Parasites and symbionts of wild mud crabs Scylla serrata (Forskal) of potential significance in aquaculture

A total of 130 wild mud crabs, Scylla serrata, from Moreton Bay, Queensland, were examined to determine if there were parasites or symbionts that could cause problems in aquaculture. Of 10 parasites and symbionts found, four were potential pathogens. The blood protozoan, Haematodinium sp. is similar to parasites that have killed crabs else-where. The ciliates, Epistylis sp. and Acineta sp., and the barnacle, Octolasmis cor. were common on the gills and may increase stress through respiratory distress. Both of the ciliates showed seasonality, with Epistylis sp. abundant in summer and Acineta sp. abundant in winter. Haematodinium sp. was present at low prevalence throughout the year. The prevalence of Octolasmis cor was significantly higher in female crabs than in males. Mean intensity was positively correlated with crab size for O. cor and for the metacestode of Polypocephalus sp. The prevalence of Epistylis sp. and the barnacle, Chelonibia patula, varied with the moult stage of the crab.     

Adding value to live, commercial size soft-shell clams (Mya arenaria L.) in Maine, USA: results from repeated, small-scale, field impoundment trials

The soft-shell clam (Mya arenaria L.) fishery in the state of Maine, USA, is worth $5–10 million annually and is primarily based on the sale of live individuals. More than 80% of the catch is sold for the “steamer clam” market that is highly seasonal due to supply and demand. Prices paid to harvesters for live clams throughout the year increases by as much as 70% during a 4-month period between early spring and late summer. If clams harvested in the spring could be held until late summer, a value-added product could be developed in this fishery. From April to August 1996, at an intertidal and subtidal location in eastern Maine, I tested whether it was biologically feasible to impound commercially harvested clams (shell LENGTH=44–75 mm) at densities between 720 and 850/m² using cages and nets (1.12 m²) to protect animals from predators. Survival was 91.7% (n=12) and 91.6% (n=6) at the intertidal and subtidal site, respectively. There was no discernible shell growth during this period and there was no difference between initial and final clam weights. The methodology was transferred to commercial harvesters in two Maine communities: Wiscasset (1997 and 1998) and Perry (2000). Impounded clams ( =21.8 kg/cage; n=18) lost, on average, 5 kg/cage in 1997 and 1998, whereas no significant net loss in wet weight occurred in cages deployed in 2000 ( =23.2 kg/cage; n=3). Differing harvesting and handling methods of individual clammers, prior to impounding clams, likely explains the variation in weight lost during the impounding periods. The difference in price per live kilogram between the beginning of the impounding period and the August sale date resulted in an average gain of $13.60/cage for clammers in 1997 and 1998 and $57.73/cage in 2000. Simple culture techniques can be used to increase the value of the live harvest of soft-shell clams along the coast of Maine. Clam impoundments may be a way for communities that co-manage the public clam resource with the state's marine resource agency to generate funds to pay for traditional management schemes, as well as enhancement programs that employ hatchery-reared juveniles. In addition, an indirect benefit of clam impoundments might be to create spawner sanctuaries since animals are impounded during the time when gamete release occurs.     

Seasonality of QPX disease in the Raritan Bay (NY) wild hard clam (Mercenaria mercenaria) population

Quahog Parasite Unknown (QPX) is a potentially lethal pathogen of the hard clam Mercenaria mercenaria along the northeastern coast of the United States. In the Raritan Bay wild clam population, QPX prevalence and infection intensity at three sites were examined by both histology and quantitative PCR in 2006. At two of three sites, QPX infection showed a seasonal pattern, with prevalence and weighted prevalence increasing through the spring, peaking in the summer and declining in the fall, while at the other site, the highest QPX prevalence and infection intensity were observed in the spring although overall prevalence at this site was generally low. Our data suggested that temperature may be an important environmental factor regulating the seasonal pattern of QPX disease in wild clams but also demonstrated that seasonal patterns vary from site to site, possibly linked to the clam density or other environmental factors. Over‐winter sampling and further investigations focusing on environmental factors, clam density and clam mortality as related to QPX infection are needed to better characterize and understand the seasonality of QPX disease.     

