Feeding opportunities of larval and juvenile cod (Gadus morhua) in a Greenlandic fjord: temporal and spatial linkages between cod and their preferred prey

Feeding of fish depends on a spatial and temporal match with prey, and since larval and juvenile feeding can be highly selective, their preferences for given prey sizes and taxa should be considered when quantifying the actual availability of potential prey. We investigated the diet and prey preferences of the early-life stages of Atlantic cod (Gadus morhua) to quantify the availability of prey during a spring-summer season in a West Greenlandic fjord. We hypothesized that abundances of larval and juvenile cod at size were synchronized to optimal availability of preferred prey in space and time. The present analysis is based on nine cruises each covering 5 stations visited between 24 May and 5 August 2010 comparing zooplankton abundance, cod gut content and distribution patterns. Cod 4–25 mm in length preferred prey of about 5 % of their own length. During ontogeny, their preferences changed from calanoid nauplii towards Pseudocalanus spp. and Calanus spp. copepodites. The larvae/juvenile had an exceptionally high dietary contribution from cladocerans, which were highly preferred by cod larger than 9 mm, while the abundant Metridia longa and the non-calanoid copepods contributed less. These findings stress the importance of focusing on abundance of preferred prey when assessing the actual prey availability to young fish. We found a spatio-temporal overlap between cod and their preferred prey, and observations suggest that advection of both zooplankton and cod contributed to this overlap. Hence, the larval feeding opportunities might be sensitive to climate-related changes affecting the circulation patterns in this fjord.     

The role of the generally unrecognised microprey source as food for larval fish in the Irish Sea

Few studies have studied the food supply to larval fish in the Irish Sea; thus, we have assessed the full prey-field available to larval fish, ranging from protozoa to copepods. Specifically we assessed if fish larvae feed on protozoa, as suggested by others, and if densities of the protozoa and the appropriate size of metazoan prey were previously underestimated. By examining the gutcontents of fish larvae, the prey available to them, and the potential accessibility of prey to fish, we develop a simple food web, presented as a box-model. By doing so, we indicate that the lack of focus on small metazoa and protozoa has underestimated the food available to fish larvae; without these, we might have concluded that prey levels were too low to support the growth of the larval fish assemblage. Our methods were as follows. Sampling was at two sites, off the Isle of Man, with distinct physical and biological structures, soon after fish spawning: the southwest coast, where many species occur in spring-summer (23 April; 6, 19 May; 1, 22 June; 12 July) and the east coast, where only herring larvae occur in September–November (12, 28 October). Microplankton (15–200 μm), mesozooplankton, and larval fish were collected at 1, 15, and 25 m: microplankton with 1.5 L bottles and a 64 μm-mesh net; mesozooplankton and larval fish with a Gulf VII high-speed sampler (280 μm mesh). The 64 μm mesh net, mounted on the Gulf VII, provided simultaneous hauls. Fixed samples were evaluated to determine species composition, abundance, and biomass. Larval fish diet was determined from fish collected by short net hauls: fixed guts were examined and prey, including protozoa, analysed. Using physical data as a guide, plankton data were integrated through the water column to determine standing stocks. Size-based food availability to larval fish was estimated from the gut contents. The role of protozoa was examined, assuming that they are digested at the same rate as metazoan and if they are digested 2.5–10 times faster; increased digestion rates indicated that they contributed substantially to the larval fish diet.     

Essential fatty acid (docosahexaenoic acid, DHA) availability affects growth of larval herring in the field

Larval fish growth and survival depends not only on prey quantity, but also on prey quality. To investigate effects of prey fatty acid concentration on larval herring growth, we collected different prey organisms and larval herring (Clupea harengus L.) in the Kiel Canal during the spring season of 2009. Along with biotic background data, we analysed fatty acids both in prey organisms and in the larvae and used biochemically derived growth rates of the larvae as the response variable. Larval herring reached their highest RNA/DNA derived growth rates only at high docosahexaenoic acid (DHA) concentration. When the ratio of copepodids to lesser quality cirriped nauplii was low, larval growth and larval DHA concentration were both significantly negatively affected. This was true even as prey abundance was increasing. This finding indicates that even in mixed, natural feeding conditions, growth variations are associated with DHA availability in larval fish.     

