Prey Ingestion Rates Research Articles

Polychaete Worm Density Correlates with Aggregate Shorebird Density, Prey Intake Rate and Foraging Success at an Intertidal Mudflat in Bangladesh

Polychaete Worm Density Correlates with Aggregate Shorebird Density, Prey Intake Rate and Foraging Success at an Intertidal Mudflat in Bangladesh

Spawning, larval morphological ontogeny and first feeding of yellow-striped cardinalfish Ostorhinchus cyanosoma (Bleeker, 1853)

The yellow-striped cardinalfish (Ostorhinchus cyanosoma) can be found in the Indo-West Pacific region. It possesses great potential for the ornamental aquarium market due to its vibrant body coloration and tendency of group swimming. This study aims to investigate the spawning frequency and larval morphological ontogeny, and develop first feeding techniques of O. cyanosoma reared in capacity. From April 2021 to August 2022, a total of 70 spawning events were documented for the broodstock. These spawning events did not exhibit specific patterns related to seasonal or lunar cycles. The effects of four live prey treatments (mono-feeding and co-feeding of SS-type rotifer Brachionus sp. and nauplii of copepod Bestiolina coreana) on the mouth opening rate, prey ingestion rate, and prey selectivity of the 0 day-post-hatch (dph) larvae were investigated. The O. cyanosoma larvae displayed significantly higher mouth opening rate, ingestion rate and prey selectivity on the copepod nauplii over the rotifer. In the 7-d larviculture experiment, the larvae showed significant higher survival rate in the copepod feeding treatment (68%) than the rotifer feeding treatment (0%). For ontogeny, O. cyanosoma larvae appeared to be altricial (at pre-flexion stage with no gas bladder and poor-developed fins) and small-mouthed (235.94 μm) when leaving the mouthbrood. The standard length increased from 2.64 to 6.51 mm during 0 to 21dph. Ntochord flexion initiated at 12 dph, and the metamorphosis was completed by 21 dph. This study is the first attempt to document the broodstock management, first feeding protocol and larval ontogeny of O. cyanosoma. The findings could have implications for larviculture of cardinalfishes.

Weaver ants regulate the rate of prey delivery during collective vertical transport.

The collective transport of massive food items by ant teams is a striking example of biological cooperation, but it remains unclear how these decentralized teams coordinate to overcome the various challenges associated with transport. Previous research has focused on transport across horizontal surfaces and very shallow inclines, disregarding the complexity of natural foraging environments. In the ant Oecophylla smaragdina, prey are routinely carried up tree trunks to arboreal nests. Using this species, we induce collective transport over a variety of angled surfaces with varying prey masses to investigate how ants respond to inclines. We found that weight and incline pose qualitatively different challenges during transport. Prey were carried over vertical and inclined surfaces faster than across horizontal surfaces, even though inclines were associated with longer routes and a higher probability of dropping the load. This additional speed is associated with more transporters being allocated to loads on steeper inclines and not from the persistence of individual ants. Ant teams also regulated a stable prey delivery rate (rate of return per transporter) across all treatments. Our proposed constrained optimization model suggests a possible explanation for these results; theoretically, prey intake rate at the colony level is maximized when the allocation of transporters yields a similar prey delivery rate across loads.

Microplastics in biotic and abiotic compartments of high-mountain lakes from Alps

Microplastic (MP) pollution is a major environmental concern for mountain ecosystem and for high-mountain lakes in particular, which are recognized indicators of global change. In this study, the presence of MPs was assessed in abiotic (water and sediment) and biotic (zooplankton, tadpoles, fish) compartments of two high-mountain lakes (Upper Lake Balma and Lower Lake Balma) in the Cottian Alps (northwest Italy). No MPs were found in water and zooplankton samples, whereas the mean MPs in sediment samples was 1.33 ± 0.67 items/m3 and 1.75 ± 0.62 items/m3 in Lower and Upper Lake Balma, respectively. The mean MPs in tadpoles of Rana temporaria was 0.33 ± 0.58 items/individual and 0.66 ± 0.58 items/individual in Lower and Upper Lake Balma, respectively. The mean number of MPs items found in the gastrointestinal tract (GIT) of fish (Salvelinus fontinalis) was considerably higher in specimens from the Lower (0.45 items/g GIT) than in those from the Upper Lake (0.20 items/g GIT). There was a negative relationship between fish size (weight and age) and MPs abundance in the GIT of fish, indicating that young fish accumulated more MP items probably due to the high prey ingestion rate compared to adults. The same MPs color (blue, white, black), shape (fibers and fragments), and chemical type (polypropylene and polyethylene) were found in the compartments of both lakes. Our findings suggest the use of S. fontinalis as an indicator of MP pollution in high-mountain lakes. Further studies are needed to better understand the sources and the effects of MPs in these remote ecosystems.

