Biological Observations on the Mudskipper Pseudapocryptes elongatus in the Mekong Delta,Vietnam

Aspects of the population biology of the mudskipper, Pseudapocryptes elongatus, (Cuvier, 1816) were studied in Bac Lieu Province in the Lower Mekong Delta, Vietnam, including sex ratio, length–frequency distribution, sexual dimorphism, hepatosomatic index (HSI), female gonadosomatic index (GSI), gonad histology, fecundity and some observations on post‐larvae and juveniles. In the population studied the sex ratio was skewed toward males. Females had a shorter mean length and a lower mean growth rate than males. No mature males or females were observed among mudskippers sampled in estuarine canals from February to May 2004 during the dry season. It is suggested that P. elongatus migrates to the sea to spawn later in the year after onset of the wet season. Salinity tolerance was evaluated in a 96‐h experiment. This showed that P. elongatus is remarkably euryhaline and able to survive in salinities ranging from freshwater to 50 ppt. Gut content analysis revealed that P. elongatus is a herbivore, feeding mainly on pennate diatoms (93% of the diet).     

Interspecific comparison of experimental and natural routes of Lepeophtheirus salmonis and Caligus elongatus challenge and consequences for distribution of chalimus on salmonids and therapeutant screening

Although the gills have been reported as a site for settlement by sea lice attached stages in experimental challenges, the prevalence and abundance of infection on gills have not been reported in farmed fish and have been reported only rarely in wild salmonids. Disproportionate attachment to the gills may be significant in evaluation of new therapeutants and this study asks whether the efficacy data from studies of therapeutants based on experimental challenge can be extrapolated to the fish farm arena. The distribution of chalimus on gills, body and fins of farmed first sea year Atlantic salmon were examined in two Scottish sea lochs, each on two occasions (30 fish on each sampling date), fish from one farm with infection with Lepeophtheirus salmonis only and the other farm with exclusively Caligus elongatus. The dorsal fin was the primary attachment site of L. salmonis (50% and 33% in two samples respectively) with 29% and 32% on other fins and 20% and 29% on the body and head. Only one of 56 chalimus (1.8%) and four of 75 chalimus (5.3%) were attached to the gills. In contrast, the chalimus distribution of C. elongatus was predominantly on fins other than the dorsal, including the tail, 74% and 71%, on two sampling occasions, respectively, with only 4.2% and 3.6% of chalimus located on the dorsal fin. The pectoral fins were the most common location with 32%, followed by the tail 23%, pelvic fins 14% and only 3% on the anal fin. None of 287 and 111 chalimus in two samples, respectively, of C. elongatus was attached to the gills. The low abundance of lice on the gills indicated that non‐destructive sampling methods adequately represented lice infection. In three experimental challenges with L. salmonis, the gills were a major attachment location with 19.9–36.3% of chalimus, 9.9–26.9% on the body, 22–27% on the dorsal fin and 19–23% on the other fins. These differences with natural challenge in L. salmonis reported here and also in the literature suggest that the importance of the gills for chalimus location in the experimental model is atypical and an artefact of reduced host swimming speed in tanks that permits copepodids to attach to the gills. In natural infection, the flow over the gills is high and reduced flow is required for copepodid settlement. Greatest reductions (96%) in chalimus bath treated with cypermethrin occurred on the gills with no significant reductions in those located on the dorsal and other fins. It is possible that the efficacy of therapeutants may be higher in chalimus attached to the gills because of the higher concentration encountered of bath therapeutant and extrapolation of efficacy to farms may be misleading and progression to field testing is essential.     

A review of the use of ivermectin as a treatment for sea lice [Lepeophtheirus salmonis (Krøyer) and Caligus elongatus Nordmann] infestation in farmed Atlantic salmon (Salmo salar L.)

