morphology by changing the mode of variability during development (e.g. caudal characters, maximum body height, belly length and ventral head length). The tail certainly undergoes the most consistent and most durable change, its characters being the most variable and the most discriminative for developmental periods from larvae to adults. The most prominent alteration in the overall shape development occurs at the transition from larva to juvenile, both regarding the number of characters involved into the change and their variability. This transition takes place at about 28 mm SL, which corresponds to a significant change in microhabitat use in both the rivers Lee and Frome, characterized by shifts by 0+ juveniles to significantly deeper waters than those occupied by larvae, with increased amounts of either submerged filamentous algae (Lee) or vegetation (Frome).     

Effects of body size, age and maturity stage on diet in a large shark: ecological and applied implications

Ontogenetic diet shifts are a widespread phenomenon among vertebrates, although their relationships with life history traits are poorly known. We analyzed the relative importance of body size, age and maturity stage as determinants of the diet of a marine top predator, the copper shark, Carcharhinus brachyurus, by examining stomach contents using a multiple-hypothesis modeling approach. Copper sharks shifted their diet as size and age increased and as they became sexually mature, incorporated larger prey as they grew, and had a discrete shift in diet with body size, with only individuals larger than ≈200 cm total length able to prey on chondrichthyans. Body size was the most important trait explaining the consumption of chondrichthyans, while age determined the consumption of pelagic teleosts. Pelagic teleosts were consumed mostly by medium-aged sharks, a result, probably, of a risk-reducing feeding strategy at young ages coupled with either a senescence-related decline in performance or a change in sensory capabilities as sharks age. Copper sharks of all sizes were able to cut prey in pieces, implying that gape limitation (i.e., the impossibility of eating prey larger than a predator’s mouth) did not play a role in producing the diet shift. Our results suggest that, contrary to the current practice of setting minimum but not maximum size limits in catches, any plan to conserve or restore the ecological function of sharks, through their predatory control of large prey, should aim to maintain the largest individuals.     

Diet-shifts by an estuarine goby (Pomatoschistus microps) in the face of variable prey availability

Ontogenetic shifts in diet provide a mechanism for maximising fitness throughout development and are common where predators exhibit large increases in size. In order to maximise their fitness throughout development, benthic feeding fish can show diet-shifts that centre on the transition from meiofaunal to macrofaunal prey. Here we assessed whether such a shift was influenced by natural variation in prey-size availability by comparing the sizes of prey consumed by naturally foraging common gobies (Pomatoschistus microps). We tested explicitly for the presence of an ontogenetic shift by analysing the length of prey consumed and an index for prey importance for gobies of different lengths. We also tested the match between actual diets and those predicted by a foraging model. The goby size at which the diet-shift occurred was consistent among locations that differed in their availability of prey and through temporal changes in densities and types of prey. The mean sizes of ingested prey increased for gobies > 35 mm in length and the relative importance of macrofauna increased at 30 mm. The foraging model predicted that gobies > 30 mm would eat larger prey than would smaller gobies which differed from the observed changes in prey-size at 35 mm. Availability of prey did not appear to influence the lengths at which gobies changed diet but did affect the size of prey taken after the diet-shift. A relatively large abundance of large-bodied chironomids at two sites was reflected in the mean size of prey consumed by gobies > 30 mm at these sites. Our study indicates that intrinsic mechanisms can be more important than fluctuating environments in determining prey-choice and shifts in diet, although for the common goby, variability in prey-size may have implications for prey-choice later in ontogeny.     

Light and scanning electron microscopic study of the tongue in the estuarine dolphin (Sotalia guianensis van Bénéden, 1864)

The importance of the tongue during feeding, and the limited information on the tongue of most aquatic mammals led us to investigate its morphological aspects in sexually immature and mature Sotalia guianensis. Six tongues were measured and photo-documented after their removal from the oral cavity. The samples were divided into rostral, middle, and caudal regions, and examined using light microscopy and scanning electron microscopy (S.E.M.). Sotalia guianensis tongue presented lateral grooves from the apex to the middle portion, while the anterolateral region presented marginal papillae. Histological characteristics revealed the presence of a keratinized stratified epithelium, salivary glands in the middle and caudal portions of the tongue, and filiform papillae in the caudal region. S.E.M. images revealed the presence of filiform papillae and ducts of salivary glands in the middle and caudal portions of the tongue. We can conclude that the characteristics found in this study may reflect an adaptation to changes in diet after weaning.     

Morphometry of the Stone Loach, Barbatula Barbatula: Do Mensural Characters Reflect the Species' Life History Thresholds?

