Free amino acid distribution in plasma and liver of juvenile cobia (Rachycentron canadum) fed increased levels of lizardfish silage

Juvenile cobia (Rachycentron canadum) (100 g) were fed four moist diets (447–476 g kg^?1 dry wt) where 0, 130, 260 or 390 g kg^?1 of concentrated lizardfish (Saurida undosquamis) silage replaced fresh lizardfish, respectively. Blood and livers were sampled at 0, 6, 12, 24 and 48 h postfeeding at the end of the 3‐week experiment. At 6 h postfeeding in all groups, maximum concentrations of most plasma essential amino acids were observed, while significantly lower levels of most non‐essential amino acid levels were recorded compared to the other sampled times. At 6 and 12 h after feeding, the concentration of most plasma free amino acid (FAA) increased with an increase in dietary fish silage levels. Most FAA in livers of all groups peaked at 12 and 24 h postfeeding. However, at 48 h postfeeding, concentrations of most plasma FAA were significantly higher in fish fed 0% silage‐based diet than in fish fed the other diets (4999 versus 3390–4339 nmol AA mL^?1 plasma). Growth rates and feed utilization were significantly lower in cobia fed 26% or 39% silage‐based diets than in fish fed 0% or 13% silage‐based diets. Different levels of silage protein thus seemed to have effects on growth and feed utilization efficiency of juvenile cobia. Results from this study support the premise that fish silage can be included until 130 g kg^?1 in cobia diets.     

Rainbow trout silage as immune stimulant and feed ingredient in diets for Mozambique tilapia (Oreochromis mossambicus)

Peptides and free amino acids resulting from protein hydrolysis could act as stimulants of fish non‐specific immunity. The study aimed to determine the potential of rainbow trout viscera silage as immune stimulant and feed ingredient for Mozambique tilapia, and to establish whether formic acid used during silage preparation contributed to any effects. Four diets were evaluated: a reference diet (R), one containing 6.5 g kg^?1 formic acid (FA) and two silage diets containing 160 g kg^?1 (SL, low inclusion) and 285 g kg^?1 (SH, high inclusion) silage. Low silage inclusion improved phagocytic activity of leucocytes compared with the reference, while high inclusion showed no improvement. No other non‐specific immunity parameters or haematology were affected by any treatments. High silage inclusion significantly decreased growth and led to higher mortality, while formic acid had no effect on growth. It is concluded that rainbow trout viscera silage can stimulate the cellular non‐specific immunity of Oreochromis mossambicus, and that protein hydrolysis products (and not formic acid) is responsible for the stimulation. The silage can also serve as source of dietary protein and essential amino acids in tilapia diets. However, both fish growth performance and improvement in cellular immunity are dependent on silage inclusion level.     

Effects on digestibility and growth of juvenile cobia (Rachycentron canadum) fed fish or crab silage protein

The study was conducted in Cam Ranh, Vietnam, in 1000‐L tanks supplied with recirculated and biofiltered saltwater (33‰ and 28.4 °C) to evaluate the potential use of lizard fish (Saurida undosquamis) or blue crab (Portunus pelagicus) acid silage protein for juvenile cobia (23–25 g). Six isoenergetic test moist diets (4915–5125 kcal kg^?1), using either raw fish diet, fish silage diet (FSD), raw crab diet, crab silage diet (CSD), mixed raw fish/raw crab diet or mixed fish/crab silage diet (MSD), as part of the protein sources in the steam‐cooked diets, were fed to satiety to triplicate groups of 20 fish each for a 6‐week growth trial. Y₂O₃ was added as an inert indicator to determine the apparent digestibility coefficients (ADC) for macro nutrients and gross energy. Weight gain (185–286%) and specific daily growth rate (2.5–3.2% per day) were significantly higher in cobia fed the raw‐based diets and FSD than in fish fed CSD and MSD (34–90 and 0.7–1.5% per day). Feed conversion ratios (FCR) were significantly higher in the groups fed CSD and MSD diets (2.1–6.5) than the groups fed the other diets (1.0–1.2), resulting in significantly lower protein productive values (0.1–0.2) in the groups fed CSD and MSD than in the other groups (0.3–0.4). The FCR results were confirmed by significantly lower ADC values in fish fed CSD and MSD than those in fish fed the other diets. We thus conclude that the present raw‐based diets were better utilized by juvenile cobia than silage‐based diets, particularly the diet made from crab silage.     