Nitrogen balance in marine fish larvae: influence of developmental stage and prey density

The utilization and fate of nitrogen in larvae of plaice (Pleuronectes platessa), blenny (Blennius pavo) and herring (Clupea harengus), from the stage of first-feeding to metamorphosis, was examined under laboratory conditions. Rates of ammonia excretion, primary amine defaecation, and growth in terms of protein-nitrogen were monitored throughout larval life. Data were used to calculate daily ration, the coefficient of nitrogen utilization (absorption efficiency), and gross and net growth efficiencies. The developmental pattern of nitrogen balance was similar for plaice and blenny larvae. These species showed increasing growth efficiency (k₁: 55 to 80%) with decreasing weight-specific waste nitrogen losses with age. Absorption efficiencies. were high (83 to 98%) in plaice and blenny larvae, and tended to increase with development in the former species. Ration relative to body weight decreased with growth in both species. Herring larval development, although at a slower rate than blenny and plaice, appeared normal up to 33 d, after which high mortality occurred. Absorption efficiency in this species tended to decline (83 to 43%) with age, until metabolic costs exceeded the absorbed ration and growth ceased. Artemia sp. nauplii proved a suitable food source for the rearing of plaice and blenny larvae, but this diet may have long-term toxicity or deficiency effects on herring. Availability and density of food affected nitrogen balance in the larvae of all three species. Feeding stimulated the output of wastes in excretion and defaecation by a factor of up to ten times the 12-h non-feeding basal rates. Waste nitrogen output reached a peak some 2 to 3 h after commencement of feeding and returned slowly to the baseline in 5 to 10 h after cessation of feeding. There was an asymptotic increase in ration, ammonia output and growth of larvae as prey density increased. Ration saturated at a higher prey density (>4 prey ml^-1) than either growth or excretion rate (1 prey ml^-1). Thus the efficiency with which food is absorbed and utilized for growth must eventually decline in response to high prey density. The idea that larval fish are adapted to maximize ingestion and growth rate, rather than optimize growth efficiency and thus to respond to prey occurring in either low density or in occasional patches, is supported by these results.     

Feeding selectivity in larvae of the European hake (<Emphasis Type="Italic">Merluccius merluccius</Emphasis>) in relation to ontogeny and visual capabilities

Feeding ecology was analysed for the first time in the larvae of the European hake (Merluccius merluccius) to determine whether their diet and selectivity were constrained by environmental conditions and how these feeding characteristics were related to ontogeny, prey availability and visual capabilities. Larvae collected during both day and night were analysed, and it was found that feeding incidence was high, regardless of the time of day. Examination of the visual system corroborated the hypothesis that hake larvae should be able to cope with a wide range of photic conditions and to forage even at low light intensity. A clear preference for adult calanoid copepods and, especially, for Clausocalanus spp. was observed in all sizes analysed. Prey number increased with larval size, but prey size did not. This finding indicates that hake larvae behave as selective and specialist predators that consume an increasing number of prey rather than larger prey during larval growth.     

Determining the diet of larvae of the red rock lobster (Jasus edwardsii) using high-throughput DNA sequencing techniques

The diet of Jasus edwardsii during its ~2-year larval (phyllosoma) phase is largely unknown. High mortalities experienced during larviculture might be reduced if their diet were nutritionally modelled on the natural diet. Here, prey species were identified from phyllosoma midgut glands using 454 pyrosequencing of 18S rDNA. We found that gelatinous zooplankton, particularly Siphonophora and Ctenophora, occurred frequently in the midgut glands of phyllosomas, resolving previous conjecture that these animals are in the diet of J. edwardsii phyllosomas. A high occurrence of sequencing reads from unicellular microbes may also reflect a reliance on scavenging detritus.     

