Susceptibility of juvenile Atlantic cod Gadus morhua to viral haemorrhagic septicaemia virus isolated from wild-caught Atlantic cod

A European strain of viral haemorrhagic septicaemia virus (VHSV) isolated from wild-caught cod Gadus morhua (H16/7/95) was shown to cause clinical disease and mortality in excess of 80% in juvenile Atlantic cod when administered by the intra-peritoneal (i.p.) route. No virus was recovered from cod cohabiting with experimentally infected fish at a ratio of 1:1, and no VHSV-associated mortality was demonstrated following immersion infection. External signs of disease in cod were the presence of exophthalmia and ascites. Virus was identified as VHSV by enzyme-linked immunosorbent assay (ELISA) and was recovered from both brain and organ pools (kidney, liver and spleen) of 100% of i.p. infected cod mortalities. Virus was also detected using an indirect immunofluorescence test on tissue imprints of kidney, liver, spleen and brain taken from moribund fish. The fact that cod were not susceptible to VHSV following waterborne exposure raises important questions surrounding the propagation, maintenance and impact of a naturally occurring reservoir of virus in the marine environment.     

Effects of Body Size and Food Ration on Over-winter Survival and Growth of Age-0 Atlantic cod, Gadus morhua

This study examined the effects of body size and food ration on over-winter survival of age-0 Atlantic cod. Cod were divided into two groups based on standard length (large=76.95±1.10 mm SL; small=57.65±1.02 mm SL; mean±1 SE) and wet weight (large=4.02± 0.21 g; small=1.52±0.09 g). Replicate tanks (n=2) of 10 large and 10 small cod were exposed to one of two food rations (0.25% and 1.0% body weight day^-1) for the entire experiment (December to June). Tanks were examined daily for mortalities and feeding was adjusted accordingly. The experiment was run under ambient light and seawater conditions. All but large age-0 cod exposed to the low food ration grew over the course of the experiment. The specific growth rate (SGR) of small cod was significantly higher (0.2425% wet weight day^-1) than that of the large fish (0.0443% wet weight day^-1). Food ration had no significant influence on SGR or over-winter survival. Significantly more of the large age-0 cod survived the winter (58.5% of those originally introduced) compared to the smaller fish (14%). Our results are consistent with those from studies of several other fish species, and are discussed in relation to the ecology of age-0 cod, and their potential use in aquaculture in Newfoundland.     

Francisella noatunensis subsp. noatunensis is the aetiological agent of visceral granulomatosis in wild Atlantic cod Gadus morhua

During the 1980s and 1990s wild-caught cod displaying visceral granulomatosis were sporadically identified from the southern North Sea. Presumptive diagnoses at the time included mycobacterial infection, although mycobacteria were never cultivated or observed histologically from these fish. Farmed cod in Norway displaying gross pathology similar to that identified previously in cod from the southern North Sea were recently discovered to be infected with the bacterium Francisella noatunensis subsp, noatunensis. Archived formalin-fixed paraffin-embedded tissues from the original North Sea cases were investigated for the presence of Mycobacterium spp. and Francisella spp. using real-time polymerase chain reaction, DNA sequencing and immunohistochemistry. Whilst no evidence of mycobacterial infection was found, F. noatunensis subsp. noatunensis was identified in association with pathological changes consistent with Francisella infections described from farmed cod in recent years. This study shows that francisellosis occurred in wild-caught cod in the southern North Sea in the 1980s and 1990s and demonstrates that this disease predates intensive aquaculture of cod.     

Intracellular localisation and innate immune responses following Francisella noatunensis infection of Atlantic cod (Gadus morhua) macrophages

The facultative intracellular bacterium Francisella noatunensis causes francisellosis in Atlantic cod (Gadus morhua), but little is known about its survival strategies or how these bacteria evade the host immune response. In this study we show intracellular localisation of F. noatunensis in cod macrophages using indirect immunofluorescence techniques and green fluorescent labelled bacteria. Transmission electron microscopy revealed that F. noatunensis was enclosed by a phagosomal membrane during the initial phase of infection. Bacteria were at a later stage of the infection found in large electron-lucent zones, apparently surrounded by a partially intact or disintegrated membrane. Immune electron microscopy demonstrated the release of bacterial derived vesicles from intracellular F. noatunensis, an event suspected of promoting phagosomal membrane degradation and allowing escape of the bacteria to cytoplasm. Studies of macrophages infected with F. noatunensis demonstrated a weak activation of the inflammatory response genes as measured by increased expression of the Interleukin (IL)-1β and IL-8. In comparison, a stronger induction of gene expression was found for the anti-inflammatory IL-10 indicating that the bacterium exhibits a role in down-regulating the inflammatory response. Expression of the p40 subunit of IL-12/IL-17 genes was highly induced during infection suggesting that F. noatunensis promotes T cell polarisation. The host macrophage responses studied here showed low ability to distinguish between live and inactivated bacteria, although other types of responses could be of importance for such discriminations. The immunoreactivity of F. noatunensis lipopolysaccharide (LPS) was very modest, in contrast to the strong capacity of Escherichia coli LPS to induce inflammatory responsive genes. These results suggest that F. noatunensis virulence mechanisms cover many strategies for intracellular survival in cod macrophages.     