Apparent digestibility coefficient of protein and amino acids of acid, biological and enzymatic silage for Nile tilapia (Oreochromis niloticus)

Using the fish silage to partially replace proteic feedstuff in aquafeeds is an alternative to mitigate sanitary and environmental problems caused by the lack of adequate destination for fisheries residues. It would also lower feed costs, consequently improving fish culture profitability. However, using fish silages in aquafeeds depends on determination of its apparent digestibility coefficients (ADC). This work aimed to determining the ADC of crude protein and amino acids of acid silage (AS), biological silage (BS) and enzymatic silage (ES) for juvenile Nile tilapia (94.5 ± 12.7 g). The ADC_CP was: 92.0%, 89.1% and 93.7% for AS, BS and SE respectively. The average ADC of amino acids was: 91.8%, 90.8% and 94.6% for AS, BS and ES respectively. Results encourage the use of AS, BS and ES to partially replace protein sources in balanced diets for neotropical fish.     

Characterization and expression analysis of the transferrin gene in Nile tilapia (Oreochromis niloticus) and its upregulation in response to Streptococcus agalactiae infection

In this study, full-length tilapia transferrin (OnTF) isolated from liver cDNA of Nile tilapia (Oreochromis niloticus) was found to have an open reading frame of 2,091-bp encoding 696 amino acid residues. Two additional amino acids: Gly³⁶⁹ and Gly³⁷⁰ were observed compared with the reported Nile tilapia transferrin protein sequence. Pre-mature protein has a predicted molecular weight of 78.2 kDa, while mature protein is 73.28 kDa in size. Comparative sequence analysis with transferrin from other species revealed two major putative iron-binding domains designated as the N-lobe and the C-lobe in accordance with the transferrin protein characteristics. The predicted tertiary structure of tilapia transferrin confirmed the presence of iron and anion-binding sites on both lobes that are conserved among transferrins from other species. Quantitative real-time PCR analysis showed significantly higher expression of tilapia transferrin gene in liver than in other tissues (p < 0.05). Transferrin expression in tilapia experimentally infected with 10⁶ and 10⁸ colony-forming units mL^?1 of Streptococcus agalactiae was significantly upregulated at 24 and 12 h post-infection (hpi), respectively, and decreased afterward. Iron-deficiency in serum of bacterially infected fish was detected at 48 and 24 hpi, respectively. The expression pattern of the transferrin gene and the iron levels of infected tilapia in this study were consistent with the function of transferrin in innate immunity.     

Chemical and Nutritional Characteristics of Dried Sardine Silage

Abstract

Acid silage was prepared by mixing fresh minced sardines with a mixture of formic and propionic acid. The silage was stored outside at ambient temperatures (range 28-39°C) during liquefaction and samples were taken for measurement of viscosity, pH and non-protein nitrogen. After 48 h the mrxtures were liquefied and were then dried separately or co-dried with wheat bran. Samples of dried silage were analyzed for proximate composition and amino acid content. Trials were conducted on tilapia, Oreochromis aureususing test diets to compare the digestibility of sardine silage with that of fish meal. The study showed that sardine silage, dried in a solar cabinet, could be prepared in 4-5 days. Essential amino acids were present at levels exceeding the requirements for tilapia. Digestibility trials showed the apparent digestibility coefficients (ADC) of crude protein, dry matter and gross energy for sardine silage to be equivalent to those of fishmeal (P > 0.05).     