Evidence of abalone (<Emphasis Type="Italic">Haliotis rubra</Emphasis>) diet from combined fatty acid and stable isotope analyses

Abalone are common herbivores throughout temperate and tropical waters, and yet the contribution of red and brown macroalgae to the diet of wild abalone remains unclear. In the northern hemisphere, adult abalone are considered to consume predominantly brown algae, but in the southern hemisphere abalone are thought to prefer red algae. Conventional methods such as gut content analysis and feeding trials provide some insight into diet choice, but the associated biases of these techniques create uncertainty surrounding the aforementioned variability in abalone diet. We use combined stable isotope and fatty acid analysis to determine the relative contribution of red algae, brown algae and detritus/microalgae to the diet of wild abalone in Tasmanian waters. Stable isotopes of carbon suggest that brown algae and/detritus are a more important source of carbon than red algae. Fatty acid analysis confirmed the larger contribution of brown algae to the diet of abalone, and also identified the bacterial and diatom component of detritus to be an important contributor to abalone diet. These results show combined use of chemical tracers to be a promising technique for resolving abalone diet, and challenge current perceptions regarding spatial variability in abalone diet choice.     

Prey selection by the hogchoker, Trinectes maculatus (Pisces: Soleidae), along summer salinity gradients in Chesapeake Bay, USA

We investigated feeding by the hogchoker, Trinectes maculatus (Bloch and Schneider), in freshwater, oligohaline, mesohaline, and polyhaline regions of Chesapeake Bay, USA, and examined prey selection in relation to food availability. Otter trawling for fish and Van Veen grab sampling for benthic macrofauna occurred in July and August 1992 and August and September 1993. Hogchokers exhibited both opportunistic and selective feeding patterns along the estuarine salinity gradient in four tributaries (Potomac, Rappahannock, York, and James Rivers) and in the mainstem Chesapeake Bay. Major prey taxa included annelids, arthropods, and tellinid siphons. In polyhaline habitat, polychaetes dominated both the benthos and gut contents numerically and gravimetrically. On the other hand, oligochaetes were numerically dominant in freshwater/oligohaline areas but were rarely eaten, perhaps because of their burial depth. Arthropods (mostly amphipods) occurred at most salinities, were common in gut contents in low-salinity areas, and were replaced as prey by larger proportions of polychaetes in polyhaline regimes. Although hogchokers ate tellinid siphons, they rarely consumed whole bivalves or gastropods. These diet patterns (and especially the importance of siphon nipping) are similar to those of juvenile or small flatfish elsewhere in Europe, Africa, and North America. A size–salinity relationship for hogchokers occurred along the summer salinity gradient, with smaller fish predominating upstream and larger fish downstream. It was not clear from our data if variation in diet composition reflected changes in prey composition along the salinity gradient rather than changes in fish size. Received: 14 June 1997 / Accepted: 27 June 1997     

Feeding habits of the Madeira rockfish <Emphasis Type="Italic">Scorpaena maderensis</Emphasis> from central Mediterranean Sea

In order to assess diet composition and niche breadth of this species, we analysed the stomach content of 182 specimens collected monthly along the eastern coast of Sicily (Central Mediterranean Sea). Overall, 50 prey taxa belonging to five major groups (algae, gastropods, crustaceans, polychaetes, fishes) were identified in 102 full stomachs. Benthic or epibenthic crustaceans, such as decapods, amphipods and isopods were the most important prey, whereas algae, gastropods, polychaetes and fishes were only occasionally ingested. In terms of composition by species, the diet of Scorpaena maderensis was characterized by a variety of rare or unimportant prey, which was consumed by few individuals only, although sometimes in large amount. As a result, S. maderensis can be considered a generalized and opportunistic feeder. The feeding intensity followed roughly a seasonal trend, with a minimum food intake in summer. The individual fish size was the most important factor affecting diet. According to the observed ontogenetic shift, small-sized individuals fed primarily on small crustaceans (i.e. amphipods and isopods), whereas large-sized specimens consumed preferably bigger and more vagile prey, such as walking and swimming decapods. No significant differences in diet were observed in relation to sex of predator and sampling season.     

