Wild Zooplankton Research Articles

Nutritional Quality of the Wild Zooplankton as a Living Feed for Fish Larvae

Nutritional Quality of the Wild Zooplankton as a Living Feed for Fish Larvae

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.

Closing the reproductive cycle: Growth of the seahorse Hippocampus reidi (Teleostei, Syngnathidae) from birth to adulthood under experimental conditions

The seahorse Hippocampus reidi is one of two seahorse species occurring in Brazil, and is in demand for the ornamental aquarium and curio trades. Juvenile H. reidi siblings ( n = 615) born in captivity were raised on a diet of wild zooplankton, enriched Artemia and wild mysid from birth to 109 days. Growth averaged 0.77 ± 0.01 mm day − 1 for the whole period. However, after the onset of sexual maturation at about 60 days of age, growth decreased to 0.31 ± 0.10 and 0.53 ± 0.09 mm day − 1 in males and females respectively. The first births of F3 juveniles were observed after 81 days of growth. All individuals were adult at the end of the study and sex ratio was 1M:1.2F. Total mortality was 11.7%, with mortality peaks apparently caused by changes in the offered diet; gradual dietary shifts may reduce the occurrence of such mortality peaks. This study indicates that H. reidi may be raised in captivity from birth to adulthood with relatively fast growth rate, low mortality and early maturity, thus, increasing the potential application of this seahorse species to commercial aquaculture and conservation initiatives.

Feeding preferences of the dusky grouper ( Epinephelus marginatus, Lowe 1834) larvae reared in semi-intensive conditions: A contribution addressing the domestication of this species

Despite research efforts addressing artificial propagation, dusky grouper aquaculture still relies on experimental trials owing to the low survival rates of larvae and juveniles, thus rendering mass rearing difficult to attain. Although some authors suggested that the preys offered to the early larval stages represent the major problem facing the rearing of all grouper species, very little information is available in literature on larval development and the behaviour of dusky grouper. The objective of this study was to investigate the trophic preferences of larval stages of the dusky grouper by rearing using a semi-intensive technology. Larvae were reared in pilot scale mesocosms (60 m 3) from 3 to 35 days post hatching. The rearing system was supplied with wild zooplankton collected in a natural coastal lagoon in order to augment the chances of satisfying the larvae's feeding requirements; the feeding preferences were analyzed during ontogenesis. The relationship between mouth gape and dimensions of preys, considered as 3-D objects, was investigated to identify possible critical factor. The results of this study suggested a sequence of suitable food items from the beginning of exogenous feeding until day 35 post hatching. In particular, dusky grouper larvae actively select copepod nauplii in the size range between 2 and 12 mm in TL, and Artemia salina nauplii for TL larger than 9 mm. The rearing approach applied seemed to be promising. Indeed, while previous rearing trials of dusky grouper failed within ten days or finished at 30 days post hatching with a lower percent of survivors, this paper reports the best survival rate reported for this species (10% ± 7.0 survivors at 35 days post hatching). Finally, mouth width was observed to be the limiting factor in the selection of prey, while the smallest prey size plays a critical role in determining ingestion. Our results contribute to clarifying some aspects of the larval ecology of this species, furnishing some suggestions for its cultivation.

Nutritional evaluation of live food organisms and commercial dry feeds used for seed production of amberjackSeriola dumerili

To improve the nutritional quality of live foods and dry feeds ordinarily used for the seed production of amberjack Seriola dumerili, the nutrient contents of rotifers, Artemia nauplii and commercial feeds used in two larval production stations were evaluated. For comparison of the nutrient contents, artificially produced larvae, wild-caught juveniles and wild zooplankton samples were also analyzed. The proportions of 22∶6n-3 in the polar lipid of the cultured larvae increased by feeding the dry feeds. The taurine contents of the cultured larvae reflected the contents of their foods (rotifers<dry feed<Artemia nauplii). The taurine content and the proportion of 22∶6n-3 in Acartia spp. were higher than in foods fed to the larvae. These parameters in the wild juveniles were higher than the cultured ones. The A/E ratios [(each essential amino acid/total essential amino acids)×1000] of the total amino acids of the live foods and dry feeds were similar to those of the cultured larvae, except for the lower ratios of histidine, arginine, threonine and lysine in the live foods. The mucosal folds of the intestine of the cultured larvae did not show typical signs of dietary phospholipid deficiency. These results suggest that requirements of nutrients such as 22∶6n-3 and taurine should be determined for mass production of amberjack seeds.

