Effect of mixed feeding schedules with varying dietary protein levels on the growth of sutchi catfish, Pangasius hypophthalmus (Sauvage) with silver carp, Hypophthalmichthys molitrix (Valenciennes) in ponds

A 6‐month feeding trial was conducted in field condition using 10 farm ponds (400–600 m²) to investigate the effect of mixed feeding schedules on the growth of sutchi catfish, Pangasius hypophthalmus with silver carp, Hypophthalmichthys molitrix. Fish were stocked at a ratio of 80:20 (sutchi catfish, 4.9±0.5 g: silver carp, 12.0±0.8 g) at the total rate of 25 000 ha^?1. Two diets of high protein (30%, HP) and low protein (16%, LP) were prepared using locally available feed ingredients. Five different feeding schedules of high‐protein diet continuously (HP), low‐protein diet continuously (LP), 1‐day low–protein/1‐day high‐protein diet (1LP/1HP), 7 days low–protein/7 days high‐protein diet (7LP/7HP) and 14 days low–protein/14 days high‐protein diet (14LP/14HP) were tested. The fish were fed twice daily at the rate of 15%, 10%, 8% and 5% of their body weight for first, second, third month and rest of experimental period respectively. Feeding rate was calculated only on the basis of sutchi catfish weight only and was adjusted every 2 weeks according to weight gain. Fish fed LP and HP on alternate day (1LP/1HP) resulted in significantly (P<0.05) higher growth rate, feed utilization and production among the treatments. However, there were no significant differences (P>0.05) between the growth rates and production of fish fed HP regularly and fish fed 7 days LP followed by 7 days HP (7LP/7HP). Fish maintained on LP grew the least. The feed conversion ratio (FCR) values for sutchi catfish ranged between 2.04 and 2.79 with feeding schedule 1LP/1HP showing the best FCR. The total production of fish (including silver carp) ranged between 8310 and 12 422 kg ha^?1 6 months^?1 with 1LP/1HP feeding schedule resulting in the highest production and net profit. The study demonstrated that feeding fish continuously with HP is less economical. Thus, for profitable sutchi catfish culture with silver carp, farmers can use the mixed feeding schedule of alternate day feeding of LP and HP as a means of reducing feed costs.     

Genotyping of Streptococcus dysgalactiae strains isolated from Nile tilapia,Oreochromis niloticus (L.)

Streptococcus dysgalactiae is an emerging fish pathogen that is responsible for outbreaks of disease on fish farms around the world. Recently, this bacterium was associated with an outbreak at a Nile tilapia, Oreochromis niloticus (L.), farm in Brazil. The aim of this study was to evaluate the genetic diversity, best genotyping method and aspects of molecular epidemiology of S. dysgalactiae infections in Nile tilapia farms in Brazil. Twenty‐one isolates from four farms located in different Brazilian states were characterized genetically using pulsed‐field gel electrophoresis (PFGE), ERIC‐PCR, REP‐PCR and sodA gene sequencing. The discriminatory power of the different typing methods was compared using Simpson's index of diversity. Identical sodA gene sequences were obtained from all isolates, and ERIC‐PCR and REP‐PCR were unable to discriminate among the isolates. PFGE typing detected three different genetic patterns between the 21 strains evaluated; thus, it was the best genotyping method for use with this pathogen. The strains from Ceará State were genetically divergent from those from Alagoas State. The S. dysgalactiae isolates analysed in this study constituted a genetically diverse population with a clear association between geographical origin and genotype.     