Habitat variability in young-of-the-year winter flounder, Pseudopleuronectes americanus, in Maine estuaries

The Magnuson-Stevens Fishery Conservation and Management Act defines essential fish habitat (EFH) as “the waters and substrate necessary to fish for spawning, breeding, feeding, and growth to maturity” and the protection of nursery areas has become a key element in US Federal fisheries management. Distribution and abundance by habitat for age-0, young-of-the-year (YOY) winter flounder, Pseudopleuronectes americanus, were compared for 28 Maine estuaries to help define essential fish habitat for this life history stage. The Maine coast was divided into three broad geographic zones based upon geological features and sampled over 5 consecutive years; during April–November of 2000 in the Mid-coast, in 2001 and 2002 along the Southwest coast and in 2003 and 2004 along the Eastern Maine coast. One beam trawl (2.0 m width, 3 mm mesh) sample was collected in one to four habitats in estuaries: eelgrass (Zostera marina), kelp (Laminaria longicruris), drift algae (Phyllophora sp.) and unvegetated sand/mud. Fish were sampled every 2 weeks, April–November 2000–2004. Abundance of YOY winter flounder was greatest in Mid-coast estuaries between Casco and Penobscot Bays and was significantly lower in Southwest and Eastern estuaries. Abundance was similar across all four habitats in Mid-coast estuaries in 2000. In the other years, YOY were found in higher abundance in eelgrass relative to other habitats. A logistic regression model based on nearshore habitat characteristics was developed to predict the distribution of this species along the three broad geological zones of the Maine coast with the physical and biological variables most important in discriminating between habitats with and without individual fish identified. This logistic regression model correctly classified winter flounder 72.4% of the time based on the year, zone, the physical habitat variables (temperature, salinity, depth) and the presence–absence of submerged aquatic vegetation (eelgrass, kelp or algae). These results indicate that the type of habitat most important to YOY winter flounder varies among estuaries and EFH for this species and life stage must be defined with care.     

Hemocyte characteristics in families of oysters, Crassostrea gigas, selected for differential survival during summer and reared in three sites

High variability among individuals is often encountered when hemocyte characteristics are measured in bivalves. Such variability is suspected to result partly from genetic factors. In this study, hemocyte characteristics of six families of Crassostrea gigas were compared by flow cytometry at one sampling date in October 2001. These families were obtained from a nested, half-sibling cross design, and reared from July to October 2001 at three sites distributed along the French Atlantic coast from north to south: Baie des Veys (Normandy), Rivière d'Auray (Brittany) and Ronce (Marennes-oléron Basin, Poitou Charentes).

Among the 15 measured hemocyte characteristics, production of reactive oxygen species (ROS) of untreated hemocytes (maintained in filtered sterile seawater) and treated hemocytes (zymosan at 20 particles per hemocyte, and with Vibrio sp. S322 at 50 bacteria per hemocyte) was the most notable differences between families. This supports the existence of a genetic basis, at least partly, for the hemocyte characteristics of oysters, and especially for ROS production.

Among the six families analyzed, three have shown high survival during summer (named as “resistant”, mean mortality 5.2%) and three experienced high mortality during summer (named as “susceptible”, 30.6% mean mortality). Families showing high or low survival to summer mortality had similar hemocyte characteristics, regardless of the environmental conditions or reproductive state. Resistant families were observed to have higher total hemocyte counts and lower production of ROS than susceptible families. Moreover, ROS production of hemocytes from susceptible families was diminished significantly more by pathogenic Vibrio than that of resistant families. However, this study demonstrates also that rearing site strongly affected the hemocyte characteristics of all families of oysters, most notably hemocyte concentration and morphology (size and granularity), production of reactive oxygen species (ROS), and susceptibility to the cytotoxic activity of the pathogenic Vibrio sp. S322 (50 bacteria/hemocyte). Food availability and reproductive state are the most probable explanations for the site differences observed. Finally, it appeared difficult to link oyster survival during summer mortality to hemocyte profiles evaluated at one sampling date; other relevant indicators would probably help explaining oyster survival during summer mortality events.     