Utilização de diferentes dietas na primeira alimentação do mandi-amarelo (Pimelodus maculatus, Lacépéde)

One of the greatest problems in larviculture of native species is first feeding and cannibalism in the some species since the first day of life. This experiment aims at evaluating different diets in the first feeding of Pimelodus maculates , popularly known as the yellow mandi. Post-larvae were stocked in a density of 15 post-larvae/L, in 3 L-glass aquariums with artificial aeration. Treatments were composed of wild zooplankton in the proportion of 300 organisms/post-larvae/day (T1); 600 organisms/post-larvae/day (T2); 900 organisms/post-larvae/day (T3); 9 nauplii of Artemia /post-larvae/day (T4) and artificial microdiet between 150 and 250 µm (>50% PB) (T5). At the end of five days the Artemia treatment had the best survival values (39,3%) and the least rate of cannibalism (28,2%). Since it registered the best final weight and length (P Pimelodus maculates was consequently demonstrated

Ontogeny of the digestive tract of thick lipped grey mullet ( Chelon labrosus) larvae reared in “mesocosms”

This work describes the ontogeny of the digestive tract in thick lipped grey mullet ( Chelon labrosus) larvae reared until day 36 post-hatching with the semi-extensive technology in mesocosms. Diet was constituted by live preys, rotifers, Artemia and wild zooplankton, then compound diet was added from day 20 (p. h.). Linear growth, weight growth and digestive enzymes specific activities were studied during larval ontogeny. Pancreatic enzymes (trypsin and amylase) and intestinal enzymes (leucine-alanine peptidase “Leu-ala”, aminopeptidase N “AN” and alkaline phosphatase “AP”) were assayed in larvae sampled throughout the rearing trial to evaluate gastrointestinal maturation along the development. The trypsin specific activities were very high during the first two weeks and then declined as observed in marine fish species. A following increase in trypsin specific activity from day 20 was attributed notably to ingestion of particle compound diet. In contrast to the pattern generally described in fish larvae, amylase specific activity showed a continuous increase. This could be attributed to the fact that C. labrosus is an omnivorous species and suggests that the fish might be able to use efficiently diets containing higher levels of starch or other carbohydrates since the end of larval development. Relative expression of intestinal brush border membrane enzymes (AP and AN) and cytosolic enzyme (Leu-ala), showed an abrupt increase of both AP/leu-ala and AN/leu-ala ratios at day 8 (p. h.), indicating that maturation of intestinal tract in C. labrosus larvae is particularly precocious. It is assumed that larvae of C. labrosus might support early co-feeding and weaning strategies, which could reasonably be initiated since mouth opening.

Changes in Lipid and Fatty Acid Composition of Wild Freshwater Zooplankton during Enrichment and Subsequent Starvation

AbstractConcentrated wild zooplankton harvested from freshwater ponds have been used to feed the larvae of hybrid striped bass (female white bass Morone chrysops × male striped bass M. saxatilis) in tanks. However, larval growth and survival have been superior when cultured rotifers and brine shrimp Artemia spp. nauplii have been offered as first feeds. We hypothesized that enrichment with highly unsaturated fatty acids (HUFAs), which is common for cultured zooplankton, would enhance the nutritional value of wild freshwater zooplankton. Wild zooplankton were enriched for 24 h with Super Selco, a formula rich in HUFAs. Lipid and fatty acid composition changes in wild freshwater zooplankton were monitored during the enrichment period and the subsequent 72 h. Total lipids in wild zooplankton increased from a preenrichment level of 38 mg/g dry weight (DW) to 72 mg/g DW after enrichment. Although wild zooplankton were initially deficient in HUFAs, the HUFA level after enrichment was 10.41 mg/g DW, which was above the recommended level for good growth and survival of hybrid striped bass larvae. The nutritional state of wild zooplankton returned to the preenrichment level 24 h after enrichment was terminated. Therefore, enriching wild zooplankton from culture ponds for 24 h after harvesting and concentration offers producers another nutritional option for hybrid striped bass larvae during the early life history stages.