Growth variability in 23 mensural characters was examined in 387 specimens of stone loach, Barbatula barbatula, from England. The standard length (SL) of the specimens ranged from 15.3 to 115.4 mm. We tested the hypothesis that body proportions change abruptly, rather than gradually, at certain intervals of ontogeny by fitting linear, quadratic and split linear curves to plots of each variable against SL. Based on patterns of allometric growth, two groups and two subgroups of mensural characters have been found. Three characters were best explained by a linear regression, indicating isometric growth. Eight characters were best explained by a quadratic curve, indicating gradual allometry. The remaining 12 characters were best explained by a split regression, indicating mainly isometric growth with abrupt allometry occurring at a specific SL (breakpoint). The first shift in morphometric values (a transformation of the head; breakpoints in three characters) occurred at 26-35 mm SL, the second (a change in fin shape and size as well as body form; breakpoints in six characters) at 36-47 mm SL. The coincidence of shifts in body morphology with those in microhabitat use (between the respective size classes) suggests that thresholds (though not as sudden as those between embryo and early larva steps) do occur during this interval of stone loach life history. We suggest that the larva period ends with the completion of the first shift in relative growth (i.e. not later than at 35 mm SL, depending on individual variability), and that the second shift in morphometric values reflects a threshold between the first and the second step of juvenile period. The importance of changes in external morphology decreased as the fish grew bigger and older.     

Prey profitability for adult Grey Herons Ardea cinerea and the constraints on prey size when feeding young nestlings

Patterns of handling time and profitability are examined for adult Grey Herons feeding on carp, eels and catfish. Handling times generally increased with prey size but were influenced markedly by the morphology and behaviour of the prey. Profitability was highest for carp (max. 0.9 g/s for 15–20 cm fish), lowest for catfish (max. 0.05 g/s) and intermediate for eels (max. 0.1 g/s). Nestlings were unable to ingest the sizes of fish most profitable for the adults to consume until aged 20 days; by the age of 30 days, they could consume the full size-range of prey taken by the adults. In order to feed their young chicks, adults must therefore either select smaller prey, or break their large prey into smaller pieces. The diet of nestling Grey Herons in the Camargue is examined for evidence to support or refute the former hypothesis.
Young nestlings (≤20 days) regurgitated smaller carp than old nestlings (> 20 days). Comparison of prey types in the diet of the two groups showed that small prey species occurred significantly more often in the diet of young chicks, while the converse was true for larger prey species. The occurrence of particular prey types in the diet only of young chicks suggests that adults may forage in more marginal, shallower water (where small prey are probably more abundant) to meet the requirements of their brood during the early part of the nestling phase. The second hypothesis, that the adults break down large prey into smaller pieces, was not examined, although evidence from other studies suggests that this does occur; both mechanisms may therefore be important.     

Active selection for large guppies, Poecilia reticulata, by the pike cichlid, Crenicichla saxatilis

Size-selective predation has been proposed to be one important evolutionary force shaping life-history traits in guppies ( Poecilia reticulata ). Populations living in the presence of the ring-tailed pike cichlid ( Crenicichla saxatilis ) are smaller, mature earlier, allocate more energy to offspring and get more and smaller young than guppies in localities without Crenicichla . We investigated if Crenicichla saxatilis is a size-selective predator, if the selectivity is a result of active choice and if the optimal prey size can be explained according to an optimal foraging model. In single-prey experiments we quantified the predators' pre-capture costs (time), capture success, and post-capture costs (time) for four different prey sizes spanning from 10 to 40 mm total length. To see which of the components of the prey cycle the predator takes into account for its choice, we then predicted prey values and optimal prey size with 6 different models that included one or more of the prey cycle components.
In two multiple prey experiments, the cichlids were given the choice of the two and four different prey sizes simultaneously. Crenicichla saxatilis actively selected the largest guppies in both cases. The three prey-value functions that included handling time (post-capture cost) did not accurately predict the prey choice. Instead the prey-value functions that took into account pre-capture cost (approach and attack time) were able to correctly predict the choice of the largest guppy size, suggesting that pre-capture costs may be more important than post-capture costs for prey choice in Crenicichla saxatilis . The study confirms that Crenicichla saxatilis is a size-selective predator selecting large guppies, while earlier evidence for selectivity for large prey in Crenicichla cichlids has been weak and equivocal. Our result strengthen the possibility that size-selective predation is a mechanism in life-history evolution in guppies.     