Depletion of florfenicol amine in tilapia (Oreochromis sp.) maintained in a recirculating aquaculture system following Aquaflor®‐medicated feed therapy

Aquaflor^® [50% w w^?1 florfenicol (FFC)], is approved for use in freshwater‐reared warmwater finfish which include tilapia Oreochromis spp. in the United States to control mortality from Streptococcus iniae. The depletion of florfenicol amine (FFA), the marker residue of FFC, was evaluated after feeding FFC‐medicated feed to deliver a nominal 20 mg FFC kg^?1 BW d^?1 dose (1.33× the label use of 15 mg FFC kg^?1 BW d^?1) to Nile tilapia O. niloticus and hybrid tilapia O. niloticus × O. aureus held in a recirculating aquaculture system (RAS) at production‐scale holding densities. Florfenicol amine concentrations were determined in fillets taken from 10 fish before dosing and from 20 fish at nine time points after dosing (from 1 to 240 h post‐dosing). Water samples were assayed for FFC before, during and after the dosing period. Parameters monitored included daily feed consumption and biofilter function (levels of ammonia, nitrite and nitrate). Mean fillet FFA concentration decreased from 13.77 μg g^?1 at 1‐h post dosing to 0.39 μg g^?1 at 240‐h post dosing. Water FFC concentration decreased from a maximum of 1400 ng mL^?1 at 1 day post‐dosing to 847 ng mL^?1 at 240 h post‐dosing. There were no adverse effects noted on fish, feed consumption or biofilter function associated with FFC‐medicated feed administration to tilapia.     

Efficacy of an experimentally inactivated Streptococcus agalactiae vaccine in Nile tilapia (Oreochromis niloticus) reared in Brazil

Tilapia aquaculture is one of the fastest‐growing segments of fish production in Brazil. Nile tilapia (Oreochromis niloticus) is largely cultivated in the state of Parana, where Streptococcus agalactiae is the cause of severe disease outbreaks. The objective of this paper was to evaluate an inactivated S. agalactiae vaccine in tilapia for the control of streptococcal disease outbreaks. Tilapia, weighing approximately 20 g each, were intraperitoneally (i.p.) inoculated with 0.1 mL of the vaccine at a dose of 2.0 × 10⁸ colony‐forming unit (CFU) mL^?1. One group of tilapia (treatment 1) received one vaccine dose, and the other group of tilapia (treatment 2) received two doses, with an interval of 21 days. The control group was i.p. inoculated with 0.1 mL tryptic soy broth fish^?1. Immunized and control tilapia were i.p. challenged with 0.1 mL of 3.0 × 10⁷ CFU mL^?1 at 30 days post vaccination. The fish were monitored daily for disease signs and for mortality for 16 days post challenge. A statistically significant difference (P=0.0045) was found between the mortality of treatments 1 and 2. The value of relative per cent of survival of 83.6% and 96.4%, respectively, indicate that this vaccine was efficient in Nile tilapia.     

Substitution of fish meal with sunflower seed meal in diets for Nile tilapia (Oreochromis niloticus L.) reared in earthen ponds

This study investigated sunflower seed meal (SSM) as dietary protein replacement of fish meal (FM) for Nile tilapia (Oreochromis niloticus) juveniles (initial mean weight of 19.8 ± 6.3 g) reared in earthen ponds for 210 days. SSM replaced 25%, 50%, 75%, and 100% (i.e., D₂₅, D₅₀, D₇₅, and D₁₀₀, respectively) of FM in an isonitrogenous and isocaloric diet. The optimum FM replacement level of D₂₅ as predicted by the equation y = 177.5 + 26.5x – 7.9x² yielded an optimum weight of about 184 g within 100 days. Thus D₂₅ was the most effective SSM level for Nile tilapia growth in earthen ponds. Fish growth declined as SSM exceeded D₂₅. The length-weight relationship (r ≥ 90%) depicted an isometric fish growth. Nutrient utilization parameters were similar in all the diets except for D₁₀₀. The protein content of fish carcass was highest in the control and D₂₅, while fiber level was highest in D₁₀₀. Amino acids imbalance and high fiber content could have reduced the fish growth at D₅₀, D₇₅, and D₁₀₀.     