Prey ingestion rates revealed by back-mounted accelerometers in Eurasian spoonbills

BackgroundQuantifying foraging success in space and time and among individuals is essential for answering many ecological questions and may guide conservation efforts. However, collecting this information is challenging for species that forage on mobile prey and are difficult to observe visually, for example, because they forage in inaccessible areas or at night. In such cases, the use of tracking devices that simultaneously collect location and acceleration data may provide a solution if foraging success can be extracted successfully. The aim of this study was to assess how well searching for and ingesting prey, among other behaviours, could be distinguished from 20 Hz acceleration data collected by GPS/ACC-trackers mounted on the back of Eurasian spoonbills Platalea leucorodia. Upon capturing a prey, spoonbills make a distinct movement with their head and back to throw the prey from the tip of the bill into the throat.MethodsWe compared the behavioural classification performance of random forest models that were trained and tested on video-annotated acceleration data segments of different (fixed or flexible) lengths. The best-performing model was then applied to 4 years of data of spoonbills foraging in the Wadden Sea during the breeding season, to explore seasonal and annual variation in prey ingestion rates.ResultsHighest classification accuracies (as indicated by the F-measure, a balanced measure of precision and sensitivity) of foraging behaviours were achieved by analysing short fixed-length segments (0.4–0.8 s) or “flexibly-cut” segments. The F-measure was very high (> 0.90) for searching, standing, sitting and flying (distinguishing active and passive flight), 0.73 for ingesting prey and 0.65 for walking. False positive and negative prey ingestions were equally likely and most often confused with searching, resulting in a close match between the predicted and observed prey ingestion rates. Application of the best-performing model revealed strong seasonal patterns in prey ingestion rates in the Wadden Sea that varied between years.ConclusionsWe show that prey ingestion rates of spoonbills can be fairly accurately estimated from acceleration data. These results are promising for the use of spoonbills equipped with GPS/ACC-trackers as monitors of spatial and temporal variation in the availability of small fish and shrimp, which is key to understand the foraging and migratory movements of spoonbills and provides information on the quality of (coastal) wetlands.

Impaired grazing of marine protozoa in sub-lethal exposure to the water accommodated fraction of crude oil and dispersant

Despite the advances in safety technology and the improved implementation of precautionary measures, crude oil pollution has been occurring in the oceans globally. The water accommodated fraction (WAF) of crude oil and chemical dispersant are hypothesized to cause sub-lethal adverse effects on marine protists that are pivotal consumers of primary production. Exposure experiments were conducted to investigate the effects of crude oil and dispersant pollutants on the growth and grazing, separately, of protozoa species in cultures. In exposure to 0–30 μL L−1 of chemically enhanced WAF (CEWAF), the heterotrophic dinoflagellate Protoperidinium sp. and the ciliate Metacylis sp. showed slower positive population growth or negative population growth even at low concentrations. The dose-response model showed that Protoperidinium sp. and Metacylis sp. were highly susceptible to the CEWAF toxicity (median inhibition concentrations (IC50) at 1.1 and 5.9 μL L−1, respectively) while one algal species Ditylum brightwellii was relatively tolerant to the toxicity (IC50 at 168.7 μL L−1). With suppressed growth and impaired grazing of the protozoan species at high CEWAF concentrations, accumulation of their algal prey in culture containers was observed, as reflected by higher final:initial prey ratios at high CEWAF concentrations. Additionally, exposure experiments to the treatments of WAF, dispersant alone (Disp), and CEWAF of the same concentration revealed that the heterotrophic dinoflagellate Oxyrrhis marina had reduced bulk grazing impact towards its algal prey population in all three treatments when compared to the control treatment (i.e., grazing mortality of prey at 1.05 d−1). Similarly, Protoperidinium sp. and Metacylis sp. had reduced per capita prey ingestion rates in exposure to WAF and CEWAF when compared to the control treatments. This study provides experimental evidence for the potential link between impaired grazing activities and the formation of algal blooms in sub-lethal exposure to crude oil pollutants.

