Uptake and Clearance of Edwardsiella ictaluri in the Peripheral Blood of Channel Catfish Ictalurus punctatus Fingerlings during Immersion Challenge

Uptake and clearance of Edwardsiella ictaluri in the peripheral blood of channel catfish Ictalurus punctatus fingerlings were monitored for 216 h after exposure to E. ictaluri for 4 h and 8 h under static conditions. Most fish exposed to E. ictaluri developed bacteriemia 24 h post-exposure, and by 72 h post-exposure E. ictaluri was recovered from all the blood of all exposed fish. The number of E. ictaluri colony forming units (CFU) in the blood of moribund fish ranged between 1.7 × 10³ to 1.6 × 10⁵ CFU/50 μL whole blood. Clearance of bacteria from the blood was observed by 216 h post-exposure and all fish surviving bacterial exposure developed agglutinating antibody against E. ictaluri . The pathogenesis of the infection was accompanied by the shedding of viable E. ictaluri into the water which may serve as a mechanism by which fish to fish transmission occurs.     

Vaccination of channel catfish, Ictalurus punctatus (Rafinesque), by immersion and oral booster against Edwardsiella ictaluri

Abstract. A commercially prepared vaccine against Edwardsiella ictaluri was used to vaccinate 12-day-old channel catfish fry by immersion, or by immersion plus an oral booster 2 months later. One month after the fish were fed the booster vaccine, they were challenged by waterborne exposure to 2·1 × 10⁶ cells ml⁻¹ of E. ictaluri. Immersion only vaccinated fish suffered 6·7% mortality and immersion plus oral-boosted fish had a 3·3% mortality. Mortality among non-vaccinated controls was 96·7% and was significantly ( P < 0·01) above the vaccinated mortality. The relative per cent survival for the immersion-only fish was 93·1, while it was 96·6 for the immersion plus oral-boosted fish. Agglutinating antibody titres of the vaccinated fish were significantly ( P < 0·05) higher than the control fish. When the ponds were drained 6 months after stocking, 42·7% of non-vaccinated, 56·3% of immersion-only and 70·8% of immersion plus oral-boosted fish were harvested. Survival of immersion plus orally-boosted fish was significantly ( P < 0·05) higher than the controls of immersion-only fish. Duplicate populations of immersion plus oral-booster-vaccinated fish grew 34% and 56% faster, respectively, on an average daily gain than the control fish, while immersion-only fish in one pond grew 20% less per day and fish in the second pond grew 48% faster.     

Susceptibility of five species of fish to Edwardsiella ictaluri

Abstract. Five species of fingerling fish, channel catfish, Ictalurus punctatus , tilapia, Sarotherodon aureus , golden shiner, Notemigonus crysoleucas , largemouth bass, Micropterus salmoides and bighead carp, Aristichthys nobilis , were tested to determine their susceptibility to the bacterium, Edwardsiella ictaluri , at 26°C, Channel catfish demonstrated high susceptibility to E. ictaluri as 100% of those fish injected with 1.5 × 10³ cells died within 10 days. Tilapia demonstrated slight susceptibility to the pathogen while golden shiner, bighead carp and largemouth bass were not susceptible. E. ictaluri was isolated from a higher percentage of peritoneal cavities, livers and kidneys of channel catfish than of other species. Sequential growth of E. ictaluri in the liver of channel catfish is described.     

Vaccination of Full‐sib Channel Catfish Families Against Enteric Septicemia of Catfish with an Oral Live Attenuated Edwardsiella ictaluri Vaccine

This study evaluated the efficacy of an oral live attenuated Edwardsiella ictaluri vaccine against enteric septicemia of catfish in 20 full‐sib fingerling channel catfish families. The vaccine was delivered orally by feeding fish a diet coated with an attenuated E. ictaluri isolate. Sixty‐nine days postvaccination, control and vaccinated fish were challenged with virulent E. ictaluri and mortality was examined for 21 d postchallenge. Vaccinated fish had significantly lower mortality than nonvaccinated fish following challenge (P < 0.001). Mortality of vaccinated fish was 1.7 ± 1.4% as opposed to 47.8 ± 28.7% in nonvaccinated fish. Relative percent survival ranged from 87.7 to 100% with an average of 95.2 ± 4.0% (±SE) among the 20 families of fish. There were significant differences in mortality among families in nonvaccinated fish (P < 0.01) while there were no differences among vaccinated families of fish. Results indicate that the live attenuated E. ictaluri vaccine is effective at reducing mortality in channel catfish exposed to virulent E. ictaluri. These data demonstrate that genetic differences among healthy families of channel catfish are not major considerations in developing an effective vaccination program utilizing the oral vaccination platform described in this study.     

