Swimbladder inflation,growth and survival of yellowtail kingfish Seriola lalandi (Valenciennes, 1833) larvae under different temperature,light and oxygen conditions

Poor swimbladder inflation leads to low fish survival due to resulting spinal deformities and the inability to feed and develop normally. Failure of swimbladder inflation may be attributed to the inappropriate range of abiotic conditions. This study investigated the effects of temperature, light source and intensity, and oxygen conditions on initial swimbladder inflation, growth and survival of yellowtail kingfish Seriola lalandi larvae. The study consisted of four separate trials including low (21.5°C) and high (24.5°C) temperatures, natural and artificial light sources, low (1000 lux) and high (32 000 lux) light intensities, and ambient and supersaturated dissolved oxygen. Initial swimbladder inflation was only significantly affected by light source, with the highest inflation rate (97.5 ± 3.5%) under artificial light. Fish growth was improved at the higher temperature and at the higher light intensity. Survival was only significantly influenced by light intensity, with the highest survival (11.0 ± 2.3%) at the high intensity (32 000 lux). This study indicates that light source affects swimbladder inflation timing and high artificial light intensity improves larval fish growth and survival.     

Modelling of nitrogen loads from the farming of yellowtail kingfish Seriola lalandi (Valenciennes, 1833)

Farming of yellowtail kingfish ( Seriola lalandi , Valenciennes, 1833) in the coastal waters of Australia is a relatively new aquaculture industry, and little is known about the magnitude of nutrient discharges from individual pens. In this work, we modelled the flow of nitrogen for each of two commercial pens in Fitzgerald Bay, upper Spencer Gulf, South Australia. The fish were fed commercial pellets with feed conversion ratios (FCRs) between 3.0 and 3.2 (dry weight feed/wet weight growth). These high values of FCR were reflected in the high nitrogen loads to the environment (176–195 kg N tonne⁻¹ growth) and a small retention of nitrogen in fish growth (14–16% of feed inputs). Considering an annual production of 2000 tonnes, total loads to the environment can reach 391 tonnes N year⁻¹. Eighty-two per cent of these loads are expected to be lost to the water column as dissolved wastes. The high nutrient loads and the importance of dissolved wastes compared with other aquaculture species, such as salmon and trout, reflect the distinctly higher metabolic rates of this pelagic predatory species. The nature of the wastes suggests low localized impacts at current production levels, but regional effects remain unknown.     

Responses of digestive enzymes and body lipids to weaning times in yellowtail kingfish Seriola lalandi (Valenciennes, 1833) larvae

The optimal time to wean fish larvae from live feed to artificial feed was explored in yellowtail kingfish Seriola lalandi (YTK). The same weaning regime started at five different days post hatching (DPH), namely 10 DPH (W10), 13 DPH (W13), 16 DPH (W16), 19 DPH (W19) and 22 DPH (W22). The activities of trypsin, lipase and alkaline phosphatase were detected in fish from 8 DPH throughout the experiment, but pepsin activity was first detected in fish on 15 DPH in the W10 and W13 treatments. The increase in pepsin activity was concomitant with the decrease in trypsin activity. Total fish lipids after weaning reduced by 40% in the W10 and W13 treatments, and increased by 20% in the W19 and W22. Fish survival rate in the W22 treatment was significantly higher than that in the W10 and W13 treatments. The results suggest that 16 DPH is the earliest day to wean and the best weaning window for YTK larvae should be 19–22 DPH. This study provides enzymatic evidence to guide the weaning process for YTK larvae, and offers a useful approach to explore optimal weaning time for fish larvae.     

Temperature and dissolved oxygen influence growth and digestive enzyme activities of yellowtail kingfish Seriola lalandi (Valenciennes, 1833)

A 5 week experiment was carried out with juvenile yellowtail kingfish Seriola lalandi to investigate the interactive effects of water temperature (21, 24, or 27°C) and dissolved oxygen regime (normoxic vs. hypoxic) on the growth rate, feed intake and digestive enzyme activity of this species. Specific growth rate (SGR) was highest at 24°C, regardless of oxygen regime, but the SGRs of the fish exposed to hypoxia at 21, 24 and 27°C were 13%, 20% and 17% lower, respectively, than the SGRs recorded for the fish reared under normoxic conditions. The digestive enzyme activities (i.e. trypsin, lipase and α‐amylase) were influenced by temperature but did not appear to be affected by dissolved oxygen concentration. Information about the effects of water temperature and dissolved oxygen on feeding, growth and digestive capacity of juvenile yellowtail kingfish could contribute to improving feed management decisions for production of this fish species under different environmental conditions.     