Spawning, early development, and first feeding in the lemonpeel angelfish Centropyge flavissimus

The present study investigates, for the first time, critical processes in early life history in captive rearing of the lemonpeel angelfish Centropyge flavissimus. Video recordings of courtship behaviors in the 300 L spawning tank showed several courtship behaviors to be similar to those described for wild fish, but courtship activities associated with harem maintenance were not observed in the captive fish. Typical courtship activity was seen on every day of observation, but gamete release occurred on only 7 of 10 observation days. The reason for the lack of gamete release was not obvious. Tank spawning began when temperature and photoperiod conditions were changed to spring and reached a maximum peak (in terms of egg number) during the summer, when the temperature was 28 °C. Egg production and fertilization rate remained relatively constant day to day and spawning continued for months at a time as long as temperature and photoperiod were maintained at optimum levels. Once the embryo hatched (15 h post fertilization) and the yolk sac stage was reabsorbed (48 h post hatching), larvae were ready to feed. Cultured Parvocalanus sp. nauplii or size-sieved wild zooplankton were suitable first food. Proper environmental conditions included rearing the larvae at 28 °C. This study provides the first data on the influence of temperature and first food offered on larval survival in the lemonpeel angelfish.

Cymothoa indica (Crustacea, Isopoda, Cymothoidae) parasitizes the cultured larvae of the Asian seabass Lates calcarifer under laboratory conditions

Parasitic disease in fishes is one of the most important factors limiting aquaculture production and its economic viability. Cymothoa indica, a cymothoid isopod, is reported here for the first time parasitizing cultured larvae of the Asian seabass Lates calcarifer in India. Fourteen-day-old L. calcarifer larvae of mean weight 8.73 +/- 0.03 mg were fed with wild zooplankton in the laboratory. On Day 14 of rearing, larvae were found parasitized by cymothoids. Infected larvae reached a mean (+/- SE) weight of 98.86 +/- 0.30 mg, while uninfected specimens weighed 117 +/- 0.43 mg at the end of the experiment (Day 21). C. indica occurred in the branchial and anterodorsal regions of infected fish, where resultant skin lesions were red, hemorrhagic, without scales and with abundant secreted mucus. The cumulative mortality over the 3 wk period was 16.54 %. These parasites are transferred to the host via the zooplankton used as food; this could easily be overcome, either by filtering wild zooplankton to remove the infectious swimming larvae of C. indica or by using cultured copepods.

Influência da primeira alimentação na larvicultura e alevinagem do yamú Brycon siebenthalae (Characidae)

Two experiments were carried out at Instituto de Acuicultura de Los Llanos/Universidad de Los Llanos (Villavicencio, Colômbia) to evaluate first feeding influence on yamú larval rearing. In the first experiment, the following prey types were tested to first feeding for yamú larvae during 24 hours: Artemia sp. nauplii, wild zooplankton and pirapitinga (Piaractus brachypomus) larvae. In the control yamú larvae were subjected to starvation. Pirapitinga larva was the prey item that offered the best results for weight and total length gain (p < 0.05). In the second experiment, yamú larvae performance was compared when stocked into fertilized ponds during 15 days either at the onset feeding or after first feeding with pirapitinga larvae prey item. Survival rate was lower in the larvae that were stocked into rearing ponds at the onset of feeding (13.4%) when compared to survival of yamú larvae that were stocked after receiving pirapitinga larvae (74.1%). These results indicate that the use of pirapitinga larvae at first feeding increases performance of yamú larval rearing.

Manejo de la primera alimentación del bocachico (Prochilodus magdalenae)