Cryptic prey colouration increases search time in brown trout (<Emphasis Type="Italic">Salmo trutta</Emphasis>): effects of learning and body size

Little is known about how cryptic colouration influences prey search in near-surface aquatic habitats, although such knowledge is critical for understanding the adaptive value of colour crypsis as well as the perceptive constraints influencing foraging behaviour in these environments. This study had two main aims: (1) to investigate how background colour matching by prey affects foraging efficiency by brown trout parr and (2) to investigate how foraging ability on cryptic and conspicuous prey is affected by fish size at age (reflecting dominance). We addressed these questions by training wild brown trout parr to forage individually on live brown-coloured maggots on a cryptic (brown) or conspicuous (green) background. A separate experiment confirmed the absence of trout preference for brown or green substrate. The results show that prey background colour matching increases search time in brown trout. Search time generally decreased by learning, but conspicuous prey remained an easier prey to find throughout the six training trials. Thus, perceptive constraints appear to limit search efficiency for cryptic prey, suggesting that cryptic colouration can confer survival benefits to prey in natural environments. Smaller fish generally found conspicuous prey faster than larger individuals, whereas search time for cryptic prey was not influenced by body size. This suggests that smaller individuals compensate for inferior competitive ability by increasing foraging activity rather than improving cognitive ability. The technique of varying cognitive demands in behavioural tasks could be used more in future studies aimed at distinguishing motivational effects from cognitive explanations for variation in behavioural performance.     

Contrasting feeding patterns among species of fish larvae from the tropical Andaman Sea

Feeding habits of tropical fish larvae were analysed in a comparative study of four species (Scorpaenodes sp., Carangoides sp., Acanthocepola sp. and Cynoglossus sp.) from the Andaman Sea. We investigated morphological characteristics and their potential influence on larval feeding, and looked for common patterns in larval prey preference. Gut contents of a total of 300 larvae were examined and compared with local zooplankton composition. The feeding habits of the investigated larvae shared a number of characteristics. During ontogeny both the preferred prey size and the number of prey in the gut increased, and across all larval size classes the relative prey size spectrum stayed constant, of approximately the same magnitude for all four species. On the other hand, larval feeding also differed in a number of aspects, especially differences in the taxonomic composition of preferred prey were apparent. Scorpaenodes sp. preferred abundant and large prey taxa, Acanthocepola sp. and Carangoides sp. preferred large, but less common prey taxa, while Cynoglossus sp., which had the relatively smallest mouth size, preferred smaller sized prey groups. Hence, the findings indicate that from an offset of common characteristics, especially related to prey size preference, larvae have their individual feeding patterns related to specific morphology and patterns of distribution.Communicated by M. Kühl, Helsingør     

Distributional ecology and food habits of the banded blennyParaclinus fasciatus (Clinidae): a resident in a mobile habitat

An ecological study of the banded blennyParaclinus fasciatus (Steindachner, 1876) showed that the fish is tolerant of physical/chemical conditions in the shallow subtidal area of Apalachee Bay, Florida (USA). Population density ofP. fasciatus was unrelated to physical/chemical characteristics and seagrass biomass at field stations. A strong correlation was found between numbers ofP. fasciatus captured and abundance of unattached red algae (drift algae) at the field sites, although seasonality in population density was related to summer spawning. Spawning began at approximately 28°C. The life span of the clinid is probably little longer than 1 yr. Long-term variation in population size was also demonstrated.P. fasciatus was found to be a highly specialized carnivore which consumes only a few species of amphipods and shrimps; however, major seasonal variation in diet occurred. Variations in food habits were related to known patterns of prey abundance in Apalachee Bay. The most important prey species taken byP. fasciatus, includingCymadusa compta andHippolyte zostericola, are associates of red algae masses, further substantiating the close functional relationship of the clinid with its algal habitat which moves about the bay. The driftalgae habitat is probably an important source of shelter, food, and dispersal for the banded blenny. The eurytolerance ofP. fasciatus makes it particularly well adapted for life in an unusual, mobile habitat.Gontribution No. 173 of Harbor Branch Foundation, Inc.     