Prevalence and influence of Loma branchialis (Microspora) on growth and mortality in Atlantic cod (Gadus morhua) in coastal Newfoundland

A study was conducted to determine the prevalence, geographical distribution, and effects of a microsporan parasite, Loma branchialis, in Atlantic cod in coastal Newfoundland. The parasite was identified microscopically as opaque, ovoid xenomas, primarily in the gills, heart, and spleen. Although widely distributed in young fish inhabiting 7 coastal localities, prevalence was 1-7% in general, <0.01% in older and large fish, but 78% in market-size cod from a commercial farm. Some of the latter fish were emaciated, and approximately 2 mo after the parasite was detected, 63% succumbed with a massive infection. Condition factor and blood values were significantly lower than those in reference fish. An experimental study conducted over 2 yr revealed that mass gained and condition factor were significantly lower in infected cod than in controls at the termination of the study. Feed consumption, mass gained, feed conversion efficiency, and condition factor also were significantly lower in another group of infected cod of marketable size studied over 12 wk. Xenomas also were observed in the heart and gills of year class II cod reared in a hatchery. Loma branchialis is considered to be a potential pathogen in both hatchery-reared and net pen-cultured cod in Newfoundland.     

The use of oxytetracycline as a marker for juvenile cod (Gadus morhua L.)

We examine aspects of marking large numbers of juvenile cod ( Gadus morhua L.) by oxytetracycline (OTC) for cod enhancement experiments. Reared cod were given OTC through the feed and released in a fjord. Control groups were kept in net pens. A total of 93% of the cod became marked. The size of the cod and the percentage marked were inversely related. The location of the mark within each vertebra gradually changed towards the centre as the cod grew and new bone tissue was added. The percentage of marked cod in the control group did not decrease during a period of almost 3 years after marking, although the intensity of the mark gradually declined. After release, we found no indication that the OTC affected growth or survival of the cod. Cod marked as O-group could be distinguished from cod marked as 2-group, from the position of the mark within the vertebra.     

Lernaeocera branchialis: a potential pathogen to cod ranching

A study was carried out to determine the effect of Lernaeocera branchialis on Atlantic cod infected in the laboratory and in the field and also to ascertain its effect on cod-ranching. Sixty-four percent (308) of 481 cod acquired infections in the laboratory and 33% (159) of the infected fish died over a 4-yr period. About 74% of the deaths occurred within 4 mo of the infection. Monthly samples of cod collected adjacent to a cod-ranching operation showed an initial prevalence of 30% that subsequently decreased in the following 2 mo to 15%. Prevalence of the infection also decreased among the initial field sample of cod that were kept alive, from 30 to 17% during the same 2-mo period and to 9% after 8 mo and was associated with death caused by the parasite. Cod examined at intervals after infection showed evidence of reduced weight gain, lower liver somatic index, liver lipid, and blood values than controls. A field sample taken from the same area during the summer of the following year indicated a prevalence of 12%. This higher than usual prevalence (4-6%) was associated with retention of the intermediate host, Cyclopterus lumpus, that provided an additional source of infective stages. It is likely that increased parasitism could affect the success of the cod-ranching operation in view of the parasite's devastating effects on its hosts.     

Impacts of egg and larval size on survival and growth of Atlantic cod under different feeding conditions

Large eggs (1·38 mm) of Atlantic cod reared in the laboratory produced large larvae. However, large larvae had low survival rates. Results also indicated that the first few days' of growth of cod larvae mainly resulted in an increase in mass. Exogenous feeding tended to result in faster growth than endogenous feeding. In the delayed feeding groups (larvae not fed until 67 degree-days), larvae from large eggs grew faster than those from small eggs (1·28 mm) after feeding commenced, while there was no significant difference in growth rate within feeding groups. Compensatory growth was detected in the delayed feeding groups.     