En el Centro de Investigación Piscícola de la Universidad de Córdoba se realizaron dos experimentos; en el primero, el objetivo fue evaluar el efecto de diferentes presas en el manejo de la primera alimentación del bocachico Prochilodus magdalenae y en el segundo, con la mejor presa del ensayo anterior, determinar el tiempo mínimo de manejo de la primera alimentación. En ambos casos las post-larvas fueron sembradas a 50/ L en acuario de 5 L. En el primer experimento se evaluaron las siguientes presas: nauplios de Artemia (NA), cistos descapsulados de Artemia (CA), zooplancton silvestre tamizado entre 125-160 μm (Z125-160) y zooplancton silvestre tamizado entre 250-400 μm (Z250-400), ofrecidas durante cinco días a razón de 5 Presa/mL. Además, un grupo de post-larvas fue sometida a ayuno. Las mayores ganancias de peso y longitud total la registraron las post-larvas alimentadas con NA (2.0±1.8 mg, 1.8±0.9 mm) y Z250-400 (2.0±1.1 mg, 1.6±0.5 mm). La sobrevivencia de las post-larvas alimentadas con NA (73.2±10.7%) fue mayor a las registradas en los demás tratamientos (P&lt;0.05). La baja sobrevivencia de las post-larvas alimentadas con Z250-400 μm (49.9±23.8%) se debió a la mortalidad ocasionada por la presencia de copépodos predadores en el zooplancton ofrecido (Mesocyclops sp, Thermocyclops decipiens ). El pobre crecimiento de las post-larvas alimentadas con Z125- 160 (0.8±0.4 mg, 1.1±0.2 mm) y CA (0.5±0.9 mg, 0.7±0.5 mm) fue debido en el caso del zooplancton porque resultó pequeño para la abertura bucal del bocachico (671±12.8 mm) y por tanto ineficiente en el balance energético; mientras que en el caso de CA este se comportó como un alimento inerte que no estimuló su consumo. En el segundo ensayo se ofreció NA durantes diferentes periodos: 1, 3, 5 y 7 días, a razón de 5 presas/mL. No se observaron diferencias significativas en la sobrevivencia final entre los diferentes tratamientos; la cual osciló entre 86.0±9.8% (1 día) y 91.1±7,7% (3 días). Las post-larvas alimentadas por 7 días presentaron la mejor ganancia en peso (5.3±3.8 mg) y ganancia en longitud (2.8±1.6 mm). Las post-larvas alimentadas entre 3 y 5 días no presentaron diferencias significativas en la ganancia en peso (P&gt;0.05). Mientras que la tasa de crecimiento específico (G) osciló entre 17.1 (5 días) y 22.3 %/día (3 días), sin observarse diferencias estadísticas significativas (P&gt;0.05). Las post-larvas alimentadas durante 3, 5 y 7 días presentaron las mejores sobrevivencias (92.2±6.2, 93.3±4.3, 95.6±3.9% respectivamente) cuando fueron sometidas a la prueba de resistencia al estrés. Las post-larvas alimentadas por un solo día mostraron la menor sobrevivencia al ser sometidas a esta prueba (87±1%). Los resultados del estudio sugieren que nauplios de Artemia o zooplancton silvestre en el rango de 250-400 mm, libre de predadores, son presas adecuada para el manejo de la primera alimentación al menos durante 3 días antes de ser sembradas en estanques en tierra.

Different iodine and thyroid hormone levels between Atlantic halibut larvae fed wild zooplankton or Artemia from first exogenous feeding until post metamorphosis

The iodine concentration in wild zooplankton was 700 times higher than in Artemia and threefold higher in Atlantic halibut Hippoglossus hippoglossus larvae fed wild zooplankton than in those fed Artemia. In larvae fed wild zooplankton thyroxine (T4) and triiodothyronine (T3) concentrations were significantly higher than in those fed Artemia at the commencement of metamorphosis, 68–77 days post hatching. There was no difference in the amino acids phenylalanine and tyrosine levels in larvae fed Artemia in comparison with those fed wild zooplankton. The selenium level was significantly higher in larvae fed Artemia than in those fed wild zooplankton, but concentrations in the prey were not different. The results indicate sufficient amounts of phenylalanine, tyrosine and selenium in Artemia, but indicated a lower thyroid status due to an insufficient iodine supply at commencement of metamorphosis in larvae fed Artemia. This may partly explain the higher frequency of juveniles with complete eye migration and asymmetric pigmentation in the group fed wild zooplankton.