Quantitative genetic properties of four measures of deformity in yellowtail kingfish Seriola lalandi Valenciennes, 1833

The main aim of this study was to estimate the heritability for four measures of deformity and their genetic associations with growth (body weight and length), carcass (fillet weight and yield) and flesh‐quality (fillet fat content) traits in yellowtail kingfish Seriola lalandi. The observed major deformities included lower jaw, nasal erosion, deformed operculum and skinny fish on 480 individuals from 22 families at Clean Seas Tuna Ltd. They were typically recorded as binary traits (presence or absence) and were analysed separately by both threshold generalized models and standard animal mixed models. Consistency of the models was evaluated by calculating simple Pearson correlation of breeding values of full‐sib families for jaw deformity. Genetic and phenotypic correlations among traits were estimated using a multitrait linear mixed model in ASReml. Both threshold and linear mixed model analysis showed that there is additive genetic variation in the four measures of deformity, with the estimates of heritability obtained from the former (threshold) models on liability scale ranging from 0.14 to 0.66 (SE 0.32–0.56) and from the latter (linear animal and sire) models on original (observed) scale, 0.01–0.23 (SE 0.03–0.16). When the estimates on the underlying liability were transformed to the observed scale (0, 1), they were generally consistent between threshold and linear mixed models. Phenotypic correlations among deformity traits were weak (close to zero). The genetic correlations among deformity traits were not significantly different from zero. Body weight and fillet carcass showed significant positive genetic correlations with jaw deformity (0.75 and 0.95, respectively). Genetic correlation between body weight and operculum was negative (?0.51, P < 0.05). The genetic correlations' estimates of body and carcass traits with other deformity were not significant due to their relatively high standard errors. Our results showed that there are prospects for genetic selection to improve deformity in yellowtail kingfish and that measures of deformity should be included in the recording scheme, breeding objectives and selection index in practical selective breeding programmes due to the antagonistic genetic correlations of deformed jaws with body and carcass performance.     

Size heterogeneity, growth potential and aggression in juvenile yellowtail kingfish (Seriola lalandi Valenciennes)

The ontogenetic development of size heterogeneity and aggression was monitored in commercial culture tanks of yellowtail kingfish (Seriola lalandi Valenciennes). Size heterogeneity increased substantially with the introduction of Artemia as a food source at 12 days post‐hatch (DPH), and was correlated with the appearance of aiming behaviour, a precursor to more direct aggressive interactions. Chasing behaviour started at approximately 19 DPH (10–12 mm total length), with the main aggressors being large‐grade individuals comprising 8% of the population. At any one time, only 10–30% of this grade carried out all of the chases, meaning that a very small proportion of the entire population (1%) was responsible for most of the aggressive interactions. The main recipients of aggression were the small grade, comprising 42% of the population, while the medium grade (50% of the population) were generally not aggressive and received only a low to moderate level of aggression. A grading trial showed that large‐grade juveniles only displayed aggressive behaviour in the presence of size heterogeneity, and that medium‐ or small‐grade juveniles did not establish an agonistic hierarchy in the absence of large individuals. A high level of aggression in the ungraded control treatment was associated with mortality of most of the small individuals. Even in the absence of aggression, the small‐grade juveniles failed to gain weight or show an increase in the RNA/DNA ratio after 12 days. The large‐ and medium‐grade larvae showed an isometric increase in RNA/DNA ratio during development, indicating that faster‐growing individuals are likely the result of better food capture or processing traits rather than better protein synthesis rates. Decreasing size heterogeneity and aggression via grading mostly benefits the medium‐grade individuals, as the majority of small individuals within a batch appear to be on a degenerative developmental trajectory irrespective of an aggressive environment.     