Transmission of Cryptocotyle lingua cercariae in natural environments: a field experiment

Atlantic cod were kept at two different depths in three cages placed at increasing distance from a rocky shore where natural infections of the digenean Cryptocotyle lingua occurred in the snail Littorina littorea . During a 6-month period the fish kept at 0–2 m accumulated significantly more parasites than those kept at 2–4 m. Distance from the shore did not affect infection rates. We conclude that host location behaviour of cercariae influences the infection pattern under natural conditions, and that transmission from snail to fish can take place over large areas of coastal waters.     

An impact of seasonal squid migrations and fishing on the feeding spectra of subantarctic notothenioids Patagonotothen ramsayi and Cottoperca gobio around the Falkland Islands

Seasonal changes in the diet of two abundant notothenioid fishes, rock cod Patagonotothen ramsayi (Nototheniidae) and frogmouth Cottoperca gobio (Bovichthidae) were studied on the Falkland Island shelf. The rock cod is a near-bottom browser, feeding mostly on gelatinous and crustacean plankton. The diet of juveniles [10–16 cm total length (TL)] includes mostly copepods in summer and comb jellies Mnemiopsis leydyi in winter. Adult rock cod (17–34 cm TL) in summer consumed mainly plankton, with comb jellies and salps being of primary importance. In winter they mainly took benthos, primarily ophiuroids and lobster krill ( Munida spp.), and the comb jelly. During the fishing season, rock cod also scavenge on fishing discards. The frogmouth is a benthic ambush predator; both small (11–30 cm) and large (31–80 cm) representatives feed on the rock cod throughout the year. During the seasonal offshore migration of the squid Loligo gahi (June–August), this prey predominated in the large frogmouth diet, with crab, Peltarion spinosulum , of secondary importance. The region is seasonally exploited by the squid trawl fishery. As the fishing season commences (August–September), discarded rock cod also becomes an important food item for the frogmouth. Seasonal offshore migrations of L. gahi into the common depth range of both notothenioids have a significant direct (change in the C. gobio diet) and indirect (scavenging on fishing discards and targeting bottom scavengers by P. ramsayi and C. gobio during the squid fishing) impact on their diets over the Falkland shelf. The niche breadth of rock cod varied from 2.63 to 3.21 and that of frogmouth varied from 2 to 6.29, depending on fish size and season.     

Transport of North Sea cod larvae into the Skagerrak coastal populations

The Atlantic cod (Gadus morhua) is economically one of the world's most important marine species--a species presently suffering from heavy overexploitation throughout its range of distribution. Although not fully understood, the Atlantic cod is believed to be structured into populations in a rather complex manner, whereby both highly migratory and more confined ocean-spawning stocks coexist with stationary coastal populations. Owing to the complex population structure, little is presently known about how overexploitation of offshore stocks may affect other segments of the species. Here, we use microsatellite DNA analyses of coastal and offshore cod in combination with oceanographic modelling to investigate the population structure of Atlantic cod in the North Sea-Skagerrak area and evaluate the potential for larval transport into coastal populations. Our results suggest an extensive but temporally variable drift of offshore cod larvae into coastal populations. In a year (2001) with high inflow of North Sea waters into the Skagerrak we find that juvenile cod caught along the Skagerrak coast are predominantly of North Sea origin, whereas in a year (2000) with low inflow juveniles appear to be of local origin. These findings indicate that offshore cod may influence coastal cod populations over large distances.     

Long-term studies on genetic interaction between wild and ranched cod Gadus morhua by use of a genetic marked strain

Releases of farmed fish, whether accidental from commercial aquaculture facilities or intentional as part of stock enhancement/ranching activities, are considered to pose a risk to native gene pools. Stock enhancement studies of Atlantic cod, Gadus morhua , based on artificially produced juveniles, were initiated in 1984 in western Norway, and genetic aspects were incorporated. In order to investigate potential interbreeding between released and wild cod, a genetically marked cod strain was developed, being homozygotic for a rare allele ( GPI‐1*30 ) expressed in white muscle tissue. In the period from 1990 to 1994, juveniles from the genetic marked strain were released in large quantities in three locations (Masfjord, Øygarden, Heimarkspollen), giving a significant increase of the marker allele in the local wild cod populations. Recently, studies have been conducted in the same areas to estimate the extent of interbreeding between the wild and released cod. The results, however, revealed no permanent increase of the frequency of the marker allele and/or GPI‐1*30 heterozygotes as would be expected from interbreeding. The recent data are compared with comprehensive genetic data of the cod populations in the areas before the actual releases, covering the full period from 1994 to 2003. The present results are also discussed in relation to fishing pressure on coastal cod, migration information and reproductive success of released, genetically marked cod.     