Different iodine and thyroid hormone levels between Atlantic halibut larvae fed wild zooplankton or Artemia from first exogenous feeding until post metamorphosis

Different iodine and thyroid hormone levels between Atlantic halibut larvae fed wild zooplankton or Artemia from first exogenous feeding until post metamorphosis

Nutrient composition and metamorphosis success of Atlantic halibut (Hippoglossus hippoglossus , L.) larvae fed natural zooplankton or Artemia

Atlantic halibut larvae were fed docosohexanoic acid- (DHA) selco enriched Artemia (RH-cysts) or wild zooplankton in duplicate tanks from first-feeding and 60 days onward. The zooplankton were collected from a fertilized sea water pond and consisted mainly of different stages of Eurytemora affinis and Centropages hamatus. There were no differences in survival, or in growth during the first 45 days of feeding, between larvae fed the two prey items, but the larvae fed Artemia showed much higher incidence of malpigmentation and impaired eye migration than larvae fed zooplankton. The prey organisms contained similar amounts of dry matter and protein, but Artemia was higher in lipid and glycogen than the zooplankton. Larvae fed Artemia were higher in both glycogen and lipid than the zooplankton-fed larvae towards the end of the feeding period. There were large differences between the prey organisms in the concentrations of essential fatty acids (% of total fatty acids) which was reflected in the fatty acid composition of the larval body. It is concluded that the macronutrient composition of Artemia in the present study was probably within the optimal range for promotion of growth and survival in young Atlantic halibut. The concentration of n-3 HUFA, and especially DHA, is however, very much lower in enriched Artemia than in copepods, and may be one of the factors triggering developmental errors in Atlantic halibut.

Optimizing rearing conditions of hatchling loliginid squid

Eggs laid by the California market squid (Loligoopalescens) were collected from spawning grounds and reared in the laboratory. The eggs were maintained in a rearing tank that was part of a closed, recirculating system. The system included seven 220-l circular tanks with attached filtration. Five experiments were conducted to test hatchling survival. One of them evaluated survival on three different food types: (a) enriched Artemia sp. nauplii, (b) wild zooplankton and (c) a mixture of a and b plus mysid shrimp. This mixture of food types (c) was offered to the hatchlings in the other four experiments. High mortality occurred in all experiments between days 1 and 15 post-hatching. However, survival over the entire time span of the experiments (45–60 days) was between 36% and 60%. These survival rates are well above previously reported survival rates for the same time period, and overall are up to 35% better than any survival results ever attained for the routine culture of Loligo spp. squid. Results suggest that high survival can be achieved by: (1) rearing hatchlings in a recirculating system consisting of small round tanks designed to maintain water quality and pH within narrow limits (8.1–8.4), (2) maintaining low current speed (1.0–1.4 cm s–1) to reduce skin damage and to enhance hatchling–prey interactions, (3) increasing feeding rate by feeding small amounts of food at regular intervals (every 2–3 h) during day time hours and keeping prey densities above 50 prey l–1, (4) feeding hatchlings with enriched Artemia nauplii during days 1–30 post-hatching and (5) feeding a variety of prey types and sizes to match the different sizes and hunting abilities of same-aged but heterogeneously developing hatchlings. The results from this study will enhance future culturing efforts for the commercially important loliginid squid.

Growth and Culture of Larval and Juvenile Tautogs in a Closed Recirculating-Seawater System

Field-captured and laboratory-held adult tautogs Tautoga onitis were allowed to spawn naturally under laboratory conditions. Embryos were cultured to hatching and raised successfully through the larval stage to 490-d-old posthatch juveniles. A large (8,500-L), closed, recirculating-seawater system, designed with a biological filter containing air diffusers that passed a strong air−water mixture through a high-surface-area substrate, was used to culture the larvae and juveniles. The biological filter provided biofiltration, aeration, degassing, and foam fractionation without disturbing the larvae. Water quality measures within the system were maintained at acceptable values. Enrichment of rotifers and brine shrimp Artemia sp. with algal strains containing highly unsaturated fatty acids, plus wild zooplankton, provided an adequate diet for larval growth. Also, a commercial feed was given at 2 weeks and larval tautogs were observed eating it. Juvenile tautogs were fed enriched adult brine shrimp along with wild zooplankton and increasingly larger sizes of a commercial feed every 4–7 weeks. At 23 weeks of age, live foods were discontinued and the fish received only commercial feed for the remainder of the study. These feeds provided adequate diets for growth through the larval and juvenile stages. Growth in the length of the juvenile tautogs was linear from 172 to 490 d of age. The estimated growth rate, based on direct measurements of growth, was 0.29 mm/d, which is similar to the rates reported for other laboratory-reared juvenile tautogs and for other labrids.

