Visual Feeding Research Articles

Water quality characteristics of Vanderkloof Dam and its potential for rainbow trout farming

Vanderkloof Dam was periodically sampled between November 2014 and April 2016 for a range of water quality parameters to assess the potential for cage culture of rainbow trout, Oncorhyncus mykiss. The dam is strongly stratified in summer and autumn, although upper water column temperatures remain relatively cool, largely <25°C. The dam is considered oligotrophic, characterised by phosphorus limitation of primary production, particularly during summer. Light is strongly attenuated in the epilimnion and heterotrophic community metabolism is observed through much of the water column. Surface nutrients are depleted through the summer with phytoplankton assemblages dominated by the green algae, Oocystis lacustris, Desmodesmus bicaudatus and Coelastrum microporum. The water column turns over during the winter and the nearly isothermal impoundment approaches 11°C. Nutrients are re-introduced into surface waters following winter mixing coincident with an increase in diatoms. The preferred environmental window for rainbow trout becomes severely contracted during the summer in that epilimnion temperatures exceed 21°C and thermocline/hypolimnion oxygen concentrations fall below 3 mg‧L−1. The coincidence of relatively high pH >9.5 could exacerbate these physiological challenges. Hydrological conditions for fish farming are most suitable in the proximity of the dam wall as opposed to further upstream. Here the hypoxic conditions that develop in the thermocline/hypolimnion during summer and autumn are less pronounced, particularly towards the southern bank, where concentrations of total suspended solids are generally <10 mg‧L−1 and water clarity is more favourable for visual feeders such as fish. Although rainbow trout may survive the adverse conditions prevalent during the summer, growth is likely to be compromised and susceptibility to disease may increase.

Olfactory behavioural and neural responses of planktivorous lacustrine sockeye salmon (Oncorhynchus nerka) to prey odours.

Fish use a variety of sensory systems when foraging. Salmonids are generally considered visual feeders. However, some species feed on zooplanktons under dark conditions, suggesting they also detect prey using nonvisual cues. Under experimental conditions, hatchery-reared rainbow trout (Oncorhynchus mykiss) have been shown to use olfaction when searching for food pellets, but olfactory foraging has not been documented in wild salmonids. In the present study, to examine their behavioural response and neural activity in the olfactory nervous system using c-fos expression as a neural molecular marker, immature wild-caught lacustrine sockeye salmon (Oncorhynchus nerka) in a flow-through aquarium were exposed to zooplanktons (Daphnia spp.) extract including zooplanktons odorant and to dimethyl sulfide. The salmon exposed to zooplanktons odour increased their total swimming distance and time, numbers of turns and ascents, and c-fos expression in the olfactory bulb, suggesting that they can detect zooplanktons extract to locate prey in the laboratory experiments. However, no response was seen in those exposed to dimethyl sulfide. The results of this study suggest that prey odour may serve as a chemosensory cue for wild immature salmonids.

All day-long: Sticklebacks effectively forage on whitefish eggs during all light conditions.

The three-spined stickleback Gasterosteus aculeatus invaded Lake Contance in the 1940s and expanded in large numbers from an exclusively shoreline habitat into the pelagic zone in 2012. Stickleback abundance is very high in the pelagic zone in winter near the spawning time of pelagic whitefish Coregonus wartmanni, and it is hypothesized that this is triggered by the opportunity to consume whitefish eggs. Field sampling has qualitatively confirmed predation of whitefish eggs by stickleback, but quantification has proven difficult due to stormy conditions that limit sampling. One fundamental unknown is if freshwater stickleback, known as visual feeders, can successfully find and eat whitefish eggs during twilight and night when whitefish spawn. It is also unknown how long eggs can be identified in stomachs following ingestion, which could limit efforts to quantify egg predation through stomach content analysis. To answer these questions, 144 individuals were given the opportunity to feed on whitefish roe under daylight, twilight, and darkness in controlled conditions. The results showed that stickleback can ingest as many as 100 whitefish eggs under any light conditions, and some individuals even consumed maximum numbers in complete darkness. Furthermore, eggs could be unambiguously identified in the stomach 24 hours after consumption. Whitefish eggs have 28% more energy content than the main diet of sticklebacks (zooplankton) based on bomb-calorimetric measurements, underlining the potential benefits of consuming eggs. Based on experimental results and estimates of stickleback abundance and total egg production, stickleback could potentially consume substantial proportions of the total eggs produced even if relatively few sticklebacks consume eggs. Given the evidence that stickleback can feed on eggs during nighttime spawning and may thereby hamper recruitment, future studies aimed at quantifying actual egg predation and resulting effects on the whitefish population are urgently needed.

