Ontogenetic shifts in prey selection and foraging behaviour of larval and early juvenile alligator gar (Atractosteus spatula)

The early foraging ecology of alligator gar (Atractosteus spatula) is poorly documented, with little information available on young‐of‐year food habits or ontogenetic diet shifts. We conducted laboratory experiments to quantify prey selection and foraging behaviours of larval and early juvenile alligator gar (16–80 mm TL) simultaneously offered zooplankton, chironomid larvae and one of three densities of fish prey. The smallest size groups of alligator gar consumed zooplankton almost exclusively, but with increasing size, selection for zooplankton declined and selection for fish prey increased. At higher densities of fish prey, alligator gar exhibited lower selection for zooplankton and positively selected for fish at smaller sizes. Ingestion efficiencies for chironomids were considerably lower than for zooplankton or fish prey, resulting in low rates of consumption and negative selection for chironomids by all size groups of alligator gar. Fish prey elicited a different foraging response from alligator gar than zooplankton or chironomids, as alligator gar pursued and struck at fish over longer distances than for other prey types. With increasing size, alligator gar used a wider vertical range of the water column for foraging, changed their strike tactics and exhibited decreased handling times for zooplankton and fish. These observations indicate that alligator gar undergo several functional and behavioural changes during early ontogeny that facilitate a rapid transition to piscivory, but fish prey density strongly affects prey consumption patterns and the size at which alligator gar transition to piscivory.     

Alligator gar (Atractosteus spatula,Lacépède 1803) vitellogenin: purification,characterization and establishment of an enzyme‐linked immunosorbent assay

Alligator gar (Atractosteus spatula) is a non‐teleost bony fish distributed in North America. Gar populations have drastically declined as a consequence of habitat deterioration and the lack of regulation for their capture. Control of reproduction is critical for recovering their natural populations. The impossibility to distinguish genders and the determination of sexual maturity have hindered their successful reproduction. This research was aimed at developing an enzyme‐linked immunosorbent assay (ELISA) for the quantitative estimation of vitellogenin (VTG), a female‐specific protein. Plasmatic VTG from 17β‐estradiol (E2)‐induced juveniles and ovary vitellin from adult females were purified and characterized. Polyclonal antibodies against both proteins were produced to develop an ELISA. The immunoassay was validated by quality tests such as sensitivity, parallelism, recovery, reproducibility and specificity. Vitellogenin was determined in different tissues (plasma, mucus, liver and gills) of alligator gar. Vitellogenin and E2 concentrations in female breeders were found to be higher in November, before the spring spawning season. This approach represents a quick, reliable and non‐invasive practical alternative to distinguish genders and evaluate gonad maturation.     

Seasonal habitat use of alligator gar in a river–floodplain ecosystem at multiple spatial scales

Alligator gar (Atractosteus spatula) populations are declining throughout their range in the United States, even where considered stable. Similar declines in other riverine species are occurring worldwide due to alteration of habitat in river–floodplain systems. Most rivers in the world are highly regulated, resulting in departures from historic hydrology that provides connection to the floodplain habitats used by riverine fishes. Knowledge of the seasonal habitat requirements of alligator gar and similar riverine fishes at a watershed scale is limited, hindering management practices. We used radiotelemetry to monitor movements and habitat use of adult alligator gar (N = 32; 25.0–84.5 kg; 149–224 cm) at macro‐ and microhabitat scales during 2 years of varying hydrology. Fish showed seasonal differences in their use of main channel and floodplain macrohabitats. Floodplain tributaries and their upper reaches were especially important to reproductive ecology. Minimum distance travelled per day varied seasonally. While using the main channel Fourche LaFave River, gar selected low velocity, shallow depth and complex structure along channel margins. The Fourche LaFave River is unique in its connection to floodplain habitats on a relatively predictable basis, with frequent flooding during the spring and summer reproductive period. Our study emphasises the importance of connectivity between the main river channel and floodplain habitats to a floodplain‐obligate riverine species. An intact, heterogeneous riparian zone creates essential microhabitat for the species. Understanding habitat requirements of alligator gar at multiple spatial scales in a river–floodplain system is crucial to management of many other species and river systems.     