Mechanisms and fluid dynamics of foraging in heterotrophic nanoflagellates

AbstractHeterotrophic nanoflagellates are the main consumers of bacteria and picophytoplankton in the ocean. In their microscale world, viscosity impedes predator–prey contact, and the mechanisms that allow flagellates to daily clear a volume of water for prey corresponding to 106 times their own volume is unclear. It is also unclear what limits observed maximum ingestion rates of about 104 bacterial preys per day. We used high‐speed video microscopy to describe feeding flows, flagellum kinematics, and prey searching, capture, and handling in four species with different foraging strategies. In three species, prey handling times limit ingestion rates and account well for their reported maximum values. Similarly, observed feeding flows match reported clearance rates. Simple point force models allowed us to estimate the forces required to generate the feeding flows, between 4 and 13 pN, and consistent with the force produced by the hairy (hispid) flagellum, as estimated using resistive force theory. Hispid flagella can produce a force that is much higher than the force produced by a naked (smooth) flagellum with similar kinematics, and the hairy flagellum is therefore key to foraging in most nanoflagellates. Our findings provide a mechanistic underpinning of observed functional responses of prey ingestion rates in nanoflagellates.

The importance of nonnative Pacific oyster reefs as supplementary feeding areas for coastal birds on estuary mudflats

Abstract A combined empirical and modelling approach was used to investigate the value of a Pacific oyster reef to feeding shorebirds and to observe and predict the impact of reef clearance on bird populations in the Colne Estuary, a protected area in south‐east England. Macro‐invertebrate biomass and numbers of feeding birds were measured on a Pacific oyster reef, an adjacent uncolonized mudflat, and an area of mudflat that had been cleared of oysters 6 months previously. These data were used to parameterize an individual‐based model (MORPH) to predict the impact of clearance of the reef on winter bird survival. Feeding success and intake rates of Eurasian oystercatcher, Eurasian curlew, and Eurasian common redshank were also recorded during the course of a winter. The macro‐invertebrate diversity and biomass within both the oyster reef and the cleared area were significantly greater than the adjacent uncolonized mudflat. The density and biomass of large invertebrate prey in the mudflat were low, yet the Pacific oyster reef had much higher densities and biomass of large prey, especially annelids and shore crabs. The winter assemblage of feeding birds differed significantly between each of the areas. The mean total number of feeding birds was significantly greater on the uncolonized mudflat; however, mean peak counts, feeding success rate and prey intake rate of Eurasian oystercatcher were greater on the reef. Significantly greater intake rates and feeding success rates were also observed on the reef for Eurasian curlew, a species of conservation concern. Field data and model predictions show that Pacific oyster reefs can provide valuable supplementary feeding areas for some shorebirds, yet other species avoided the reef. However, as estuaries vary in available feeding resources, it is important that the value of reefs and their management is determined regionally.

