Fish larvae – zooplankton relationships in microcosm simulations of earthen nursery ponds. I. Freshwater system

To address the commercial nurseries problem of obtaining enough zooplankton of adequate species composition and size when fish larvae start to feed, a simulation of the effects of predation intensity on zooplankton composition in freshwater nursery ponds was carried out in an experimental system of twelve 130-liter containers. Treatments consisted of two densities (1 or 2 larvae l⁻¹) of common carp stocked on the 4th day after filling the containers and a control without fish. Zooplankton–environment relationships were explored using factor analysis. Several groups of zooplankters that respond in different ways to fish larvae predation pressure were identified. The first factor was a general measurement of zooplankton abundance that mainly reflects allochthonous rotifer species that entered the system with the filling water, replaced in time by autochthonous species that hatched from resting eggs in the sediment. The second factor identified the direct effects of size selective fish predation, and the third showed indirect effects of high fish density. Larvae growth rate matched the plankton availability: high just after stocking, when they fed on the abundant but declining numbers of allochthonous rotifers; then decreased drastically and increased again when hatching of resting eggs of autochthonous rotifers and the availability of the crustacean preys preferred by the fish, increased. It was concluded that in commercial nurseries increased larvae production can be achieved just by stocking fish earlier than the 4th day after pond filling, to match the rotifer peak.     

Fish larvae: zooplankton relationships in microcosm simulations of earthen nursery ponds. II. brackish water system

A simulation of the effects of predation intensity on zooplankton composition in brackish water nursery ponds was carried out in order to address the problem that commercial fish nurseries encounter in obtaining enough zooplankton of adequate species composition and size when fish larvae start to feed. The experimental system consisted of twelve 130 l containers with treatments of four densities (0, 1, 2, or 4 larvae l⁻¹) of common carp (Cyprinus carpio L.), stocked on the 6th day after filling the containers. Zooplankton-environment relationships were explored using factor analysis. Factor analysis allowed identifying several groups of zooplankters that responded in different ways to fish larvae predation pressure. The first factor represented a general measurement of rotifer abundance, and the second identified the direct effect of size-selective fish predation. Since no rotifers were present in the filling water, all these species were autochthonous populations that hatched from resting forms in the sediment and reproduced. In the absence of fish predation, this led to a steep rotifer increase. Fish predation started when the rotifer concentration was just starting to increase and their direct predation reduced and delayed the rotifer abundance peak. This effect increased with the increase in fish larvae density. Estimations of rotifer consumption by fish larvae in this experiment were higher than similar calculations from data of the literature, which led us to test the hypothesis that factors other than direct predation were affecting rotifer population dynamics. The mechanisms involved in rotifer population regulation are discussed. It was concluded that in commercial nurseries, increased larvae production can be achieved by keeping the larvae density at an intermediate level and stocking fish to match the increasing phase of the rotifer peak. Under reasonable larvae density (up to 2 l⁻¹) it seems that the direct predation effect of fish larvae on rotifer dynamics is minor, compared to fish induced self regulation.     

Natural zooplankton as larval feed in intensive rearing systems for juvenile production of Atlantic cod (Gadus morhua L.)

The growth potential of cod larvae is not fully achieved when rotifers (Brachionus spp.) are used as live feed. In this experiment, we studied the effect of natural zooplankton (mainly copepods) on the growth of cod (Gadus morhua L.) larvae reared in intensive systems. Using a growth model developed for cod larvae, the growth rates observed could be evaluated and compared with growth rates reported previously. The cod larvae showed optimal growth rates until age 19 days post hatch (DPH) when they reached 9.77 ± 0.25 mm standard length (SL). Early weaning (20–25 DPH) resulted in significantly longer larvae at age 30 DPH compared with late weaning (25–32 DPH); however, in this period, the zooplankton concentrations were low. The experimental larvae showed considerably higher growth rates compared with rotifer (Brachionus spp.)‐reared cod larvae in previous experiments. The nutritional composition of cod larvae was analysed and compared with published results on rotifer‐reared larvae. The levels of iodine, manganese, selenium and n‐3 PUFA were considerably higher in larvae fed copepods compared with larvae fed rotifers. The differences in nutritional status may well explain the differences in growth observed between copepod and rotifer‐reared larvae.     