Experimental transmission of Enteromyxum leei Diamant, Lom and Dykova, 1994 in sharpsnout sea bream, Diplodus puntazzo C. and the effect on some innate immune parameters

Enteromyxum leei has been implicated in outbreaks in sharpsnout sea bream, causing a serious economic impact on Mediterranean fish farming. Information about transmission and life cycles of marine myxosporea is limited to only a few fish species. In the present study, direct transmission of the infection was achieved by cohabitation of infected and healthy Diplodus puntazzo C. for the first time. The progress of the infection was monitored by the detection of different developmental stages of the parasite in the posterior intestine. Infected fish with E. leei were firstly observed in day 5 (prevalence of 20%). The total prevalence reached 100% in day 19 and was maintained in this high level until the end of the experimental period (day 55). In addition, the initial intensity of the different parasite stages was low, but on day 26 the intensity of trophozoites was suddenly increased, followed by the increased intensity of sporoblasts and mature spores. Also, the experimental transmission of E. leei in recovered D. puntazzo previously treated for parasite elimination proved to be unsuccessful, suggesting of a possible development of protective immunity against the parasite.     

Investigations into the temporal development of epitheliocystis infections in brown trout: a histological study

Epitheliocystis in Swiss brown trout (Salmo trutta) is a chlamydial infection, mainly caused by Candidatus Piscichlamydia salmonis and Candidatus Clavichlamydia salmonicola. To gain a better understanding of the temporal development of infections in wild brown trout, we investigated epitheliocystis infections during the course of the summer and autumn months of a single year (2015), and compared this to sampling points over the span of the years 2012–2014. The survey focused on tributaries (Venoge and Boiron) of the Rhone flowing in to Lake Geneva. When evaluated histologically, epitheliocystis infections were found throughout the period of investigation with the exception of the month of June. Fifty to 86 animals per sampling were investigated. Highest prevalence and infection intensities were seen in September. A correlation between epitheliocystis infection and water temperatures was not evident. Interyear comparison revealed consistent levels of prevalence and infection intensities in late summer. The absence of infections in June, combined with the consistent interyear results, indicates seasonal fluctuation of epitheliocystis infections in brown trout with a reservoir persisting during winter months from which infections can re‐initiate each year. This could either be at levels below detection limits within the brown trout population itself or in an alternative host.     

Gametogenic cycle and variations in oocyte size of Tapes philippinarum from the Lagoon of Venice

The gametogenic cycle of the clam Tapes philippinarum was studied in two populations from different areas of the Lagoon of Venice: S. Angelo (central basin) and Chioggia (southern basin). Samples were collected at regular intervals from July 2000 to July 2001. The study was performed by microscopic observations of histological sections of gonadic tissue and by measuring follicular and oocyte sizes by image analysis. Gametogenesis began in January and was clearly evident until May. Ripe clams appeared in April, and most active spawning occurred in May, continuing during summer until September. Measurements performed using image analysis (i.e., follicular area, percentage of reproductive tissue, oocyte diameter, percentage of resorbing area) highlighted environmental differences between the two sampling sites, demonstrating the better condition of clams from Chioggia.     

An assessment of the aquaculture potential of the tropical sponges Rhopaloeides odorabile and Coscinoderma sp.