OPTIMUM DENSITY OF Nannochloropsis sp. FOR MASS SCALE CORAL TROUT, Plectropomus leopardus (Lacepède, 1802)

Coral trout, Plectropomus leopardus (Lacepède, 1802)is a visual feeder. Turbidity caused by phytoplankton or clay particle in the water will affect the visual foraging of coral trout larvae. Addition of Nannochloropsis sp. has been included in standard operational procedure for marine fish larval rearing as green water. However, the density of Nannochlorposis sp. in coral trout larval rearing system has not been evaluated. This study aimed to evaluate the optimal of Nannochloropsis sp. required for rearing of coral trout larvae. Nannochloropsis sp. was given to two days old larvae (D-2), with the densities of 2 x 105, 4 x 105, and 6 x 105cell/mL. After 50 days rearing period (D-50), evaluation on the average size and total harvest were recorded. The results showed that the density of 2 x 105 cell/mL Nannochloropsis sp. was the best in survival rate (2.35 ± 1.05%) than other densities, but they were not significantly different (P>0.05) than those of 4 x 105 cell/mL (1.67 ± 0.70%) and 6 x 105 cell/mL (1.26 ± 1.05%). The lower densities, 2 x 105 and 4 x 105cell/mL, were dominated by more than 50% of > 2.7 cm sized juvenile. Histological analysis of fish eyes supported that the two lower densities produced dominant cone shape as the receptor cells in the retina observed. From an economical aspect, addition of 2 x 105 cells/mL resulted on the higher profit, hence optimum density of Nannochloropsis sp. added in coral trout larval rearing on a mass scale was 2 x 105 cells/mL.

Evidence for tactile foraging in pterosaurs: a sensitive tip to the beak of Lonchodraco giganteus (Pterosauria, Lonchodectidae) from the Upper Cretaceous of southern England

The sense of touch is important for hunting and feeding in vertebrates, especially when visual cues are unreliable. Foramina in the jaws and face, associated with nerves and sensory organs, may provide information about feeding. Pterosaurs, many of which had large, well-developed eyes, are often assumed to have been visual feeders. Here, we show that the lonchodectid pterosaur Lonchodraco giganteus (Bowerbank, 1846) has clusters of circular foramina at the anterior mandibular symphysis (the odontoid) and on the lateral margins of the rostrum that indicate enhanced sensitivity of the rostrum tip. This pattern implies tactile foraging. The foramina were likely occupied by Herbst corpuscles or similar integumentary sensory micro-organs (ISO). They presumably served a sensory function at the jaw tip to enhance food gathering. A similar morphology occurs in some avians that feed using tactile cues, including probe feeders such as kiwis, sandpipers, and ibises, tactile hunters such as spoonbills, and filter feeders such as ducks and flamingos. The beak morphology of L. giganteus does not closely resemble that of these birds, and thus its modus operandi for feeding remains speculative, however tactile foraging for fish or invertebrates in shallow water seems likely. Like birds, pterosaurs evolved a diverse range of feeding strategies.

Overwinter thermal habitat use in lakes by anadromous Arctic char

Anadromous Arctic char (Salvelinus alpinus) migrate back to fresh water in late summer to spawn and (or) overwinter. Upon freshwater entry, feeding is reduced or absent, and movement activity is restricted. While the physiological responses to low temperatures (e.g., growth, metabolism) are understood, specifics of the use of thermal habitat for overwintering remains poorly characterized. This study used acoustic and archival telemetry data from two lakes in southern Labrador, Canada, to study thermal habitat use during the ice-covered period. Results showed that lake-dwelling anadromous Arctic char predominantly occupied a narrow range of temperatures (0.5–2 °C) and used cooler temperatures available within the middle and upper water column. Use of the selected temperatures is likely a strategy that lowers metabolic costs and minimizes energy expenditure, preserving stored lipids for overwinter survival and the energetic costs of preparation for seaward migration. As Arctic char are visual feeders, use of the upper water column is also thought to aid foraging efficiency by increasing the likelihood of prey capture.

