Size-dependent predation on marine fish larvae by Ctenophores, Scyphomedusae, and Planktivorous fish

Size-dependent predation rates on marine fish larvae by the ctenophore Mnemiopsis leidyi , scyphomedusa Chrysaora quinquecirrha , and planktivorous fish Anchoa mitchilli were estimated via experiments in 3.2 m³ me-socosms. Predation rates on larvae of the goby Gobio-soma bosci were estimated in relation to 1) length of larval prey, 2) presence or absence of alternative < 1 mm zooplankton prey, and 3) a predator-prey interaction between the scyphomedusa and ctenophore. The consumption rate of larvae by the three predators generally declined as larval length increased. The ctenophore ate fewer (1.0 d^-1 per predator) larvae than did the medusa (7.8 d ^-1 per predator) or bay anchovy (4.0 d ^-1 per predator), but it consumed larvae in all size classes tested (3.0–9.5 mm SL). Predation by bay anchovy and the medusa was more size-dependent: 3.0–5.0 mm larvae were most vulnerable. Fewer larvae were eaten by the ctenophore and bay anchovy when zooplankton alternative prey was available, but predation on larvae by the medusa was not influenced by alternative zooplankton prey. Consumption rate of fish larvae by the medusa was reduced 20–25% when the ctenophore was present as its alternative prey. An encounter-rate model was developed and its parameters estimated from the experimental results. Model simulations indicated that the relationship between larval size and vulnerability is dependent on size, swimming speed, and behavior of both predators and larvae, and that bigger or faster-growing fish larvae are not always less vulnerable to predation.     

RNA–DNA ratio of herring and sand lance larvae from Port Moller, Alaska: Comparison with prey concentration and temperature

A key assumption of hypotheses that link the production of prey for larval fish with year-class strength of fish is that larval growth and condition is food-limited. We tested this assumption by comparing whole-body RNA-DNA ratios of individual Pacific herring, Clupea pallasi , larvae and Pacific sand lance, Ammodytes hex-apterus , larvae from Port Moller, a subarctic Alaskan estuary, with prey concentration and temperature. RNA-DNA ratios were correlated with larval length, but not with prey concentration or temperature. Ratios were not significantly different between a warm, well-mixed station with low prey concentrations and a colder, stratified station with higher prey concentrations. Using RNA-DNA ratios, we classified as starving 11 to 23% of first-feeding (< 13 mm long) herring larvae and 45% of first-feeding (< 7 mm long) sand lance larvae. However, starvation could not have been caused by low concentrations of prey because micro-zooplankton prey concentrations were high enough (16 to 84 prey L^_l) to support relatively high rates of growth. Therefore, starving larvae were either abnormal or they were stIII learning to forage. We conclude that the magnitude of starvation among first-feeding herring and sand lance larvae, and, therefore, the total contribution of starvation to year-class strength, is dependent not only on prey concentration, but on the proportion of a population of larvae that can feed effectively.     

A general biophysical model of larval cod (Gadus morhua) growth applied to populations on Georges Bank

Field-derived growth rates (RNA-DNA based) of cod (Gadus morhua) larvae collected on the southern flank of Georges Bank were higher on average in May 1993 than May 1994, despite the apparent higher abundance of potential prey in 1994. A biophysical modeling study is presented here in which factors are examined that may have led to the difference in population mean growth. A one-dimensional physical model, forced by winds and tides, was used to simulate the vertical structure (of currents, temperature field, and turbulent kinetic energy dissipation rate) following a column of water in a Lagrangian sense at a site on the southern flank of Georges Bank over 5-day periods in late May of 1993 and 1994. The biophysical model and observed zooplankton abundance allowed us to explore the vertical structure and temporal (hourly) evolution of feeding and growth for cod larvae in relation to environmental conditions. Our trophodynamic model is improved over previous versions and now includes the effect of light on larval feeding response, as well as the effect of temperature on larval metabolic costs, ingestion, and digestion. Larval prey profiles, comprising four copepod species, were used from a time series of 1/4-m² MOCNESS tows to define the prey field. Data from a collateral time-series of larval gut contents (1-m² MOCNESS tows) was used to define maximum ingestion (satiation level) and prey selection. Model outputs provide depth-dependent estimates of growth, prey biomass ingested, larval length, and larval weight. Water-column growth-rate profiles were made for four size classes of larvae (5, 6, 7 and 9 mm) under the environmental conditions observed in May 1993 and 1994. A weighted-mean growth rate based on the mean vertical distribution of larvae was estimated for each size class. In all cases, when using all available potential prey, the model-derived 1994 growth rates were higher (by 3–6% day⁻¹) than those for 1993. However, simulations in which 7-mm larvae followed the field-derived weighted mean depth over the sampling period, and were limited to their preferred Pseudocalanus prey, resulted in average growth of 12.2% day⁻¹ for 1993 and 9.7% day⁻¹ for 1994. These compared closely to the field growth means of 11.3% day⁻¹ in 1993 and 9.8% day⁻¹ in 1994. Thus, the lower observed growth in May 1994 may have resulted from depth-dependent food limitation and prey-selectivity coupled with the greater metabolic costs induced by the higher temperature that year.     