Improving production efficiency of farmed Atlantic salmon (Salmo salar L.) by isoenergetic diets with increased dietary protein‐to‐lipid ratio

The effects of isoenergetic diets with high (HP) and low (LP) protein‐to‐lipid ratios on feeding rate (SFR), feed conversion (FCR), growth (TGC) and relative‐ and absolute nutrient retention were investigated using both whole‐body weight (BW) and carcass weight (CW) to assess the production efficiency. Three different feeding trials in seawater were conducted: two large‐scale trials with yearling smolt (S1) and under‐yearling smolt (S0) and one small‐scale with S1 smolt. The initial body weights in the trials were 105, 319 and 978 g, respectively, and the fish were fed and monitored until they reached harvest weights. In all three trials, the dietary HP group attained significantly higher (p < .05) CW at harvest based on fish with equal BW. Also, fish fed the HP diets significantly improved FCR (p < .05) when based on CW. In the small‐scale trial, fish fed HP diet, especially during late autumn and spring, significantly (p < .001) improved FCR_BW and FCR_CW. Improved FCR coincided with significantly higher (p < .05) relative energy retention in the dietary HP group. In all three trials, the HP groups had significantly higher (p < .05) TGC with regard to both BW and CW. Taken together, the present studies indicate that growth performance and feed utilization in modern salmon farming has the potential to be further improved by increasing the dietary protein‐to‐lipid ratio. In addition, dietary influence is more precisely assessed when using carcass as the weight denominator when analysing feed utilization and growth performance.     

Rapid dispersion of escaped meagre (Argyrosomus regius) from a coastal Mediterranean fish farm

Meagre (Argyrosomus regius) is an emerging species in aquaculture considered a locally absent species in the Western Mediterranean. Little is known about meagre escape incidents from farms. To evaluate escape impacts and develop mitigation measures, knowledge about the behaviour of escaped meagre is necessary. In this study, a rapid dispersion of escaped meagre was observed following a simulated escape incident in a coastal Mediterranean farm, using acoustic telemetry (acoustic tag: AT) and mark‐and‐recapture techniques (external tag: ET). A small proportion of AT meagre (22.7%) remained within the farm boundaries 24 h after the simulated escape. A total of 84 meagre (9.2%) were captured by local trammel netters nearby the farm facility within the first 48 h after escape. Only two AT individuals (15.3%) were detected by acoustic receivers located in coastal areas within 2 days after dispersal from the farm and eight ET meagre (0.8%) were recaptured along the shoreline during the first week. Two AT individuals (15.3%) were considered to be dead by predation few hours after escape. The rest of the AT individuals (N = 6, 46.2%) left the facility alive, and they were never detected by receivers nor reported back by fishermen. Thus, our results highlight the necessity of establishing escape mitigation strategies at Mediterranean fish farms, i.e. recapture programmes 24–48 h after the escape incident in collaboration with local fishermen, to diminish potential impacts related to meagre escapees.     

Development,distribution and expression of a DNA vaccine against nodavirus in Asian Seabass,Lates calcarifier (Bloch, 1790)

Fish nodavirus (betanodavirus), a viral pathogen responsible for viral nervous necrosis (VNN) was isolated from infected Asian sea bass (Lates calcarifer). The distribution, clearance and expression of nodavirus vaccine, on the basis of DNA vaccine (pFNCPE42 DNA‐pcDNA3.1) construction, were analysed in tissues of the Asian seabass by PCR, RT‐PCR, ELISA and Immunohistochemistry. Fish immunized with a single intramuscular injection of 20 μg of the pFNCPE42‐DNA vaccine showed a significant increase in the serum antibody level in the 3rd week after vaccination, compared to control eukaryotic expression vector pcDNA3.1 vaccinated fish. Results from PCR studies indicated that the vaccine‐containing plasmids were distributed in heart, intestine, gill, muscle and liver 10 days after vaccination. Clearance of pFNCPE42‐DNA vaccine was studied at 10, 25, 50, 75 and 100 days of post vaccination (d p.v). At 100 days p.v. pFNCPE42‐DNA was cleared from muscle of vaccinated sea bass. In vitro and in vivo expression of fish nodavirus capsid protein gene (FNCP) was determined by fluorescent microscopy. Asian seabass was immunized with pFNCPE42‐DNA vaccine at a dose of 20 μg per fish and were challenged with betanodavirus by intramuscular injection. The vaccinated seabass was protected from nodaviral infection and 77.33% of relative percent survival (RPS) was recorded.     

