Microsporidian parasites in the gills of salmonid fishes*

Abstract. A microsporidian parasite was found in the gills of the brook trout, Salvelinus fontinalis (Mitchill), which was similar to Loma morhua Morrison and Sprague, a microsporidian parasite in the gills of cod. It was described and designated as Loma sp. Plistophora (=Pleistophora) salmonae Putz, Hoffman and Dunbar in gills of rainbow and steelhead trout, Salmo gairdneri Richardson, was also observed and was transferred to the genus Loma. All microsporidia known in gills of Gadidae and Salmonidae were judged to belong to the genus Loma but the data were not sufficient to provide a basis for a definitive separation of species. Provisionally, Loma morhua, L. branchialis, L. salmonae and Loma sp. were recognized.     

Reducing bycatch impacts in recreational fisheries: Case study examining terminal tackle in the multispecies Gulf of Maine groundfish fishery

Terminal tackle regulations can be a valuable tool for fisheries management, especially in multispecies fisheries where bycatch and discards are common issues. In the Gulf of Maine, recreational anglers frequently discard critically depleted Atlantic cod Gadus morhua L. as bycatch when targeting the abundant haddock Melanogrammus aeglefinus (L.) stock. The present study investigated species catch composition, catch rates and animal welfare across various terminal tackle setups, and aspects of capture and handling with the goal of reducing bycatch and discard mortality. Overall, 2558 cod and 4266 haddock were captured with six terminal tackle setups in the western Gulf of Maine from April to October 2018. Along with angler experience and capture depth, lure type primarily influenced species catch composition and catch-per-unit-effort; hook types additionally influenced hook-removal times and physical injuries to fish. Results indicate that using baited hooks can both promote haddock catch and reduce cod bycatch, with specific hook types promoting increased survival.     

Effect of acclimation temperature on the acute stress response in juvenile Atlantic cod, Gadus morhua L., and haddock, Melanogrammus aeglefinus L.

Juvenile Atlantic cod (Gadus morhua L.) and haddock (Melanogrammus aeglefinus L.) were subjected to 30 s air exposure stressors following acclimation to 4, 10 and 14 and 4, 8 and 14°C respectively. Both species responded to the stressor with increases in plasma cortisol at all temperatures tested. At 14°C cortisol levels peaked within 1 h post‐stressor, and returned to pre‐stressor levels within 24 h. In contrast, at 4°C, peak cortisol levels were not attained until 6 h post‐stressor in haddock and remained elevated beyond 24 h in both species. The rate of plasma glucose accumulation was greater at higher temperatures in both species and no increase was seen at 4°C. Lysozyme activity in cod, in response to the stressor, was lower than the values reported for some other species and increased slightly at 14°C. The results show that cod and haddock acclimated to different temperatures respond to common, acute stressors in a manner similar to other teleosts.     

Efficacy of egg surface disinfectants in captive spawning Atlantic cod Gadus morhua L. and haddock Melanogrammus aeglefinus L.

In an effort to optimize the efficiency of high‐density incubation of Atlantic cod Gadus morhua and haddock Melanogrammus aeglefinus eggs, the per cent hatch of eggs treated with four disinfectants (3% hydrogen peroxide, 1% polyvinylpyrrolidone iodine, 0.1% sodium hypochlorite and a 0.005% antibiotic solution – penicillin/streptomycin) was compared in both species. The per cent hatch of eggs of each species was greatest after a 24 h treatment with the antibiotic solution. The hatching success of eggs treated within the different disinfectant treatments depended upon the embryonic developmental stage in both species. The sodium hypochlorite treatment had the lowest % coverage of colony growth after disinfected haddock eggs were plated onto sterile agar media, highest survival rates to the end of the embryonic period, but the lowest per cent hatch.     

Recruitment variability and oceanographic stability

We examine the hypothesis that recruitment is more variable in populations on isolated offshore banks than nearby shelf populations. Recruitment of cod (Gadus morhua) and American plaice (Hippoghssoides plates-soides) on Flemish Cap is more variable than in any comparable population. Recruitment of haddock (Melanogrammus aeglefinus) on Rockall Bank is also more variable than in surrounding populations. These results are confirmed both by estimates obtained by virtual population analysis and by research surveys. Recruitment of haddock and herring (Clupea harengus) on Georges Bank is also more variable than in surrounding populations; however, the results for two other groundfish populations, cod and yellowtake flounder (Limanda ferruginae), on Georges Bank are ambiguous. We conclude that marine fish populations on isolated banks are more variable than those on nearby shelf regions.     