Ectoparasitic sea lice, Lepeophtheirus salmonis (Krøyer) and Caligus elongatus (Nordmann) browse on the skin of farmed Atlantic salmon, Salmo salar L.. The resulting lesions cause stress and increase the susceptibility of the fish to secondary infections. In extreme infestations, fish can suffer from osmoregulatory failure and death. The most immediate treatment for the relief of sea lice infestations at fish farms is the use of chemotherapeutants, either by bath or oral administration. One compound which has been investigated for use as a chemotherapeutant is ivermectin. Ivermectin is a member of the avermectin group of macrocyclic lactones. The avermectins are neurotoxins, which have been used successfully in the treatment of helminthic parasitic infections in a number of terrestrial farm animals and also in the treatment of river blindness in humans. Owing to the low solubility of the compound, ivermectin has been administered as an oral treatment with the feed and has been found to be effective for the treatment of sea lice on Atlantic salmon. Ivermectin is poorly absorbed by fish with a high percentage of the administered dose being excreted in the faeces. The highest concentrations of the absorbed ivermectin were found in lipid‐rich organs. The ivermectin remained in the tissues of the treated fish for a prolonged period of time and was excreted mainly in the unchanged form. Ivermectin can reach the marine environment via excretion from the bile, unabsorbed via the fish faeces and by uneaten food pellets and has a strong affinity to lipid, soil and organic matter. Risk assessments have shown that ivermectin is likely to accumulate in the sediments and that the species therein would be more at risk than the species in the pelagic environment. Ivermectin has been shown to be toxic to some benthic infaunal species in single species tests, but there is no evidence that treatment of fish with ivermectin has affected multispecies benthic communities in the field situation. This review paper ends with details of the development and the status of the use of ivermectin in the treatment of sea lice infestations at Scottish fish farms.     

Toxicity of the aquaculture pesticide cypermethrin to planktonic marine copepods

The acute and sublethal toxicity of cypermethrin, the active ingredient in the sea lice treatment formulation Excis^®, to non‐target planktonic marine copepods was determined. The comparative sensitivities of three life stages (nauplii, copepodites, adults) of four common marine copepods (Acartia clausi, Pseudocalanus elongatus, Temora longicornis and Oithona similis) were assessed in 48‐h exposures, followed by a recovery period in toxicant‐free sea water. The cyclopoid copepod, O. similis, was most affected by cypermethrin, with EC₅₀ values ranging from 0.14 to 0.24 μg L^?1 for nauplii and adults respectively. With the exception of T. longicornis nauplii, the calanoid copepods (A. clausi, P. elongatus and T. longicornis) responded similarly to cypermethrin. Overall, 48‐h EC₅₀ values ranged from 0.12 μg L^?1 (T. longicornis nauplii) to >5 μg L^?1 (P. elongatus adults). For all species, nauplii and copepodite EC₅₀ values were lower than those of the adults. The primary toxic effect, immobilization, was generally irreversible. A sublethal test with adult A. clausi females, involving pulse exposures over 4 days measured a significant increase in egg production at the higher concentrations (1.58 and 5 μg L^?1). Concentrations causing acute toxicity to planktonic copepods were lower than the recommended sea lice treatment concentration of 5 μg L^?1 cypermethrin, indicating the potential for toxic effects in the field. However, acute toxicity values were higher than the Environmental Quality Standard of 0.016 μg L^?1 for dispersing treatment plumes, suggesting that cypermethrin released to the marine environment following sea lice treatments is unlikely to affect adversely planktonic copepods.     