Finfish vs jellyfish: complimentary feeding patterns allow threespine stickleback Gasterosteus aculeatus and common jellyfish Aurelia aurita to co-exist in a Danish cove

The threespine stickleback Gasterosteus aculeatus and the common jellyfish Aurelia aurita are keystone species in many marine ecosystems, including the shallow cove Kertinge Nor, in Denmark. Both species feed on zooplankton, raising the potential for competition between them. While jellyfish are tactile filtering planktivores, sticklebacks are visual feeders that actively detect, attack and capture prey. The study compared clearance rates (Cl) and tested the hypothesis that jellyfish are more efficient in feeding on small prey and sticklebacks on larger prey animals. Individual (Clind) and population (Clpop) feeding characteristics were studied under good visual conditions. Individual sticklebacks (TL = 44 mm) demonstrated 14–51-fold higher Clind than jellyfish (d = 27 mm) when feeding on small (< 1 mm) and medium (1–4 mm) sized prey and threefold higher Clind when feeding on larger prey (4–11 mm). Clpop was calculated for both species based on their densities in the cove. When consuming small- and medium-sized prey in May–July, Clpop for stickleback was 2–20-fold higher than for jellyfish, but in August following a decrease in fish density, Clpop was higher for jellyfish. This may imply higher predation pressure from stickleback on zooplankton in Kertinge Nor at the beginning of the season, though the common jellyfish was considered earlier as a species controlling zooplankton there. The two competing species likely coexist in the cove due to different seasonal cycles of abundance and thus different seasonal patterns of plankton consumption.

PERTUMBUHAN DAN KELANGSUNGAN HIDUP LARVA IKAN BETOK (Anabas testudineus) YANG DIBERI PENCAHAYAAN DENGAN LAMA WAKTU BERBEDA

ABSTRACT Climbing perch (Anabas testudineus) is a kind of swamp fish which is potentially to be cultivated. Photoperiod is one of factor that influence growth from culture fish larvae. Light also influence growth hormone and make larvae easier to eat their feed because larvae usually visual feeder. The aim of this research is to determine the best photoperiod in larvae of age 3 – 27 days so as to generate growth and survival rate climbing perch of larvae highest in aquarium with defferent of photoperiod. The usage of the research are expected that different photoperiod can influence the growth and survival of climbing perch larvaes. The research was conducted on April - May 2016 in Laboratory of Budidaya Perairan, Faculty of Agriculture, Sriwijaya University. This research methods used completely randomized design (CDR) with use six treatments and three replications of photoperiod those are P1(24L:0D), P2(18L:6D), P3(12L:12D), P4(6L:18D), P5(0L:24D). The parameters of this research are growth, survival rate of climbing perch larvae and maintenance of water quality. The result showed that photoperiod has significant effect of survival rate, weight growth and length growth from climbing perch photoperiod for 24 hours each day (treatments P1) has the highest result for survival rate and growth 81.11%±2.01%, 30.68mm±0.14mm, 0.46g±0.04g. The value of water quality during larvae rearing were temperature 27 – 31 oC, pH 5.70 – 7.10, dissolved oxygen 4.56 – 7.58 mg.L-1 and ammonia 0.02 – 0.11 mg.L-1. Keywords: Climbing perch, Growth, Photoperiod, Survival rate