Sponge aquaculture offers the opportunity to provide a sustainable supply of bath and industrial sponges and mitigate the environmental effects of over-harvesting. The potential to develop sponge aquaculture is diverse given the variety of sponge species. However, amenability to culture and the selection of appropriate culture methods are species specific and need to be determined to provide the platform for commercial success. In this study the survival (in situ and ex situ), growth rates (in situ), and recovery processes (ex situ) were measured for explants of two sponge aquaculture candidates, Rhopaloeides odorabile Thompson et al. and Coscinoderma sp. [Phylum Porifera: Order Dictyoceratida: Family Spongiidae], cultured in the Palm Islands Group of the central Great Barrier Reef. Sponge survival was dependent on the species, experiment duration, and method of culture. For both species the highest mortality occurred within days of excision of sponge material from parent stock, and mortalities were negligible after 78 days. After 78 days, R. odorabile had highest survival ex situ (75%) and lowest in situ (60%). In contrast, Coscinoderma sp. had the lowest survival ex situ (30%) and highest in situ (90%). The total growth of R. odorabile (146.0 ± 40.3%) and Coscinoderma sp. (195.9 ± 39.8%) was not significantly different over the 21 month experimental period, but was highly variable between explants from the same individual. Both species demonstrated initial size dependent growth rates with smaller explants growing fastest over the first 78 days. Explant recovery rates were rapid for both species with a protective layer of collagen forming over the surface within 24 h. This layer was replaced by pinacoderm between 3 and 41 days after excision as the subsurface tissue reorganised to recreate a functional surface for each species, including redevelopment of the aquiferous system within 41 days for R. odorabile. R. odorabile and Coscinoderma sp. both show potential for commercial aquaculture, however, further research is required to reduce initial mortality rates and the high variability in growth rates between explants.     

Infection and mortality by the yeast Metschnikowia bicuspidata var. bicuspidata in chinook salmon fed live adult brine shrimp (Artemia franciscana)

First-feeding chinook salmon were fed either live adult Artemia franciscana or commercial feed over a 15-week period. Unexplained mortality began occurring in the Artemia-fed fish after 35 days on the diets, with cumulative mortality reaching 34.5% in the Artemia-fed fish compared with 4.3% in feed-fed fish. Necropsy examinations revealed systemic fungal yeast infections and the causative agent was cultured from kidneys. Fungal cells were observed in the transport water of Artemia shipments and within the Artemia. The Artemia were purchased from a retail supplier, and originated from salt ponds in San Francisco Bay (SFB), California. Artemia infection rates ranged from 0.5% to 37.5% (mean 16.5±3.59%) in shipments received over a 5-week period. The fungus was characterized by morphological and physiological properties and was identified as Metschnikowia sp., a pathogenic yeast of aquatic invertebrates. The yeast grew at 9–27 °C, and 0–180 ppt NaCl, and could tolerate salinity of at least 270 ppt. Sequence analysis of the divergent D1/D2 domain of ascomycetous yeast 26S ribosomal DNA identified the organism as Metschnikowia bicuspidata var. bicuspidata. The organism was found in water from the salt ponds and probably entered from the bay. Mortality due to the fungal infection stopped after changes were made in the handling of incoming Artemia shipments.     

Hatchery production of diploid and triploid clams, Tapes dorsatus (Lamarck 1818): a potential new species for aquaculture

To assess the potential for hatchery production of the venerid clam, Tapes dorsatus, diploid and triploid clams were produced and ongrown until the spat reached 5–8 mm in size. Triploidy percentages at metamorphosis that ranged from 56 to 85% were induced using a 15-min exposure to 1 mg/l cytochalasin B. No differences were observed in the growth rate of sibling diploid and triploid T. dorsatus larvae. Survival of diploid clams to pediveliger stage was however higher. Post-metamorphic growth and survival of diploid and triploid clams remained the same until the clams were removed from the hatchery to outdoor nurseries. Throughout the larval and early spat phases of production, T. dorsatus growth was similar to that reported for the Manila clam, Tapes philippinarum. Based upon its ease of culture, rapid growth and marketability, T. dorsatus are thought to have considerable aquaculture potential.     

CO2 concentration change in Nannochloropsis sp. culture medium

CO₂ concentration in a marine phytoplankton, Nannochloropsis sp., culture medium was observed to consider whether or not CO₂ defficiency for growth occurs in the culture medium of a mass culture. In the first experiment, the Nannochloropsis sp. culture was continued for 4–5 days and the CO₂ concentration was observed three times a day, namely, the initial, middle and end of the light period. While, in the second experiment, the CO₂ concentration was observed every 2–3 hr for 24 hr on the second day after inoculation. In the first experiment, the total CO₂ concentration decreased gradually throughout the culture period, repeating its decrease and increase in the light and dark periods, respectively. At the end of the light period of the third day after inoculation, the CO₂ concentration decreased to concentrations which could not be determined. In the second experiment, the CO₂ concentration decreased in the light period, and increased in the dark period. The increment in the dark period was faster at heavier aeration. It was considered that some kind of carbon source should be supplied to obtain high density of Nannochloropsis sp. ealier under strong light intensity.     