Stimulatory effect of thyroid hormones improves larval development and reproductive performance in alligator gar (Atractosteus spatula) and spotted gar (Lepisosteus oculatus)

Development of alligator gar (Atractosteus spatula) and spotted gar (Lepisosteus oculatus) larvae was evaluated after exposure of embryos to thyroid hormones (TH) by two different methods of administration. For the first method, alligator gar embryos were placed in a water bath with triiodothyronine (T₃; 1 ppm for 2 h), and control treatment embryos were placed in a water bath without T₃. For the second method, spotted gar adult males and females were injected with different TH during induced spawning: a group was injected with T₃ [20 mg kg^?1 body weight (BW)], a second group with thyroxine (T₄; 20 mg kg^?1 BW), a third group with thyroid‐stimulating hormone (TSH; 4 IU kg^?1BW) and a control group with dimethyl sulfoxide. Both methods resulted in increases in the concentrations of T₃ and T₄ in embryos of both species. An increase in the alkaline enzymatic activity of TH‐treated larvae was observed as well as an increase in the RNA:DNA, RNA:Dry weight and DNA:Dry weight ratios at hatching. Some positive effects of TH application were: a significant increase in the hatching rate of spotted gar larvae from TH‐injected parents and a higher percentage of successful spawns. Evenness of larval sizes and a higher survival rate were observed in alligator gar larvae. Snout development was accelerated by TH treatment in both species.     

Digestive enzymatic activity on Tropical gar (Atractosteus tropicus) larvae fed different diets

Digestive enzymatic activity and growth performance on tropical gar (Atractosteus tropicus) larvae fed Artemia nauplii (LF), frozen adult Artemia (AB), an artificial diet (AF) with 46% protein and 16% lipids and a starvation group (SG) from first feeding (5?days after hatching-5 DAH) to 34 DAH were studied. All larvae under starvation (SG) died at 15 DAH. By the end of the experimental period, morphological variables (total length, wet weight and specific growth rate) were significant in larvae fed AF compared to LF and AB. All enzymes studied in the experiment were present since the start of exogenous feeding (including pepsin) and the enzymatic activity varied with the diets. Low levels of enzymatic activity were observed until the 29 DAH; however, after this moment, there was a significant increase (eightfold), particularly for the AF treatment. In vitro protein digestibility tests performed with enzymatic extracts showed that artificial diets with 52% protein and 14% lipids were better digested by larvae before 30 DAH, while diets with 45% protein and 11% lipids were better digested after this age. Taking into account the better growth performance, higher enzymatic activity and better protein digestibility obtained, artificial diets can be used since the start of exogenous feeding on tropical gar larvae, as in other lepisosteids.     

Digestive enzyme activity during early larval development of the Cuban gar Atractosteus tristoechus

The ontogenesis of digestive enzymes (proteases, amylases, lipases, and phosphatases) in Cuban gar Atractosteus tristoechus was determined in larvae between 5 and 18 days after hatching (DAH). Variations in specific activities of most enzymes were related to the transition from endogenous to exogenous feeding and to the transition from the larval to the juvenile stage. Alkaline protease activity was not detected until 8 DAH in contrast to acid protease activity, which was quantifiable at 5 DAH. Acid protease activity was consistently higher than alkaline protease activity, indicating the presence of a functional stomach in the early stages of larval development. The acid protease activities of larvae and adults were compared by means of zymogram analysis. Four acid protease bands were found in adults (two more than in larvae). This result is the first time that more than one band of acid proteolytic activity has been found in Lepisosteidae. High lipase activity indicated the importance of lipid utilization, particularly during yolk-sac absorption. In contrast to the other enzymes studied, amylase activity was consistently low, probably due to the strictly carnivorous diet of gar larvae and their low capacity for carbohydrate digestion. High activities of aminopeptidase and acid and alkaline phosphatases suggest intestinal absorption. This result, together with the existence of a short gut and a lower proteolytic activity in the intestine than in the stomach, suggest that most of the proteolytic activity takes place in the stomach, while the primary function of the intestine is nutrient uptake.     