Ecological and Ethological Perspectives in Larval Fish Feeding

ABSTRACT

Low survival rates during larval stages constitute a major bottleneck in the successful culture of many marine and some freshwater fish. The availability of live food is recognized as a critical factor influencing larval survival. Live food is still superior to the best larval diets in terms of larval survival and growth. This paper reviews important ecological and ethological aspects of feeding, from hatching tothe weaning stage, and relates them to problems in larval culture. In general, freshwater fish larvae are easier to raise than marine fish larvae, because at hatching they are larger and endowed with more yolk reserves, are less sensitive to starvation, and canbe weaned to artificial diets sooner. The feeding behavior of the larvae can be analyzed in terms of the sequential components of predation: search, encounter, pursuit, attack, capture, and ingestion. The searching efficiency and encounter rates of the visual predator are influenced by prey parameters such as body size, conspicuousness, and evasiveness. Turbidity of the water and light intensity also affect prey detection. To changing prey densities, the larvae show typical Type II functional responses, which are influenced by prey handling time, which in turn is largely a function of prey size. Knowledge of larval functional responses is helpful in providing the right concentrations oflive food for larval culture. The larvae are initially gape-limited and exhibit prey size selectivity but gradually widen their prey size range as they grow. An aquacultural application of this is the commonly employed feeding protocol, prey size sequencing, in which progressively larger live food items are offered as the larvae grow. A thorough knowledge of the feeding behavior is also essential in the formulation of acceptable larval diets.     

Factors affecting the feeding response of larval southern bluefin tuna,Thunnus maccoyii (Castelnau, 1872)

Southern bluefin tuna, Thunnus maccoyii, are cultured in Australia following collection of wild juveniles. Hatchery culture from egg is in the experimental stage. High early mortality has hindered the production of quality juveniles in the hatchery. This study investigated the visual capacity of T. maccoyii during early larval ontogeny in order to describe the best larval rearing conditions to produce high‐quality seed stock. Functional visual ability, determined through behavioural experimentation, identified the effect of light intensity, prey density, turbidity, tank colour and turbulence on the feeding response. Larvae were visually challenged to feed under a range of conditions in short‐duration (4 h) feeding experiments. Feeding performance was measured as the proportion of larvae feeding and the intensity of feeding. First‐feeding performance was positively affected by increasing prey density and lower turbidities and unaffected by light intensity, tank colour, turbulence, prey size and larval density. The key findings from feeding experiments on 6 and 9 dph larvae was that as T. maccoyii aged, lower light intensities and higher prey densities significantly increased feeding performance. In addition, the study has identified that high light intensity and high air‐driven turbulence induced significant mortality. The proficient first‐feeding response indicated that early mortality common in culture is unlikely to be associated with a failure to initiate feeding. Our findings show the use of low light intensity has the potential to significantly improve survival and feeding response during the first two critical weeks of culture, when the major bottleneck in hatchery production is currently experienced.     