The performance of larval Seriola lalandi (Valenciennes, 1833) is affected by the taurine content of the Artemia on which they are fed

This study describes the effects of feeding taurine‐supplemented Artemia on the growth, survival, whole body taurine content and jaw malformation rate of larval yellowtail kingfish Seriola lalandi. Larvae were fed rotifers containing no supplemental taurine from 3 to 15 day post hatch (dph) and Artemia co‐enriched with taurine from 12 to 22 dph. Artemia were supplemented at concentrations of either 0, 0.8, 1.6, 2.4, 3.2 or 4.0 g of taurine L^?1 during the 18 h HUFA enrichment process. Taurine content in the Artemia increased from 0.76 ± 0.04% DW in those without supplementation to 3.95 ± 0.17% DW in those supplemented at 4.0 g L^?1. Survival rates of larval yellowtail kingfish were significantly lower in all taurine‐supplemented treatments compared to the unsupplemented control. Growth was significantly improved in those larvae fed taurine‐supplemented Artemia; however, we cannot attribute this improvement solely to taurine, as improved growth may have been a function of the reduced survival, and therefore increased prey availability, in these treatments. The whole body taurine content of larvae fed unsupplemented Artemia was significantly lower (1.85 ± 0.03% DW) than those fed supplemented Artemia, which did not differ from each other (pooled average 2.48 ± 0.03% DW), suggesting either a functional excretion mechanism is in place or that this represents the saturation value for larvae of this age. Jaw malformation rates were not affected by Artemia taurine content. The results of this research suggest yellowtail kingfish larvae may have a lower requirement and/or a reduced tolerance to excess dietary taurine than juveniles.     

The first‐feeding response of larval southern bluefin tuna,Thunnus maccoyii (Castelnau, 1872), and yellowtail kingfish,Seriola lalandi (Valenciennes, 1833), to prey density,prey size and larval density

We investigated the first‐feeding success of two species: southern bluefin tuna (Thunnus maccoyii) and yellowtail kingfish (Seriola lalandi) to determine if similar culture parameters can be used for both, especially when S. lalandi are held in the same tanks as prey for T. maccoyii. The feeding performance (proportion and intensity) was examined in three short‐duration (4 h) experiments: prey density, prey size and larval density. Increasing prey density from 0.5 to 25 rotifers mL^?1 increased the proportion of T. maccoyii and S. lalandi larvae feeding. Prey size alone did not affect feeding in either species. Seriola lalandi had a decreased proportion of larvae feeding when larval density reached 50 larvae L^?1 concurrent with a gradual increase in feeding intensity between 2 and 50 larvae L^?1. In T. maccoyii, there was no pattern to the effect of larval density on the proportion of larvae feeding. The overall feeding performance of larvae was higher in T. maccoyii than S. lalandi. Increased prey density improved the first‐feeding ability of T. maccoyii and S. lalandi larvae. The effect of larval density on S. lalandi feeding requires further investigation, to ensure that they remain feeding when provided as prey in T. maccoyii culture. The identification of factors in this study, which increase first‐feeding success, will improve the culture of both species.     

Ambient salinity and osmoregulation,energy metabolism and growth in juvenile yellowtail kingfish (Seriola lalandi Valenciennes 1833) in a recirculating aquaculture system

The effects of salinity on plasma osmolality, branchial chloride cell density, feed consumption and conversion and growth performance of yellowtail kingfish (Seriola lalandi) were evaluated. Fish (11.6 ± 0.6 g) were kept for 29 days at 14, 18, 22, 26 (experimental) and 30 g L^?1 (control) salinity in independent, pilot‐scale recirculation aquaculture systems. No differences in plasma osmolality or chloride cell numbers in gills were observed, pointing to a strong osmoregulatory capacity in the juveniles. Fish at 14, 18 and 22 g L^?1 (7.61 ± 0.19, 7.61 ± 0.01 and 7.61 ± 0.13% day^?1, respectively) had higher growth rates than fish at 26 and 30 g L^?1 (7.10 ± 0.05 and 6.97 ± 0.06% day^?1 respectively). The higher growth rate at lower salinity resulted from increased feed intake; feed conversion was not different. An evaluation of the impact of salinity on growth rate of on‐growing stages (till market size) seems warranted to assess whether the profitable effects of low salinity persist in later stages of this important aquaculture species.     

Nutritional parameters of dorsal and ventral fillets in summer and winter from farmed yellowtail kingfish Seriola lalandi (Valenciennes, 1833)

The nutritional profile of dorsal and ventral portions of fillets from farmed yellowtail kingfish (YTK) (Seriola lalandi, Valenciennes, 1833) was assessed in winter and summer months to evaluate if there were differential distributions of proximate constituents, fatty acid composition and mineral content seasonally and through the dorso‐ventral axis. The proximate composition of YTK fillets varied between the two anatomical locations examined. In particular, a relatively large variation was observed in crude fat content, which decreased significantly from ventral to dorsal portions of the fillet and was inversely proportional to moisture and protein content. Higher crude fat content was also observed in the fish sampled in summer, compared to those sampled in winter. The omega 3/6 ratio remained constant between fillet portions but was significantly different between seasons (summer > winter). The index of nutritional quality for EPA and DHA was markedly different regarding to fillet portion and season (summer > winter, ventral > dorsal), and significant differences in the levels of potassium, phosphorus, magnesium and iron were also evident between the seasons examined. This study clearly shows differences in nutritional profiles of fillets from farmed YTK in South Australia relative to anatomical location and season. The increase in knowledge gained from this study may lead to improved farm management practices of YTK, which may extend to other commercially relevant species to enhance the production of premium farmed products.     