Common bottlenose dolphin (Tursiops truncatus) interaction with fish farms in the Gulf of Ambracia,western Greece

1. Marine aquaculture, and its fast-growing development, has the potential for wider environmental, ecosystem, and biodiversity impacts. This study assesses the impact of fish farming on a bottlenose dolphin (Tursiops truncatus) population within the Gulf of Ambracia (western Greece) between 2007 and 2018. Two different study areas were defined in order to investigate differences on occurrence, abundance, behaviour, and seasonal fluctuations between them: a ‘control area’, in the north-western side of the Gulf; and a ‘fish farm area’, in the south west, where the highest density of fish farm cages is found.
2. A total of 169 daily surveys were dedicated to the control area and 74 days were dedicated to the fish farm area, yielding 104 and 37 sightings, respectively. Both the probability of detecting dolphins (U = 6,763.000, P = 0.002) and the group sizes (U = 578.000, P < 0.001) were smaller around fish farms.
3. Seasonality analyses were restricted to 2007–2008, as this was the period with year-round effort. Results revealed that dolphins were more frequently seen around fish farms in the winter (Kruskal–Wallis test, P = 0.036).
4. From the 40 identified individuals that were re-sighted at least 10 times during the study period, 21 used the fish farm area less than expected according to sampling effort, and 10 of them were never observed in that area (binomial test, P < 0.05).
5. The results revealed an impact of fish farms on the distribution of bottlenose dolphins in the Gulf of Ambracia. This information should be taken into consideration when defining ecosystem-based management measures within the management plan, which is currently in preparation for this Natura 2000 site.

     

Diet,size and location as determinants of n‐3 long‐chain polyunsaturated fatty acid content in farmed Atlantic Salmon (Salmo salar)

We explored how currently manufactured feeds, under real‐world conditions and across geographically distinct locations, promoted flesh n‐3 long‐chain polyunsaturated fatty acid (LC‐PUFA, i.e. 20:5n‐3 + 22:6n‐3) levels in various life stages of farmed Atlantic Salmon (Salmo salar). Potential effects on flesh LC‐PUFA included: (1) diet and fish weight at one Canadian east coast farm, (2) diet and farm location across six east coast farms, and (3) diet and farm location between east and west coast farms. For objectives 1 and 2, salmon were fed a currently manufactured feed (labelled as feeds A, B or C) and harvested at 1, 3 and 5 kg. LC‐PUFA levels in 5 kg (harvest size) fish were then compared to previously published values for west coast farmed Atlantic Salmon (Obj. 3). Combined results revealed that variability in LC‐PUFA levels was better explained by diet than by fish weight or farm location. Fish size, however, was also important for two reasons. First, feeding a high LC‐PUFA diet early in life appeared important for ensuring high LC‐PUFA levels at harvest size. Second, salmon flesh LC‐PUFA levels increased with fish size, but only when dietary LC‐PUFA was provided above an apparent threshold value (~3000 mg per 100 g or 10% of total fatty acids) that likely promoted LC‐PUFA incorporation and storage. Overall, our comparison makes new recommendations for feed manufacturers and demonstrates that farmed Atlantic Salmon reared under real‐world conditions on currently available salmon feeds were good sources of n‐3 LC‐PUFA to consumers.     