Incubation duration effects on copepod naupliar grazing estimates

Copepod naupliar grazing estimates often appear to represent an insignificant fraction of prey community mortality, despite high naupliar abundances and weight-specific ingestion rates. To address this seeming paradox, the impact of incubation time on grazing by nauplii of the subtropical copepod Parvocalanus crassirostris on natural prey assemblages was evaluated. Measurements of prey removal during feeding experiments were taken every 6-h over a 24-h period during two experiments (E1, E2), where the initial 2–35μm natural prey biomass differed by 3-fold, i.e., 66μgCL−1 (E1) vs. 198μgCL−1 (E2). Results showed that total prey ingestion rate estimates decreased over the course of incubation by up to 75% after 24-h, with highest ingestion rates obtained during the initial 6-h. In no predator controls, total prey biomass also decreased significantly during the 24-h incubation particularly in the smallest prey size groups despite the absence of nauplii, and positive non-significant trends were observed in prey >10μm in the experiment with higher initial prey abundances. Predator density effects in this experiment provided additional evidence for trophic cascades. Results of this work suggest that in communities with rapidly changing natural prey assemblages and predators with short development times, short incubations minimize bottle effects and reduce the risk of measuring grazing rates on prey communities that no longer resemble the in situ populations.

Co-occurrence and commensal feeding between Little Egrets Egretta garzetta and Eurasian Spoonbills Platalea leucorodia

ABSTRACTCapsule: The spatial distribution and feeding efficiency of Little Egrets Egretta garzetta wintering in the gulf of Gabès, Tunisia, are affected by a commensal association with the Eurasian Spoonbills Platalea leucorodia.Aims: To investigate the role of the interspecific interaction between Little Egrets and Eurasian Spoonbills in shaping the spatial distribution and feeding efficiency of Little Egrets.Methods: Using count and behavioural data, we examined the co-occurrence of these species in flocks, and compared the foraging efficiency of Little Egrets feeding with Eurasian Spoonbills with that of solitary Little Egrets.Results: We found that the presence of Eurasian Spoonbills doubled the chance of Little Egrets being present. Within mixed flocks, the number of Little Egrets increased with the number of Spoonbills. Moreover, Little Egrets foraging in association with Eurasian Spoonbills took fewer steps, had higher pecking rates and higher prey intake rates than solitary Little Egrets.Conclusion: Little Egrets appear to obtain foraging efficiency benefits by following Eurasian Spoonbills. This interaction seems to play a role in determining the spatial distribution of Little Egrets.

Starving seabirds: unprofitable foraging and its fitness consequences in Cape gannets competing with fisheries in the Benguela upwelling ecosystem

Fisheries are often accused of starving vulnerable seabirds, yet evidence for this claim is scarce. Foraging energetics may provide efficient, short-term indicators of the fitness status of seabirds competing with fisheries. We used this approach in Cape gannets (Morus capensis) from Malgas Island, South Africa, which feed primarily on small pelagic fish in the southern Benguela upwelling region, thereby competing with purse-seine fisheries. During their 2011–2014 breeding seasons, we determined body condition of breeding adult Cape gannets and measured their chick growth rates. In addition to these conventional fitness indices, we assessed the daily energy expenditure of breeding adults using a high-resolution time-energy budget derived from GPS-tracking and accelerometry data. For these same individuals, we also determined prey intake rates using stomach temperature recordings. We found that adult body condition and chick growth rates declined significantly during the study period. Crucially, most birds (73 %) studied with electronic recorders spent more energy than they gained through foraging, and 80–95 % of their feeding dives were unsuccessful. Our results therefore point to unprofitable foraging in Cape gannets, with a longer-term fitness cost in terms of adult body condition and reproductive performance that corresponds to a local population decline. Based on this evidence, we advocate a revision of regional fishing quotas for small pelagic fish and discuss the possibility of an experimental cessation of purse-seine fishing activities off the west coast of South Africa. These measures are needed for the ecological and socio-economical persistence of the broader southern Benguela upwelling ecosystem.

Summing the strokes: energy economy in northern elephant seals during large-scale foraging migrations.