Pathology of experimental enteric septicaemia in channel catfish, Ictalurus punctatus (Rafinesque), following immersion-exposure to Edwardsiella ictaluri

Abstract. Fingerling channel catfish, Ictalurus punctatus (Rafinesque), were exposed experimentally to Edwardsiella ictaluri by immersion for 1 h in water containing 5 × 10⁸ colony-forming units (CFU) of the bacterium per ml. Ninety per cent of the fish developed lesions typical of enteric septicaemia of catfish (ESC), 93% of affected fish developed the acute form of ESC and 7% developed chronic ESC. Acute disease was characterized grossly by cutaneous haemorrhage and ulceration, and microscopically by enteritis, olfactory sacculitis, hepatitis and dermatitis. The earliest lesions of acute ESC, i.e. enteritis and olfactory sacculitis, were observed microscopically at 2 days post-exposure (PE); gross lesions, primarily mild subcutaneous haemorrhage at the base of fins, were first apparent at 4 days PE. Chronic ESC, seen most commonly 3–4 weeks PE, was characterized by dorsocranial swelling and ulceration, granulomatous olfactory neuritis/perineuritis, and meningoencephalitis involving the olfactory bulbs, olfactory tracts and olfactory lobes of the brain. Gross and microscopic lesions in the acute and chronic forms of experimental ESC were similar to the lesions reported in naturally occurring ESC. Definitive pathogenesis of acute and chronic ESC remains to be determined.     

The Relationship Between Vaccine Dose and Efficacy in Channel Catfish Ictalurus punctatus Vaccinated as Fry with a Live Attenuated Strain of Edwardsiella ictaluri (RE‐33)1

Channel catfish Ictalurus punctatus were vaccinated at 12 d of age (post‐hatch) by a 2‐min bath immersion with attenuated Edwardsiella ictaluri RE‐33 at doses of 2.5 × 10⁵, 2.5 × 10⁶, and 2.4 × 10⁷ colony‐forming units CFU/mL of water. Following vaccination, RE‐33 was recovered from a greater percentage of fry that were vaccinated at the high and intermediate doses compared to fry vaccinated at the lowest dose. Independent of dose, the greatest percentage of RE‐33 positive fry occurred between 1 and 6 d post‐vaccination with a significant decrease in positive fry observed on day 12. A significant increase in mortality occurred 6 to 12 d post‐vaccination in fry vaccinated at the highest dose. No differences in post‐vaccination mortalities occurred between the other treatments. Following virulent E. ictaluri challenge, mortalities of fish vaccinated at doses of 2.5 × 10⁶ and 2.4 × 10⁷ CFU/mL were significantly less than those of fish vaccinated at 2.5 × 10⁵ CFU/mL and sham‐vaccinated control fish. These data show that vaccination with RE‐33 can offer protection against subsequent virulent E. ictaluri infection.     

Protective properties of egg yolk IgY containing anti-Edwardsiella tarda antibody against paracolo disease in the Japanese eel, Anguilla japonica Temminck & Schlegel

Abstract. Immunized hens are known to contain a high level of immunoglobulin Y (IgY) in their egg yolk. In this study, the present authors obtained anti- Edwardsiella tarda IgY (containing 20% specific IgY, agglutination titre, 1:128) from hens vaccinated by injection with formalin-killed bacterin. The IgY was stable against eel digestive factors, and therefore, was orally administered with viable E. tarda to the Japanese eels and the efficacy of protection against E. tarda infection was evaluated. Orally administered IgY at a dose of 400mg fish^-1 cleared E. tarda inoculated simultaneously at 10^5–6 CFU fish^-1 from the intestine within 24h. Moreover, orally administered IgY at doses of 200 and 400mg fish^-1 inhibited the penetration of E. tarda inoculated simultaneously at 10^4–6 CFU fish^-1 into the liver and kidney via the damaged intestine. The fish orally administered with IgY showed reduced mortality. These results suggest that egg yolk containing anti- E. tarda IgY is effective in preventing edwardsiellosis.     