Early weaning of winter flounder (Pseudopleuronectes americanus Walbaum) larvae on a commercial microencapsulated diet

Like most small marine fish larvae, the stomachs of winter flounder Pseudopleuronectes americanus are undeveloped at first feeding and have relatively reduced digestive capacity. This work was undertaken to test whether larvae at the onset of stomach differentiation (larval size about 5.5 mm) could be early weaned onto a commercial microencapsulated diet. We assessed the effect of early weaning by first comparing growth performance (standard length, total protein content and age at metamorphosis) of larvae fed enriched live prey from first feeding to a size of 5.5 mm and then reared on three different feeding regimes until metamorphosis: (1) live prey (LP) as a control group; (2) mixed feeding of live prey and microencapsulated diet (LP‐ME); (3) exclusively microencapsulated diet (ME) after fast weaning over 4 days (to a larval size of 6.2 mm). No differences were observed between larval development in the two first groups, which began metamorphosis at 40 days old. The larvae of the third group showed significantly slower growth that resulted in a delay of 4 days in the onset of metamorphosis. Differences in live prey availability between the treatments and the short transition period to allow the larvae to adapt to the new diet were identified as possible contributing factors to the slower growth and to the delay in metamorphosis of early weaned larvae. In a second experiment, the transitional weaning period was increased until the larvae were 6.6 mm in length. Weaning at that size resulted in no slowing of growth or delay in metamorphosis, suggesting that the feeding schedule was adequate.     

Effect of light intensity,prey density,and ontogeny on foraging success and prey selection of larval yellow perch (Perca flavescens)

Light intensity has been shown to influence the foraging success of larval fish. However, the effect of light intensity on larval foraging is likely variable and influenced by both the density and characteristics of planktonic prey. In this study we examined the influence of light intensity of 0.1, 2.0, and 60 μmol·s^?1·m^?2 Photosynthetically Active Radiation (PAR) on foraging of yellow perch (Perca flavescens) larvae at two prey densities. We fed them with a mixture of zooplankton taxa common to lakes inhabited by yellow perch. In addition to light intensity and prey density, the effect of larval yellow perch size was examined by using fish ranging from 9 to 15 mm. The results of our study indicated that yellow perch larvae are well adapted to feed at a wide range of light intensities, as there was no difference in foraging success at investigated light intensities. Increasing prey density from 25 to 150 (zooplankton·l^?1) significantly improved the foraging success of larval yellow perch. However, the influence of prey density on foraging success was dependent on fish length. Improved foraging success at increased prey densities occurred only for individuals with a total length >10 mm. Overall, prey selection by fish larvae was influenced by light intensity, prey density, and fish length. However, the factors that influenced selection for specific prey types differed. Our study, combined with evidence from other field and laboratory work, highlight the need for a better understanding of the influence of prey density on foraging throughout ontogeny.     

Environmental factors influencing larval sprat Sprattus sprattus feeding during spawning time in the Baltic Sea

The management of Baltic sprat is challenged by highly variable recruitment success and hence large stock fluctuations. Recent studies have identified the larval and early juvenile life stages to be critical for the survival rate of a sprat year class. Although prey abundance was found to be linked to larval survival success, an analysis identifying the functional relationship and relative importance of other environmental factors is still missing. Sprat larval feeding was investigated in 2002 during three cruises, covering the main spawning time in the Bornholm Basin, Baltic Sea. The aim of the study was to identify the key environmental factors determining the feeding success of larval sprat taking their potential interactions explicitly into account. An extension of generalized additive models (GAMs) was adopted that allows the inclusion of interaction terms in a non‐parametric regression model. The final model of sprat larval feeding success explained ～80% of the variance in the data and was based on the following environmental factors: bottom depth, cubed wind speed as proxy for small‐scale turbulence rates, degree of cloudiness as proxy for light conditions and prey density in combination with a feeding period–cloudiness interaction term. Our study demonstrates that the feeding success of sprat larvae in the Baltic Sea is controlled by a number of simultaneously acting key environmental factors.     

An individual-based model for the direct conversion of otolith into somatic growth rates

Otolith increment width and larval fish data (length and weight) were used to develop an individual‐based model (IBM) to describe daily resolved growth rates of North Sea herring (Clupea harengus) larvae (Autumn Spawners) caught during International Herring Larvae Surveys in the ICES area IVa from 1990 to 1998. The model combines sagittal otolith readings (core and individual increment measurements), larval standard length and weight data, and solves an over‐determined set of linear system equations for all parameters using the method of least square residuals. The model consists of a matrix, which describes the increment width formation of 119 larvae, a vector containing their length/weight measurements, and a vector describing residuals. The solution vector yields age‐dependent maximum somatic growth rates of herring larvae up to an age of 41 days with sizes ranging from 10 to 25 mm. The observed environmental temperature in which larvae dwelled was relatively uniform. Therefore, measured increment width was individually used to determine daily growth from any single larva in relation to their potential maximum growth under optimal feeding conditions. The results are discussed with respect to the spatial and temporal variability of larval occurrence. Finally, an analysis of error estimation of the larval growth characteristics is presented.     