BackgroundThe energy requirements of free-ranging marine mammals are challenging to measure due to cryptic and far-ranging feeding habits, but are important to quantify given the potential impacts of high-level predators on ecosystems. Given their large body size and carnivorous lifestyle, we would predict that northern elephant seals (Mirounga angustirostris) have elevated field metabolic rates (FMRs) that require high prey intake rates, especially during pregnancy. Disturbance associated with climate change or human activity is predicted to further elevate energy requirements due to an increase in locomotor costs required to accommodate a reduction in prey or time available to forage. In this study, we determined the FMRs, total energy requirements, and energy budgets of adult, female northern elephant seals. We also examined the impact of increased locomotor costs on foraging success in this species.ResultsBody size, time spent at sea and reproductive status strongly influenced FMR. During the short foraging migration, FMR averaged 90.1 (SE = 1.7) kJ kg−1d−1 – only 36 % greater than predicted basal metabolic rate. During the long migration, when seals were pregnant, FMRs averaged 69.4 (±3.0) kJ kg−1d−1 – values approaching those predicted to be necessary to support basal metabolism in mammals of this size. Low FMRs in pregnant seals were driven by hypometabolism coupled with a positive feedback loop between improving body condition and reduced flipper stroking frequency. In contrast, three additional seals carrying large, non-streamlined instrumentation saw a four-fold increase in energy partitioned toward locomotion, resulting in elevated FMRs and only half the mass gain of normally-swimming study animals.ConclusionsThese results highlight the importance of keeping locomotion costs low for successful foraging in this species. In preparation for lactation and two fasting periods with high demands on energy reserves, migrating elephant seals utilize an economical foraging strategy whereby energy savings from reduced locomotion costs are shuttled towards somatic growth and fetal gestation. Remarkably, the energy requirements of this species, particularly during pregnancy, are 70–80 % lower than expected for mammalian carnivores, approaching or even falling below values predicted to be necessary to support basal metabolism in mammals of this size.Electronic supplementary materialThe online version of this article (doi:10.1186/s40462-015-0049-2) contains supplementary material, which is available to authorized users.

Prey type and foraging ecology of Sanderlings Calidris alba in different climate zones: are tropical areas more favourable than temperate sites?

Sanderlings (Calidris alba) are long-distance migratory shorebirds with a non-breeding range that spans temperate and tropical coastal habitats. Breeding in the High Arctic combined with non-breeding seasons in the tropics necessitate long migrations, which are energetically demanding. On an annual basis, the higher energy expenditures during migration might pay off if food availability in the tropics is higher than at temperate latitudes. We compared foraging behaviour of birds at a north temperate and a tropical non-breeding site in the Netherlands and Ghana, respectively. In both cases the birds used similar habitats (open beaches), and experienced similar periods of daylight, which enabled us to compare food abundance and availability, and behavioural time budgets and food intake. During the non-breeding season, Sanderlings in the Netherlands spent 79% of their day foraging; in Ghana birds spent only 38% of the daytime period foraging and the largest proportion of their time resting (58%). The main prey item in the Netherlands was the soft-bodied polychaete Scolelepis squamata, while Sanderlings in Ghana fed almost exclusively on the bivalve Donax pulchellus, which they swallowed whole and crushed internally. Average availability of polychaete worms in the Netherlands was 7.4 g ash free dry mass (AFDM) m−2, which was one tenth of the 77.1 g AFDM m−2 estimated for the beach in Ghana. In the tropical environment of Ghana the Sanderlings combined relatively low energy requirements with high prey intake rates (1.64 mg opposed to 0.13 mg AFDM s−1 for Ghana and the Netherlands respectively). Although this may suggest that the Ghana beaches are the most favourable environment, processing the hard-shelled bivalve (D. pulchellus) which is the staple food could be costly. The large amount of daytime spent resting in Ghana may be indicative of the time needed to process the shell fragments, rather than indicate rest.

Larval fish feeding ecology, growth and mortality from two basins with contrasting environmental conditions of an inner sea of northern Patagonia, Chile