Loma salmonae (Protozoa: Microspora) infections in seawater reared coho salmon Oncorhynchus kisutch

Loma salmonae (Putz et al., 1965) infections were observed in five groups of coho salmon, Oncorhynchus kisutch, reared in seawater net-pens in Washington State, U.S.A. in 1984–1986. Ultrastructural characteristics, size of spores, tissues and host infected, and geographical location identified the microsporidium as Loma salmonae. Preserved spores measured 4.4×2.3 (4–5.6×2–2.4) μm and exhibited 14–17 turns of the polar filament. Infections were evident in the gills of some fish before seawater entry, but few parasites were observed and they caused little tissue damage. Infections observed in fish after transfer to seawater were associated with significant pathological changes in the gills. A mixed inflammatory infiltrate was associated with ruptured microsporidian xenomas within the vessels and interstitium of the primary lamellae. Microsporidian spores were dispersed throughout the lesions and were often seen inside phagocytes. The parasite was also observed in the heart, spleen, kidney and pseudobranchs; however, the inflammatory lesions were common only in the heart.

Monthly examination of fish after transfer to seawater showed peak prevalences (33–65%) of gill infections during the summer. Although moribund fish were often infected with other pathogens, the high prevalence of L. salmonae infections and the severity of the lesions it caused, suggested that this parasite significantly contributed to the recurrent summer mortalities observed at this net-pen site.     

Waste excretion characteristics of Manila clams (Tapes philippinarum) under different temperature conditions

Water recirculating systems have been used in the shellfish industry for depuration and wet-storage. Knowledge of shellfish excretion characteristics is critical to recirculating system design. In this study, the excretion rate of total ammonia nitrogen (TAN), total Kjeldahl nitrogen (TKN), and 5-day biochemical oxygen demand (BOD₅) from Manila clams (Tapes philippinarum) were investigated under both laboratory and commercial conditions. The laboratory tests were conducted under temperatures ranging from 3 to 30°C. The experimental results showed that temperature was a key factor in determining the excretion rate of all the above parameters. The relationship between TAN excretion rate (R_TAN) and temperature (T) can be represented by an exponential function (R_TAN=0.57×1.25^T). For the temperature range between 3 and 20°C, the daily mean excretion rates of TAN, TKN and BOD₅ ranged between 1.5–46.1, 4.8–131.0 and 57.4–219.4 mg per kilogram of the clams (wet weight with shell on), respectively. There were linear correlations between TAN, TKN and BOD₅ production rates. The data presented in this paper can be used to estimate waste generation from a given shellfish processing operation and to size the waste treatment components for a recirculating depuration (or wet-storage) system.     

Vibrio harveyi and other bacterial pathogens in cultured summer flounder, Paralichthys dentatus

A monitoring program for Vibrio harveyi and other potential bacterial pathogens of summer flounder was conducted at two facilities in the Northeast United States. Bacterial samples were collected from larval and juvenile fish and live-feed, and identified using API 20E biochemical profiles and 16S rDNA sequencing. Histopathological examinations were conducted in order to relate histological changes with the presence of potential bacterial pathogens. V. harveyi, Vibrio ichthyoenteri, and Photobacterium damselae subsp. damselae, three known pathogens of flatfish, were isolated from diseased summer flounder. Although a high proportion of juvenile summer flounder showed microscopic signs of disease, the presence of these potential bacterial pathogens in fish was not associated with large-scale mortalities. An outbreak of flounder infectious necrotizing enteritis with 30% cumulative mortality occurred when juveniles were transported to a new facility. Isolates of V. harveyi from the disease outbreak were genetically different from the isolates from the commercial hatchery. V. harveyi isolates from both facilities were pathogenic to summer flounder by intraperitoneal injection. P. damselae subsp. damselae, V. ichthyoenteri, and Vibrio scophthalmi, also found in fish with gross lesions, were not pathogenic to juvenile summer flounder by intraperitoneal injection. Our research shows that several potential bacterial pathogens are associated with morbidity and mortality in summer flounder larvae and juveniles, especially in situations of stress. Increased knowledge about the environmental conditions that lead to disease, as well as the interactions of V. harveyi with other microbial species, could lead to the development of management strategies in summer flounder farms.     