Cod, Gadus morhua L., populations identified by mitochondrial DNA

Isozyme and haemoglobin analysis has shown that the cod, Gadus morhua L., in the NE Atlantic can be regarded as two populations: Arctic and coastal cod. One hundred and one individual cod from nine different locations were sampled and restriction fragment analysis uncovered 14 different mtDNA clones. Calculation of sequence divergence between localities displayed large divergence between samples from coastal and Arctic areas (l.77–5.62%), as opposed to the low intrapopulation divergence (∼0.10%) and the divergence between localities along the coast (∼0.17%) and in the Barents Sea (∼ 1.00%).     

TaqMan DNA technology confirms likely overestimation of cod (Gadus morhua L.) egg abundance in the Irish Sea: implications for the assessment of the cod stock and mapping of spawning areas using egg-based methods

Recent substantial declines in northeastern Atlantic cod stocks necessitate improved biological knowledge and the development of techniques to complement standard stock assessment methods (which largely depend on accurate commercial catch data). In 2003, an ichthyoplankton survey was undertaken in the Irish Sea and subsamples of 'cod-like' eggs were analysed using a TaqMan multiplex, PCR (polymerase chain reaction) assay (with specific probes for cod, haddock and whiting). The TaqMan method was readily applied to the large number of samples (n = 2770) generated during the survey and when combined with a manual DNA extraction protocol had a low failure rate of 6%. Of the early stage 'cod-like' eggs (1.2-1.75 mm diameter) positively identified: 34% were cod, 8% haddock and 58% whiting. As previous stock estimates based on egg surveys for Irish Sea cod assumed that the majority of 'cod-like' eggs were from cod, the TaqMan results confirm that there was probably substantial contamination by eggs of whiting and haddock that would have inflated estimates of the stock biomass.     

A field and experimental evaluation of the effect of data storage tags on the growth of cod

The effect of electronic data storage tags (DSTs) on the growth of cod Gadus morhua was evaluated in laboratory and field experiments. In the first laboratory experiment, large DSTs (60 × 18 mm, 3 g in water ) attached externally for 3 months did not have any effect ( anova , P  > 0·05) on the growth of adult cod (mean ±  s . d . 65 ± 4·5 cm total length) relative to untagged adult cod. In a second experiment, small DSTs (34 × 11 mm, 1·5 g in water) implanted into young cod (48·1 ± 4·4 cm) for an 8 month period did not have any effect upon the growth relative to untagged controls ( anova , P  > 0·05). Length data returned from tagging experiments conducted on adult cod (57·3 ± 7·5 cm) in the North Sea showed that the growth of fish tagged either externally or internally with large DSTs was not different ( t ‐test, P  > 0·05). Attachment wounds, however, provided evidence that external attachment of DSTs should be avoided unless sensor configuration requires access to the external environment, and that internal implantation should be preferred whenever possible.     

Diet of Atlantic cod in the southern Gulf of St Lawrence as an index of ecosystem change, 1959–2000

Major temporal changes in the importance of euphausiids and Atlantic herring Clupea harengus in the diet of Atlantic cod Gadus morhua , 10–75 cm L _F, occurred in shallow waters (<100 m depths) of the southern Gulf of St Lawrence between 1959 and 2000. Euphausiids represented 6–70% of prey mass for Atlantic cod 31–60 cm L _F from 1959 to 1987 but only trace amounts were detected in stomachs collected from 1990 to 2000. Mysids and gammarid amphipods (for Atlantic cod ≤45 cm L _F), and Atlantic herring (for Atlantic cod >45 cm L _F) largely replaced euphausiids in the Atlantic cod diets from 1990 to 2000. This diet change suggested there has been a major perturbation of the food web of the southern Gulf of St Lawrence. The importance of fishes (mostly Atlantic herring) in the diet of Atlantic cod >45 cm L _F increased significantly between the periods 1959–1980 and 1987–2000. Atlantic herring comprised 0–4% (mean 1.3%) of prey mass of Atlantic cod 46–60 cm L _F from 1959 to 1980 and increased to 2–42% (mean 19.6%) of the diet from 1987 to 2000. Atlantic herring comprised 0–25% (mean 9.4%) of the prey mass of Atlantic cod 61–75 cm L _F from 1959 to 1980 and increased to 42–81% (mean 54.2%) of prey mass from 1987 to 2000. This increased consumption of Atlantic herring was consistent with observed changes in abundance of Atlantic herring in the ecosystem between the late 1970s and 2000. The large changes in consumption of euphausiids and Atlantic herring represent highly significant changes and would need to be included in the development and interpretation of ecosystem-based management models for this ecosystem.     