Strategies for Development of Rotifers as Larval Fish Food in Ponds

Strategies to sustain rotifer peak biomass, distribution of rotifer resting eggs in the sediment, and relationship between rotifers and larval fish growth were studied in a series of pond experiments. After the ponds were filled with water, herbivorous rotifers (e.g., Brachionus calyciflorus ) developed first, but were gradually replaced by predatory rotifers (e.g., Asplanchna ). Subsequently, herbivorous cladocerans (e.g., Moina sp) eventually replaced rotifers and dominated the zooplankton community. The occurrence of Asplanchna and Moina indicated the decline of B. calyciflorus . Peak rotifer biomass developed 8–10 d after the ponds were filled with water at 20–25 C, 10–15 d at 17–20 C, 15–20 d at 15–17 C, 20–30 d at 10–15 C, and >30 d at < 10 C. The abundance of resting eggs in the top 5-cm sediment varied from 6 to 83/cm². About 25% of resting eggs were buried in the top 5-cm sediment but the number of resting eggs decreased with increased sediment depth. Optimum rotifer biomass for silver carp Hypophthalmichthys molitrix larvae stocked at 1,500,000/ha was 20–40 m/gL. High rotifer biomass (>20 mg/L) usually lasted 3–5 d, but could be prolonged by pond fertilization or cladoceran controls. A weekly application of dipterex at 0.05 mg/L reduced cladoceran biomass but enhanced rotifer biomass. Our results indicate with a careful management plan it is possible to synchronize the rotifer development with larval fish stocking.     

Effect of prey type and size on the growth,survival and pigmentation of cod (Gadus morhua L.) larvae

The aim of this study was to investigate the effect of natural zooplankton versus rotifers and the effect of prey size on the growth and survival of cod larvae. At 20 days post hatch (DPH) myotome height, standard length and dry mass were significantly higher in larvae fed zooplankton compared to those fed rotifers. The dry mass at age 25 DPH was 135 μg (±45), 331 μg (±114), 391 μg (±121) for larvae fed rotifers, small size and large size plankton, respectively. At 25 days post hatch, the survival rates were 41.8% (±10.5), 90.7% (±2.3) and 91.4% (±1.7) for larvae reared on rotifers, small size and large size plankton, respectively. The limited growth and survival of cod larvae reared on rotifers were not mainly caused by the small size of rotifers. Large differences in skin coloration between larvae in the rotifer group and the two zooplankton groups were observed, probably caused by the large difference in astaxanthin levels in rotifers and natural zooplankton. We suggest that the nutritional composition of rotifers is a limiting factor for cod larvae growth and survival.     

锯缘青蟹幼体饵料的营养强化

用酵母、水球藻、鱼油强化和豆油强化四种不同方式培养轮虫，再分别投喂锯缘青蟹幼体，分析测定轮虫和体的生化组成，结果显示，（1）不同方式培养的轮虫之间以及摄食这些轮虫的锯缘青蟹幼体之间的蛋白质含量都没有显著差异；（2）轮虫的脂类含量和脂肪酸组成与培养方式密切相关，小球藻轮虫的脂类含量最高，20：5n-3(EPA)占总脂肪酸的比例也最高 ，为18.05%，鱼油轮虫则含有最多的22：6n-3(DHA)，占总脂肪酸3.16%，脂类含量仅次于小球藻轮虫；（3）锯缘青蟹幼体的脂类含量和脂肪酸组成受相应饵料营养成分的影响。另外，幼体培育实验也发现，饵料营养成分影响幼体的存活率，结果表明，提高轮虫的EPA和DHA含量，尤其晨DHA含量，将有利于锯缘青蟹幼体的存活和发育。     