Ontogeny of head and caudal fin shape of an apex marine predator: The tiger shark (Galeocerdo cuvier)

How morphology changes with size can have profound effects on the life history and ecology of an animal. For apex predators that can impact higher level ecosystem processes, such changes may have consequences for other species. Tiger sharks (Galeocerdo cuvier) are an apex predator in tropical seas, and, as adults, are highly migratory. However, little is known about ontogenetic changes in their body form, especially in relation to two aspects of shape that influence locomotion (caudal fin) and feeding (head shape). We captured digital images of the heads and caudal fins of live tiger sharks from Southern Florida and the Bahamas ranging in body size (hence age), and quantified shape of each using elliptical Fourier analysis. This revealed changes in the shape of the head and caudal fin of tiger sharks across ontogeny. Smaller juvenile tiger sharks show an asymmetrical tail with the dorsal (upper) lobe being substantially larger than the ventral (lower) lobe, and transition to more symmetrical tail in larger adults, although the upper lobe remains relatively larger in adults. The heads of juvenile tiger sharks are more conical, which transition to relatively broader heads over ontogeny. We interpret these changes as a result of two ecological transitions. First, adult tiger sharks can undertake extensive migrations and a more symmetrical tail could be more efficient for swimming longer distances, although we did not test this possibility. Second, adult tiger sharks expand their diet to consume larger and more diverse prey with age (turtles, mammals, and elasmobranchs), which requires substantially greater bite area and force to process. In contrast, juvenile tiger sharks consume smaller prey, such as fishes, crustaceans, and invertebrates. Our data reveal significant morphological shifts in an apex predator, which could have effects for other species that tiger sharks consume and interact with. J. Morphol. 277:556–564, 2016. © 2016 Wiley Periodicals, Inc.     

Ontogenetic patterns of relative growth in young roach Rutilus rutilus: within-river basin comparisons

To address the lack of information on the ecomorphology of young of the year juveniles (0+) freshwater fishes despite its importance to understanding geographical variation in life history, we compared the morphology of 0+ juvenile roach Rutilus rutilus from eight sub-regions of the River Great Ouse basin (U K) We tested for variation due to water retention structures, which could act to isolate upstream stocks from the general population Differences in mensural characters between the eight different regions appeared in ventral fin length and eye diameter only, which both were strongly affected by variable relative growth during early juvenile development, thus the discrepancies between regions were not due to geographical location Morphological development in roach ≥ 15 mm standard length (SL) consists of two different intervals during the first (∼ 15-40 mm SL). in contrast to adults, several mensural characters continue to develop allometrically. either positively or negatively suggesting a continuance of the larval period Whereas, m the second interval (> ∼40 mm SL). all the mensural characters except head length and eye diameter grew close to lsometrically with body length, and the rate of variation in relative growth became as strongly limited as that previously reported for adults In contrast to previous studies, which are based on few specimens we suggest this second interval to be the true start of the juvenile period     

Behavioural and life history effects of predator diet cues during ontogeny in damselfly larvae

A central issue in predator–prey interactions is how predator associated chemical cues affect the behaviour and life history of prey. In this study, we investigated how growth and behaviour during ontogeny of a damselfly larva (Coenagrion hastulatum) in high and low food environments was affected by the diet of a predator (Aeshna juncea). We reared larvae in three different predator treatments; no predator, predator feeding on conspecifics and predator feeding on heterospecifics. We found that, independent of food availability, larvae displayed the strongest anti-predator behaviours where predators consumed prey conspecifics. Interestingly, the effect of predator diet on prey activity was only present early in ontogeny, whereas late in ontogeny no difference in prey activity between treatments could be found. In contrast, the significant effect of predator diet on prey spatial distribution was unaffected by time. Larval size was affected by both food availability and predator diet. Larvae reared in the high food treatment grew larger than larvae in the low food treatment. Mean larval size was smallest in the treatment where predators consumed prey conspecifics, intermediate where predators consumed heterospecifics and largest in the treatment without predators. The difference in mean larval size between treatments is probably an effect of reduced larval feeding, due to behavioural responses to chemical cues associated with predator diet. Our study suggests that anti-predator responses can be specific for certain stages in ontogeny. This finding shows the importance of considering where in its ontogeny a study organism is before results are interpreted and generalisations are made. Furthermore, this finding accentuates the importance of long-term studies and may have implications for how results generated by short-term studies can be used.     