Toxicity of Four Chemotherapeutic Agents to Rabbitfish Siganus rivulatus

As aquaculture evolves from extensive pond culture to intensive tank and cage systems, chemical dips and baths are increasingly being used to treat a concomitant increase in ectoparasitic and bacterial infestations. Some of the main disease‐causing agents are ectoparasites on the skin and gills of fish. Consequently, application of chemotherapeutics is increasing in aquaculture industries in order to control outbreaks of parasitic infestations. However, the toxic effect of commonly used chemotherapeutic agents on specific aquacultured fish species is often unknown. The present work was performed to test the effect of four commonly used chemical treatments on rabbitfish, Siganus rivulatus. The lethal concentrations for 50% of population (LC₅₀) of formalin, copper sulfate, potassium permanganate, and hydrogen peroxide for S. rivulatus juveniles treated for 1 h were assessed. Formalin and potassium permanganate tolerance values were determined by calculating 72‐h LC₅₀ values through probit analysis. The 72‐h LC₅₀ values for the formalin toxicity tests were 551.0 and 1.68 mg/L, respectively. LC₅₀ of copper sulfate and hydrogen peroxide could not be determined from the concentrations tested but were found to be >3 and >700 mg/L, respectively. Accordingly, treatment concentrations of formalin and potassium permanganate used for other fish species could be lethal to S. rivulatus, but the species appears to be quite tolerant to copper sulfate and hydrogen peroxide.     

Larval feeding behaviour of angel fish Pterophyllum scalare (Cichlidae) fed copepod Eucyclops serrulatus and cladoceran Ceriodaphnia quadrangula

Three different live diets, Eucyclops serrulatus, Ceriodaphnia quadrangula and equal combination of E. serrulatus copepodid and C. quadrangula, were offered to angelfish (Pterophyllum scalare) larvae viz 1‐week, 2‐week and 3‐week old at prey densities of 2, 5 and 10 individuals mL^?1. Results showed that 1‐week‐old P. scalare larvae consumed E. serrulatus copepodid at a rate of 31.3–56.7 ind. h^?1, C. quadrangula at 8.0–12.0 ind. h^?1, and mixture of E. serrulatus and C. quadrangula at 20.7–40.7 ind. h^?1. For 2‐week‐ and 3‐week‐old larvae, consumption rate increased accordingly. The electivity indices (E) of P. scalare (1‐week‐old larvae) for E. serrulatus copepodid were +0.18, +0.23 and +0.22 at prey densities of 2, 5 and 10 ind. mL^?1 respectively. Tendency towards E. serrulatus copepodid consumption reduced by aging P. scalare as indicated by the E values for 2‐ and 3‐week‐old larvae. However, growth and survival of P. scalare larvae was greatest when fed on combination of copepod E. serrulatus and C. quadrangula.     

Vibrogen‐2 vaccine trial in lumpfish (Cyclopterus lumpus) against Vibrio anguillarum

Lumpfish (Cyclopterus lumpus), a native fish of the North Atlantic Ocean, is utilized as cleaner fish to biocontrol sea lice infestations in Atlantic salmon aquaculture. However, bacterial infections are affecting cleaner fish performance. Vibrio anguillarum, the aetiological agent of vibriosis, is one of the most frequent bacterial infections in lumpfish, and effective vaccine programmes against this pathogen have been identified as a high priority for lumpfish. Vibrogen‐2 is a commercial polyvalent bath vaccine that contains formalin‐inactivated cultures of V. anguillarum serotypes O1 and O2, and Vibrio ordalii. In this study, we evaluated Vibrogen‐2 efficacy in lumpfish against a local isolated V. anguillarum strain. Two groups of 125 lumpfish were bath‐immunized, bath‐boost‐immunized at four weeks post‐primary immunization, and intraperitoneally (i.p.) boost‐immunized at eight weeks post‐primary immunization. The control groups were i.p. mock‐immunized with PBS. Twenty‐seven weeks post‐primary immunization, the fish were i.p. challenged with 10 or 100 times the V. anguillarum J360 LD₅₀ dose. After the challenge, survival was monitored daily, and samples of tissues were collected at ten days post‐challenge. Commercial vaccine Vibrogen‐2 reduced V. anguillarum tissue colonization and delayed mortality but did not confer immune protection to C. lumpus against the V. anguillarum i.p. challenge.     