Nutrient evaluation of rotifers and zooplankton: feed for marine fish larvae

The objective of this study was to examine the biochemical composition of intensively reared rotifers after enrichment with three commonly used enrichment media, Multigain, Ori‐Green and DHA‐enriched freshwater algae Chlorella, using standard enrichment protocols at a local cod larvae producer and compare it with that of natural zooplankton from Lofilab AS, a cod larvae producer using semi‐intensive rearing techniques. Unenriched rotifers were analysed to examine whether the enrichment procedures were successful in increasing the content of essential nutrients to level requirements for marine fish larvae. Neither total lipids nor proteins were affected by enrichment. Unenriched rotifers were significantly lower in highly unsaturated fatty acids (HUFAs) and significantly higher in linoleic acid (LA, C18:2, n‐6), than were zooplankton. Enrichment with Chlorella and Multigain increased the HUFAs significantly, while they were slightly reduced after enrichment with Ori‐Green. Total amino acids and mineral content were unaffected by enrichment. Zooplankton was rich in taurine and selenium, whereas rotifers were devoid of it, both prior to and after enrichment. Using zooplankton as a reference for the nutritional requirements of marine fish larvae, results from this study demonstrate that enrichment media currently in use are not effective for enhancing the nutritional quality of rotifers.     

Production and Collection of Copepod Nauplii from Brackish Water Ponds

Copepod nauplii are a nutritious food item for first-feeding marine fish larvae. Unfortunately, mass culture techniques for producing copepod nauplii are not well established. Copepod nauplii can be collected from wild zooplankton populations or specially prepared ponds and transferred to larval fish tanks for feeding. This study evaluated the use of two trapping methods for harvesting zooplankton, particularly copepod nauplii, from fertilized ponds and the impact on the zooplankton population. Nine, 0.11 ha brackish-water (～2-7 ppt salinity) ponds were filled and fertilized with organic and inorganic fertilizers. The change in zooplankton abundance, mainly rotifers, nauplii and adult copepods, was monitored in the ponds for 22 d following initial pond filling. Beginning on day 8, three ponds were trapped with a large plankton net (Trap I), three with a pump and bag trap method (Trap II), and three ponds were not trapped. The ponds were trapped with the corresponding method for 1 h per day, for 15 d. The two trapping methods were similar in their efficiency to harvest nauplii, averaging 8,383,400 ± 2,508,378/h and 6,695,822 ± 433,533/h for Traps I and II, respectively. The zooplankton harvested by Trap I was not correlated to the densities in the ponds. However, the number of rotifers and nauplii harvested by Trap II was correlated to the rotifer and nauplii densities in the ponds. Both trapping methods were similar in terms of labor requirements and ease of use. Both methods were effective in collecting zooplankton without negatively impacting pond abundance.     

Food ingestion,consumption and selectivity of pompano,Trachinotus ovatus (Linnaeus 1758) under different rotifer densities

The growth, survival, food selection and consumption of pompano larvae under different rotifer densities as well as their colour preference during the rotifer feeding stage were examined in this study. Growth and survival of fish larvae were not significantly affected when rotifer density was between 10 and 20 mL^?1. Fish larvae grew slower at 1 and 40 rotifers mL^?1 than at 10 and 20 rotifers mL^?1, and higher fish survival was achieved when fish larvae were exposed to 10 and 20 rotifers mL^?1. The rotifer density of 1 mL^?1 not only reduced food ingestion during the early stage, but also delayed diet switch from rotifer to copepod nauplii. On 5 days post hatching (DPH), larval pompano ingested more rotifers in dark‐coloured tanks and ingested more rotifers when prey colour was green. Based on the results obtained in the present study, the culture of larval pompano larvae is recommended using dark wall tanks with a feeding density of 10–20 rotifers mL^?1 during the initial feeding stage. This study proposes a management protocol to use appropriate type and quantity of live food to feed pompano larvae in a hatchery rearing condition, which could be applicable to the culture of fish larvae in other marine fish species.     

Relationship between prey utilization and growth variation in hatchery‐reared Pacific bluefin tuna,Thunnus orientalis (Temminck et Schlegel), larvae estimated using nitrogen stable isotope analysis

In mass culture of Pacific bluefin tuna Thunnus orientalis, a marked growth variation is observed after they start feeding at 6–7 mm in body length (BL) on yolk‐sac larvae of other species, and the growth variation in tuna larvae is a factor leading to the prevalence of cannibalism. To examine the relationship between prey utilization and growth variation, nitrogen stable isotope ratios (δ¹⁵N) of individual larvae were analysed. A prey switch experiment was conducted under two different feeding regimes: a group fed rotifers (rotifer fed group), and a group fed yolk‐sac larvae of spangled emperor, Lethrinus nebulosus (fish fed group) from 15 days after hatching (6.87 mm BL). The fish fed group showed significantly higher growth than the rotifer fed group. Changes in the δ¹⁵N of the fish fed group were expressed as an exponential model and showed different patterns from those of the rotifer fed group. The δ¹⁵N of fast‐growing tuna larvae collected in an actual mass culture tank after the feeding of yolk‐sac larvae was significantly higher than those of the slow‐growing larvae, indicating that slow glowing larvae depended largely on rotifers rather than the yolk‐sac larvae.     