The development of live vaccines for furunculosis lacking the A-layer and O-antigen of Aeromonas salmonicida

Abstract. Mutants of Aeromonas salmonicida strains lacking either the A-protein, O-antigen or both of these major surface antigens were tested in rainbow trout, Oncorhynchus mykiss (Walbaum), for their suitability as live vaccines (LV). All of these mutants were shown to be attenuated, as fish receiving ∼5 × 10⁷ of the respective strains showed no clinical signs of furunculosis. Immersion vaccination of fish in 5 × 10⁷ cfu ml^-1 of these strains with an identical immersion dose 14 days later resulted in significant protection by all strains from challenge with a heterologous virulent strain of A. salmonicida 5 weeks later. The levels of protection conferred were all greater than or equal to that provided by an injected bacterin using the same vaccination schedule. With one exception, all LV strains that still possessed a functional O-antigen provided protective indices (PI) four- to seven-fold greater than the PI for the fish injected with bacterin. When antibody responses of vaccinated fish were compared, it was found that only vaccination by bacterin gave rise to a measurable agglutinating litre. Western immunoblots using the immune fish sera failed to reveal any major differences in antigen recognition in fish that received any of the vaccines tested. These data suggest that the immune response generated by the use of live vaccine strains is different from that generated by a bacterin, and that these useful mutations may be incorporated into existing furunculosis LVs for further attenuation.     

Efficacy of a Live‐attenuated Edwardsiella ictaluri Oral Vaccine in Channel and Hybrid Catfish

This study evaluated the efficacy of an oral live‐attenuated Edwardsiella ictaluri vaccine against enteric septicemia of catfish (ESC) in channel and hybrid catfish. The vaccine was delivered one time orally by feeding fish a diet coated with an attenuated E. ictaluri isolate at four doses to deliver between 4 × 10⁶ to 3.2 × 10⁷ viable vaccine cells/g wet feed. Thirty‐five days postvaccination, control and vaccinated fish were challenged with virulent E. ictaluri and mortality was examined for 30 d postchallenge. Mortality of nonvaccinated hybrids (85%) and nonvaccinated channel catfish (73%) was similar but significantly greater than all groups of vaccinated fish. In channel catfish, a trend toward increasing mortality with decreasing dose was observed. Mortality of channel catfish vaccinated with the lowest dose (26.6%) was significantly higher than fish vaccinated with the highest dose (14.1%) but similar to fish vaccinated at the intermediate doses (17.5 and 19.4%). In contrast, mortality of four doses of vaccinated hybrid catfish was similar and ranged between 10.4 and 14.0%. The data demonstrate that the attenuated E. ictaluri vaccine at all four doses tested is effective at reducing ESC‐related mortalities in hybrid and channel catfish.     

Fertilization efficiency of cryopreserved sperm from striped catfish, Pangasius hypophthalmus (Sauvage)

The fertilization efficiency of cryopreserved sperm was compared with fresh sperm from striped catfish, Pangasius hypophthalmus . Of the two sets of experiments carried out, the first compared four sperm doses using fresh sperm and fresh eggs. The second experiment compared six concentrations of cryopreserved sperm ranging from 6.94 × 10⁷ to 6.94 × 10¹⁰ to fertilize 100 eggs per batch. Fertilization, hatch and survival rates were compared between cryopreserved and fresh sperm. The highest fertilization rate (53.75±1.62%) was achieved with a sperm dose of 6.94 × 10⁸. Increasing the sperm dose to 3.47 × 10⁹ did not increase the fertilization rate, indicating that the optimum sperm:egg ratio lies between 6.94 × 10⁶ and 3.47 × 10⁷ sperm per egg. Both highest (6.94 × 10¹⁰) and the lowest (6.94 × 10⁷) sperm doses resulted in lower fertilization rates (2.04% and 16.90% respectively). No significant differences were found among four fresh sperm doses compared. Mean hatch and survival rates resulting from fresh and cryopreserved sperm were similar. The experiment shows that while only 1.89 × 10⁶ fresh spermatozoa was required to fertilize a fresh egg, 6.94 × 10⁶ (or 3.67 times more) cryopreserved sperm was required to achieve the same level of fertilization. This provides important information for making decision to cryopreserve sperm for commercial and/or conservation purposes.     