Prey selection by flounder, Platichthys flesus, in the Douro estuary, Portugal

Prey selection by the flounder, Platichthys flesus (Linnaeus, 1758), in an estuarine nursery was investigated and the major factors influencing food choice by this species were assessed. Diet breadth was narrow, reflecting the low prey diversity observed in the benthos. A gradual ontogenetic shift from small prey such as amphipods to larger prey like polychaetes and bivalves was observed. Amphipods had positive electivity values in the upper estuary and negative values in the lower estuary.Polychaetes showed the inverse pattern. Bivalve electivity values were always positive. Differential selectivity throughout the estuary was mainly related to spatial segregation of flounders according to size, with the smaller individuals concentrating in the upper estuary and larger individuals concentrating in the lower estuary. Amphipods such as Corophium spp. play a crucial role in the flounder diet because of their small size, low mobility and diel activity pattern. As prey, the polychaete value increases throughout flounder ontogeny since the flounder size range is compatible with the larger mouth gapes and detection ability of larger fish. Bivalve electivity values for flounder are mainly related to high calorific values. The absence of Crangon crangon (Linnaeus, 1758) in the diet may be due to low water temperature since the cost–benefit involved in the capture of highly mobile prey is too high at low temperatures. It was concluded that flounder must use several sensory features to detect and capture prey in turbid estuarine waters and that field studies provide important background information on the actual predator preferences under natural conditions.     

The diet of coastal breeding hooded crows corvus corone comix

Three methods of analysis were used to determine the diet of territorial hooded crows at Lough Hyne Marine Reserve, Co Cork, Ireland The regular collection of prey Items from these sites at Lough Hyne was integrated with pellet and stomach analysis to determine diet Intertidal organisms occurred in over 80% of pellets and 43% of stomachs and occupied over 77% of the total wet weight of foods identified in pellets All prey items recovered from drop sites originated from the intertidal habitat, involved either large-sized species or larger individuals of smaller-sized species, and were only dropped during October to February Twenty-five intertidal species were identified but only a few of these species contributed to the bulk of the diet Hooded crows were shown to consume a wide range of intertidal species throughout the year, though the species composition in the diet was seasonally influenced Depletion and weight loss of intertidal molluscs through the winter was shown to have a minimal effect on selection suggesting that prey switching was driven by the birds nutritional requirements     

Riparian contributions to the diet of terapontid grunters (Pisces: Terapontidae) in wet–dry tropical rivers

The diets of terapontid assemblages in 22 catchments across Australia's wet–dry tropics were investigated in relation to the direct use of terrestrial‐riparian inputs, as well as the role of ontogeny and morphology in mediating consumption of allocthonous material. The diet of several species was restricted almost entirely to instream trophic resources throughout their life history. In contrast, ontogenetic diet shifts towards increasing consumption of terrestrial prey types were a prominent feature of the dietary ecology of some terapontids, with collective allocthonous dietary items making a significant contribution (up to 42%) to diet in larger size classes of several species. For those species consuming terrestrial‐riparian material in their diet, terrestrial invertebrates were the most common prey item; however, terrestrial vegetation, principally riparian fruits, and terrestrial vertebrates were also important dietary inclusions in the larger size classes of particular species. A large mouth gape was the morphological feature most strongly associated with consumption of terrestrial food resources within the Terapontidae. Results indicate that the direct consumption of terrestrially derived food sources in northern Australian aquatic systems may be more important than previously asserted, and that additional research is required to better clarify the role of terrestrial subsidies to these ecosystems.     

Feeding habits of juvenile <Emphasis Type="Italic">Trematomus newnesi</Emphasis> (Pisces,Nototheniidae) at Potter Cove,South Shetland Islands,Antarctica

The dietary composition of juvenile Trematomus newnesi, trawled at Potter Cove, South Shetland Islands, in the summers 2004–2005 and 2005–2006 was analysed using frequency of occurrence (F%) and dietary coefficient Q (%) methods. The samples consisted exclusively of immature specimens in the range 4–15.4 cm (total length) and are complementary to those of larger fish from the same site, including adults, which were analysed in previous work. Benthic-demersal organisms such as gammaridean amphipods and harpacticoid copepods were the main (coefficient Q) and most frequent (F%) prey. The importance of the smaller and larger main prey diminished and increased, respectively, during ontogeny. Pelagic krill, being negligible in the diet of the small and medium size fish categories, became secondary food, but only for fish larger than 12 cm. Other taxonomic groups occurred scarcely and constituted occasional food. They were mostly benthic, such as gastropods, bivalves, isopods, cumaceans, and algae, with the exception of an insignificant number of pelagic ostracods and calanoid copepods. Unlike the more pelagic/planktivorous mode of life known for late juvenile-adult stages of T. newnesi, including cryopelagy, present results indicate that early juvenile fish remain sheltered among macroalgae beds preying on the associated community of demersal-benthic organisms.