Growth,feed utilization,and liver histology of juvenile Nile tilapia (Oreochromis niloticus) fed diets containing increasing levels of swine fat

Due to predictions that the expanding aquaculture sector cannot continue to rely on finite marine stocks for fish oil, there have been several research works within the aquafeed industry to find sustainable alternatives for dietary lipid sources. This study was conducted to evaluate the effects of a range of inclusion levels of rendered swine fat on the growth performance and liver histology of juvenile Nile tilapia, Oreochromis niloticus. Fish (initial weight ~ 20 g) were randomly stocked into twelve 1 m³ nylon mesh hapas and fed isoenergetic and isonitrogenous diets containing rendered swine fat at varying inclusion levels of 0% (Control), 5% (SF5), 7% (SF7), or 10% (SF10) for 84 days. The control diet contained 5% fish oil as the lipid source. At the end of the trial, there were no significant differences between the final weights of the fish that were fed the control (38.01 ± 0.76) and SF5 diets (37.85 ± 0.68), and both groups recorded significantly higher (P < 0.05) specific growth rates relative to the SF7 and SF10 groups. Specific growth rates ranged from 0.59% day^?1 (SF10) to 0.75% day^?1 (Control and SF5). The fish that were fed the SF7 and SF10 diets had significantly (P < 0.05) larger livers relative to the control and SF5 fish. Histological examination further revealed significant liver alterations in the SF7 and SF10 fish, as evidenced by hepatocyte nuclei migration, hepatic vacuolation, and varying degrees of steatosis. Calculations of profit indices showed that the SF5 diet offers the best economic viability, taking into consideration both the monetary values of fish produced and cost of feed fed over the trial period. The results of this study indicate that rendered swine as a sole lipid source in juvenile Nile tilapia diets can be included at 5% without depressing fish growth.     

Ultraviolet-induced androgenesis in Nile tilapia, Oreochromis niloticus (L.), and hybrid Nile × blue tilapia, O. aureus (Steindachner)

Fertilization of ultraviolet (UV) irradiated oocytes of Nile tilapia, Oreochromis niloticus (L.), with sperm from O, niloticus or blue tilapia, O, aureus (Steindachner), and subsequent suppression of the first cleavage of fertilized eggs successfully induced androgenesis in Nile and blue tilapia. The optimal doses of UV irradiation to denucleate a female genome of Nile tilapia prior to androgenesis ranged from 5940 to 6930 erg mm^?2 for 54-63 s at a fixed intensity of 110 erg mm^?2 s^?1. Putative androgenetic fish were created from eggs which were irradiated at various times and several durations of heat-shock. Eggs which were treated for 5 min at 41.6 ^oC at 2 7.5 min after fertilization were the most successful at suppressing the first cleavage and producing viable androgenetic diploids in Nile or hybrid Nile X blue tilapia. The maximal survivals of putative androgenetic diploids in relation to the control were 1.60% and 0.90% in Nile and hybrid Nile X blue tilapia, respectively. The androgenetic offspring established exhibited active feeding and normal growth.     

Faecal coliforms in pond water, sediments and hybrid tilapia Oreochromis niloticus×Oreochromis aureus in Saudi Arabia