During austral spring 2011, a survey was carried out in the inland sea (41°30′–44°S) of north Patagonia, South Pacific, studying a northern basin (NB: Reloncaví Fjord, Reloncaví Sound and Ancud Gulf) characterized by estuarine regime with stronger vertical stratification and warmer (11–14 °C) and most productive waters, and a southern basin (SB: Corcovado Gulf and Guafo mouth), with more oceanic water influence, showed mixed conditions of the water column, colder (11–10.5 °C) and less productive waters. Otolith microstructure and gut content analysis of larval lightfish Maurolicus parvipinnis and rockfish Sebastes oculatus were studied. Larval M. parvipinnis showed similar growth rates in both regions (0.13–0.15 mm d−1), but in NB larvae were larger-at-age than in SB. Larval S. oculatus showed no differences in size-at-age and larval growth (0.16 and 0.11 mm d−1 for NB and SB, respectively). M. parvipinnis larvae from NB had larger number of prey items (mostly invertebrate eggs), similar total volume in their guts and smaller prey size than larvae collected in SB (mainly calanoid copepods). Larval S. oculatus had similar number, volume and body width of prey ingested at both basins, although prey ingestion rate by size was 5 times larger in NB than in SB, and prey composition varied from nauplii in NB to copepodites in SB. This study provides evidence that physical–biological interactions during larval stages of marine fishes from Chilean Patagonia are species-specific, and that in some cases large size-at-age correspond to increasing foraging success.

Predation by Apocyclops royi (Cyclopoid: Copepod) on Ciliates and Rotifers

We studied the prey consumption rates in Apocyclops royi on neonate and adult rotifers, Brachionus rotundiformis and ciliate, Euplotes sp. in the presence and absence of algae, the smaller Isochrysis galbana and the larger Tetraselmis chui. Both males and females of A. royi were able to successfully ingest neonate and adult rotifers and ciliate. The prey consumption rates were significantly lower in males than in females of A. royi. Regardless of sex and reproductive state of the copepod, prey consumption rates were higher on neonates rotifer than on adults. However, the prey consumption rates were significantly lower on ciliate than rotifer in the female copepods. Further, the prey ingestion rates did not differ statistically between ovigerous and non-ovigerous adults of A. royi females. The ovigerous rotifer was ingested at lower rates than either non-ovigerous or neonates rotifer by both males and females of A. royi. The presence of algae (no matter I. galbana or T. chui) negatively influenced the rotifer and ciliate consumption rates. The present study provides first information on predatory efficiency of A. royi, and attests its omnivory feeding habit. Our results suggest that A. royi can utilize heterotrophic food efficiently and can derive nutrients during periods of low primary production. The present study points to the role of A. royi in forming a link between the microbial loop and classical food chain, which expedites the flow of bacterial carbon to higher trophic levels in estuarine ecosystems.

Ciliate (Euplotes sp.) predation by Pseudodiaptomus annandalei (Copepoda: Calanoida) and the effects of mono-algal and pluri-algal diets

AbstractBackgroundPseudodiaptomus annandaleiis an estuarine species and being cultured as live feed for grouper fish larvae and other planktivores. We examined the predation behavior ofP. annandaleiadults when preying on ciliated protists (Euplotessp.) and the effects of mono- and pluri-algal diets on ciliate predation byP.annandaleiunder laboratory conditions. The algal food comprised the pigmented flagellateIsochrysis galbana(4 ~ 5 μm) andTetraselmis chui(17 ~ 20 μm).ResultsMales and females ofP.annandaleiconsumed 8 ~ 15 ciliate cells/h. The probability of ciliate ingestion following an attack was a direct function of the copepod's hunger level. Conversely, the probability of prey rejection after capture was a negative function of the copepod's hunger level. Starved and poorly fed females showed a significantly lower rate of prey rejection compared to similarly treated males. The duration of handling a ciliate prey did not significantly differ between males and females ofP.annandalei. Starved copepods spent less time handling a ciliate prey than fed copepods. Prey ingestion rates showed a negative relation with the feeding duration, whereas the prey rejection rate increased as the feeding duration increased. The ciliate consumption rate ofP. annandaleiwas significantly lower in the presence of mixed algae. NeitherI.galbananorT.chuialone had any significant effect on ciliate consumption byP. annandalei.ConclusionsThe results confirmed thatP. annandaleiingests bacterivorous heterotrophic protists even in the presence of autotrophic protists. Therefore, our results point to the role ofP. annandaleiin the transfer of microbial carbon to the classical food chain in estuarine and brackish water ecosystems.