Comparative sensitivity of carp, Cyprinus carpio L. and rainbow trout, Salmo gairdneri Richardson, to Renibacterium salmoninarum

Abstract. The pathogenicity of Renibacterium salmoninarum to carp, Cyprinus carpio L., and rainbow trout, Salmo gairdneri Richardson, was investigated. All carp injected with 4·8 × 10⁸ cells/fish, or 4·8 × 10⁷ cells/fish survived for 38 days. R. salmoninarum was isolated from all moribund fish, but not from the kidney of surviving fish, although R. salmoninarum antigen was detected in several of these fish by the dot blot assay. On the other hand, mortality in rainbow trout was 95% in the fish injected with 4·8 × 10⁸ cells/fish, and 15% in those which received 4·8 × 10⁷ cells/fish. R. salmoninarum antigen was detected by the dot blot assay in all surviving rainbow trout. The number of R. salmoninarum cells was immediately decreased by carp or rainbow trout serum, and the serum bactericidal activity of carp was higher than that of rainbow trout. Carp blood leucocytes had higher phagocytic activity than those of rainbow trout.     

Effects of the probiotic, Lactobacillus acidophilus, on the growth performance, haematology parameters and immunoglobulin concentration in African Catfish (Clarias gariepinus, Burchell 1822) fingerling

This experiment was carried out to evaluate the effects of the probiotic, Lactobacillus acidophilus , on the growth performance, haematology parameters and immunoglobulin concentration in African catfish Clarias gariepinus fingerling. Two experimental diets were formulated to contain 35 g kg⁻¹ crude protein and 10 g kg⁻¹ lipids accordingly and fed three times daily for 12 weeks to 25 C. gariepinus fingerlings per fibreglass tank in 12 replicates each. The control diet was prepared with no probiotic supplementation whereas the second diet was prepared supplemented with a probiotic, L. acidophilus , containing about 3.01 × 10⁷ colonies/g of diet. The results show that growth performance [specific growth rate (SGR) and relative growth rate (RGR)], nutrient utilization [protein efficiency ratio (PER) and feed conversion ratio (FCR)] and survival were significantly ( P <0.05) higher in fish maintained on the probiotic-supplemented diet compared with those on the control diet. Haematology parameters (packed cell volume, haemoglobin, erythrocyte sedimentation rate, red blood cell and white blood cell, total serum protein, Ca²⁺, Mg²⁺, Cl⁻, glucose and cholesterol) and total immunoglobulin concentrations were also significantly better in fish fed the probiotic-supplemented diet than in the control. Although the water quality parameters monitored were better in the fish fed the probiotic-supplemented diet than in the control, the parameters were not significantly different ( P >0.05). From the results of this experiment, we conclude that L. acidophilus can be used as a probiotic agent in African catfish culture, to enhance fish health, survival and better feed efficiency and growth performance.     

Haematological and biochemical alterations in Korean catfish, Silurus asotus, experimentally infected with Edwardsiella tarda

This study determined the haematological changes in Korean catfish, Silurus asotus , experimentally infected with Edwardsiella tarda . Twenty-four or 48 h after infection with 1 × 10⁴ colony-forming units (CFU) mL⁻¹ of E. tarda , the fish were anaesthetized and the blood was collected. The haematological and biochemical tests included haematocrit (Ht), haemoglobin (Hb), leucocyte counts (Lc), total calcium (Ca), total protein (TP) concentration, alkaline phosphatase (ALP), glucose (GLU), aspartate aminotransferase, lysozyme and differential leucocyte populations. After 24- and 48-h infection, the values of Ht, Hb, Lc, TP, ALP and GLU were significantly decreased with respect to the control. Aspartate aminotransferase and lysozyme activity in both the 24- and 48-h groups exhibited higher levels compared with those in the control. However, the bacterial infection did not induce any significant alteration in the Ca concentration. After the 48-h infection, neutrophils and macrophage populations were reduced whereas lymphocytes were increased with respect to those in the control and the 24-h infection groups. The present study demonstrates that E. tarda infection causes haemato-biochemical responses that are related to the general physiological condition and defence system in catfish.     

Effects of dietary organic acids on growth, nutrient digestibility and gut microflora of red hybrid tilapia, Oreochromis sp., and subsequent survival during a challenge test with Streptococcus agalactiae

A 14-week feeding trial was conducted to determine the effects of dietary organic acids. The experimental diets were added with 0, 1, 2 or 3 g kg⁻¹ of a novel organic acid blend or with 2 g kg⁻¹ of potassium diformate and fed to triplicate groups of red hybrid tilapia ( Oreochromis sp.). Upon completion, tilapia were challenged by immersion with Streptococcus agalactiae . There was no significant difference ( P >0.05) in the growth, feed utilization and nutrient digestibility among treatment groups despite a trend towards improved results with fish fed organic acid-supplemented diets. Diet pH decreased, causing a reduction in the digesta pH of the stomach and gut. Total bacteria per gram of faeces were significantly ( P <0.05) reduced from 1.81 × 10⁸ colony-forming units (CFU) (control group) up to 0.67 × 10⁸ CFU in the fish fed organic acid diets. A similar trend was observed for adherent gut bacteria. Cumulative mortality of fish fed no organic acids was higher compared with fish fed organic acid-supplemented diets at 16 days post challenge. The data showed that dietary organic acids can exert strong anti-microbial effects and have the potential to exert beneficial effects on growth, nutrient utilization and disease resistance in tilapia.     