Effect of antiprotozoal drugs on the proliferation of the bivalve parasite Perkinsus olseni

The protozoan parasite Perkinsus olseni causes severe losses among Ruditapes decussatus clams. The development of an in vitro culture of this parasite together with the use of a proliferation assay has provided the opportunity to screen for drug sensitivity of this parasite. Xenobiotics known for their antimalarial and antiprotozoal properties were tested against P. olseni. Only four of these drugs, namely cycloheximide, pyrimethamine, deferoxamine (DFO) and 2,2-bipyridyl (BIP), showed in vitro inhibitory effect on the parasite proliferation. Two in vivo experiments were designed to determine the effect of iron chelators on reducing P. olseni infection in clams. For this purpose, naturally infected clams from the Ria Formosa, Portugal, were exposed to DFO and BIP at various concentrations. In the first experiment, hemolymph samples were taken from each clam before and after treatment to determine the infection intensity and in the second experiment, clams were randomly distributed in groups of five and the parasite burden in treated and untreated groups was determined at the end of the experiment on the whole clam wet tissues. Only DFO was found to be effective in reducing in vivo P. olseni infections. In addition, acute toxicity of DFO and BIP has been determined and no mortality of Perkinsus-free clams was observed.     

Dietary impact of algal and artificial diets, fed at different feeding rations, on the growth and fatty acid composition of Tapes philippinarum (L.) spat

The effect of lipid supplementation and algal ration on growth and fatty acid composition of juvenile clams, Tapes philippinarum was investigated. A diet of Tetraselmis suecica was supplemented with a lipid emulsion and fed at a daily ration of 0.5, 1.0 and 1.5% WW DW⁻¹ day⁻¹. A mixed algal diet of Isochrysis galbana (clone T-Iso) and T. suecica (1:1 on DW basis) and starvation functioned as positive and negative control treatments, respectively. T. suecica, which contains eicosapentaenoic acid (20:5n−3) but no docosahexaenoic acid (22:6n−3), was supplemented with 50% (% of algal dry weight) of a 22:6n−3-rich emulsion. Higher growth rates of T. philippinarum were associated with higher T. suecica feeding rations. Lipid supplementation resulted in significantly better growth rates of T. philippinarum at a T. suecica ration of 0.5, 1.0 and 1.5% WW DW⁻¹ day⁻¹. The growth promoting effect of lipid supplementation was significantly reduced during 4 consecutive weeks or by increasing the algal feeding ration. Increasing T. suecica rations were associated with a decrease in the % composition of 22:6n−3 while the 20:5n−3% composition was hardly affected in the total lipids of T. philippinarum. On the contrary, the absolute concentration of 22:6n−3 (μg per gram dry weight, μg g DW⁻¹) in the total lipids of T. philippinarum was similar at all T. suecica rations while the 20:5n−3 concentration significantly increased. The major effect of lipid supplementation on the fatty acid composition of the clams, was a significant increase of the 22:6n−3% composition in the total lipids, polar lipids and triglycerides. In contrast with 20:5n−3, the effect of lipid supplementation on the absolute 22:6n−3 content (μg g DW⁻¹) was more pronounced at a higher T. suecica ration. After a starvation period of 4 weeks, the concentration (μg g DW⁻¹) of all fatty acids was significantly reduced, except the n−7 monoenoic fatty acids (MUFA) and the non-methylene interrupted dienoic fatty acids (NMID). The concentration (μg g DW⁻¹) of the NMID in starved clams and clams fed different diets was similar to the concentration in the initial sample. This indicated that growing spat was actively synthesising NMID and they were selectively retained in unfed animals. The results indicated that either dietary lipid or 22:6n−3 was a growth-limiting factor when T. suecica was fed at a ration of 0.5, 1 and 1.5% WW DW⁻¹ day⁻¹. A selective retention in starved clams and a preferential accumulation in fed clams was observed for 22:6n−3 but not for 20:5n−3 which indicated a greater importance of 22:6n−3 compared to 20:5n−3.     