Optimum temperature and food-limited growth of larval Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) on Georges Bank

We estimated recent growth of Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) larvae collected on the southern flank of Georges Bank in May 1992–94 from the ratio of RNA to DNA (R/D) and water temperature. Growth of both species increased with water temperature to about 7°C and then decreased. The highest growth rates were observed in May 1993 at water temperatures around 7°C. These data confirm an earlier observation of comparable temperature optima for growth of Atlantic cod and haddock larvae in the north‐west Atlantic. Comparisons of field growth rates and temperature optima with data for larvae cultured at high temperatures and prey densities in the laboratory suggest that growth may have been food‐limited at higher temperatures on Georges Bank. Given that 7°C is the long‐term mean water temperature on the southern flank in May and that climate models predict a possible 2–4°C rise in water temperatures for the western North Atlantic, our findings point to a possible adverse effect of global warming on Atlantic cod and haddock.     

The growth of larval cod and haddock in the Irish Sea: a model with temperature,prey size and turbulence forcing

We applied a physiological individual‐based model for the foraging and growth of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) larvae, using observed temperature and prey fields data from the Irish Sea, collected during the 2006 spawning season. We used the model to estimate larval growth and survival and explore the different productivities of the cod and haddock stocks encountered in the Irish Sea. The larvae of both species showed similar responses to changes in environmental conditions (temperature, wind, prey availability, daylight hours) and better survival was predicted in the western Irish Sea, covering the spawning ground for haddock and about half of that for cod. Larval growth was predicted to be mostly prey‐limited, but exploration of stock recruitment data suggests that other factors are important to ensure successful recruitment. We suggest that the presence of a cyclonic gyre in the western Irish Sea, influencing the retention and/or dispersal of larvae from their spawning grounds, and the increasing abundance of clupeids adding predatory pressure on the eggs and larvae; both may play a key role. These two processes deserve more attention if we want to understand the mechanisms behind the recruitment of cod and haddock in the Irish Sea. For the ecosystem‐based management approach, there is a need to achieve a greater understanding of the interactions between species on the scale a fish stock is managed, and to work toward integrated fisheries management in particular when considering the effects of advection from spawning grounds and prey–predator reversal on the recovery of depleted stocks.     

Atypical Aeromonas salmonicida infection in naturally and experimentally infected cod, Gadus morhua L.

Cod, Gadus morhua L., of wild origin, were reared at different temperatures for 12 months. During this period, moribund and newly dead fish were examined and samples collected for bacteriology and histopathology. Atypical Aeromonas salmonicida was isolated from 10 individuals reared at or above 7 °C. The isolates were homogeneous with respect to biochemical and antibiogram characters and similar to the ssp. achromogenes National Collection of Industrial and Marine Bacteria, UK, type strain 1110 and reference strains that have been isolated from salmonids and haddock in Iceland. Histopathological analysis of the naturally infected cod showed typical ulceration associated with atypical A. salmonicida infection and also widespread granulomatous formations. One‐year‐old cod of farmed origin, kept at 9 °C, received intraperitoneal or intramuscular injection with different doses of atypical A. salmonicida, isolated from the above wild cod. Mortalities were monitored for 28 days and the LD₅₀ calculated. The route of bacterial injection influenced the mortality rate and LD₅₀ value and affected, to some extent, the pathological changes observed and humoral immune parameters. Pathological changes, including haemorrhage, early stages of granuloma formation and necrotic changes, were seen in several organs. Infection appeared to induce non‐specific antibody activity against trinitrophenyl (TNP)‐haptenated protein and may have activated the complement system. Specific antibody response against atypical A. salmonicida was not detected.     

Effect of trawling with traditional and ‘T90’ trawl codends on fish size and on different quality parameters of cod <Emphasis Type="Italic">Gadus morhua</Emphasis> and haddock <Emphasis Type="Italic">Melanogrammus aeglefinus</Emphasis>

The effect of trawling on fish size and on different quality parameters of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) was evaluated after conducting 16 valid hauls using two trawls in a double rig fitted with a traditional and a novel ‘T90’ codend, respectively. The total catch volume during the fishing period was 47.6 metric tons, with an average catch per codend of 1.5 (range 0.5–2.9) tons. The mean haul duration was 5 h. The catch was assessed according to fish size, mortality, external damage, initial white muscle pH and development of rigor mortis. Fillet quality (colour, blood spots, gaping) was assessed after 1 week of freeze-storage. Our results showed there was no difference between the two types of nets in terms of catch volume, but significantly slightly bigger fish were caught with T90 than with the traditional trawl net (p < 0.05). Haddock caught with the traditional trawl net had more external injuries related to the trawl gear than haddock caught with the T90 gear (p < 0.05). The gaping frequency for cod caught with the traditional trawl net tended to be higher than cod caught with the T90 gear, but the difference was not significant (p = 0.07). No other differences in fish quality between fish caught in the trawl nets were observed.     