Effects of E/Z Isomers and Coating Materials of Astaxanthin Products on the Pigmentation and Antioxidation of Rainbow Trout,Oncorhynchus mykiss

The objective of this study was to compare the effects of four astaxanthin preparations with different ratio of E/Z (trans/cis) isomers and different coating materials on the pigmentation and antioxidation properties of rainbow trout. Five diets were designed as basal diet (without astaxanthin supplementation) and four astaxanthin diets (100 mg/kg diet) supplemented with four astaxanthin products, BASF (79% all‐E, gelatin coated), Wisdom‐B (85% all‐E, gelatin coated), Wisdom‐C (94% all‐E, carrageenan coated), and Wisdom‐D (94% all‐E, gelatin coated). After 4 wk feeding, the flesh astaxanthin content, redness, and yellowness of astaxanthin‐supplemented groups were higher than those of control group at the second and fourth week (P < 0.05). Among these astaxanthin‐supplemented groups, the Wisdom‐B group had the highest flesh redness and astaxanthin content. The astaxanthin‐supplemented groups also had the lower flesh frozen loss and malondialdehyde level than control group at the fourth week (P < 0.05). The results indicated that dietary 100 mg/kg astaxanthin could improve the flesh redness and antioxidation abilities of rainbow trout. The ration of E/Z isomers and properties of coating materials of astaxanthin preparations influenced the pigmentation and antioxidation properties, and Wisdom‐B had a better pigmentation effect on the flesh than other astaxanthin preparations.     

Precision of age estimates derived from scales and pectoral fin rays of blue sucker

Abstract Evaluating the response of fish populations to environmental influences requires precise age estimation. Pectoral fin rays and scales were compared to determine which is more precise for estimating age of blue sucker, Cycleptus elongatus (Lesueur), from the Missouri River, USA. Mean coefficient of variation was significantly lower for fin rays both among three novice readers (rays 19.9%; scales 23.1%) and among three reads by a single experienced reader (rays 11.8%; scales 18.6%). Additionally, exact per cent agreement (rays 20.0%; scales 14.3%) and within‐1‐year per cent agreement (rays 89.1%; scales 73.9%) were higher for fin rays compared with scales for multiple reads by the same experienced reader. Age–bias plots revealed that scale ages were lower than fin‐ray ages >7. The use of fin rays is recommended for estimating ages of blue sucker until validation procedures are conducted.     

The Three‐spined Stickleback,Gasterosteus aculeatus Linnaeus 1758, plays a minor role as a host of Lepeophtheirus salmonis (Krøyer 1837) in the Gulf of Maine

The sea louse, Lepeophtheirus salmonis (Krøyer 1837), is a significant parasite of farmed salmon throughout the Northern Hemisphere. Management of on‐farm louse populations can be improved by understanding the role that wild fish play in sustaining and providing refuge for the local population of sea lice. In this study, 1,064 sticklebacks were captured. Of these animals, 176 individuals were carrying a total of 238 sea lice, yielding a prevalence and intensity of 16.5% and 1.4 lice per fish, respectively. Detailed examination of the sea lice on the three‐spined sticklebacks captured in Cobscook Bay found two L. salmonis individuals using three‐spined sticklebacks as hosts. A 2012 survey of wild fish in Cobscook Bay, Maine, found multiple wild hosts for Caligus elongatus (von Nordmann 1832), including three‐spined sticklebacks (Gasterosteus aculeatus L.), but no L. salmonis were found in this earlier study.     