The integrated culture of seaweed, abalone, fish and clams in modular intensive land-based systems: II. Performance and nitrogen partitioning within an abalone (Haliotis tuberculata) and macroalgae culture system

A pilot-scale system for the intensive land-based culture of abalone was established using an integrated design aimed at eliminating the dependence on external food sources, whilst reducing water requirements and nutrient discharge levels. The system was the first and simplest trial in a series of progressive complexity of the concept of integrated culture of seaweed, abalone, fish and clams in modular and intensive land-based facilities. Relative sizes of the modules, their stocking densities and the rate of nutrient supply were determined based on earlier results to be optimal. Effluents from two abalone (Haliotis tuberculata) culture tanks drained into macroalgae (Ulva lactuca or Gracilaria conferta) culture and biofilter tanks, where nitrogenous waste products contributed to the nutrition of the algae; net algal production from each algal tank was harvested and used to provide a mixed diet for the abalone. Excess algal yield was used elsewhere. The system was monitored to assess productivity and nitrogen partitioning over a year, while improvements were made based on the accumulating results. Total annual N-budgets were combined with mean production figures to determine a suitable ratio of abalone biomass to algal culture vessel productivity, towards commercial application of the concept. The abalone grew on average 0.26% and 0.25% body weight/d in the two culture tanks; reduced growth and increased food conversion ratios (food eaten/biomass gain; w/w) were associated with high summer water temperatures (max. 26.9°C). U. lactuca showed reliable growth and filtration performance (mean production of 230 g fresh weight/m²/d, removing on average 58% of nitrogen supplied). Conversely, G. conferta growth was highly erratic and was deemed unsuitable for the current application. It is estimated that 1 kg of abalone biomass would require food supplied by 0.3 m² of U. lactuca culture, reducing N inputs required by 20% and N in effluent by 34% when compared to the two organisms grown in monoculture.     

Recruitment and effects of Discocotyle sagittata (Monogenea) infection on farmed trout

Farmed rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta were monitored over 3 years for infection with the blood-feeding gill fluke Discocotyle sagittata. Parasite transmission is seasonal: new infections take place during summer/autumn, and transmission is generally negligible during winter/spring. There are 2 sources of infection for naïve fish-of-the-year: limited invasion when fish are in the raceways by riverborne larvae originating external to the farm; and internally, within the farm, when 0+ fish are transferred to ponds previously occupied by older cohorts of infected fish. Thereafter, infection levels continue to increase in rainbow trout primarily through transmission within the farm. Prevalence rose to 100% in 1+ fish by the end of their second summer. In O. mykiss, mean abundance reached 194 worms/host for 1+ fish (up to 489 worms/host) and 160 worms/host for 2+ fish. By contrast, in S. trutta, parasite prevalence never exceeded 85% and, after the first year's invasions, infection levels decreased over time: in 1+ and 2+ brown trout, parasite mean abundance was < 4 (maximum 15) worms/host. We present evidence of the detrimental effects of D. sagittata on the host: high burdens are associated with pale gills, decreased body condition and host mortality. Parasite burdens become overdispersed during the warmer part of the year, as prevalence and mean abundance increase. However, the degree of parasite overdispersion decreases over winter; we cannot distinguish whether decreased aggregation is due to parasite losses from infected fish (including immune-mediated parasite mortality) or parasite-induced host mortality.     

Gametogenesis of the European flat oyster (Ostrea edulis) and Pacific oyster (Crassostrea gigas) in warm water in Israel

The gametogenic cycle of Ostrea edulis and Crassostrea gigas was studied in the outflow water of fish ponds in Eilat, Israel. Despite relatively high water temperatures (14–28°C) and high salinity (41 ± 2 ppt), both species developed gonads. O. edulis spawned from March to May, when water temperature ranged from 18 to 22°C, with a dormant period during the summer months. C. gigas spawned between May and August, when water temperature ranged between 20 and 26°C, with a dormant period between August and April. Spat never developed in the fish pond system.