Fructooligosaccharide supplementation in diets for tropical gar (Atractosteus tropicus) juvenile: Effects on morphophysiology and intestinal barrier function

Supplementary fructooligosaccharides (FOS) were evaluated on growth, survival, gut morphology, digestive enzyme activity and expression of intestinal barrier genes in tropical gar (Atractosteus tropicus). Four treatments were designed to include FOS (5, 10, 15 and 20 g/Kg) and were compared with a control (0 g/Kg) and the current commercial trout diet. Ten fish were stocked (0.25 ± 0.01 g and 4.2 ± 0.4 cm total length) per experimental unit by triplicate in a 45‐day experiment. Fish fed with 5 and 10 g/Kg of FOS had the highest average weight and total average length. Fish fed 5 g/Kg FOS showed the best growth indexes (absolute weight gain: 2.16; specific growth rate: 4.96; and feed conversion rate: 1.23) and survival (96.66%) and also had the highest acid protease, chymotrypsin and leucine aminopeptidase activities. Fish fed 5 g/Kg FOS had a high absorption area and accumulation of lipids in the liver. Finally, relative overexpression of occluding (occ), nucleotide‐binding oligomerization domain‐2 (nod2) and mucin 2 (muc2) was detected in fish fed 10 and 15 g/Kg FOS. The supplementation between 5 and 10 g/Kg FOS in diets for A. tropicus juveniles could be beneficial for growth, digestive enzyme activities, gut morphology and intestinal barrier function.     

Development and evaluation of microparticulate diets for early weaning of Atlantic cod Gadus morhua larvae

Early weaning trials were conducted with cod larvae to investigate the effectiveness of microparticulate diets (microbound and microcoated) with and without lipid-walled capsules (LWCs). The microparticulate diets were evaluated by measuring physical parameters of the diet in the water column (leaching and settling rate), palatability (intestinal fullness), performance of the diet (survival and growth), and examination of the diet in the larval intestine (histological analysis). A feeding trial was conducted using four experimental diets (carrageenan microbound diet, carrageenan microbound diet with LWCs, zein microcoated diet and zein microcoated diet with LWC), one commercial diet (BioKyowa: A-250) and a live feed control (rotifers and Artemia ). Survival of cod larvae to 39 days post-hatch ranged from 5 to 10% with the experimental diets, 22.9% with the BioKyowa diet, and 36.5% with live prey. There was evidence of food absorption with all diets in the form of lipid vacuoles in the midgut and supranuclear vacuoles in the hindgut. Large vacuoles in the midgut were more abundant in the enterocytes of larvae fed the experimental diets compared with larvae on the BioKyowa diet and the live feed control. Based on observations of intestinal fullness, the experimental diets appeared to be less palatable than the BioKyowa diet. As a result, it took longer to wean the larvae and higher mortality was experienced during weaning. Once successfully weaned, the experimental diets yielded growth rates equivalent to larvae feeding on the commercial diet for the remainder of the experiment.     

Weaning Chinese perch Siniperca chuatsi (Basilewsky) onto artificial diets based upon its specific sensory modality in feeding

Chinese perch are one of the most valuable food fish in China, but the sole source of feed for intensive culture is live prey fish. Our previous studies on systematic sensory physiology revealed that this species have a mechanism for this peculiar feeding habit. In the present study, a specific training procedure was designed, and both experimental (initial body weight 171.0 g; 120 days) and commercial (initial body weight 52.4 g; 240 days) net-cage cultures were conducted to investigate the training success, growth performance and survival of the trained yearlings fed with nonlive or Oregon-type moist diet. The training successes of minced prey fish and the Oregon moist diet were 100 and 89.9%, respectively, in experimental culture, and 92.2 and 83.5% in commercial culture. In an experimental trial, the fish fed minced prey fish or the Oregon moist diet attained final body weights of 472.7 g or 344.7 g, although the specific growth rates of these groups were significantly lower than that of the fish fed live prey fish (final body weight 560.0 g). Mortality was not significantly related to dietary treatment. In commercial culture, the final body weights were as follows: 750 g on live prey fish, 705 g on minced prey fish and 651 g on the Oregon moist diet. Feed costs to produce 1 kg fish were estimated to be US$6.59 for live prey fish, US$1.76 for minced prey fish and US$2.07 for the Oregon moist diet. The results of the present study confirmed that sensory modality and associative learning appear to be critical factors in determining food discrimination of Chinese perch, indicating that both minced trash fish and Oregon-type moist diet can be substituted for live prey fish in intensive commercial production.     