Sensory Biology as a Risk Factor for Invasion Success and Native Fish Decline

AbstractNative freshwater fish populations are among the world's most threatened taxa due to the combined effects of habitat degradation and invasive alien species. Habitat degradation negatively impacts native species, whereas invasive species tend to possess adaptations, such as thermal and salinity tolerance, that are more suited to the degraded environment. Sensory ecology may also be a contributing factor. Most threatened native species are visual feeders, whereas invasive species found in degraded systems often have nonvisual specializations. Behavioral and distributional characteristics of the invasive Western Mosquitofish Gambusia affinis and the New Zealand native Inanga Galaxias maculatus illustrate the potential for sensory biology to influence foraging success, distribution, and species interaction between degraded and clear habitats. Behavioral trials measured the change in feeding rate in clear (0 NTU) and turbid (100 NTU) water over 30 min for Inanga and Western Mosquitofish feeding on brine shrimp Artemia salina nauplii. These experiments showed that Western Mosquitofish maintained similar feeding rates between clear and turbid water, whereas the native species exhibited a marked decline in feeding efficiency in turbid water. Across a strong natural turbidity boundary, the alien species was found to dominate turbid habitat less than 1 m from clear water, where both species were found. Accounting for sensory biology as a potential contributing factor in the establishment of invasive fish species in degraded habitat may help to identify invasive species risk and to shape strategies for rehabilitating native species.

Summer use of the tidal freshwaters of the River Seine by three estuarine fish: Coupling telemetry and GIS spatial analysis

Acoustic telemetry and GIS-based spatial analysis were used to investigate the summer habitat use and movement patterns of three fish species in the tidal freshwaters of the Seine estuary (France). Experimental displacement of tagged individuals of thin-lipped grey mullet (Liza ramada), European eel (Anguilla anguilla), and common bream (Abramis brama) were conducted to test for their spatial fidelity and home range establishment. Most tagged individuals (95%) successfully returned to their previously occupied capture site, showing spatial homing abilities. The studied upstream tidal freshwater segment of the Seine estuary was regularly used by grey mullet as a part of its larger summer home range, while European eel and common bream were resident in this segment. The fidelity of eel to small nocturnal refuges and the regular use of intertidal waterbodies at high tide by grey mullet and bream suggested that they possess a capacity of acquiring spatial memory of habitats in a fluctuating environment. Importantly, the scale of movements travelled by each species was positively related to tidal phase. Grey mullet and bream, both visual feeders, exhibited short-term tidal movements to known habitats, providing food resources and contiguous resting habitat suggesting that they have shown behavioural strategies adaptive to fluctuating environments. Eel, in contrast, was found to have a different strategy strongly related to diel dynamics: it stayed in subtidal habitats rich in refuges that remained available at low tide. The results of this study emphasize the importance of restoring intertidal waterbodies and the relevance of considering the availability of adjacent subtidal habitats providing refuge at low tides.

Histological and allometric growth analysis of eye in Caspian kutum, Rutilus kutum Kamensky, 1901 (Teleostei: Cyprinidae) during early developmental stages

Fish larvae have several sensory systems that are functional at or soon after hatching and then are developed further during larval and juvenile stages. This study was conducted to investigate development of the eye in Rutilus kutum , based on histological and allometric growth analysis during early developmental stages up to 35 day post hatching with emphasis on retinal morphology. For this purpose, the histological sections were prepared and allometric growth pattern of the eye was calculated. The results showed that the most eye’s structures along with the retina of the newly hatched larvae, as the inner sensory (photosensitive) tissue were completely differentiated. Allometric growth pattern of the eye diameter up to the inflexion point (7 dph) was somewhat positive and then it became negative. The results revealed that the Caspian kutum is dependence on visual capability as visual feeder during their larval period which itself explains completion of eye structures and the high growth rate of eye before 3 dph i.e. beginning of mixed feeding.

Effects of light intensity on growth, immune responses, antioxidant capability and disease resistance of juvenile blunt snout bream Megalobrama amblycephala.