Ecological success of the cladoceran <Emphasis Type="Italic">Penilia avirostris</Emphasis> in the marine environment: feeding performance,gross growth efficiencies and life history

Marine cladocerans are important contributors to the zooplankton community of tropical and temperate coastal ecosystems during the warmer months, when they show explosive population growth. Despite this fact, little information is available on their ecology compared with the extensive studies on their freshwater relatives. The main objective of this study was to determine the in situ feeding and growth rates, and life history parameters of Penilia avirostris in São Sebastião Bay (Brazil) during austral summer 2004, as a premise to understand the advantages of this cladoceran in oligotrophic waters. Culture development experiments, monitored for a period of 12 days, showed that maximum juvenile release occurred after 2 days, and that the development duration of a complete cohort was around 9 days. From bottle incubation grazing experiments, significant ingestion rates upon flagellates, ciliates, dinoflagellates and diatoms were detected. Flagellates were the most important contributors to P. avirostris diet (ca. 80%). P. avirostris ingested between 28 and 97% of its own carbon biomass per day (daily ration) and individual growth rates of this marine cladoceran (0.10–0.24 d^?1) increased with prey availability. The combination of ingestion rates of natural prey and growth rates provided gross growth efficiencies (GGE) of 15–53%, on a carbon basis. Our results suggest that P. avirostris has similar GGE to copepods, although at low food conditions the values for the marine cladocerans seems slightly higher. However, this characteristic alone does not explain the explosive growth and community dominance shown by P. avirostris. Therefore, other traits related to the reproductive biology of the species, such as short generation time, parthenogenetic reproduction, and continuous somatic growth, seems to be mostly responsible for the success of P. avirostris in many marine ecosystems during their seasonal occurrence.     

珠江流域野生淡水鱼中有机紫外线吸收剂的组织分布、生长稀释和物种特异性生物蓄积

以华南珠江流域9种野生淡水鱼为材料,本文研究了12种有机紫外线吸收剂（UVA）的组织分布、个体体型依赖性和物种特异性生物蓄积。鱼组织样品中12种UVA的浓度为109～2320 ng/g脂质重量（lw）。在超过一半的鱼组织样本中检测到2-羟基-4-甲氧基二苯甲酮（BP-3）、八甲基丙烯（OCR）、UV531和5种苯并三唑类紫外线稳定剂（UVP、UV329、UV234、UV328和UV327）。UV531在野生淡水鱼体内具有明显的生物蓄积潜力,其生物蓄积因子（Log BAF）和生物沉积物蓄积因子（BSAF）分别为4.54±0.55和4.88±6.78。一般来说,肝脏（989±464 ng/g lw）的UVA含量最高,其次是腹部脂肪（599±318 ng/g lw）、膀胱（494±282 ng/g lw）、背肌（470±240 ng/g lw）和卵（442±238 ng/g lw）。淡水野生鱼体内UVA的生物积累表现出物种特异性和化合物依赖性。摄食杂食性鱼类的底栖食碎屑者含有明显较高的UVA浓度,表明摄食碎屑/沉积物是野生淡水鱼暴露于UVA的重要途径。UV531和BP-3表现出生长稀释的潜力。代谢可能在消除鱼组织中的UVA方面发挥重要作用,其中肝脏的代谢率最高。在珠江流域的淡水生态系统中,UVA没有显示出明显的营养级放大作用。有必要进行更多的研究来阐明UVA在生物体内的母体转移。     

Relation of food preference to fitness for the green sea urchin,Strongylocentrotus droebachiensis