A novel genotyping technique for distinguishing between Flavobacterium psychrophilum isolates virulent and avirulent to ayu, Plecoglossus altivelis altivelis (Temminck & Schlegel)

We developed a simple genotyping method for Flavobacterium psychrophilum for analysing two single nucleotide polymorphisms (SNPs) in the gyrA gene and to distinguish between isolates that are virulent and avirulent to ayu, Plecoglossus altivelis altivelis (Temminck & Schlegel). The genotyping method is an on/off switch assay and is based on the polymerase chain reaction technique with phosphorothioated primers. We classified 232 isolates from four families of fish (i.e. Plecoglossidae, Osmeridae, Cyprinidae and Salmonidae) into four genotypes (G‐C, A‐T, A‐C and G‐T). The G‐C type isolates exhibited strong pathogenicity to ayu, whereas the A‐T and G‐T types did not show any pathogenicity to this species. The A‐C type exhibited no or weak pathogenicity to ayu. These results indicate that genotyping F. psychrophilum isolates with two SNPs from gyrA can clearly distinguish between isolates potentially harmful to ayu (G‐C type) and those that are potentially not harmful or less harmful (A‐C, A‐T and G‐T type). The on/off switch assay provides a quick, simple, and very powerful DNA genotyping technique for F. psychrophilum isolates.     

Comparisons of age,growth, and maturity between male and female,and diploid and triploid individuals in Carassius auratus from Okinawa‐jima Island,Japan

• 1. Carassius auratus, a primary freshwater fish with bisexual diploid and unisexual gynogenetic triploid lineages, is distributed widely in and around the Eurasian continent and is especially common in East Asia. East Asian C. auratus diverged genetically to form local endemic populations in different regions, and those distributed in the Ryukyu Archipelago form a local endemic population that can be regarded as an evolutionarily significant unit because of its high phylogenetic independence and evolutionary distinctiveness. Although the evolutionary uniqueness of this population should be conserved, its distribution area and population size are decreasing rapidly, and some island populations are currently considered endangered or already extinct.
• 2. To develop effective conservation measures to stop the current decline of Ryukyuan C. auratus, ecological data need to be collected. In this study, life history data for a C. auratus population distributed in the Hiji River system were collected by estimating age, growth, and spawning season.
• 3. The spawning season of C. auratus in the Hiji River extended from March to September, peaking during March–May. Females became sexually mature in their second year, but males reached maturity and were able to spawn as early as in the late spawning season of their year of hatching. Once having reached sexual maturity, males probably continuously stay ripe throughout their life.
• 4. Sagittal otoliths of C. auratus proved to be useful ageing structures because one annual increment is formed on the sagittal otolith before the spawning season in each year. The oldest fish observed were a 10‐year‐old female and an 11‐year‐old male. Females showed faster somatic growth and higher final standard length than males, and a sexual size dimorphism was observed.
• 5. The standard length at each age class did not differ between diploid and triploid C. auratus, suggesting that triploid growth rates were almost equal to those of diploids. Copyright © 2009 John Wiley & Sons, Ltd.

     

Potential use of Artemia biomass by‐products from Artemia cyst production for the nursing of goby Pseudapocryptes elongatus in Vietnam: effects on growth and feed utilization

The present study was performed to evaluate the effect of using Artemia biomass, by‐product from Artemia cyst production on growth and feed utilization of goby Pseudapocryptes elongatus fingerlings. A control diet containing fishmeal as main protein source was compared with four experimental diets in which fishmeal protein was replaced by increasing dietary levels of Artemia protein, namely 25%, 50%, 75% and 100%. The five test diets were compared with a commercial diet and dried Artemia. All diets were formulated to be equivalent in crude protein (360–370 g kg^?1) and lipid (58–65 g kg^?1). The experiment was conducted in 80 L plastic tanks filled with water at a salinity of 15 g L^?1. Goby fingerlings with 0.21 g initial weight were fed the test diets for 30 days. The results showed that weight gain and specific growth rate of goby were positively correlated with total feed intake. Moreover, growth performances and feed utilization in the fry receiving the commercial feed and fishmeal control diet were similar, both were inferior to the groups fed dried Artemia and the based formulated diets. These results illustrate that both dried Artemia and Artemia‐based feeds can be used for feeding goby fingerlings, indicating the high potential of using locally produced Artemia biomass, which could contribute to reduce the reliance on fishmeal and improve profits for Artemia producers.     