Acute dermatitis in farmed trout: an emerging disease

A new skin condition, known as puffy skin disease (PSD), emerged in farmed rainbow trout Oncorhynchus mykiss (Walbaum) in 2002. The number of new cases increased considerably from 2006. Clinical signs include white or grey skin patches, which become raised and red with excessive mucous production and scale loss. Fish are inappetant and lose condition. Histologically, the key feature is epithelial hyperplasia. We undertook a questionnaire study of trout farmers in England and Wales to investigate prevalence and risk factors. PSD was reported on 37% (n = 49) of rainbow trout sites, located in 28 river catchments. The increase in cases from 2006 onwards was mirrored by the increase in red mark syndrome (RMS). Prevalence and severity of PSD were highest in the summer months. The presence of PSD was associated with RMS (OR = 9.7, P < 0.001). Sites receiving live rainbow trout in the previous 12 months were considerably more likely to have PSD (OR = 5.3. P < 0.01), which suggests an infectious aetiology. The size of affected fish and prevalence varied between farms, indicating that farm‐level factors are important. Future research should further investigate the aetiology of PSD and practices to manage the disease.     

Depletion study and estimation of withdrawal periods for florfenicol and florfenicol amine in pacu (Piaractus mesopotamicus)

The intensive production of farmed fish is at a global all‐time high, and the control of bacteria proliferation in fish farms requires the frequent use of antimicrobials. This practice raises important environmental concerns related to the emergence of antimicrobial‐resistant bacteria strains. Only a few antimicrobial drugs have been approved for use in aquaculture, one of which is florfenicol. This work studies the depletion and withdrawal period of florfenicol and its main metabolite, florfenicol amine, in pacu (Piaractus mesopotamicus), a neotropical characin widely farmed in the southern hemisphere. Juvenile pacu (average weight of 724 g) were stocked in a closed‐loop laboratory system with controlled water temperature (25.8°C), and were fed for 10 consecutive days with a diet containing an intended dose of 10 mg/florfenicol per kg bw. Muscle and skin tissues were collected at 1, 3, 6, 8, 10, 12 and 16 days post‐treatment, and florfenicol and florfenicol amine were quantified using a validated ultra‐high performance liquid chromatography‐tandem mass spectrometry (UPLC‐MS/MS) method. The limits of quantitation for florfenicol and florfenicol amine were 10 ng/g in muscle and 50 ng/g in skin. Considering a maximum residue limit of 1000 ng/g for the sum of florfenicol and florfenicol amine in muscle with skin in natural proportions a withdrawal period of 5 days (water temperature 25.8°C) or 129 degree days was calculated on the basis of the upper limit of the one‐sided 95% confidence interval for the 99^th percentile derived from the residue depletion study.     

Infectious agent detections in archived Sockeye salmon (Oncorhynchus nerka) samples from British Columbia,Canada (1985–94)

In response to concerns that novel infectious agents were introduced through the movement of eggs as Atlantic salmon aquaculture developed in British Columbia (BC), Canada, we estimated the prevalence of infectious agents in archived return‐migrating Sockeye salmon, from before and during aquaculture expansion in BC (1985–94). Of 45 infectious agents assessed through molecular assays in 652 samples, 23 (7 bacterial, 2 viral and 14 parasitic) were detected in liver tissue from six regions in BC. Prevalence ranged from 0.005 to 0.83 and varied significantly by region and year. Agent diversity ranged from 0 to 12 per fish (median 4), with the lowest diversity observed in fish from the Trans‐Boundary and Central Coast regions. Agents known to be endemic in Sockeye salmon in BC, including Flavobacterium psychrophilum, Infectious haematopoietic necrosis virus, Ceratonova shasta and Parvicapsula minibicornis, were commonly observed. Others, such as Kudoa thyrsites and Piscirikettsia salmonis, were also detected. Surprisingly, infectious agents described only recently in BC salmon, Ca. Branchiomonas cysticola, Parvicapsula pseudobranchicola and Paranucleospora theridion, were also detected, indicating their potential presence prior to the expansion of the aquaculture industry. In general, our data suggest that agent distributions may not have substantially changed because of the salmon aquaculture industry.     