Development of testis and digestive tract in longnose gar (Lepisosteus osseus) at the onset of exogenous feeding of larvae and in juveniles

The aim of this study was to describe the ontogenetic development of the testis and the alimentary tract in longnose gar (Lepisosteus osseus) related to fish size and age at the onset of exogenous feeding and late ontogenesis. Using light microscopy, testes were first detected histologically by the appearance of primordial germ cells 9 days after the first exogenous feeding [31–31.5 mm total body length(TL)] and presumptive seminiferous tubules (maleness characteristic) in fish of 107 mm TL. The present histological studies indicated that the alimentary tract of lepisosteids is completely functional at the beginning of exogenous feeding, several days before the completion of yolk absorption. Based on these results, we have concluded that garfish larvae/juveniles can be effectively trained to consume formulated diets at early stages, after an initial feeding of live food for 2–3 days (23.5 mm TL). Our findings provide evidence of the first controlled rearing of longnose gar using live and formulated diets, providing the possibility of experimental work with this non‐teleost fish.     

Assessment of dietary lipid sources in tropical gar,Atractosteus tropicus larvae: Growth parameters and intermediary lipogenic gene expression

Five experimental diets containing different lipid sources, fish oil (D1), soybean lecithin (D2), corn oil (D3), canola oil (D4) and olive oil (D5), were evaluated in Atractosteus tropicus larvae for the relative gene expression of the enzymes fatty acid synthase (fas), acetyl‐CoA carboxylase 1 (acc1) and carnitine palmitoyltransferase 1C (cpt1c), in addition to their effects on larval growth, survival and cannibalism during a 30‐day feeding trial. Higher growth and survival were obtained in treatments D1 and D2, and lower performance in diets D3, D4 and D5. The highest levels of expression of fas and acc1 occurred in larvae fed with D1, which contained high amounts of n‐3 long‐chain polyunsaturated fatty acids (LC‐PUFA), mainly DHA and EPA FA are regulators of lipogenesis. The higher cpt1c expression in plant‐based diets is attributed to the fact that these diets are rich in α‐linolenic acid (ALA) and low DHA, EPA and ARA levels that favour ß‐oxidation. In conclusion, the diets with fish oil (D1) and soybean lecithin (D2) were the best treatments for larval growth, survival and cannibalism and thus appear to meet both lipid and energy requirements of A. tropicus larvae, meanwhile the use of vegetable oils influences the expression of intermediary lipogenic genes.     

Migratory dynamics of stream-spawning longnose gar (Lepisosteus osseus)

Abstract– Literature evidence suggests that lake-dwelling longnose gar (Lepisosteus osseus) enter tributary streams to spawn, Until the present study, the dynamics of this breeding migration had never been investigated quantitatively. During the summers of 1991 and 1992, longnose gar were captured as they entered Weaubleau Creek, Missouri, a tributary of Harry S. Truman Reservoir. The in-stream spawning migration began in early April and ended in late May, and was positively correlated with stream flow and water level, and negatively correlated with water temperature. In-stream residence times ranged from 15 to 94 days, with males exhibiting longer residence times than females. Once in-stream, longnose gar travelled as far as 10 km upstream and occupied certain pools at greater relative frequencies. Although the reason for this preferential utilization is not completely understood, it may relate to pool depth and riffle proximity. Longnose gar disperse from the spawning stream great distances, with gar captured in Weaubleau Creek being recaptured up to 48 km away. This information should provide fisheries biologists the means to consider the reproductive ecology of this species in their conservation and management decisions.     