Feeding high inclusion of whole grain white lupin (Lupinus albus) to rainbow trout (Oncorhynchus mykiss): effects on growth,nutrient digestibility,liver and intestine histology and muscle fatty acid composition

The effect of dietary inclusion of whole grain white lupin (Lupinus albus) on growth performance, histology, muscle fatty acid composition and nutrient digestibility was investigated in an 11‐week growth and a 4‐week digestibility trial with rainbow trout (initial body weight of 54.0 ± 6.2 and 181.9 ± 3.4 g respectively). Four experimental extruded diets were formulated to contain 0%, 30%, 40% and 50% of whole grain lupin and fed to triplicate groups of fish twice a day until apparent satiation. Faeces were collected daily from each digestibility tank by decantation. No significant trends were observed with respect to growth, feed utilization, apparent digestibility coefficients or whole‐body composition (P>0.05). Conversely, increasing levels of dietary lupin led to significant decreases in the Hepatosomatic index (R²=0.75, P<0.05) and slight lipid infiltration into hepatocytes and enterocytes. Muscle fatty acid compositions were slightly affected by the dietary treatment. Polynomial regression of dietary inclusion of lupin and muscle fatty acid concentrations showed an increase in C18:1n‐9, C18:2n‐6 and C18:3n‐3 and a decrease in C20:5n‐3 with increasing dietary lupin level. These results demonstrated that whole grain lupin can be included up to 50% in commercial rainbow trout diets without negative effects.     

Relationship between sea lice levels on sea trout and fish farm activity in western Scotland

The relationship between aquaculture and infestations of sea lice on sea trout, Salmo trutta L., is controversial. Here, the association between sea lice infestations on wild sea trout and characteristics of local Atlantic salmon, Salmo salar L., farms were investigated using data collected on the Scottish west coast. The proportion of sea trout with louse burdens above a critical level was positively related to the fork length of the sea trout and the mean weight of salmon on the nearest fish farm, and negatively related to the distance to that farm. The distance to the nearest fish farm did not influence the probability of infestations above the critical level beyond 31 km although there was considerable uncertainty around this cut‐off distance (95% limits: 13–149 km). The results support a link between Atlantic salmon farms and sea lice burdens on sea trout in the west of Scotland and provide the type of information required for marine spatial planning.     

Influence of temperature on growth and survival of juvenile winter flounder,Pseudopleuronectes americanus (Walbaum) reared under continuous light

Winter flounder, Pseudopleuronectes americanus, has emerged as a promising candidate flatfish for cold‐water aquaculture and restocking. Here, juveniles were reared for 8 weeks at three temperatures: 10, 15, and 20°C under 24‐hr light. All fish were imaged at stocking and at 2‐week intervals, where growth was measured as changes in standard length (S_L) and body area (B_A). By week 2, fish reared at 15 and 20°C were larger than those grown at 10°C. At weeks 4 to 6, fish at 15°C were larger than fish at 20°C. Linear regressions were used to model growth dynamics over time at each temperature. Highly significant linear growth trajectories were detected over time for S_L and B_A. S_L and B_A regressions also showed a significant difference among the slopes across temperatures, where comparing slopes showed the best temperature to rear the flounder was 15°C. Weights of fish held at 15°C and 20°C were greater than at 10°C at the termination of the experiment. Within each temperature, the growth rate of malpigmented fish was not different from that of the normally pigmented fish. Overall, growth of winter flounder was comparable to that of other commercially produced flatfish species, providing strong evidence for this flatfish species as a potential species for aquaculture.     