Low stress response exhibited by juvenile yellowtail kingfish (Seriola lalandi Valenciennes) exposed to hypercapnic conditions associated with transportation

Transportation of yellowtail kingfish (Seriola lalandi Valenciennes) juveniles from hatchery to on‐growing operations in New Zealand exposes the fish to significantly elevated carbon dioxide (CO₂) concentrations. Experiments were undertaken to assess metabolic and haematological stress responses after a 5‐h period of hypercapnia, followed by recovery in normocapnia. Mortality was low (0.5%) and secondary stress indices (blood glucose, blood lactate, muscle pH and muscle lactate) remained largely unchanged during a simulated transportation and recovery, despite juveniles being exposed to CO₂ concentrations as high as 75 mg CO₂ L^?1 (38 mm Hg partial pressure). There was some haematological disturbance midway through simulated transports, where water was maintained at fixed CO₂ concentrations of 8 and 50 mg CO₂ L^?1 (4 and 26 mm Hg respectively). Persistent erythrocyte swelling continued during transport at 50 mg CO₂ L^?1, whereas at 8 mg CO₂ L^?1, haematological variables had returned to control levels. There was no mortality recorded for any of the treatments, and haematological variables were restored to pre‐manipulation levels after 31 h. The results indicated that juvenile yellowtail kingfish have a robust physiology and can cope with the stressors imposed by acute exposure to moderate to high levels of CO₂ associated with live transport.     

Replacement of fresh algae with commercial formulas to enrich rotifers in larval rearing of yellowtail kingfish Seriola lalandi (Valenciennes, 1833)

This study compared the efficacy of four products that are commonly used in hatchery for nutritional enhancement of rotifer Brachionus plicatilis as the starter food for yellowtail kingfish Seriola lalandi larvae. This experiment consisted of one fresh algae and three enrichment products: (1) Fresh algae were a mixture of Nannochloropsis and Isochrysis at 2:1 on a cell concentration basis; (2) S.presso, (Selco S.presso ^®, INVE Aquaculture); (3) Algamac 3050^® (Aquafauna, USA); (4) Nutrokol ^® (Nutra‐Kol, Australia). Survival rates of the fish fed rotifers enriched with fresh microalgae (40.69%) and S.presso (31.21%) were higher than those fed Algamac 3050 (10.31%). On 3 day post hatch (DPH), fish feeding incidence in the fresh algae treatment was significantly higher than that in other treatments. On 6 DPH, fish showed the lowest feeding incidence in the Algamac 3050 treatment. The methods of enrichment did not affect total lipid levels in either rotifer or fish larvae, but Algamac 3050 enrichment achieved the highest DHA/EPA ratio and lowest EPA/ARA ratio in both rotifers and fish larvae. This study indicates that fresh algae can be replaced by S.presso, but Algamac 3050 is not as good as other formula for rotifer enrichment in rearing yellowtail kingfish larvae in this system.     

黄条鰤生长性状全基因组关联分析

利用2b-RAD技术对119尾黄条鰤(Seriola lalandi)个体进行测序,共获得黄条鰤SNP分子标记26665个,对黄条鰤个体的体质量和全长这2个重要生长性状进行全基因组关联分析,筛选与体质量和全长性状相关的SNP位点和候选基因。结果显示,黄条鰤体质量性状中共筛选到17个体质量显著关联的SNP位点,找到17个可能的候选基因,全长性状共筛选到12个潜在显著关联位点,找到12个可能的候选基因。利用KEGG数据库对可能的候选基因进行Pathway分析,得知候选基因主要参与了细胞或组织生长发育相关的代谢通路调控过程,可能是影响黄条鰤生长性状密切相关的重要候选SNP位点和功能基因,结果可为今后黄条鰤种质资源可持续利用和育种提供遗传信息资料积累。     

Qualitative analysis of cartilaginous jaw element malformation in cultured yellowtail kingfish (Seriola lalandi) larvae