Francisella noatunensis in Atlantic cod (Gadus morhua L.); waterborne transmission and immune responses

This is the first report that confirms waterborne transmission of francisellosis in Atlantic cod. To investigate the transmission of disease, particle reduced water was transferred from a tank with intraperitoneally infected cod to a tank with healthy cod. Waterborne transmission of Francisella noatunensis was confirmed in the effluent group using immunohistochemistry and real-time quantitative PCR (RT-qPCR). The bacteria were located inside the accumulated macrophage-like cells. Specific and high antibody responses against live and inactivated bacteria were observed. Oil adjuvant had no effect on the antibody responses against inactivated F.?noatunensis compared to saline formulation. The antigen epitope was a 20-25?kDa component of F.?noatunensis suggested to be lipopolysaccharide detected by Western blot, Sypro Ruby and Silver staining. Systemic immune reactions were investigated by measuring the expression of IFN-γ, IL-1β and IL-10 genes with RT-qPCR. After i.p. injection of live bacteria, a significant up-regulation of IFN-γ and IL-1β expression was observed from 15 to 60 days post infection in spleen and head kidney. In intestine, IFN-γ was significantly up-regulated after 30 days whereas rectum showed no significant differences in expression. Elevated expression of IL-10 was observed in all the organs tested but was only significantly up-regulated at 60 days post infection in intestine from i.p. infected fish. For the cohabitant group, IL-1β and IFN-γ was up-regulated in spleen whereas intestine and rectum showed a down-regulation after 60 days. IL-10 was up-regulated in intestine of cohabitant fish from day 30 to day 60. These results indicate that F.?noatunensis infection provokes both specific antibody responses and long term inflammatory responses in cod. The present study provides new knowledge about infection routes and shows that both humoral and cellular defence mechanisms are triggered by F.?noatunensis in cod.     

Extrapolating predation mortalities back in time: an example from North-east Arctic cod cannibalism

Cannibalism is known to be a significant source of natural mortality of young North-east Arctic (NEA) cod. Cannibalism data, starting from 1984, have been used in NEA cod stock assessments since 1995, which has led to inconsistency in the cod abundance time series from 1946 to the present. To address this inconsistency, this study estimates the cannibalism-induced mortality (M2) of NEA cod at age 3–5 for the period 1946–1983. Combined qualitative and quantitative cod stomach content data for 1984–2010 were used to make the M2 estimations for age groups 3–5 (ICES 2014), then different factors including SSB were used to examine which covariates explained variability in M2 and thus make predictions for 1946–1983. The level of cannibalism was estimated to be high in the 1950s – early1960s. VPA-based assessment was run using these estimated M2 values. As a result, numbers of cod eaten by their conspecifics in the historical period and new increased recruitment estimates at age 3 were computed. The main factors affecting cannibalism appeared to be young cod abundance, total stock biomass (TSB) of large cod, and capelin total stock biomass (which represents an alternative prey). The problems involved in using the new recruitment time series in fishery management are discussed. The methodology presented here represents a generic approach to extending predation mortalities back in time to improve historical stock estimates.     

Diet of juvenile cod (age 0–2) in the Barents Sea in relation to food availability and cod growth

Diet investigations were carried out on 0-, 1- and 2-year-old Northeast Arctic cod (Gadus morhua) sampled in the Barents Sea during 1984–2002. Stomach-content analyses showed that the 0 and 1 group cod fed mainly on crustaceans, with krill and amphipods composing up to 70% of their diet. Krill (Thysanoessa spp. and Meganyctiphanes norvegica) and amphipods (Themisto spp.) were mainly found in cod stomachs sampled in the central and close to the Polar Front region in the Barents Sea where these prey organisms are reported to be abundant in summer. A shift in the main diet from crustaceans to fish was observed from age 1 to age 2. The diet of 2-year-old cod mainly comprised capelin (Mallotus villosus) and other fish, and to a lesser degree, krill and amphipods. Shrimp (mainly Pandalus spp.) was also an important prey in both age 1 and 2 cod. A statistically significant positive relationship was obtained between capelin stock size and the amount of capelin in the diet of 2-year-old cod. Results from this study also show that the larger age-2 cod preyed more on capelin in winter and that larger cod (>22 cm) prefer larger capelin (>12 cm). During periods of low capelin abundance, the 2-year-old cod shift their diet more to crustaceans, such as krill and amphipods. A positive significant relationship was also obtained between Total Fullness Index (TFI) and the amount of capelin in the diet and between TFI and the growth of 2-year-old cod, indicating that the growth of age-2 cod is to a large extent dependent on the amount of capelin consumed. Growth of age-1 cod was also positively correlated to TFI.