Use of GABA to enhance rotifer reproduction in enrichment culture

Gamma‐aminobutyric acid (GABA) has been shown to enhance the reproduction of the rotifer Brachionus plicatilis Muller in stressful culture conditions. During the enrichment of rotifers for feeding to marine fish larvae, they are usually stressed as a result of exposure to different marine oils and high population densities. This typically results in decreased rotifer survival, reproduction and swimming activity. In the present study, we used GABA to increase rotifer reproduction and the swimming activity of rotifers in enrichment cultures. GABA treatment 24 h before high density enrichment enhanced reproduction during enrichment culture, but not when carried out simultaneously with enrichment. Swimming activity was not significantly affected by GABA treatment 24 h before or simultaneously with nutrient enrichment.     

Evaluation of nutritional and environmental variables during early larval culture of pigfish Orthopristis chrysoptera (Linnaeus)

Pigfish (Orthopristis chrysoptera Linnaeus) are a commonly used baitfish in the southeastern United States. Aquaculture methods for broodfish spawning and juvenile grow‐out have been developed but there is still a paucity of information regarding larval culture methods. Five, short duration (10 days) experiments were conducted to determine effective strategies to yield high larval survival and growth during early development. Experiment one examined the rotifer enrichments Ori‐Green, DHA Protein Selco, and AlgaMac 3050 as well as a non‐enriched control along with corresponding fatty acid levels in the enriched rotifers and pigfish larvae. Experiment two evaluated three, once daily feeding frequencies of either 5, 10 or 20 rotifers mL^?1. Experiment three compared feeding 20 rotifers mL^?1 once daily to feeding 5 rotifers mL^?1 twice daily. Experiment four examined four different larval stocking densities: 50, 75, 100, or 125 larvae L^?1. Experiment five examined green water strategies using either live Tahitian strain Isochrysis galbana (Parke) or Nannochloropsis oculata (Hibberd) paste at either 250 000 or 500 000 cells mL^?1 as well as a clear water control. Results indicated rotifer enrichment with DHA Protein Selco and green water application using live T‐ISO at 500 000 cells mL^?1 had the highest survival of pigfish during early stages of larval culture. A once daily rotifer feeding regime of 20 rotifers mL^?1 and stocking density of 50 larvae L^?1 also improved survival. These results provide producers with methods to improve efficiency for pigfish larval culture and provide researchers with new foundational data, such as potential fatty acid requirements.     

Short- and long-term differences in growth, feed conversion efficiency and deformities in juvenile Atlantic cod (Gadus morhua) startfed on rotifers or zooplankton

Growth, feed conversion efficiency and frequencies of skeletal deformities were studied in juvenile Atlantic cod (Gadus morhua) that had been startfed on either rotifers (rotifer group) or zooplankton (zooplankton group). After metamorphosis, the fish were reared at four constant temperatures (7, 10, 13, 16°C) or moved successively from 16 to 13 and 10°C (T‐step, average 13.2°C). The zooplankton group had a consistently higher growth rate at all the temperatures studied. Further, the zooplankton group had higher food intake (20%) and higher feed conversion ratio (1.65 vs. 1.31). In addition, a significantly higher incidence of skeletal deformities was found in the rotifer group (14.2%) compared with the zooplankton group (4.1%). After termination of the laboratory study, the fish were reared in sea pens under ambient conditions for 17 months. Final weights of the zooplankton group were consistently larger (between 12% and 14% larger depending on original temperature groups). To verify the growth results, we conducted a follow‐up study where a single egg group was divided into two parts and fed either on rotifers or zooplankton. This study indicated similar growth differences as found in the first study. Overall, our data suggest that different startfeeding diets may be important for long‐term growth, incidence of deformities and quality of juvenile cod. The use of zooplankton can greatly improve long‐term growth and quality of cod juveniles. The study also highlights the advantage of using elevated temperatures in the juvenile phase as this will lead to significantly higher final weights in the adult stage.     