Effects of an Extended Hatchery Phase and Vaccination against Enteric Septicemia of Catfish on the Production of Channel Catfish, Ictalurus punctatus, Fingerlings

The present study was conducted to evaluate production management methods to improve overall survival of channel catfish, Ictalurus punctatus, fry to the fingerling stage by incorporating the use of a live, attenuated vaccine against Edwardsiella ictaluri and employing an extended hatchery phase. In this experiment, four treatments were used. In Treatment 1, 10‐d posthatch (PH) fry were vaccinated and then directly stocked into earthen ponds. In Treatments 2 and 3, 10‐d PH fry were sham‐vaccinated (control) and vaccinated, respectively, kept in nursery tanks for 22 d, and then stocked into earthen ponds. Fry in Treatment 4 were sham‐vaccinated at 10 d PH, kept in nursery tanks for 22 d, and then vaccinated prior to stocking into earthen ponds. Mean fingerling yield at harvest ranged from 4716 kg/ha in Treatment 1 to 8112 kg/ha in Treatment 4. Mean individual fish weight ranged from 38.8 g in Treatment 1 to 40.9 g in Treatment 4, and feed conversion ratios (FCR) ranged from 1.15 in Treatment 4 to 1.51 in Treatment 1. Mean survival ranged from 47.5% in Treatment 1 to 73.4% in Treatment 4. In specific comparisons to evaluate the nursery effect (Treatments 1 and 3), yield and overall survival were significantly different (P < 0.05) between these two treatments. In specific comparisons to evaluate the effect of the use of the vaccine (Treatments 2, 3, and 4), overall survival was significantly different (P < 0.05) between Treatment 2 (sham‐vaccinated control with nursery phase) and Treatment 4 (vaccinated at 32 d PH with nursery phase). No significant differences (P > 0.05) in yield, average weight, and FCR were observed between treatments. Results indicate that implementing an extended hatchery phase and vaccination strategy with older fry can improve overall survival of fingerling fish.     

Histopathology, electron microscopy and isolation of channel catfish virus in experimentally infected European catfish, Silurus glanis L.

Abstract Two groups of European catfish, Silurus glanis L., fingerlings were infected with channel catfish virus (CCV) by either intraperitoneal injection with 10⁵ TCID₅₀ of CCV, or bathing in water containing 10⁵ TCID₅₀ of CCV per 1·0 ml. The virus was isolated from spleen, intestine and brain of CCV-injected fish at day 1 and the titres ranged from 10^2·1 to 10^3·3 TCID₅₀/g. However, the tissue distribution of CCV was irregular and no virus was isolated after day 3 post-exposure. In CCV-bathed fish, the virus was isolated only from the liver of one specimen at day 3 post-exposure. No clinical signs of CCV disease developed in any of the fish. Specimens in each regime from all sampling periods showed some minor histopathological changes, but there were no differences between treatments. Lesions included oedema and focal haemorrhage in the liver and the spleen was congested. Electron micrographs of tissue samples showed the presence of a few virus particles around the nuclei of kidney, spleen and intestinal cells, and in or around a myelinated nerve within the optic lobes of infected fish during the first 4 days of infection.     

The susceptibility of salmonid fish to an atypical strain of Aeromonas salmonicida that infects goldfish, Carassius auratus (L.), in Australia

Abstract. An enzootic, Australian, atypical strain of Aeromonas salmonicida isolated from diseased goldfish, Carassius auratus (L.), was inoculated into Atlantic salmon, Salmo salar L., brown trout, S. trutta L., rainbow trout, S. gairdneri Richardson, and brook trout, Salvelinus fontinalis (Mitchill), fingerlings by intraperitoneal injection (i.p.) and by bath challenge, the latter with and without prior abrasion of skin. The 10-day LD₅₀ (i.p.) was estimated to be 7·4 × 10^-3 colony forming units (cfu) for Atlantic salmon, 3·0 × 10^-2 cfu for brown trout, 3·7 × 10² cfu for brook trout and 6·4 × 10³ cfu for rainbow trout. Brown, rainbow and brook trout succumbed to bath challenges with between 10⁵–10⁶ cfu/ml, developing ulcers of the skin and septicaemia. The organism was trasmitted from inoculated fish to five of 195 within-tank control fish via water and established a carrier state in one of 14 Atlantic salmon. It was concluded that the organism poses a significant threat to the salmonid farming industry and wild salmonid fisheries in Australia.     