Detection of tilapia lake virus (TiLV) infection by PCR in farmed and wild Nile tilapia (Oreochromis niloticus) from Lake Victoria

Tilapia lake virus disease (TiLVD) has emerged to be an important viral disease of farmed Nile tilapia (Oreochromis niloticus) having the potential to impede expansion of aquaculture production. There is a need for rapid diagnostic tools to identify infected fish to limit the spread in individual farms. We report the first detection of TiLV infection by PCR in farmed and wild Nile tilapia from Lake Victoria. There was no difference in prevalence between farmed and wild fish samples (p = .65), and of the 442 samples examined from 191 fish, 28 were positive for TiLV by PCR. In terms of tissue distribution, the head kidney (7.69%, N = 65) and spleen (10.99%, N = 191), samples had the highest prevalence (p < .0028) followed by heart samples (3.45%, N = 29). Conversely, the prevalence was low in the liver (0.71%, N = 140) and absent in brain samples (0.0%, N = 17), which have previously been shown to be target organs during acute infections. Phylogenetic analysis showed homology between our sequences and those from recent outbreaks in Israel and Thailand. Given that these findings were based on nucleic acid detection by PCR, future studies should seek to isolate the virus from fish in Lake Victoria and show its ability to cause disease and virulence in susceptible fish.     

Growth and fry production of Nile tilapia, Oreochromis niloticus (L.), on different feeding schedules

The effect of scheduled use of high‐protein and low‐protein diets on body weight and fry production of Nile tilapia, Oreochromis niloticus (L.), was determined. A preliminary feeding trial was first conducted on fingerlings. These were fed a high‐protein diet (H, 25% protein) or a low‐protein diet (L, 18% protein) daily, or diet H for 1–3 days followed by diet L for 1–4 days. Final body weight was significantly higher (P < 0.05) in fish fed diet H daily and in fish fed diet H for 2–3 days followed by diet L for 1 day (2H?1L and 3H?1L). Fingerlings on 1H?1L and 3H?2L had slightly lower growth. Based on the response of the fingerlings, five feeding schedules were tested with the broodstock. A high‐protein diet (HP, 40%) and a low‐protein diet (LP, 25%; same as H for fingerlings) were used. Feeding schedules significantly influenced body weight of female but not the male fish. Fry production was not significantly affected by the feeding schedule for broodstock. When growth, fry production and saving in feed cost were all considered, the broodstock on 1HP?1LP and 3HP?2LP feeding schedules both gave the highest overall performance. These findings give fish farmers an option in the management of feeding of tilapia broodstock.     

Efficacy of a polyvalent injectable vaccine against Flavobacterium psychrophilum administered to rainbow trout (Oncorhynchus mykiss L.)

Flavobacterium psychrophilum is one of the most important pathogens affecting cultured rainbow trout (Oncorhynchus mykiss). Recent information from UK salmonid farms showed country‐wide distribution of genetically and serologically divergent clones, which has hampered the development of a vaccine for rainbow trout fry syndrome. The current study assessed the efficacy of an injectable polyvalent vaccine containing formalin‐inactivated F. psychrophilum in rainbow trout. The vaccine was formulated with an oil adjuvant (Montanide ISA 760VG) or formalin‐killed cells alone. Duplicate groups of trout (60 ± 13 g) were given phosphate‐buffered saline or vaccine formulated with Montanide by intra‐peritoneal (i.p.) injection and challenged by intra‐muscular (i.m.) injection with a homologous and a heterologous isolate of F. psychrophilum at 525 degree days post‐vaccination (dd pv). Significant protection was achieved in vaccinated fish (p = 0.0001, RPS 76% homologous, 88% heterologous). Efficacy of the adjuvanted vaccine was also demonstrated by heterologous challenge at 1155 dd pv resulting in 100% protection, whereas survival in the un‐adjuvanted group was not significantly different from control fish. Levels of specific antibody at 1155 dd pv, as measured by ELISA, were significantly higher in the fish vaccinated with adjuvant when compared with unvaccinated fish.     