Using commercial landings data to identify environmental correlates with distributions of fish stocks

We examined the efficacy of using commercial landings data to identify potential environmental correlates with fish distributions. Historical landings data for two commercially important species, Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus), were used along with historical conductivity, temperature, and depth (CTD) data to infer monthly mean spatial distributions of catch per unit effort (CPUE), temperature, salinity, density, and stratification over Georges Bank. Relationships between CPUE and these environmental variables plus bottom sediment type and bottom depth were examined on seasonal, annual, and interannual time scales. Empirical analysis suggests that both cod and haddock are found preferentially in water temperatures of approximately 5°C in winter/spring, and as high as 10–11°C during late fall. Both species are also found preferentially over coarse sand and gravel as opposed to fine sand, and in water depths between 60 and 70 m. These preferences appear to vary seasonally. The above results are consistent with findings of previous investigators using semi‐annual research trawl survey data, and suggest that commercial landings data, despite their known errors and biases, can be used effectively to infer associations between fish and their environment.     

Growth, product quality and immune status of Atlantic salmon, Salmo salar L., fed wet feed with alginate

By-products, by-catch and offal from the fishing industry represent valuable resources of high-quality protein and energy. In order to utilize some of these resources, a method for preparing wet feed for Atlantic salmon, Salmo salar L., using a binder produced from seaweed, Ascophyllum nodosum L., was developed. The feed was formulated with offal from lean fish (haddock, Melanogrammus aegleflnus L., saithe, Pollachius virens L., pollack, Pollachius pollachius L., and cod, Gadus morhua L.) and fatty fish [mackerel, Scomber scombrus L., and argentine, Argentina silus (Ascanius)], fish meal extruded wheat, capelin oil, vitamin and mineral premix, astaxanthin, and seaweed binder. The feed was gellified in a solution of calcium chloride. Equipment for the production of such feed was developed and the feed tested on a large scale at a commercial salmon farm. As control, a high-fat, extruded, commercial dry feed was used. On a dry matter basis, the two feeds were comparable with respect to protein, fat and carbohydrate. Growth rates, appetite and feed conversion were similar for the two experimental groups. Fish fed dry feed seemed to be more active when handled before slaughter and had a higher incidence of skin damage than fish given wet feed. Chemical and sensory analyses of the fish showed no difference (P > 0.05) between the two groups. Fifty fish from each group were analysed for haemoglobin, protein, lysozyme, complement and haemolytic activity in the blood. There was a significantly higher level of lysozyme in the wet feed group than in the dry feed group, implying an immunostimulating effect of alginate. It was clearly shown that producers have an excellent opportunity to improve the utilization of the resources and decrease feed production costs in places where the natural conditions are favourable, i.e. locations where there is an integrated production of fish farming and processing of wild catch.     

Stocking location and predation by marine fishes affect survival of hatchery‐reared Atlantic salmon smolts

Abstract Release strategies of hatchery‐reared Atlantic salmon, Salmo salar L., smolts were compared by studying survival and migration of smolts (n = 99) and their predators (Atlantic cod, Gadus morhua L., n = 8; and saithe, Pollachius virens (L.), n = 2) during the first 37 km of the marine migration using acoustic telemetry. Survivorship was higher in smolts released at the river mouth (30%) compared with smolts released in the river (12%). This was likely due to mortality or reduced migratory behaviour in fresh water. The marine mortality was 37% during the first 2 km after leaving the river (at least 25% mortality because of predation from marine fishes), and total marine mortality over 37 km was 68%. Detection‐depth data were useful for evaluating whether the tagged smolts were alive or predated; mortality during the first 2 km of outward migration would have been underestimated at 26% instead of 37% without the analysis of depth detection. Transmitters from consumed post‐smolts remained in predators for up to 47 days (average 29 days).     