We maintained 30 to 35 mm sea urchins (Strongylocentrotus droebachiensis) on 17 different macroalgal diets in the field over a 130-d period and correlated the resulting changes in somatic and gonadal tissues with measures of food preferences. The varions algae fell into there distinct categories according to their ability to support growth. The most striking effect of diet was on gonadal mass, there being a 14-fold increase for the best algal diet, compared to initial gonadal mass, and nearly a threefold decrease for the worst diet. The relation of food preference to somatic growth was nonlinear. A sharp increase in the growth of test diameter, total mass and test mass occurred between the nonpreferred and intermediately preferred algae, but growth rates were similar for intermediate and preferred algae. Gonadal growth tended to increase exponentially with an important measure of food preference, feeding rate (g ingested d^–1). These observations suggested that urchins only channel food resources into somatic growth up to a given point, additional reserves being used for producing gametes. The positive relationship of food preference to growth, and especially the strong correlation with gonadal production, indicates that the highly selective feeding of the urchin contributes to ils fitness.     

Integration of a fish bioenergetics model into a spatially explicit water quality model: Application to menhaden in Chesapeake Bay

Although fish are usually thought of as victims of water quality degradation, it has been proposed that some planktivorous species may improve water quality through consumption of algae and sequestering of nutrients via growth. Within most numerical water quality models, the highest trophic level modeled explicitly is zooplankton, prohibiting an investigation of the effect a fish species may be having on its environment. Conversely, numerical models of fish consumption do not typically include feedback mechanisms to capture the effects of fish on primary production and nutrient recycling. In the present study, a fish bioenergetics model is incorporated into CE-QUAL-ICM, a spatially explicit eutrophication model. In addition to fish consumption of algae, zooplankton, and detritus, fish biomass accumulation and nutrient recycling to the water column are explicitly accounted for. These developments advance prior modeling efforts of the impact of fish on water quality, many of which are based on integrated estimates over an entire system and which omit the feedback the fish have through nutrient recycling and excretion. To validate the developments, a pilot application was undertaken for Atlantic menhaden (Brevoortia tyrannus) in Chesapeake Bay. The model indicates menhaden may reduce the algal biomass while simultaneously increasing primary productivity.     

Prey selection by larval weakfish (Cynoscion regalis): the effects of prey size,speed, and abundance

We examined feeding by larval weakfish, Cynoscion regalis (Bloch and Schneider), in laboratory experiments conducted during the 1991 spawning season. under natural conditions weakfish larval development is ca. 3 wk, and we ran separate experiments with larvae of five different ages (5, 8, 11, 14, and 17 d post-hatching). We used two different size classes of rotifers (Brachionus plicatilis) and brine shrimp nauplii (Artemia sp.) as prey organisms. Contrary to results of previous research, weakfish larvae did not select prey based on size alone. When prey abundance was above 100 itemsl^-1 weakfish, larvae always chose large rotifers (length = 216 m) over small rotifers (length = 160 m). At 11 d post-hatching, larvae switched their diet from large rotifers to small brine shrimp nauplii (length = 449 m); however, when fed small rotifers and small brine shrimp nauplii the change in diet occurred at 14 d post-hatching. This pattern of selectivity was maintained in each larval age class. Early-stage larvae (5 and 8 d post-hatching) did not feed selectively when prey abundance was less than 100 itemsl^-1. Late-stage larvae (17 d post-hatching) fed selectively at abundances ranging from 10 to 10000 items^-1. Lwimming speeds of prey items, which ranged from 1 to 6 mms^-1, had no consistent effect on prey selection. These results suggest that weakfish larvae are able to feed selectively, that selectivity changes as larvae age, and that selectivity is also influenced by prey abundance.     

Foraging behavior in early Atlantic cod larvae (Gadus morhua) feeding on a protozoan (Balanionsp.) and a copepod nauplius (Pseudodiaptomussp.)