The enrichment of rotifers (Brachionus plicatilis) and Artemia franciscana with taurine liposomes and their subsequent effects on the larval development of California yellowtail (Seriola lalandi)

Taurine is an essential or conditionally essential nutrient for many species of marine fish, especially during early development. There is growing evidence that marine fish larvae benefit from taurine‐enriched rotifers; however, it is unknown if larvae benefit from taurine‐enriched Artemia. We investigated the effects of taurine‐enriched rotifers (Brachionus plicatilis) and Artemia franciscana on the growth and whole‐body taurine concentrations of California yellowtail (Seriola lalandi; CYT) larvae. The approach used in this study was to encapsulate taurine within microparticles (liposomes), which were then fed to rotifers and Artemia. We found that feeding taurine liposomes to rotifers and Artemia resulted in taurine concentrations in these prey species that were similar to or above those previously reported in copepods. At the end of the rotifer phase, CYT larvae fed taurine‐enriched rotifers showed increased growth (final dry weights; DW) and had higher whole body taurine concentrations when compared to larvae fed unenriched rotifers. At the end of the Artemia phase, CYT whole body taurine concentrations varied among dietary treatments. Larval lengths and DWs were not significantly different among treatments at the end of the Artemia phase, suggesting that the taurine concentrations of unenriched Artemia were sufficient to support the growth of CYT larvae.     

Effects of fish stocking on the zooplankton community structure in a shallow lake in China

Abstract  Changes in the zooplankton community structure in relation to fishery practices in Lake Donghu, Wuhan, China were examined. The number of Protozoa species increased slightly, whereas the number of rotifers and crustaceans decreased from the 1960s to the 1990s. The total annual average densities of zooplankton increased 15–20 times in the 1990s compared with the 1960s. This increase was largely attributed to Protozoa, which contributed 93.4% by number of the total zooplankton density in 1991. Cladoceran densities decreased markedly from 1987. Changes in densities of rotifers and copepods were not evident. Trends in zooplankton biomass were similar to density. Large changes in zooplankton community structure coincided with markedly changes in concentration of chlorophyll a and transparency in Lake Donghu in 1987. The year 1987 seems to be the threshold year when the zooplankton community structure changed considerably. These changes were related to continuously increasing fish stock biomass in the lake. It was suggested that fish stocking and fish biomass should be a better managed for improvement of the quality of the lake's environment.     

Essential fatty acid enrichment of cultured rotifers (Brachionus plicatilis, Müller) using frozen-concentrated microalgae

There is a growing interest in preserving microalgal preparations to maintain constant properties over a long period. The aim is to ensure sufficient delivery of essential fatty acids (and other key nutrients) to mollusc and crustacean larvae and to zooplankton used as live prey in the first feeding of fish larvae. For example, the rotifer Brachionus plicatilis has to be enriched with polyunsaturated fatty acids (PUFA) prior to fish feeding. We used four microalgal species [ Isochrysis galbana (T-ISO), Chaetoceros muelleri (CHGRA), Pavlova lutheri (MONO), and Nannochloropsis sp.] both as fresh culture or in a frozen-concentrated form to enrich rotifers. Overall, rotifers had similar relative fatty acid levels when fed the frozen-concentrated or fresh microalgal diets. The levels of 20:4n-6, 22:6n-3, and 20:5n-3 between B. plicatilis and the microalgal diets were linearly correlated. The fatty acid 20:4n-6 was the most readily assimilated: the content found in rotifers reached half the level measured in the microalgal diets. Our results indicate that both the fresh and frozen-concentrated forms of the four microalgal species can be used to enrich PUFA levels in rotifers. Further experiments should be conducted to test if assimilation differs when rotifers are enriched with mono- or multispecific microalgal preparations.     