Continuous exposure to infectious pancreatic necrosis virus during early life stages of rainbow trout, Oncorhynchus mykiss (Walbaum)

Rainbow trout, Oncorhynchus mykiss (Walbaum), were exposed continuously to infectious pancreatic necrosis virus (IPNV) at 0, 10¹, 10³ or 10⁵ plaque forming units (pfu) L⁻¹ of water to estimate the effects of chronic IPNV exposure on early life stages. Fish density averaged 35 fish L⁻¹ (low density) or 140 fish L⁻¹ (high density), and the tank flow rate was 250 mL⁻¹ min. Virus exposure began at 6 days before hatch and continued until fish were 44 days old. Cumulative per cent mortality, analysis of survival and hazard functions, and discrete-time event analysis were used to explore the patterns of survival and mortality. In eggs and fish exposed to IPNV, mortality significantly greater than in the 0 pfu L⁻¹ exposure did not occur until IPNV concentration was 10⁵ pfu L⁻¹ at low fish density and 10³ pfu IPNV L⁻¹ at high fish density. These results suggest that in the natural aquatic environment, where rainbow trout densities are likely to be considerably lower than in this study, mortality resulting from infection with IPNV will very likely not occur when ambient concentrations of virus are ≤10³ pfu IPNV L⁻¹. In aquaculture rearing units, trout density is likely to be as high or higher than the densities used in this study. Therefore, continuous inputs of virus at concentrations greater than 10¹ pfu L⁻¹ may result in IPN epidemics in aquaculture facilities.     

An experimental investigation on aspects of infectious salmon anaemia virus (ISAV) infection dynamics in seawater Atlantic salmon, Salmo salar L.

This study investigated infection dynamics of infectious salmon anaemia virus (ISAV) by conducting two experiments to examine minimum infective dose and viral shedding of ISAV. In terms of minimum infective dose, the high variability between replicate tanks and the relatively slow spread of infection through the population at 1 × 10¹ TCID₅₀ mL⁻¹ indicated this dose is approaching the minimum infective dose for ISAV in seawater salmon populations. A novel qPCR assay incorporating an influenza virus control standard with each seawater sample was developed that enabled the quantity of ISAV shed from infected populations to be estimated in values equivalent to viral titres. Viral shedding was first detected at 7 days post-challenge (5.8 × 10⁻² TCID₅₀ mL⁻¹ kg⁻¹) and rose to levels above the minimum infective dose (4.2 × 10¹ TCID₅₀ mL⁻¹ kg⁻¹) on day 11 post-challenge, 2 days before mortalities in ISAV inoculated fish started. These results clearly demonstrate that a large viral shedding event occurs before death. Viral titres peaked at 7.0 × 10¹ TCID₅₀ mL⁻¹ kg⁻¹ 15 days post-infection. These data provide important information relevant to the management of ISA.     

Temporal variation of seasonality of egg production and the spawning biomass of Pacific anchovy, Engraulis japonicus, in the southern waters of Korea in 1983–1994

Embryonic mortality, egg production and the spawning stock biomass of Pacific anchovy, Engraulis japonicus , off Southern Korea during 1983–1994, and their biological response to oceanographic features in spring and summer, were analysed. The instantaneous mortality rate (IMR) of embryonic stages decreased in spring and increased in summer, with a range of 0.33–1.23 day^–1 in spring and 0.78–1.69 day^–1 in summer. Egg production in summer was three times that during spring and production was low in the late 1980s. Mean lengths of yolk-sac larvae and adult females were greater in spring than in summer, whereas spawning fraction and spawning stock ratio (spawning biomass:adult biomass) were lower in spring than summer. Estimated mean spawning stock biomass ranged from 141 × 10³ to 380 × 10³ MT in spring and from 221 × 10³ to 557 × 10³ MT in summer. Statistically, the seasonal and long-term trends of embryonic mortality, egg production and spawning stock biomass of Pacific anchovy can be explained largely by spring warming, summer cooling and by less abundant zooplankton in the late 1980s.