Total bacterial load, total coliforms faecal coliforms in pond water, sediment, intestine of hybrid tilapia Oreochromis niloticus×Oreochromis aureus and pigeon Columba livia faeces were investigated monthly over a period of 1 year from July 1999 to June 2000. Fish were collected randomly by a cast net. Samples were analysed for coliforms using the multiple‐tube fermentation technique. Results showed total viable bacterial counts in the pond water, sediment, intestine of tilapia and pigeon faeces ranging from 1.8±0.9×10² to 6.0±1.2×10⁴ cfu mL^?1, 3.2±1.2×10⁵ to 2.8±1.5×10⁷ cfu g^?1, 8.2±1.6×10⁵ to 9.9±1.5×10⁷ cfu g^?11.0±0.4×10⁷to9.7±0.2×10⁹ cfu g^?1respectively. The most probable number (MPN) of coliforms and faecal coliforms ranged from 287±12 to ≥1600±0 100 mL^?1 in pond water; the MPN ranges for sediment, tilapia intestine and pigeon faeces were 257±29 to ≥1100±0 g^?1, 237±46 to ≥1100±0 g^?1 and 403±98 to ≥1100±0 g^?1 respectively. The abundance of normal bacteria coliforms was greater in the warm months than in the cold months. Ground water was free from any sort of coliform organisms, and there were no sources of human faecal matter in the pond. So, it is clear that faecal coliforms from pigeon faeces significantly contaminated (P<0.05) the ponds and tilapia intestines. Escherichia coli was the only coliform organism found in pond water, sediment, intestine of tilapia and pigeon faeces.     

Nutrient composition and sensory evaluation of fillets from wild-type Nile tilapia (Oreochromis niloticus, Linnaeus) and a red hybrid (Florida red tilapia × red O. niloticus)

A comparison of composition and sensory evaluation were performed on fillets of Oreochromis niloticus (wild type) and red hybrid tilapia (RHT) (Florida red tilapia ♂× red O. niloticus♀). Fifty male tilapia, 25 O. niloticus (initial weight 159.3 g) and 25 red hybrid (initial weight 132.4 g), were placed in each of three 2.0 m³ volume tanks. The fish were fed a commercial feed containing 35.9% crude protein during the 120‐day experimental period and then killed in cold water (3°C). All the fish were filleted. Twelve fillets from each treatment were used for proximate analyses, five fillets from each treatment were used for fatty acids analyses and the remaining fillets were used for sensory evaluation. The compositional analyses showed similar moisture, true protein and ash content in both genetic groups, but a lower crude fat content was measured in the red hybrid fillets (0.70%) compared with that of O. niloticus fillets (0.97%). No differences between O. niloticus and the red hybrid were observed in their fillet profile of fatty acids. In the sensory evaluation test, a difference in fillet flavour between both genetic groups was perceived. Of the 112 consumers in the preference test, 81.2% perceived a difference in fillet flavour between the two tilapias, with a general preference for the red hybrid over the wild‐type O. niloticus. The benefits of cultivating a RHT with a low lipid content are discussed.     

Relative contribution of alternative proteins to the growth of Juvenile Cobia,Rachycentron canadum (Linnaeus)

Five isonitrogenous and isocaloric diets were fed to juvenile cobia, to assess the relative contribution of different proteins (fish meal, soybean meal, corn gluten and beer yeast) to the growth of cobia. The dietary effects on nitrogen and carbon turnover and on the isotopic diet‐consumer discrimination factors (Δ¹⁵N and Δ¹³C) were also assessed. Growth results showed that the final body weight, growth rate, feed conversion ratio and protein efficiency ratio of cobia fed diets with alternative protein were significantly lower (P < 0.05) than cobia fed diet formulated with 100% fish meal. The estimated half‐lives of nitrogen and carbon ranged between 9–11 days and 6–8 days, respectively, with significant differences among treatments (P < 0.05). Δ¹⁵N ranged between 0.0–1.2‰ and ?0.1–1.6‰ in whole fish and muscle and Δ¹³C ranged between 3.8–5.1‰ and 4.0–5.1‰ in whole fish and muscle respectively. Diets were formulated with low levels of dietary nitrogen (10%) supplied by alternative protein sources substituting fish meal. The relative contributions of the dietary nitrogen supplied from these sources to the growth of whole fish and muscle tissue ranged between 4.9–5.2% and 5.9–7.7% respectively. Results indicated that growth accounted for the majority of observed isotopic change in animals under all treatments. In whole animals and muscle tissue, isotopic change due to metabolism occurred faster for carbon stable isotopes than for nitrogen stable isotopes. Cobia fed diets formulated with alternative proteins showed reduced nitrogen turnover rate and increased Δ¹⁵N.     