Light is necessary for many fish species to develop and grow normally since most fishes are visual feeders. However, too intense light may be stressful or even lethal. Thus, this study was conducted to evaluate the effect of light intensity on growth, immune response, antioxidant capability and disease resistance of juvenile blunt snout bream Megalobrama amblycephala. Fish (18.04 ± 0.22 g) randomly divided into 5 groups were exposed to a range of light intensities (100, 200, 400, 800 and 1600 lx) in cultures for 8 weeks. After the feeding trial, fish were challenged by Aeromonas hydrophila and cumulative mortality was recorded for the next 96 h. The results demonstrated that fish subjected to 400 lx showed the greatest weight gain (125.70 ± 5.29%). Plasma levels of glucose and lactate increased with light intensity rising from 100 lx to 1600 lx while the lowest plasma levels of cortisol was observed at 400 lx group. Post-challenged haemato-immunological parameters (including plasma lysozyme and alternative complement activities, as well as plasma nitric oxide level and globulin contents) improved with light intensity increasing from 100 lx to 400 lx, and then decreased with further increasing light intensity. However, antioxidant biomarkers such as liver catalase and malondialdehyde showed an opposite trend with immune response with the lowest values observed at 400 lx groups. The application of light intensity at 1600 lx significantly lowered liver glutathione activity to 76.78 ± 6.91 μmol g(-1). Within a range of light intensity from 100 to 400 lx, no differences were observed in liver total superoxide dismutase and glutathione peroxidase activities while they were significantly higher at 800 and 1600 lx. After challenge, the lowest mortality was observed in fish exposed to 400 lx. It was significantly lower than that of fish exposed to 100 and 1600 lx. The results of the present study indicated that high light intensities (more than 800 lx) not only produced poor growth, but also led to stress response, as might consequently result in the elevated liver oxidation rates and depress immunity of this species. Although no stress response was observed, fish subjected to low light intensities (lower than 400 lx) also showed oxidative stress, immunosuppression and reduced disease resistance. Taken together, the optimal light intensity to enhance growth and boost immunity of this species at juvenile stage is 400 lx.

Early development and allometric growth patterns in ide Leuciscus idus (Linnaeus 1758)

Allometric growth and ontogeny were studied in ide Leuciscus idus (L.) reared in laboratory conditions from 0 to 30 days post hatching (DPH), to define important steps in the species’ early life history. Based on the external morphology, eleven different stages during the early development of ide were identified (ES9a – JS1a; ES – embryonic stage, JS – juvenile stage). After a short period of rest, hatched individuals were visual feeders with a pelagic life. Organogenesis and allometric changes indicated that development priorities concerned feeding efficiency, respiration efficiency and locomotion performances. Also concluded was that the gradual changes observed in body proportions (which occurred across a wide range of body lengths) reflect the absence of abrupt changes in the behaviour and habitat and may also be considered as a gradual adaptation to life in flowing water.

Effect of feed colour on growth and feed utilization of Nile tilapia (Oreochromis niloticusL.) larvae and fingerlings

Two experiments were conducted to investigate the effects of feed colour on the performance of Nile tilapia (Oreochromis niloticus) larvae and fingerlings. In the first experiment, triplicate groups of newly hatched larvae (0.01 g fish−1) were stocked in 40 L glass aquaria at a density of 2 fish L−1. The fish were fed a test diet (400 g kg−1 crude protein) with six different colours (dark blue, dark green, red, dark brown, yellow and light brown) for 60 days. The best performance and survival were achieved in fish fed on dark-coloured diets, while light-coloured diets (yellow and light brown) resulted in inferior performance. Dark diets also produced higher body protein than light diets. Body water, lipids and ash showed irregular trends. In the second experiment, triplicate groups of Nile tilapia fingerlings (5.30 g fish−1) were stocked in 140-L aquaria, in a recirculating indoor system. The fish were fed a test diet (350 g kg−1 crude protein) with the same colours used in the larval trial, for 60 days. Growth rates, feed efficiency, survival and body composition were not significantly affected by feed colours. These results suggest that Nile tilapia larvae are visual feeders, and they prefer dark-coloured diets to light-coloured diets, while fingerling fish showed no preference to diet colours.