Survival and growth of selected marine fish larvae first fed with eggs and endotrophic larvae of the sea urchin Paracentrotus lividus

Two sets of experiments were carried out to evaluate the potential of eggs and endotrophic larvae of captive Paracentrotus lividus as alternative live prey for marine fish larvae first feeding. The first consisted in rearing sparids, Diplodus sargus and Sparus aurata, larvae until 15 days after hatching in a recirculation system. Compared with the commonly used live prey – rotifer Brachionus spp. – general lower values of survival and growth were obtained when fish larvae were fed with the alternative live prey. Among these, eggs showed to be the preferred feeding. Broodstock feed showed to play a fundamental role on prey quality and consequent fish larvae survival. In the second set of experiments, the 24‐h ingestions of the first feeding larvae in static water were determined for five currently cultured fish larvae species. Except for larger and more predatory Dicentrarchus labrax larvae, there was a trend for higher P. lividus egg ingestion, followed by pre‐plutei and prisms. Prey size, colour and movement affected food selection by fish larvae. It is concluded that, in spite of the alternative live prey being readily consumed by all tested fish larvae, they cannot however presently compete with rotifers in marine fish larvae first feeding.     

Coupling ecosystem and individual‐based models to simulate the influence of environmental variability on potential growth and survival of larval sprat (Sprattus sprattus L.) in the North Sea

To investigate the impact of changing environmental conditions in the North Sea on the distribution and survival of early life stages of a marine fish species, we employed a suite of coupled model components: (i) an Eulerian coupled hydrodynamic/ecosystem (Nutrients, Phyto‐, Zooplankton, Detritus) model to provide both 3‐D fields of hydrographical properties, and spatially and temporally variable prey fields; (ii) a Lagrangian transport model to simulate temporal changes in cohort distribution; and (iii) an individual‐based model (IBM) to depict foraging, growth and survival of fish early life stages. In this application, the IBM was parameterized for sprat (Sprattus sprattus L.) and included non‐feeding (egg and yolk‐sac larval) stages as well as foraging and growth subroutines for feeding (post‐yolk sac) larvae. Sensitivity analyses indicated that the angle of visual acuity, assimilation efficiency and the maximum food consumption rate were the most critical intrinsic model parameters. As an example, we applied this model system for 1990 in the North Sea. Results included not only information concerning the interplay of temperature and prey availability on larval fish survival and growth but also information on mechanisms underlying larval fish aggregation within frontal zones. The good agreement between modelled and in situ estimates of sprat distribution and growth rates in the German Bight suggested that interconnecting these different models provided an expedient tool to scrutinize basic processes in fish population dynamics.     

Relationships among vertically structured in situ measures of turbulence,larval fish abundance and feeding success and copepods on Western Bank,Scotian Shelf

Using vertically stratified data of the abundance of silver hake (Merluccius bilinearis) larvae and concentrations of copepods collected in the field, we examine relationships among the vertical distribution of larval fish, their potential prey, feeding success and water column turbulence. Water column turbulence and associated stratification parameters were estimated from: (i) in situ measures of turbulent kinetic energy dissipation (?) provided by an EPSONDE profiler; (ii) in situ wind speed; (iii) the Richardson number (Ri); and (iv) the buoyancy frequency (N²). Small (< 5 mm total length) silver hake were more abundant in the least turbulent waters (i.e. at a minimum in the rate of dissipation of turbulent kinetic energy, ? < 10^–7 W kg^?1; Ri > 0.25; N² > 0.001 (rad s^–1)²). Partial correlations amongst ?, N² and small hake larvae were significant only for N². The abundance of larger (> 5 mm total length) hake larvae was positively correlated with depth and was not associated with either ? or N². Vertical distributions of three potential prey (classified by stage) were variable. Early stage copepodids were positively correlated with N² and negatively correlated with ?. We found no evidence of diel distribution patterns for small (< 5 mm total length) hake larvae or for any of the developmental stages of the copepods examined. Neither estimate of water column turbulence inferred from wind speed nor from Ri was meaningfully related to in situ estimates of ? or to larval fish abundance. Feeding success, measured either as prey items (gut)^–1, average prey length, or total prey volume (gut)^–1, was not related to predicted encounter rates between days. However, the average prey length (gut)^–1 was significantly (P < 0.01) related to water column turbulence. These conflicting results suggest that the relationship between larval feeding and the environment is more complicated than assumed. We conclude that without substantial high resolution in situ examination of the relationship between the vertical distributions of turbulence, larvae and their prey, the growing acceptance in the secondary literature that turbulence has a positive and biologically meaningful effect on trophic interactions between fish and their zooplankton prey (a generalization based largely on modelling and laboratory experiments) is premature.     