Protein‐sparing capability of dietary lipid in herbivorous and omnivorous freshwater finfish: a comparative case study on grass carp (Ctenopharyngodon idella) and tilapia (Oreochromis niloticus × O. aureus)

Two, 8‐week feeding trials were conducted to compare protein‐sparing capability of dietary lipid in herbivorous grass carp (Ctenopharyngodon idella) and omnivorous tilapia (Oreochomis niloticus × O. aureus). Utilizing a 2 × 3 factorial design, experimental diets containing two levels of crude protein (380 and 250 g kg⁻¹) and three levels of lipid (0, 40 and 100 g kg⁻¹) were formulated for use in both feeding trials. Growth performances showed better response of both fish fed 380 g kg⁻¹ protein diet than those fed 250 g kg⁻¹ protein diet. Despite the dietary protein level, weight gain (WG), specific growth ratio (SGR), feed conversion ratio (FCR) and protein efficiency ratio were much higher (P < 0.05) for grass carp fed 40 g kg⁻¹ lipid diet than those fed 100 g kg⁻¹ lipid diet; however, there were no significant differences in tilapia fed the two diets. The feed intake of grass carp fed lipid‐free diet was the lowest, but it tended to decrease with increase in dietary lipids in tilapia. Lipid retention (LR) was negatively correlated with dietary lipid concentration of both fish. Viscerosomatic index (VSI), hepatosomatic index (HSI), intraperitoneal fat ratio (IPF) and whole‐body and liver lipid content positively correlated with dietary lipid concentration of both fish. Plasma parameters and liver enzymes activities were also positively correlated with dietary lipid concentration of both fish. Liver lipid contents were higher and enzymes activities were lower in grass carp when compared with tilapia. These data suggested that there was no evidence of a protein‐sparing effect of dietary lipids in grass carp. Tilapia has relatively higher capacity to endure high dietary lipid level compared to grass carp.     

Acid and Fermented Silage Characterization and Determination of Apparent Digestibility Coefficient of Crude Protein for Pacu Piaractus mesopotamicus

Three types of raw materials including commercial waste from saltwater (SW), freshwater fish (FW) and tilapia fillet residue (FR) were used to produce fish silage by either acid digestion (2% formic acid and 2% sulfuric acid) or anaerobic fermentation (5% of Lactobacillus plantarum and 15% sugar cane molasses). Six test diets were used in digestibility trials prepared with 70% reference diet and 30% of each experimental silage. These diets were fed to juvenile pacu Piaractus mesopotamicus (146 g average weight) in triplicate. Fish were kept in 500-L tanks and feces collected by manual extrusion. It was observed for both processes that SW waste always had the highest moisture content and lowest fat and ash. Highest crude protein levels were found in silages from commercial fish waste (SW and FW) made from whole fish unfit for human consumption. However, apparent digestibility coefficients did not vary among diets ( P > 0.05). Although values did not differ statistically, fermented silage consistently displayed higher digestibility coefficients compared to acid silage. The silages exhibited relatively high protein digestibility (72.5–80.0%), thus suggesting the feasibility of using fish industry by-products in aquaculture feeds.     

Effects of replacing fish meal with rubber seed meal on growth,nutrient utilization,and cholesterol metabolism of tilapia (<Emphasis Type="Italic">Oreochromis niloticus</Emphasis> × <Emphasis Type="Italic">O. aureus</Emphasis>)