Predator foraging behaviour drives food‐web topological structure

1. The structure and dynamics of prey populations are shaped by the foraging behaviours of their predators. Yet, there is still little documentation on how distinct predator foraging types control biodiversity, food-web architecture and ecosystem functioning. 2. We experimentally compared the effects of model fish species of two major foraging types of lake planktivores: a size-selective visual feeder (bluegill), and a filter feeder (gizzard shad). The visual feeder forages on individually captured consumer prey, whereas the filter feeder forages on various prey simultaneously, not only consumers but also primary producers. We ran a 1-month mesocosm experiment cross-classifying a biomass gradient of each predator type. We analysed the effect of each fish on food-web architecture by computing major topological descriptors over time (connectance, link density, omnivory index, etc.). These descriptors were computed from 80 predator-prey binary matrices, using taxa mostly identified at the species level. 3. We found that the visual feeder induced more trophic cul-de-sac (inedible) primary-producer species, lower link density and connectance, and lower levels of food-web omnivory and generalism than the filter feeder. Yet, predator biomass did not affect food-web topology. 4. Our results highlight that top-predator foraging behaviour is a key functional trait that can drive food-web topology and ultimately ecosystem functioning.

Complexity affects habitat preference and predation mortality in postlarval Penaeus plebejus: implications for stock enhancement

Global attempts to offset declines in fishery populations through stock enhancement have had varied levels of success due to the absence of preliminary studies to determine which habi- tats best support release species and the mechanisms controlling their distribution. Habitat prefer- ence was examined as a possible mechanism driving distribution of postlarval Penaeus plebejus, a current candidate prawn for stock enhancement in Australia. Occupancy of complex (artificial macro- phyte) and simple (bare sand and mud) habitats by postlarvae was compared in the presence and absence of a choice between the habitats. Predation mortality was also compared amongst these habitats. P. plebejus settled into the different habitats randomly during the night, but actively selected macrophyte over the simple habitats during the day. Mortality caused by the predatory fishes Centropogan australis and Acanthopagrus australis was higher in simple habitats than in com- plex habitats, but was similar across habitats when large penaeid prawns, Metapenaeus macleayi (which are tactile rather than visual feeders), were used as predators. Postlarvae may select macro- phyte habitats during the day to lower predation risk, but because nighttime foraging efficiency is reduced in their predators, which are primarily visual hunters, this may preclude the need of postlar- vae to obtain shelter in macrophyte habitats at night. Predation mortality of stocked P. plebejus may be minimized by releasing postlarvae directly into macrophyte habitats. Studies such as these must precede all stock enhancement attempts because they identify optimal release strategies and allow ecological and financial costs of enhancement to be weighed against projected benefits, and thereby assess the practicality of enhancement as a management option.

Morphological structure in a reef fish assemblage

Two key morphological traits, horizontal gape and eye diameter, were measured in a large representative group of coral reef fishes. These morphological traits were used concurrently to assess their utility in exploring abilities of coral reef fishes at an assemblage level. A total of 1,218 specimens from 181 species found on the Great Barrier Reef were examined. Cryptobenthic fishes were included to provide a broader representation of reef fish groups. In the analyses, a clear morphological distinction was found between nocturnal and diurnal fishes. Nocturnal fishes had larger relative horizontal gapes and relative eye diameters by factors of 1.6 and 1.5, respectively. A bivariate plot separated into quadrants was used to assess the implications of morphological variation. The morphological measures reflected distinct ecological traits in each quadrant. Whilst nocturnal fishes had large relative gapes and eye diameters, diurnal predators and detritivores had the same wide gapes, but small relative eye diameters. Highly selective, visual feeders such as the Chaetodontidae and Pseudochromidae had large eyes and small gapes, whilst non-selective feeders with low visual dependence such as the grazing herbivores (Acanthuridae, Siganidae, etc.) had both small eye diameters and gape sizes. The analysis proved to be robust enough to apply to a wide assemblage, but with enough subtlety to distinguish morphological differences within individual families. The methods used in this study may have broad applications to other fish assemblages, both fossil and extant.