Designing an in‐house ELISA to detect antibody against viral haemorrhagic septicaemia virus using recombinant N protein in Iranian farmed rainbow trout (Oncorhynchus mykiss)

Viral haemorrhagic septicaemia (VHS) is one of the most important viral diseases in rainbow trout that has caused great losses to Iranian rainbow trout aquaculture industry in the last 3 years. Therefore, rapid and reliable diagnosis of VHS virus infections is of great importance. An enzyme linked immunosorbent assay (ELISA) method was performed to study serum antibodies against viral haemorrhagic septicaemia virus (VHSV) using recombinant fragments of their N protein. For this purpose, the virus was first isolated from an infected farm. A part of the nucleocapsid (1–505 bp) gene was amplified by RT‐PCR using specific primers. The amplified fragment was ligated to pMALc2x vector and transferred to DH5α strain of Escherichia coli. Then, recombinant plasmids were tested for protein expression in E. coli Rosetta strain. SDS‐PAGE analysis indicated the production of a recombinant protein with an expected molecular weight of 61 KDa. Analysis of trout serum samples from seven previously infected farms and two VHS free farms showed that the designed ELISA method was effective in diagnosing the infected fish. The results revealed that the developed serological assay using designed ELISA based on recombinant protein (N) has the potential to be used in monitoring studies and to determine the prevalence of VHS in rainbow trout farms. The present data allow evaluating the levels of nonneutralizing antibodies without crude virus preparations.     

Influence of dietary concentrations of protein,lipid and carbohydrate on growth,protein and energy utilization,body composition,and plasma titres of growth hormone and insulin-like growth factor-1 in non-transgenic and growth hormone transgenic coho salmon,Oncorhynchus kisutch (Walbaum)

This study compared the ability of juvenile non-transgenic (NT) and transgenic growth hormone (T; gene construct OnMTGH1) coho salmon (Oncorhynchus kisutch) to metabolically utilize energy from lipid and carbohydrate for growth at two dietary protein concentrations. Triplicate groups of size-matched (initial weight, 28.2–29.1 g) NT and T salmon held in 10.5–10.8 °C well water on a natural phototocycle (12.25 h → 8.25 h) were each fed one of four isoenergetic (~ 17.5 MJ of digestible energy (DE)/kg) dry diets twice daily to satiation for 83 days. These diets contained 340 g (LP) or 430 g (HP) of estimated digestible protein (DP)/kg and either 123 g (LL) or 164 g (HL) of estimated digestible lipid (DL)/kg at each DP level (dry weight basis). Estimated digestible carbohydrate (DCHO) concentrations ranged from 45 to 269 g/kg to equalize dietary DE. Under the preceding conditions 46–66% of NT and 18.7–27.5% of T coho, depending upon diet treatment, exhibited no growth or lost weight during the study. NT coho are known to grow slowly or lose weight between the fall equinox and winter solstice but this was unexpected for T coho. Considering all fish T coho, regardless of diet treatment, exhibited significantly higher specific growth rates (SGR) than NT fish due to enhanced feed intake, feed and protein (gross and available deposited) utilization and generally improved available energy utilization. Diet treatment did not affect the growth performance of T fish. Within NT fish, the HP diets supported best gross and available protein and energy utilization. SGR values for growing fish (F_G) only followed identical trends to those for all fish in relation to diet treatment. Regardless of fish genotype, terminal hepatosomatic indices for F_G were directly related to dietary DCHO content and maximum values were noted in NT fish fed LP–LL (significantly higher than observed in NT or T fish fed HP–HL). Final whole body protein concentrations adjusted for dissimilar fish size were higher in T fish fed HP–HL than in T fish fed LP diets and NT fish fed LP–LL. This was also true for T fish fed HP–LL versus T fish fed the LP diets. Within HP groups, whole body lipid and energy contents were generally higher in NT versus T fish whereas within the LP groups energy content was significantly higher in T versus NT fish and was highest in T fish fed LP–LL. Terminal plasma titres for GH and IGF–1 (all fish) were significantly higher in T than in NT fish and were generally uninfluenced by diet treatment. Non-growing NT and T fish had elevated GH relative to NT and T fish that grew. Also, GH was higher in growing T fish relative to growing NT fish whereas the opposite was true in non-growing fish. Plasma IGF-1 levels were higher in growing NT and T fish than in their respective non-growing counterparts. Within F_G only, IGF-1 levels were higher in T versus NT fish.It is concluded that T coho have enhanced ability to effectively utilize DE from DCHO relative to NT fish especially when DP is near the bottom of the optimal range and DL is concurrently suboptimal for NT fish. This is likely due to the elevated titres of GH, IGF-1 and 3,5,3′-triiodo-l-thyronine (measured in another study) in T fish. Also, the enhanced ability of T fish to store energy under these dietary conditions suggests enzymatic improvements in their anaerobic and aerobic metabolism of glucose.     