Study on the adaptability of common carp (Cyprinus carpio) to diets from the perspective of digestion and absorption

As an ecologically sustainable aquaculture mode, the rice‐fish approach has been paid more attention in recent years. In rice paddies, there are plant and animal diet items available to fish, but it is not clear how common carp adapt to different diets. In view of this, common carp (initial weight 492.6 ± 30 g, n = 270) were randomly divided into three groups and fed with earthworms (group A), earthworms + duckweed (group M) and duckweed (group P) respectively. After 8 weeks under these feeding regimes, the intestinal digestive enzymes activities were assayed, and intestinal tissue sections stained with HE and AB‐PAS were used to observe the morphology. The results showed that the activities of trypsin and lipase were highest in group A, followed by group M and group P. Group A had greater intestinal fold height, fold width and fold absorption area than the other groups (p < .05), especially in the foregut. Meanwhile, the total number of mucous cells was largest in group P, followed by the groups M and A. Totally, activities and distributions of digestive enzymes, fold height, fold width, fold absorption area and number and distribution of mucous cells of common carp were affected by the type of diet. Based on these results, it was found that common carp has strong adaptability to diets, and it was speculated that the majority of digestion and absorption of protein was concentrated in the foregut, while for starch and cellulose, it was spread out along the whole intestine.     

Weaning methods using formulated feeds for snakehead (Channa striata and Channa micropeltes) larvae

The culture of snakehead fish (Channa striata and Channa micropeltes) in Vietnam is limited, and snakehead culture has been banned in Cambodia, because traditional practices include capture of fingerlings from the wild as seed, as well as capture of small‐size (also known as trash fish or low‐value) fish. As hatchery breeding technology has improved, we investigated the optimal weaning practices for these two species. Both laboratory experiments and farm trials were conducted. For C. striata, the optimal weaning procedure is to begin at 17 days after hatch (dah) and wean the fish at 10% replacement of live feed with formulated feed per day. However, for C. micropeltes, the optimal procedure is to wait until 40 dah to begin weaning and then to wean the fish with a 10% replacement of live feed with formulated feed every 3 days. These results should enable farmers to domesticate snakehead culture in Vietnam and Cambodia and eliminate reliance on fish captured from the wild as both seed and feed.     

Evaluation of weaning performance of California halibut (Paralichthys californicus) larvae using growth,survival and digestive proteolytic activity

This study evaluated weaning success of California halibut, Paralichthys californicus, larvae onto a microdiet at various stages of development utilizing growth, survival and digestive enzyme activity. Weaning onto a microdiet was evaluated at 16, 26, 36 and 46 days posthatch (dph). Alkaline and acid proteases and leucine aminopeptidase activities were measured after weaning and compared between the weaned treatment and Artemia‐fed controls. Survival was significantly lower in the microdiet‐fed treatments compared to the control groups. Growth was significantly reduced in all weaning treatments compared to the control, except for the 46 dph group. No differences in enzyme activities were detected between treatment and diet at 16 and 26 dph; however, activities were higher for the microdiet‐fed larvae at 36 and 46 dph. This study demonstrates that California halibut larvae possess a differentiated and effective digestive system early in development and can be weaned with relative success (>40% survival) before completion of the metamorphosis (i.e., 36 dph). The lack of weaning success at an early date cannot be entirely because of the absence of a functional stomach but could be related to, among other factors, the low‐microdiet ingestion rates observed and higher leaching of smaller microdiets.     

Weaning of bullseye puffer (Sphoeroides annulatus) from live food to microparticulate diets made with decapsulated cysts of Artemia and fishmeal

Two experiments were carried out to test microparticulate diets forweaning hatchery-produced larvae and juveniles of bullseye pufferSphoeroides annulatus. The diets were formulated with differentprotein sources: diet 1 with a combination of decapsulated cysts ofArtemia and fishmeal, and diet 2 with a combination offishmeal, squid, tuna gonad and shrimp meal. In the first experiment60-days-old fish were weaned with the microdiets over five days. Fishsurvival after 11 weeks of feeding was 92% for diet 1, 85%for diet 2, and 95% for the control fish fed Artemianauplii. Once it was determined that bullseye puffer can be adequatelyreared with artificial dry diets, diet 1 was used to test earlier timesfor weaning to reduce the period of Artemia feeding. In thesecond experiment, three different times were tested for initiation ofweaning in sibling fish larvae, i.e., at 29, 34, and 39 days post-hatch.Small differences in weight, length and survival were found amongweaning treatments after 23 days of feeding. When weaned at day 29post-hatch, fish larvae grew from an initial weight of 38.4 mg andlength of 11.1 mm to a final weight and length of 405.7 mg and 25.1 mmrespectively. Final survival in this treatment was 49.3%. Thereduced period of Artemia feeding would provide an economicalalternative for the species to take into consideration for its cultureat commercial scale.     