A central problem facing worldwide culture of yellowtail kingfish (Seriola lalandi) is the presence of skeletal malformations, including jaw deformities. This study presents a morphological characterization of normal and abnormal cartilage jaw structures during early larval development. Samples of 70–150 larvae were collected from three cohorts from 2 to 9 days post hatching, anaesthetized and fixed for cartilage staining. Cartilaginous components were defined clearly at four days post hatch (dph) (4.65 ± 0.05 mm total length), and abnormal jaw structures were detectable at this time. Jaw deformities observed included extension of Meckel's cartilage with or without ventral bending of the anterior tip, displacement of ceratohyal and hypohyal cartilage ventrally and below Meckel's cartilage, and shortening and dorsal flexion of the lower jaw. At 4 dph, between 44% and 47% of all larvae examined had jaw abnormalities. The contribution of each deformity to the total number of deformities was variable among the three cohorts examined. To compare shape difference accurately we performed an exploratory, landmark‐based geometric morphometric analysis using seven homologous landmarks. Larvae were classified into three jaw morphology groups. The geometric morphometric approach provides a useful tool to standardize classification of cartilage jaw abnormalities at early developmental larval stages. Early recognition of developing abnormalities is of importance for fish farmers in both improving fish selection efficiency and for evaluating effects of rearing parameters.     

Morphological development and allometric growth of yellowtail kingfish Seriola lalandi V. larvae under culture conditions

Morphological development and allometric growth patterns of Seriola lalandi larvae were assessed to characterize normal growth patterns under culture conditions. Early ontogenetic stages of yellowtail kingfish exhibited an exponential growth in terms of standard length as a function of age. Five development stages were characterized from hatching to the juvenile stage: larval stage I (0–2 days post hatch, dph) with endogenous feeding, characterized by a small yolk sac, unpigmented eyes, primordial finfold surrounding the body and a closed mouth; larval stage II (2–15 dph) characterized by mouth opening, complete pigmentation of eyes and the beginning of the exogenous feeding; subsequently, in the larval stage III (15–25 dph) the posterior tip of notochord of the larvae bended upward and the first rays appeared in fins, concomitant with a change in swimming behaviour; thereafter, larval stage IV (post‐flexion stage; 25–30 dph) began when larvae resembled in morphology to a juvenile organism; however, caudal and dorsal fins were not completely development. Lastly, the juvenile stage was reached 30 dph characterized by a morphology and fin structures similar to those of the adults. Growth and development of structures and organs associated with vital functions such as feeding, sensorial and breathing systems seemed to be more critical previous to 23 dph, which was reflected with a positive allometric growth of head and eyes during this period. The results from this study can be used as a tool‐guide to assess normal development in larval research with S. lalandi to improve existing rearing protocols in hatchery production.     

Karyotype analysis of the yellowtail kingfish Seriola lalandi lalandi (Perciformes: Carangidae) from South Australia

The karyotype and chromosomal characteristics of the yellowtail kingfish Seriola lalandi lalandi were investigated by examining metaphase spreads from kidney cells of 4-month-old fish artificially bred using wild-captured broodstock from the coast of South Australia. A total of 80 mitotic metaphases from 10 individual fish were analysed, with 52 spreads having a mode of 2 n =48 chromosomes, representing 65% of the metaphases observed. The diploid consisted of two metacentric, two submetacentric, six subtelocentric and 38 telocentric chromosomes, with the fundamental number of chromosome arms being 52. The total haploid chromosome length was approximately 44.412 μm. This study shows that the karyotype of S. lalandi lalandi differs from those reported previously for other Seriola species in having one pair of metacentric chromosomes.     

Food ingestion,prey selectivity,feeding incidence,and performance of yellowtail kingfish Seriola lalandi larvae under constant and varying temperatures