Invasive Chinese pond mussel Sinanodonta woodiana threatens native mussel reproduction by inducing cross‐resistance of host fish

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Effect of dietary chitosan oligosaccharide complex with Ce (IV) on growth,immunity and disease resistance against Vibrio splendidus of sea cucumber,Apostichopus japonicas

An 8‐week feeding trial was conducted to investigate the effects of dietary chitosan oligosaccharide complex with cerium (Ce IV) (COS‐Ce) on growth performance, nonspecific immunity and disease resistance of sea cucumber, Apostichopus japonicas. Five isonitrogenous (18.6%) and isolipidic (1.1%) practical diets were formulated with graded level of COS‐Ce (0, 150, 300, 600 and 1200 mg kg^?1 dry feed), which were named as COS‐Ce/0, COS‐Ce/150, COS‐Ce/300, COS‐Ce/600, COS‐Ce/1200 respectively. Each diet was allocated to four replicates of sea cucumbers (Initial weight: 6.72 ± 0.02 g). Sea cucumbers were fed to apparent satiation once daily (19:00 hours) for 56 days. During the experiment, water temperature was kept at 16 ± 0.5°C, pH 7.8–8.2, dissolved oxygen beyond 5 mg L^?1, ammonia nitrogen below 0.5 mg L^?1 and salinity from 30‰ to 31‰. Results showed that the specific growth rate of sea cucumbers was significantly higher in COS‐Ce/600 than that in other four treatments. Activities of phagocytosis, respiratory burst, acid phosphatase and alkaline phosphatase in COS‐Ce/600 were significantly higher than that in COS‐Ce/0 (P < 0.05) respectively. On the contrary, cumulative mortality was the lowest in COS‐Ce/600 following 14 days exposure to Vibrio splendidus (P < 0.05). In conclusion, these results confirmed that dietary COS‐Ce had beneficial effects on growth performance, nonspecific immunity and disease resistance of sea cucumber.     

Aquaculture potential of the basket cockle (Clinocardium nuttallii). Part 1: effects of stocking density on first year grow‐out performance in intertidal and off‐bottom suspended culture

The basket cockle (Clinocardium nuttallii) is a candidate species for aquaculture in the northeast Pacific. The aim of the current research was to assess the feasibility of C. nuttallii grow‐out, with an emphasis on growth performance and qualities affecting product marketability. In this article, we investigated the combined effects of culture mode (intertidal and off‐bottom suspended culture) and initial stocking density (1500, 3000, 10 500 and 21 000 ind m^?2) on C. nuttallii survival and growth during the first year of grow‐out (May through October). In intertidal culture, cockles exhibited low survival and poor growth rates. In suspended culture, survival was consistently high (>96%) at all stocking densities tested; growth and condition parameters had the highest values at 1500 and 3000 ind m^?2. The edible portion (meat yield) exceeded 40% of the whole wet weight at all stocking densities, occurrences of fouled and deformed cockles were <1% and no commensal species were observed. Depending on the minimum harvestable size and stocking density chosen, harvestable proportions constituted from 1.1% to 15.2% by October of the first grow‐out year in the suspended system. The effects of stocking density and depth on second year grow‐out performance of C. nuttallii are reported in a companion paper (Dunham et al. in this issue).     

Epitheliocystis in Atlantic salmon,Salmo salar L., farmed in fresh water in Ireland is associated with ‘Candidatus Clavochlamydia salmonicola’ infection

Intracellular inclusions containing chlamydia‐like organisms are frequently observed in the gill epithelial cells of Atlantic salmon, Salmo salar L., cultured in fresh water in Ireland. In this study, the causative agent was identified in four separate freshwater sites, using 16s rRNA sequencing, as ‘Candidatus Clavochlamydia salmonicola’. Histopathology and real‐time (RT) PCR were used to further assess infections. The prevalence of infection ranged from 75–100% between sites and infection intensity was highly variable. No significant lesions were associated with these infections. As a diagnostic tool, RT‐PCR proved marginally more sensitive than histopathology. The fate of ‘Candidatus Clavochlamydia salmonicola’ in Atlantic salmon post‐seawater transfer was investigated in a 12‐week marine longitudinal study. Both RT‐PCR and histopathological examination indicate that the organism disappears from the gills 4–6 weeks post‐transfer.