Food limitation is likely to be a source of mortality for fish larvae in the first few weeks after hatching. In the laboratory, we analyzed all aspects of foraging in cod larvae (Gadus morhua Linnaeus) from 5 to 20 d post-hatching using protozoa (Balanion sp.) and copepod nauplii (Pseudodiaptomus sp.) as prey. A camera acquisition system with two orthogonal cameras and a digital image analysis program was used to observe patterns of foraging. Digitization provided three-dimensional speeds, distances, and angles for each foraging event, and determined prey and fish larval head and tail positions. Larval cod swimming speeds, perception distances, angles, and volumes increased with larval fish size. Larval cod swam in a series of short intense bursts interspersed with slower gliding sequences. In 94% of all foraging events prey items were perceived during glides. Larval cod foraging has three possible outcomes: unsuccessful attacks, aborted attacks, and successful attacks. The percentage of successful attacks increased with fish size. In all larval fish size classes, successful attacks had smaller attack distances and faster attack speeds than unsuccessful attacks. Among prey items slowly swimming protozoans were the preferred food of first-feeding cod larvae; larger larvae had higher swimming speeds and captured larger, faster copepod nauplii. Protozoans may be an important prey item for first-feeding larvae providing essential resources for growth to a size at which copepod nauplii are captured. Received: 20 April 1999 / Accepted: 12 January 2000     

Optimal foraging: The influence of intraspecific competition on diet selection

Summary Three-spined sticklebacks (Gasterosteus aculeatus) showed a relative preference for a familiar prey size when hunting for two sizes of Daphnia magna in high density. This result is not compatible with the apparent size hypothesis. Ten groups were investigated, each consisting of two stickle-backs tested under three consecutive experimental conditions, to establish whether the function of the preference for a familiar prey size could be avoidance of competition.First, the relative competitive ability of each fish was determined by the proportion it consumed of three series of 60 medium-sized daphnia, offered pairwise. Second, during the consumption of five series of 30 pairs, each consisting of a large and a small daphnia, it was determined how many items each fish caught of each prey size. As handling times were equal for both prey types, the larger prey size was more profitable. There was a significant correlation between relative competitive ability and mean proportion of large daphnia in the diet. In the last series the less successful competitiors caught a higher proportion of small prey than in the first series. Finally, each fish was given the choice between large and small daphnia in the absence of its competitor. The sticklebacks chose a diet similar to the one they had been allowed to select previously with competition. The previously more successful competitors concentrated on large daphnia, whereas the poorer competitors fed as generalists but not unselectively.The fish probably learned the distance from which they had recently attacked familiar prey successfully. This sure attack distance depends on the fish's competitive ability.     

Feeding behaviour and prey size selection of gilthead seabream,Sparus aurata,larvae fed on inert and live food

Prey selection shortly after the onset of feeding by laboratory-reared gilthead seabream, Sparus aurata L., larvae was studied using larvae fed on two types of microcapsule (hard- and soft-walled) having diameters ranging from 25 to 300 m. Preferences between inert food and live prey (rotifers and Artemia sp. nauplii) were also studied. Seabream larvae were able to ingest inert food from first feeding. Larvae of all size classes ingested hard microcapsules with diameters in the range 25 to 250 m. However, larvae with a total length (TL) below 4 mm preferentially selected particles 25 to 50 m in diameter, larvae of TL 4 and 5 mm preferred particles 51 to 100 m in diameter, while larvae above 5 mm TL preferred particles 101 to 150 m in diameter. With soft microcapsules, larvae always preferred particles larger than in the previous case, and above 4.5 mm TL they preferentially selected particles 201 to 250 m in diameter. In addition, the gradual increase of preferred diameters with increasing TL was more pronounced when larvae were increasing TL was more pronounced when larvae were fed on soft particles. Mean values for prey width/mouth width ratios were approximately 0.24 and 0.30 when larvae were fed on hard-walled and soft-walled microcapsules, respectively, irrespective of the absolute value of larval length. When a mixed diet of live and inert food items was offered, live prey were always preferentially selected, even if the prey width/mouth width ratio was apparently not favourable. Only a physical constraint such as excessive prey width could counter this preference for living prey vs inert microcapsules. These results contribute to our knowledge in larval feeding behaviour, especially in the presence of inert food, and represent a fundamental step in developing prepared food for marine fish larvae.