Feeding requirements of white sharks may be higher than originally thought

Quantifying the energy requirements of animals in nature is critical for understanding physiological, behavioural, and ecosystem ecology; however, for difficult-to-study species such as large sharks, prey intake rates are largely unknown. Here, we use metabolic rates derived from swimming speed estimates to suggest that feeding requirements of the world's largest predatory fish, the white shark (Carcharodon carcharias), are several times higher than previously proposed. Further, our estimates of feeding frequency identify a clear benefit in seasonal selection of pinniped colonies - a white shark foraging strategy seen across much of their range.

Foraging mechanisms of age-0 lake trout (Salvelinus namaycush)

Reaction distances under various light intensities (0-19 uE/m2/s), angles of attack, swimming speeds, and percentage of overall foraging success were measured. Extensive efforts have been invested in restoring lean lake trout (Salvelinus namaycush) populations in the Laurentian Great Lakes, but successful natural recruitment of lake trout continues to be rare outside of Lake Superior and parts of Lake Huron. There is evidence of high mortality during the first several months after eggs hatch in the spring, but little is known about the foraging mechanisms of this age-0 life stage. We developed a foraging model for age-0 lake trout (S. namaycush) in response to amphipods (Hyalella azteca) and mysids (Mysis diluviana) by simulating underwater environmental conditions in the Great Lakes using a temperature-controlled chamber and spectrally matched lighting. Reaction distances under various light intensities (0–19 uE/m2/s), angles of attack, swimming speeds, and percentage of overall foraging success were measured. Intake rates under different light intensities and prey densities were also measured. Age-0 lake trout were non-responsive in the dark, but were equally responsive under all light levels tested. Age-0 lake trout also demonstrated a longer reaction distance in response to moving prey, particularly mysids, which had an escape response that reduced overall foraging success. We determined that prey intake rate (numeric or biomass) could be modeled most accurately as a function of prey density using a Michaelis–Menton equation and that even under low mysid densities (3 individuals/m2), age-0 lake trout could quickly satisfy their energetic demands in a benthic setting.

Visual thresholds for feeding and optimum light intensity for larval rearing of Asian seabass,Lates calcarifer(Bloch)

This study examined the retinomotor responses and prey ingestion rates of 10-, 15-, 20- and 30-day-old Asian seabass Lates calcarifer under different light intensities from 0 to 1000 lx to determine the visual thresholds. Subsequently, two age groups of seabass larvae were reared under light intensities of 10, 100 and 1000 lx to determine the optimum illumination in hatchery tanks. Retinomotor response was absent in 10-day-old larvae, but quite marked in 15- and 20-day-old seabass at 1 lx and higher. Ingestion of Artemia nauplii by 10-day-old larvae was almost zero at <1 lx, increased significantly at 1 lx, and was maximal at 10-100 lx. Artemia ingestion under dim light <1 lx improved with age, and older larvae took more prey in complete darkness due to the presence of rod cells (and also free neuromasts). Larvae from 13 to 26 days group had similar survival and growth at 10-1000 lx, however, from 5 to 10 days group showed similar survival rate with highest weight gain at 100 lx. Therefore, we recommend that hatchery rearing tanks be illuminated such that the larvae in the water are exposed to approximately100 lx.

Turbulence effects on the feeding dynamics in European sea bass (Dicentrarchus labrax) larvae

Turbulence, unless reaching a threshold level, is generally assumed to enhance ingestion rates in larval fish by increasing encounters with prey. This hypothesis is mainly based on the assumption of a prey-limited environment. In some estuarine areas characterized by high turbulence levels, this hypothesis is rarely met. This study experimentally investigated the effects of flow turbulence (flow Reynolds number Re, ranging from 0 to 55×103) on prey ingestion in European sea bass (Dicentrarchus labrax) larvae exposed to prey densities comparable to what is found in a nursery area (i.e., Seine estuary, English Channel). This paper also estimated the prey ingestion rates in sea bass larvae exposed to three different flow conditions (Re=0, 21.6×103, and 43.9×103) during various time periods. Contrary to predictions regarding limiting prey densities, the analysis of individual gut contents under different levels of turbulence revealed that increasing Reynolds number (Re) decreased prey ingestion. In addition, increased turbulence affected the feeding to satiation levels in sea bass larvae.