A production season of turbot larvae Scophthalmus maximus (Linnaeus, 1758) reared on copepods in a Danish (56°N) semi‐intensive outdoor system

Turbot were reared from yolk sack larvae to juvenile in an outdoor semi‐intensive system. Three production cycles were monitored from May to September. A pelagic food chain was established with phytoplankton, copepods and turbot larvae. Abiotic and biotic parameters of lower trophic levels together with turbot larval survival, development, prey electivity and growth were monitored. A decreasing larval survival from 18.4% in May to 13.6% in July and just 7.0% in September was observed. The overall phytoplankton and copepod abundance decreased during the productive season. The turbot larval growth showed significant differences between larvae below (isometric) and above (allometric) 7 mm. Larval fish gut content showed no differences with available prey between production cycles. Therefore, it appears that the available prey concentration is governing their growth in this outdoor system. First‐feeding turbot larvae exhibited active selection for nauplii whereas developed larvae switched to copepodites and adult copepods. Although developing turbot larva exhibited active selection towards copepod size classes, there was no evidence of selective feeding on either of the two dominant copepod species. The turbot larvae's prey ingestion was modelled together with the standing stock of copepod biomass. The model results indicated that the estimated need for daily ingestion exceeded the standing stock of copepods. Hence, the initially established food web was unable to sustain the added turbot larvae with starvation as a consequence. We therefore suggest several solutions to circumvent starvation in the semi‐intensive system.     

Modelling changes in the length–frequency distributions of fish larvae using field estimates of predator abundance and size distributions

The goal of this study is to determine if an individual-based size-dependent model can realistically simulate changes in the length–frequency distributions of several species of fish larvae collected in Conception Bay in 1993 and 1994, using field estimations of growth and predator abundance. We first model the length–frequency distribution of field samples with the best possible estimates of mean growth rate. Then, we add predation mortality given the characteristics of the predator community observed during our surveys, which was composed of macrozooplankton and adult capelin. The larval fish community is generally not affected by predation by macrozooplankton, as the average instantaneous mortality rate predicted by the model was 0.004 day^–1. Fish larvae appear to be more vulnerable to predation by the population of adult capelin. We estimate that an abundance of adult capelin ranging between 0.2 and 1.0 individuals per 1000 m^–3 may have a substantial impact on the larval fish community. The predictions of an individual-based model are directly related to the accuracy of estimates of the mean growth rates of the larval fish cohorts. We find that it is difficult to differentiate size-selective removal of individuals from random selection by analysing changes of the length–frequency distributions of the larval fish community.     

Ingestion of Artemia nauplii by Chinese mitten crab Eriocheir sinensis zoea larvae

A series of ingestion trials were conducted to determine the ingestion rate of Artemia nauplii by Eriocheir sinensis zoea larvae with increasing densities of Artemia and with or without rotifers as a co-feed. At each zoeal stage, 10 groups of 10 larvae were reared individually in glass beakers and fed with increasing densities of newly hatched Artemia nauplii (0.5, 2.5, 5, 10 and 20 individual (ind.) mL⁻¹) with or without rotifers (15–25 ind. mL⁻¹) as a co-feed. The average number of ingested Artemia was measured over 24 h. In addition, the average larval development rate (Larval Stage Index, LSI) over a longer period (time needed for the best treatment to reach 100% moult or metamorphosis to the next larval stage) was compared. The results showed that Artemia ingestion rate of E. sinensis larvae increased with increasing prey densities and larval development, and had a significantly negative correlation with rotifer consumption for all zoeal stages. Rotifers as an alternative prey significantly affected the intake of Artemia at early larval stages (Z1 and Z2) and promoted LSI at a lower Artemia density. Further experiments are needed to clarify the effect of prey density on survival and larval development when larvae are reared communally.     