Growth,Survival, and Whole‐body Proximate and Fatty Acid Composition of Haddock,Melanogrammus aeglefinus L., Postlarvae Fed a Practical Microparticulate Weaning Diet

Further development of high‐quality feeds for hatchery‐reared haddock in the North Atlantic would benefit from a standard formulation that can be used as a reference for hatcheries and laboratory studies. A practical microparticulate diet (PMD) developed and evaluated with newly metamorphosed juvenile haddock, Melanogrammus aeglefinus L., postlarvae is proposed. Survival of fish fed PMD was just as high (88–89%; P > 0.05) as those fed a high‐quality imported feed (Biokyowa). Alternatively, fish fed PMD had higher (P < 0.05) final fork lengths (39.5 vs. 35.1 mm), wet weights (851.3 vs. 580.2 mg) and weight gains (1637.2 vs. 1115.7%). No differences (P > 0.05) in whole‐body moisture (846–857 g/kg), ash (17–18 g/kg), or protein (101 g/kg) contents were found. Lipid content of fish fed PMD (26 g/kg) was higher (P < 0.05) than those fed Biokyowa (21 g/kg) despite PMD containing 15 g/kg lower dietary lipid, suggesting higher intake and/or lipid retention. The PMD formulation proved to be a highly suitable weaning diet for haddock postlarvae based on high feed acceptance, survival, and fish growth. Given the economic and logistical difficulties associated with importing commercial weaning feeds, this easily produced practical weaning diet has good potential for use by laboratory researchers and farm managers for hatchery‐based nutrition research with haddock postlarvae.     

Development of a continuous cell line from larval Atlantic cod (Gadus morhua) and its use in the study of the microsporidian,Loma morhua

In vitro cell culture methods are crucial for the isolation, purification and mass propagation of intracellular pathogens of aquatic organisms. Cell culture infection models can yield insights into infection mechanisms, aid in developing methods for disease mitigation and prevention, and inform commercial‐scale cultivation approaches. This study details the establishment of a larval cell line (GML‐5) from the Atlantic cod (Gadus morhua) and its use in the study of microsporidia. GML‐5 has survived over 100 passages in 8 years of culture. The line remains active and viable between 8 and 21°C in Leibovitz‐15 (L‐15) media with 10% foetal bovine serum and exhibits a myofibroblast phenotype as indicated by immuno‐positive results for vimentin, α‐smooth muscle actin, collagen I and S‐100 proteins, while being desmin‐negative. GML‐5 supports the infection and development of two microsporidian parasites, an opportunistic generalist (Anncaliia algerae) and cod‐specific Loma morhua. Using GML‐5, spore germination and proliferation of L. morhua was found to require exposure to basic pH and cool incubation temperatures (8°C), in contrast to A. algerae, which required no cultural modifications. Loma morhua‐associated xenoma‐like structures were observed 2 weeks postexposure. This in vitro infection model may serve as a valuable tool for cod parasitology and aquaculture research.     

Selectivity and escape percentages during three phases of the towing process for codends fitted with different selection systems

We investigated the selectivity of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) escaping from three different selection systems (a conventional diamond-mesh codend, a codend fitted with escape windows, and a codend fitted with a sorting grid) using the covered codend technique. By using a MultiSampler attached to the codend cover to partition the collection of escaping fish, we were able to determine the partitioned selectivity of each of these devices at different phases during the towing process: at depth, during the haul back, and at the surface.The percentages of escaping fish at depth from the diamond-mesh codend were significantly lower than those estimated for the escape windows and sorting grid codends: almost 62% of cod and 79% of haddock escaping from the diamond-mesh codend did so at depth, whereas approximately 75% of cod and 93% of haddock escaped from the other two systems at depth. The partitioned selectivity parameters, l₅₀ and SR, were estimated and modelled for each of the three phases of the towing process and used to compare the selective properties of the three selection systems. A comparison was also made with a traditional whole haul selectivity analysis. Finally, bulk catch was added to the model as an explanatory variable and its effect upon selectivity was measured in both, partitioned and whole haul estimates.     

Encapsulation of Bolbophorus damnificus (Digenea: Bolbophoridae) Metacercariae in Juvenile Channel Catfish,Ictalurus punctatus,Is Linked to Delayed‐onset Mortality

Bolbophorus damnificus (Digenea: Bolbophoridae) causes significant economic losses in catfish aquaculture in the southeastern USA. To determine mortality trends associated with B. damnificus exposure, three separate disease challenges were conducted. The first challenge exposed channel catfish, Ictalurus punctatus, fingerlings (7–13 cm) to approximately 285 B. damnificus cercariae/L, while the second exposed fish to approximately 215 cercaria/L. Fish were monitored for morbidity and mortality for 28 d. In both studies, mortality first occurred 8 d postchallenge (PC) and peaked at 9 and 10 d PC, respectively. Mortality had subsided by 14 d PC, with only sporadic mortality occurring >15 d PC. Dead and moribund fish presented with lesions consistent with B. damnificus infection. A third challenge was performed to record histological changes associated with parasite development from initial exposure up to the cessation of mortality. Catfish (2–5 cm) were exposed as described above to a range covering 0, 35, 70, 100, 150, 175, and 215 cercariae/L. Each day, two fish were sampled from each treatment. Mortality trends were consistent with previous challenges. The mortality window coincided with parasite encapsulation, neovascularization, and development of the host‐derived cyst, suggesting that morbidity and mortality are linked to host pathophysiological changes in response to metacercariae encapsulation.     