A feeding trial was conducted to evaluate the effects of replacing fish meal with rubber seed meal (RSM) on growth, nutrient utilization, and cholesterol metabolism of tilapia (Oreochromis niloticus × Oreochromis aureus). Five experimental diets were formulated with 0, 150, 300, 450, and 600 g kg^?1 RSM replacing graded levels of fish meal, respectively. Each diet was randomly assigned to triplicate groups of 25 fish (initial average weight 65.3 g) per aquarium in a rearing system maintained at 29 ± 1 °C for 8 weeks. Dietary 150 g kg^?1 RSM inclusion did not affect the weight gain and daily growth coefficient, whereas these were depressed by a further inclusion. Additionally, feed efficiency ratio and protein efficiency ratio were not affected by dietary RSM inclusion regardless of inclusion level. However, the inclusion of 450 and 600 g kg^?1 RSM decreased the mid-intestinal trypsin, lipase, and amylase activities; the hepatic acyl-CoA/cholesterol acyl transferase; low-density lipoprotein receptor; and 3-hydroxy-3-methyl-glutaryl-CoA reductase activities. Similarly, dietary 600 g kg^?1 RSM inclusion inhibited the plasma catalase and hepatic glutathione peroxidase activities. These results indicated that 150 g kg^?1 RSM can be included in tilapia diets, whereas higher inclusion of RSM inhibited the growth rate, digestive enzyme activity, antioxidant capacity, and cholesterol metabolism.     

Acute responses of juvenile cobia Rachycentron canadum (Linnaeus 1766) to acid stress

Fish are potentially submitted to water acidification when reared in recirculating aquaculture systems. This study evaluated the responses of juvenile cobia Rachycentron canadum after acute exposure to acid water. Juvenile cobia (12.6 ± 0.5 g; 14.2 ± 0.2 cm) were acutely exposed to four pH levels (7.9 (control), 6.5, 6.0, and 5.5). After 24 h of exposure to different pH values, fish were sampled for physiological and histopathological evaluation. Acid water affected physiological parameters and induced morphological histopathologies on gill and skin of juvenile cobia, and these effects were more conspicuous with decreasing pH values. Acid stress induced blood acidosis in juvenile cobia, coupled to a decrease in bicarbonate (HCO₃^?) and saturated O₂ (sO₂) in fish blood. On the other hand, haematocrit, haemoglobin and glucose concentration increased their values (P < 0.01) comparing to control level. Hyperplasia with completely fusion of secondary lamella was observed in all pH treatments (6.5. 6.0 and 5.5), while telangiectasia and proliferation of chloride cells were present for fish exposed to pH 6.0 and 5.5. In skin hyperplasia and hypertrophy of mucous cells, necrosis of these cells for fish exposed to pH 6.0 and 5.5 was observed. The results of this study demonstrate that acute acid water exposition affected physiology and histopathology in juvenile cobia, especially at pH values below 6.5. Accordingly, particular attention must be given to pH during cobia reared in recirculating aquaculture.     

Differential pathogenicity of five Streptococcus agalactiae isolates of diverse geographic origin in Nile tilapia (Oreochromis niloticus L.)

Streptococcus agalactiae is an emerging pathogen of fish and has caused significant morbidity and mortality worldwide. The main objective of this study is to assess whether pathogenic differences exist among isolates from different geographic locations. Nile tilapia (Oreochromis niloticus L.) were administered an intraperitoneal injection of suspension containing USA, Brazil, Honduras, Israel, or Kuwait S. agalactiae isolates at concentrations ranging from 10² to 10⁷ cfu mL^?1. The LD₅₀ values 7 days after challenge were as follows: USA (1.0 × 10² cfu mL^?1), Brazil (1.5 × 10³ cfu mL^?1), Honduras (6.8 × 10³ cfu mL^?1), Israel (1.0 × 10⁴ cfu mL^?1) and Kuwait (7.2 × 10⁵ cfu mL^?1). Fish from all groups exhibited lethargy, anorexia, exophthalmia, corneal opacity, erratic swimming, petechiae and mortality. Opercular clearing and ascites were only found after infection with certain geographic isolates. The findings in this study indicate that S. agalactiae isolates of different geographic origin can cause significant mortalities after experimental challenge and can have different pathogenic capacities. Isolates from the Americas (USA, Brazil and Honduras) were more pathogenic to Nile tilapia than isolates from the Middle East/Asia (Israel and Kuwait).