The effects of temperature and dietary fatty acids on the fatty acid composition of harpacticoid copepods, for use as a live food for marine fish larvae

Two marine harpacticoid copepod species, Amonardia sp. and Tisbe sp., were isolated from plankton collected near Halifax, Nova Scotia. The effect of two algal diets ( Dunaliella tertiolecta and Isochrysis galbana) on the essential fatty acid (EFA) composition of Amonardia for use as an alternative live food for coldwater marine finfish larvae was evaluated. This copepod was able to synthesize a significant amount of the EFAs, 20:5 n−3 (eicosapentaenoic acid; EPA, 0.5%) and 22:6 n−3 (docosahexaenoic acid; DHA, 13.6%) from shorter chain n−3 polyunsaturated fatty acids (PUFAs), when fed the algae D. tertiolecta. The marine harpacticoids, Amonardia sp. and Tisbe sp., were fed the same algal diet ( I. galbana) and cultured at 20, 15 and 6°C. The temperature effect on n−3 EFA for both species showed the same general pattern. The concentration of n−3 HUFA decreased in the following order: 6°C>20°C>15°C. The EFA values for marine fish larvae of two cultured harpacticoid copepods, Tisbe sp. and Amonardia sp., are compared with that of wild zooplankton and trochophore larvae of the mussel Mytilus edulis

Maternal, paternal and temperature effects on otolith size of young herring ( Clupea harengus L.) larvae

Investigations were carried out to clarify the timing of first check formation, and the influence of maternal, paternal and temperature effects on otolith size of young herring larvae. Three male and three female herring of the Norwegian spring-spawning-herring-stock were used as parental fish in a 3×3 factorial design, resulting in nine parental combinations. Eggs from each of the combinations were incubated at 4, 8 and 12°C. Larvae were sampled at two ontogenetic stages: newly hatched and at the end of the yolk sac (EYS) stage. In addition, some larvae were also reared with wild zooplankton and sampled 16–21 days after hatching to compare first check size with observed otolith size of unfed larvae. The first check was deposited 1–2 days after hatching and before onset of exogenous feeding at 12°C. Significant maternal and temperature effects were observed on the size of sagitta and lapillus at hatching, with smaller otoliths at higher temperatures. Larval length showed a similar temperature response as the otoliths, whereas larval dry mass showed an opposite trend. Otolith size and larval length and mass were poorly correlated, and large variability in the otolith data were evident. Temperature also influenced the measured variables at the EYS stage while the maternal influence was not as strong as at hatching. No sagittal growth between hatching and the EYS stage was observed at 4°C although larval standard length increased. Large variability limits the direct application of the first check size as a reliable tool for discrimination of units within and between fish stocks.

Effects of prey density on the growth and mortality of weakfish Cynoscion regalis (Bloch and Schneider) larvae: Experiments in field enclosures

Weakfish larvae ( Cynoscion regalis (Bloch and Schneider)) were reared in large enclosures (1.4 m 3) deployed near adult spawning grounds in Delaware Bay, USA. Larvae were fed wild zooplankton, and experimental prey levels encompassed naturally occurring densities (10, 100 and 1000 prey items 1 −1). Separate experiments were conducted for early-stage and late-stage larvae in 1990, and the late-stage experiment was repeated in 1994 using an improved statistical design. In 1990, there was a trend toward elevated growth rates with increasing prey levels. For early-stage larvae, larval notochord lengths ( NL) varied with prey density and ranged from 3.65 to 4.30 mm. Larval dry weight ( W) also varied with prey density and ranged from 39.5 to 61.3 μg. Calculated growth rates varied accordingly. Growth among late-stage larvae followed a similar trend, and NL ranged from 4.65 to 5.25 mm, while W ranged from 73.7 to 139.7 μg. In the repeated late-stage experiment (1994), a nested ANOVA showed significant effects of prey level on growth. Notochord length (4.33 mm) and dry weight (72.3 μg) were significantly depressed at the lowest prey density. Rates of instantaneous mortality were variable among individual enclosures and were not related to prey density in either year. Our results differ from previous field enclosure studies, which have shown little effect of prey density on the growth of larval fish. By contrast, our data indicate that low food levels may result in depressed growth rates among weakfish larvae in nursery areas like Delaware Bay. These lower growth rates may lead to an increased duration of larval development, potentially culminating in high mortality from increased exposure to predation or from advection from suitable juvenile habitats.