Predicting the vertical distribution of the opossum shrimp, Mysis relicta, in Lake Ontario: a test of laboratory-based light preferences

Light and temperature strongly influence the vertical distribution of the mysid shrimp, Mysis relicta . We monitored the vertical movements and depth selection behavior of mysids exposed to different light intensities and light–temperature gradients in the laboratory and derived a mysid light preference function in units relevant to mysid vision: “mylux”. Mysids preferred light levels between 10−8 and 10−7 mylux (∼10−6 to 10−5 lux) and rarely moved into waters of 10−3 mylux (∼0.1 lux) and greater. A model that assumed equal weight and independence of mysid light and temperature preference functions successfully predicted the proportion of mysids found in two different temperature–light combinations in the laboratory. This model also predicted the depth of maximum mysid density to within 2 m on two spring nights and within 5 m on two summer nights of varying moon phase and thermal conditions in Lake Ontario. This study provides novel insights into how temperature and light interact to influence the vertical distribution of mysids. Our model may be used to predict mysid vertical distribution in any deepwater system inhabited by mysids in which the primary mysid predators are visual feeders.

Influence of light intensity on feeding, growth, and early survival of leopard coral grouper ( Plectropomus leopardus) larvae under mass-scale rearing conditions

This study investigated the effect of different light intensities on feeding, growth and survival of early stage leopard coral grouper Plectropomus leopardus larvae. Four different light intensities (0, 500, 1000 and 3000 lx) were used and larvae were kept under constant light conditions from 0 day after hatching (DAH) to 5 DAH. The larvae were fed a small S-type of Thai strain rotifers at a density of 20 individuals/mL from 2 DAH. The number of rotifers in larval digestive organ and total length of larvae were examined at 3 h intervals between 04:00 and 22:00 h on 3 DAH, and thereafter at 6 h intervals until the end of the experiment (5 DAH). Four experimental trials of the larval rearing were repeated using by 60 kL mass-scale rearing tanks. The results indicate that coral grouper larvae are visual feeders and their food intake increases with increasing light intensity. Food intake of larvae reared at 3000 lx was significantly higher than those reared at 0–1000 lx on 3 DAH despite being the first-feeding day ( P < 0.01). On 4 DAH, total length of larvae reared at 3000 lx was significantly larger than those reared at the lower light intensities (0, 500 and 1000 lx), and thereafter light intensity significantly influenced larval feeding and growth until the end of the experiment. Survival on 5 DAH did not show a significant difference between light intensities, but survival rate at 3000 lx and 1000 lx had a tendency to be higher than those reared at the lower light intensities (0 and 500 lx). In contrast, larvae reared at 0 lx exhibited stagnant and/or negative growth. These results indicate that light intensity is significantly the factor affecting larval feeding, growth, and survival in coral grouper larvae under the rearing conditions.

Visual fields in Blue Ducks Hymenolaimus malacorhynchos and Pink‐eared Ducks Malacorhynchus membranaceus: visual and tactile foraging

Blue Ducks Hymenolaimus malacorhynchos (Anatidae), an IUCN Red Listed Endangered species, reside in headwaters of New Zealand rivers and feed primarily on aquatic invertebrates. However, whether such food items are detected by tactile or visual cues is unknown. That Blue Ducks may use tactile cues when foraging is suggested by the presence of specialized flaps of thickened, keratinized epidermis containing Herbst's corpuscles along the ventral margins of the upper mandibles near the bill tip. Similar bill flaps are found only in one other duck species, Pink‐eared Ducks Malacorhynchus membranaceus, that surface filter‐feed on a range of planktonic organisms. Using an ophthalmoscopic reflex technique we determined the visual fields of both species. In Blue Ducks the eyes are frontally placed resulting in a relatively wide binocular field into which the narrow tapering bill intrudes. There is a large blind area to the rear of the head. This visual field topography is similar to that of other visually guided foragers including those that take mobile prey from the water column, e.g. penguins (Spheniscidae). By contrast, Pink‐eared Duck visual fields show features found in other tactile feeding ducks: a narrow frontal binocular field with the bill falling at the periphery, and comprehensive visual coverage of the celestial hemisphere. We conclude that although Blue Ducks may take prey from rock surfaces they are primarily visual feeders of the water column and we suggest therefore that their foraging may be significantly disrupted by changes in water clarity. This introduces a previously unconsidered factor into the selection of sites for population enhancement or re‐introductions, a current conservation focus.