Rotifers fed various diets of baker''s yeast and/or Nannochloropsis oculata and their effect on the growth and survival of striped mullet (Mugil cephalus) and milkfish (Chanos chanos) larvae

The rotifer Brachionus plicatilis (S-type Hawaiian strain) was cultured with various combinations of baker's yeast and Nannochloropsis oculata. There were no significant differences in the daily rotifer production and amino acid profiles of the resulting rotifers. The significantly lower levels of fatty acids (C14, C16, C20:4n−6, C20:5n−3 and C22:6n−3) observed in the rotifers were found to correspond with the amount of yeast presented in their diet. The low survival and growth of striped mullet larvae recorded at Day 15 posthatching indicates that rotifers fed only yeast were nutritionally deficient in fatty acids. The fatty acid requirements of mullet, however, appeared to be satisfied with rotifers cultured on a combination of yeast and N. oculata. In contrast, no significant differences in larval milkfish survival and growth at Day 10 posthatching were detected when using rotifers fed the various diets in the larval rearing protocol.     

Providing harpacticoid copepods via floating sieve improves herring (Clupea harengus L.) larval feeding incidence

Harpacticoid copepods are being considered as alternative candidates for live feed in aquaculture, but their benthic affinity may pose problems for pelagic fish larvae. We compared the swimming behaviour and feeding incidence of herring larvae (Clupea harengus) in the presence of harpacticoid copepods (Tachidius discipes) and rotifers (Brachionus plicatilis). Additionally, we provided T. discipes via a floating sieve to improve the prey availability. The comparison was performed at 5 and 10 days post hatch (dph) via 2D‐video observations. Quantitative analyses of larval trajectories allowed the estimation of feeding behaviour through a series of indicators: swimming speed, straightness of trajectories, turning angles and swimming activities (break, sink, slow, normal, fast). The outcomes highlighted that the prey type had no significant effect on swimming speed or straightness of the swimming path. However, at 10 dph directly copepod‐fed larvae spent less time in slow but more time in the normal swimming‐state than rotifer‐fed larvae and larvae fed with Tachidius via sieve. This suggests higher energy expenditure of directly copepod‐fed larvae. Moreover, the feeding incidence was higher in larvae fed with Tachidius via sieve than directly Tachidius‐fed larvae. Thus, providing harpacticoid copepods via a floating sieve can improve the rearing of marine fish larvae.     

Rotifers enriched with iodine,copper and manganese had no effect on larval cod (Gadus morhua) growth,mineral status or redox system gene mRNA levels

It currently remains unclear if rotifers contain sufficient mineral levels to meet larval fish requirements. In this study, rotifers were enriched with a commercial enrichment (control), or with additional iodine, iodine and copper, or iodine, copper and manganese, and the effects of feeding these rotifers to Atlantic cod (Gadus morhua) larvae from 3 to 18 days post hatch were investigated. Rotifer enrichment with minerals was successful, but Mn enrichment also increased rotifer zinc levels. No differences were observed between treatments in larval growth or survival, or in the mRNA levels in the majority of the redox system genes analysed. Only Zn levels increased in cod larvae in response to mineral enrichment of rotifers. Apart from Zn, little evidence was found to suggest that cod larvae require increased concentrations above the control rotifer levels of the essential elements studied here.     

Cod Larviculture Using High‐density Rotifer Production with Different Enrichments

High‐density (HD) rotifer culture systems have been recently commercialized, but are not commonly used by the aquaculture industry. The aim of this study was to determine if HD systems could be used in cod hatcheries. An enrichment strategy using the commercial products, ArteMac and Protein Selco Plus (Com), was compared with the manufacturer's suggested enrichment, using Pavlova‐DHA (Pav). The Pav enrichment increased the eicosapentaenoic acid (EPA) levels in rotifers, but reduced the docosahexaenoic acid (DHA) and 22:5n‐6 (n‐6DPA, docosapentaenoic acid) levels. Larvae EPA levels in both polar and neutral lipids were relatively stable in the larvae fed with Com‐rotifers; while they were higher in early stages, they were progressively reduced through ontogeny in the Pav‐rotifers fed‐larvae. DHA levels in polar lipids decreased in larvae, particularly when fed with HD‐Pav rotifers. In all larvae, arachidonic acid (ARA) levels increased in the polar and neutral lipids, regardless of treatments. In both lipid fractions, the levels of ARA were quite stable in time, but still higher in larvae fed with Com‐rotifers. Bacterial load was lower in larvae fed with Pav‐rotifers. Denaturing gradient gel electrophoresis (DGGE) bacterial profiles of larvae and rotifers were all similar. This study shows the potential of using HD systems to produce rotifers, but highlights the necessity of adjusting the nutritional composition of rotifers prior to being fed to larvae.