This study tested the effects of constant and varying temperatures on newly hatched yellowtail kingfish Seriola lalandi larvae in two experiments. In Experiment I, four constant temperatures (21, 23, 25, and 27 °C) were tested under fed or unfed conditions with the fish age from the day of hatch to 24 days post hatch (DPH). Temperatures at 25 and 27 °C reduced the time of fish to reach irreversible starvation, but did not affect the percentage of fish that were able to ingest food. Fish survivals at 21 and 23 °C were significantly higher than those at 25 °C by 24 DPH, but all fish died at 27 °C by 24 DPH in the treatment with food. In Experiment II, three constant temperatures (21, 23, and 25 °C) and two varying temperatures (21–23 and 21–25 °C) were compared using fish from hatch to 28 DPH. On 4 DPH, fish ingested more rotifers, but from 6 to 9 DPH, fish ingested fewer rotifers at 25 °C than at other temperatures. On 19 and 23 DPH, fish ingested more Artemia at 25 °C than at other temperatures. At 25 °C, fish selected for Artemia nauplii earlier than at other temperatures. Fish length and survival between constant temperatures (21 and 23 °C) were not significantly different, but fish survival at the constant 21 °C or at the 21–25 °C varying temperature was significantly higher than that at the constant 23 °C or at the 21–23 °C varying temperature. This study indicates that within the range of temperature tested, the optimal temperature for the first feeding larvae is 21–23 °C after hatch and mortality is likely to occur at ≥25 °C in the first 10 DPH, but fish grew faster at 25 °C after they adapted to the increasing temperature.     

A comparison of the palatability of racemic praziquantel and its two enantioseparated isomers in yellowtail kingfish Seriola lalandi (Valenciennes, 1833)

The bitterness of racemic praziquantel (Rac‐PZQ) constrains its use as an in‐feed treatment against monogenean flukes in finfish aquaculture. Evidence exists in mammals that the R‐(?) enantiomer of PZQ is less bitter than the S‐(+) enantiomer. If fish exhibit this same response, then the recently described techniques for the large‐scale resolution of R‐(?)‐PZQ from Rac‐PZQ could facilitate the wide‐spread application of this effective anthelmintic compound via feed. The hypothesis that yellowtail kingfish Seriola lalandi would find R‐(?)‐PZQ more palatable than Rac‐PZQ and S‐(+)‐PZQ was tested in four trials. During the first three trials, the palatability of diets top‐coated with 10 g kg^?1 of Rac‐PZQ or its two enantioseparated isomers were compared in small (85–160 g) and large (1.2 kg) yellowtail kingfish. A fourth trial compared the palatability of R‐(?)‐PZQ and Rac‐PZQ at dietary inclusion levels of 2.5, 5.0 and 10.0 g kg^?1 in small yellowtail kingfish (170 g). Ingestion data showed that R‐(?)‐PZQ to be no more palatable than either Rac‐PZQ or S‐(+)‐PZQ to yellowtail kingfish, regardless of size. Indeed, evidence suggested that the S‐(+)‐PZQ to be slightly more palatable than both R‐(?)‐PZQ and Rac‐PZQ. From these data, we hypothesize that the strong smell of R‐(?)‐PZQ (which was not present in S‐(+)‐PZQ) is an equally important determinant to palatability as taste in yellowtail kingfish. Results demonstrate that dietary inclusion level is a more important determinant to palatability than PZQ chirality; however, administration of R‐(?)‐PZQ may still be advantageous if it is demonstrated to be the only enantiomer efficacious against monogeneans.     

Effects of the gill monogenean Zeuxapta seriolae (Meserve, 1938) and treatment with hydrogen peroxide on pathophysiology of kingfish, Seriola lalandi Valenciennes, 1833

Infections by the gill fluke Zeuxapta seriolae are a serious concern for sea cage aquaculture of kingfish, Seriola lalandi. The present study aimed to determine the pathophysiological effects of a progressive infection with Z. seriolae and the effects of treatment with hydrogen peroxide. For the progression of infection study, infected fish were taken from a sea cage farm, treated to remove parasites and then infected by cohabitation with heavily infected fish. Samples were taken at 2-week intervals for 8 weeks. Infection intensity peaked at 4 weeks post-infection (mean intensity 565.9) and the number of mature worms (2 mm fixed length or larger) peaked at 6 weeks post-infection. Attachment of Z. seriolae appeared to cause little localized pathology; however, the occurrence of hyperplastic lamellae increased as the infection progressed. Haemoglobin concentrations were negatively correlated with Z. seriolae intensity and were lower than controls at 4 weeks (35.8% decrease) and 6 weeks (57.4% decrease) post-infection. Blood lactate concentration and plasma osmolality increased throughout the course of infection. For the effect of treatment experiment, groups of infected and non-infected fish were sampled either before or after treatment with hydrogen peroxide. Treated fish from both infected and uninfected groups had increased plasma lactate, osmolality and pH compared with pre-treatment groups. Treatment with hydrogen peroxide appeared to have acute effects on fish health but the magnitude (e.g. lactate, osmolality) and extent of the effects (e.g. haemoglobin) was much less than that caused by chronic infection with Z. seriolae.