Assessment of sex and evaluation of ovarian maturation of fish using ultrasonography

Ultrasonography is a non‐invasive method that can be used to assess sex and maturity of fish. This study was conducted to examine the feasibility of using ultrasound images to sex juvenile and mature halibut Hippoglossus hippoglossus (Linnaeus), mature winter Pleuronectes americanus (Walbaum) and yellowtail flounder Pleuronectes ferruginea (Storer) and mature haddock Melanogrammus aeglefinus (Linnaeus). In females, both immature and mature ovaries were relatively easy to distinguish, but in males only mature testes were consistently distinguishable. Ovarian maturation of haddock and ovulatory cycles of halibut may also be assessed using ultrasound.     

Enhancement of image-based fecundity methods: Gravimetric sampling at sea and safer sample preservation

Collecting accurate fecundity samples in the field is challenging as field scales are often unreliable and inaccurate in rough seas. In addition, the storage and use of caustic preservatives may be difficult and dangerous. Quantifying the efficacy of both a non-caustic alternative to traditional preservatives, and the use of pre-weighed vials in sampling protocols is essential in improving the accuracy and utility of field samples. Streck Tissue Fixative (STF) is a less toxic alternative to traditional preservatives and proved effective in the short- and long-term preservation of haddock (Melogrammus aeglefinus) and cod oocytes (Gadus morhua). We found no time series trend or systematic change in the number of oocytes counted and there was stabilization in size decay by the second month, which is comparable to traditionally used preservatives. In examining the use of pre-weighed vials we compared several different vial types. Plastic vials with an o-ring showed a satisfactory predictable loss and we were able to develop a system of time dependent corrections for evaporative rate. Thermal effect was also examined, with evaporative loss decreasing at low temperatures. The effectual use of an alternative preservative and the validation of the use of pre-weighed vials have the potential to increase the efficiency and accuracy of field sampling.     

Light and electron microscopic studies on Epieimeria anguillae (Léger & Hollande, 1922), a coccidium parasitizing the European eel, Anguilla anguilla L.

Abstract. The foregut of eels naturally infected by Epieimeria anguillae (Léger & Hollande, 1922) was studied by light and electron microscopy. It has been established that this parasite, which develops in a characteristic location on the surface of epithelial cells, and was classified on this basis by Dyková & Lom (1981) as a member of the genus Epieimeria , undergoes intracellular merogony and gamogony similarly to other eimerians; however, its sporogony takes place outside the fish or intercellularly. The trophozoites and merogonic and gamogonic stages each develop in a para-sitophorous vacuole which is half embedded in the epithelial cell and protrudes into the intestinal lumen. The parasitophorous vacuote is surrounded by a single membrane; however, towards the intestinal lumen it is covered also by the cell membrane. In its location, Epieimeria anguillae resembles cryptosporidia, but differs from the latter significantly in its relationship with the host cell.     

Isolation and partial characterization of a new reovirus in the Chinese mitten crab, Eriocheir sinensis H Milne Edwards

A second type of freshwater crab reovirus has been isolated from Chinese mitten crab, Eriocheir sinensis H. Milne Edwards, in China; we named it E. sinensis reovirus (EsRV816). The negatively stained virion is a non‐enveloped icosahedral particle, 60 ± 5 nm in diameter. Its genome is composed of 10 dsRNA linear pieces exhibiting an electrophoretic pattern of 5/3/2. The largest segment (RNA‐1) was cloned and sequenced. The deduced amino acid sequence, corresponding to the RdRp of the virus, showed 26% identity with the RdRp of Operophtera brumata (L.) cypovirus 19 in the genus Cypovirus and 24% identity with RdRp of Nilaparvata lugens (Stal) reovirus in the genus Fijivirus. On the basis of its ultrastructure and physicochemical properties, this virus is quite different from other crab reoviruses, and particularly with another freshwater crab reovirus EsRV905, recently classified in a new genus Cardoreovirus. This virus (EsRV816) possesses all the characters of the members of the reoviridae family and could represent a new genus.