Improved digestibility of plant ingredient‐based diets for European seabass (Dicentrarchus labrax) with exogenous enzyme supplementation

This study evaluated the effect of supplementation of a plant ingredients‐based diet with two levels of commercially digestive enzyme products: Synergen? and Natugrain^® on the apparent digestibility coefficients (ADCs) of nutrients and on digestive enzyme activities in European seabass (Dicentrarchus labrax) juveniles. A control diet was formulated with 450 g/kg CP (200 g/kg from fish meal + 800 g/kg from plant protein) and 180 g/kg CL (fish oil), and four other diets were made similar to the control but supplemented with 0.2 g/kg or 0.4 g/kg of Synergen? and Natugrain^®. Digestive enzyme activities were not affected by dietary treatments. Nevertheless, chyme pH along the intestine of fish fed the Natugrain^®‐supplemented diets was reduced. The ADC of nutrients increased with exogenous digestive enzyme supplementation, with the highest ADC values being observed for the 0.4 g/kg Natugrain^® diet. The ADC of dry matter, protein, lipids, energy and phosphorus in this diet were 26%, 6%, 4%, 16% and 63% higher, respectively, than in the control diet. Overall, diet supplementation with exogenous enzyme complexes seems to have high potential for improving the utilization of plant feedstuff‐rich diets for seabass juveniles. This study revealed that Natugrain^® was more effective than Synergen? in improving the ADC of nutrients and energy in European seabass juveniles.     

Effects of two diets on lipid composition and reproductive performance of brill (Scophthalmus rhombus) eggs

The effects of two diets with different lipid composition (squid Loligo gahi and one commercial pellet for breeders) on reproductive performance and egg lipid composition of brill (Scophthalmus rhombus) were compared. A total of 36 adult brill weighing 1482 ± 432 g were randomly distributed in the experimental tanks. A flow‐through system composed of four tanks with 4.2 m² of bottom area (5.0 m³ × 1.2 m water depth) was used. The experiment lasted for approximately 5 months (168 days). From the four mature females fed squid, three ovulated. Five females fed pellets matured and ovulated. Total ovulations were of 10 and 44, by the three females fed squid and the five ovulating female fed pellets respectively. Egg viability was higher, although not significantly, for females fed pellets. This indicates that pellets could be a better food for breeding females. In general, the egg total lipid content and the lipid classes composition did not seem to be affected by diet. In contrast, results indicate that not only diets but also individual females, regardless of the diet consumed, are determinant for egg fatty acid composition, clearly modulating their composition.     

Rearing zebrafish (Danio rerio) larvae without live food: evaluation of a commercial, a practical and a purified starter diet on larval performance

Owing to the increasing importance of zebrafish as a vertebrate model in many fields of research, efforts must be made to breed and maintain this species in laboratory. Zebrafish larvae are traditionally reared on cultured live paramecia during the first 9 days of exogenous feeding, followed by a combination of paramecia and artemia nauplii until day 21, making larval rearing expensive, labour intensive and unpredictable. Thus, a trial was conducted with zebrafish larvae in order to evaluate the suitability of artificial diets as an alternative to live food during the first 21 days of exogenous feeding. Five dietary treatments were tested: (1) artemia nauplii; (2) a commercial; (3) a purified; (4) a practical diet, all delivered continuously; (5) the same practical diet delivered manually. The best overall larval performance was achieved in the group fed artemia nauplii (86% survival, 14.3 mm standard length, 46.1 mg wet weight). Compared with existing results obtained with the traditional live food schedule, our results suggest that paramecia might not be the most suitable first food for zebrafish, and that artemia nauplii could be used as the only live food. The present work demonstrates that zebrafish larvae can be reared without live food with a significant growth and a high survival, provided that an appropriate artificial diet is presented in a continuous way. Among the diets tested, the practical diet, if continuously delivered, led to the best performance assuring a mean standard length of 72% of that obtained with artemia and a similar survival rate. Moreover, the purified diet, supporting over 50% survival and an appreciable growth, could be useful in some toxicological studies in which a well‐defined diet is needed.