Match/mismatch predictions of spawning duration versus recruitment variability

According to the match/mismatch hypothesis, larval fish survival and eventual recruitment is dependent on the offset time between the peaks of abundance of larvae and their planktonic prey. A rudimentary larval food supply model is developed to determine the dependence of food availability on the mismatch between peaks. The model predicts that recruitment variability should increase as spawning duration decreases, a result which is moderately supported by an analysis of Atlantic cod (Gadus morhua) data.     

Temporal and spatial variability in growth and condition of dab (Limanda limanda) and sprat (Sprattus sprattus) larvae in the Irish Sea

Variability in the high mortality rate during early life stages is considered to be one of the principal determinants of year‐class variability in fish stocks. The influence of water column stability on the spatial distribution of fish larvae and their prey is widely acknowledged. Water column stability may also impact growth through the early life history of fishes, and consequently alter the probability of survival to maturity by limiting susceptibility to predation and starvation. As a test of this concept, the variability in condition and growth of dab (Limanda limanda) and sprat (Sprattus sprattus) larvae was investigated in relation to seasonal stratification of the water column in the north‐western Irish Sea. RNA/DNA ratios and otolith microincrement analysis were used to estimate nutritional status and recent growth rates of larvae captured on four cruises in May and June of 1998 and 1999. Dab and sprat larvae were less abundant in 1999 and were in poorer condition with lower growth rates than in 1998. Dab larvae of <13 mm also exhibited spatial variability with higher RNA/DNA ratios at the seasonal tidal‐mixing front compared with stratified and mixed water masses. However, the growth and nutritional status of sprat larvae was uncorrelated to water column stability, meaning the more favourable feeding conditions generally associated with the stratified pool and tidal‐mixing front in the Irish Sea were not reflected in the growth and condition of these larvae. This suggests that the link between stability, production and larval growth is more complicated than inferred by some previous studies. The existence of spatio‐temporal heterogeneity in the growth and condition of these larvae has implications for larval survival and the recruitment success of these species in the Irish Sea.     

Impact of spectral composition on larval haddock,Melanogrammus aeglefinus L., growth and survival

In a small‐scale culture experiment, larval haddock, Melanogrammus aeglefinus L., were raised under various combinations of light quality [blue (470 nm), green (530 nm) or full‐spectrum white light] and light intensity [low (0.3–0.4 µmol  s^?1  m^?2) or high (1.7–1.9 µmol  s^?1  m^?2)], and in total darkness (both fed, and starved). Larval growth (0.9% day^?1 in standard length; 2.4% day^?1 in body area) was not significantly different between any combination of coloured light. At the time of total mortality in the starved treatment, survival was significantly reduced under low intensity, full‐spectrum white light (13%) vs. all other coloured light treatments (68%). Larvae raised under both continuous dark treatments (fed and starved) exhibited morphological changes associated with irreversible starvation (point‐of‐no‐return). Lack of a pronounced effect of light quality on larval haddock growth probably results from a combination of plasticity in early larval vision, and enhanced encounter rates between larvae and prey at the relatively high prey densities used in aquaculture.     

Molecular identification of laser‐dissected gut contents from hatchery‐reared larval cod,Gadus morhua: a new approach to diet analysis

The transition between endogenous and exogenous feeding in hatchery production of fish larvae has long been a bottleneck to increased production. Identification of alternative prey species with a wider array of nutritional profiles is essential for further expansion and diversification within the aquaculture industry. Traditional morphological methods to identify dietary composition are limited. In this study, we present a novel DNA‐based methodology for identifying the gut contents of larval fish that is independent of prey retaining identifiable characteristics. Laser capture microdissection is used to collect ingested material for DNA extraction, thus limiting contaminating DNA originating from the larval fish. PCR is then conducted using universal eukaryotic primers that have the potential to detect a wide diversity of prey items. Using this approach, we identified ingested Artemia salina from hatchery‐reared Gadus morhua larvae. Differential length amplification PCR was used to evaluate the effects of DNA degradation on the sensitivity of A. salina detection. Although it was possible to detect A. salina with relatively long (626 bp) fragments, targeting smaller fragments (264 bp) resulted in the highest return of prey sequences. This combined LCM and molecular approach to diet analysis could offer a more complete assessment of the diets of naturally feeding larvae.     

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.