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.     

Tracking escaped Atlantic cod (Gadus morhua L.) aggregated around Norwegian farms: Considerations for management strategies

To examine site fidelity and potential for ecosystem impacts, 20 escapee cod (Gadus morhua L.) from fjord‐based cod farms were tagged with acoustic transmitters, released in the vicinity of the same farm, and their subsequent behaviour and survival monitored using a fixed array of acoustic receivers. Most individuals demonstrated high fidelity to their farm where tagged, and this increased during the polar night. Only five individuals (25%) were occasionally detected at another farm (4 km distance), and only three cod (15%) migrated out of the fjord during the 6‐month study period. Vertical movements were consistently greater in the hours around noon than at night, a behaviour inconsistent with that observed for wild fish. A high mortality rate (40%) was observed during the first 4 months post‐release, and actual mortality may have been even higher, limiting the potential for negative ecosystem effects from cod escapees. Findings from this study are used to discuss the development of recapture strategies for minimising potential impacts from cod escapees to ensure sustainable farming activity.     

Weaning methods using formulated feeds for snakehead (Channa striata and Channa micropeltes) larvae

The culture of snakehead fish (Channa striata and Channa micropeltes) in Vietnam is limited, and snakehead culture has been banned in Cambodia, because traditional practices include capture of fingerlings from the wild as seed, as well as capture of small‐size (also known as trash fish or low‐value) fish. As hatchery breeding technology has improved, we investigated the optimal weaning practices for these two species. Both laboratory experiments and farm trials were conducted. For C. striata, the optimal weaning procedure is to begin at 17 days after hatch (dah) and wean the fish at 10% replacement of live feed with formulated feed per day. However, for C. micropeltes, the optimal procedure is to wait until 40 dah to begin weaning and then to wean the fish with a 10% replacement of live feed with formulated feed every 3 days. These results should enable farmers to domesticate snakehead culture in Vietnam and Cambodia and eliminate reliance on fish captured from the wild as both seed and feed.     

Determination of minimal concentration of Piscirickettsia salmonis in water columns to establish a fallowing period in salmon farms

A highly sensitive real‐time PCR procedure to detect and quantify the number of Pisciricketsia salmonis units in seawater samples from affected farm sites has been developed. The purpose was to determine a fallowing period that would allow safe restocking of the target farm with new fish. Bacterial load was determined in water samples by comparing the obtained amplification values against a standard curve generated by the amplification of known concentrations of the ITS‐ribosomal component of P. salmonis DNA, cloned in a suitable vector. The standard curve was linear over the range of 10¹–10¹⁰ log units. Target samples were taken every 10 days over a 40‐day period, at 5 m depth and at the surface. In a highly affected area of southern Chile, the number of bacterial units in farm water decreased to zero at day 50. Therefore, a fallowing period of 50 days post‐removal of cages of affected fish appears to be appropriate before restocking. This procedure could be adapted to control disease problems because of other pathogens in fish farm waters.