Sea-cage Fish Research Articles

Caught in the trap: over half of the farmed Atlantic salmon removed from a wild spawning population in the period 2014-2018 were mature

Whilst aquaculture continues its global expansion, containment of fish in sea cages remains a persistent environmental challenge. Within Atlantic salmon Salmo salar farming, widespread escapes over several decades have left a legacy of ecological and genetic impacts on wild populations. Quantifying the characteristics of escapees, and how they vary in time and space, is important to understand how environmental impacts will vary and how mitigation strategies need to be tuned. Using a fish trap located in the River Etne on the west coast of Norway, we created an ecological and genetic profile for 616 escapees entering the river. The most important findings of the present study were (1) the annual number of escapees entering the trap declined in the period 2014-2018; (2) more than half of the escapees entering the river were mature; (3) the vast majority of escapees entering the river were categorised as recent escapees; (4) nearly all (96%) of the early escapees were mature upon entry to the river, while just over half (55%) of the recent escapees were mature; and (5) the escapees originated from multiple sources every year. We conclude that without the fish trap, this population would have been at risk of exposure to further spawning and introgression of domesticated salmon. Furthermore, the current findings of maturation status and escape history highlight the fact that mitigation efforts to reduce escape of smolts, post-smolts and larger fish all need continued attention from the management authorities and aquaculture industry.

İskenderun Körfezi'nde (Türkiye, Kuzey-Doğu Akdeniz) Balık Çiftliği Etrafında Aetomylaeus bovinus (Myliobatidae) Sürüsünün Varlığı

In this study, a shoal of bull rays, Aetomylaeus bovinus (E. Geoffroy Saint-Hilaire, 1817), around a sea-cage fish farm in Iskenderun Bay, Turkey is reported. Such an aggregation is probably due to ontogenetic shift but may also be a factor of reproductive process as some pregnant females were observed. The number of individuals recorded, in all more than 25, shows that the species is still present in the area but needs a conservation plan due to its k-selected life-histories and potential for extinction.

Wild fish aggregations near sea-cages rearing adult and juvenile fish in the Aegean Sea

Sea-cage fish farms cause wild fish to aggregate nearby. Nevertheless, it is still not clear to what extent the attraction effect around sea-cage fish farms varies according to the type and size of the reared species. We examined this attraction effect in six sea-cage fish farms across the Aegean Sea using an integrated approach combining these variables with seasonal and spatial factors. Six 5-minute rapid visual counts were conducted on each farm complex for comparing the wild fish aggregations around the sea-cages concerning both cultivated species (seabass and seabream) and their sizes (adult and juvenile). We counted a total of 509,061 fishes, representing 36 species and 18 families. While 292,377 fish belonging to 34 species were counted in seabass cages, 216,684 fish belonging to 27 species were counted at seabream cages. In addition, 160,192 fish belonging to 28 species were counted around the juvenile cages, while 348,869 fish belonging to 34 species were counted at adult fish cages. The study shows that wild fish communities are formed around the sea-cages where seabass and seabream are reared in both adult and juvenile sizes in the Aegean Sea. According to our results, the fish species reared in the cages do not have a statistically significant effect on the wild fish community structure around the cages, whereas fish size, which was correlated with cage biomass and a longer history of feeding, had a strong effect. Other factors affecting the wild fish community structure around the cages are the season and the location of the cages.

Influence of some physico-chemical variables on wild fish richness beneath sea-cage fish farms in the Aegean Sea, Turkey

AbstractThis study aims to determine the relationship between physico-chemical variables on a seasonal basis and wild fish assemblages beneath sea-cage fish farms. Assemblages of wild fish were counted monthly on two separate days at each of six fish farms between August 2015 and July 2017, by six rapid visual counts (RVC) in 5 minutes with scuba by two divers. Seawater samples were simultaneously taken by a Nansen bottle during the RVC from the fish farm barge. SST (°C), salinity (ppm), dissolved oxygen (mg l−1) and pH were measured by YSI multiparameter, while Secchi disk was also used for light transmittance. Wild fish species richness went up with increasing temperature and salinity in the Izmir region, however, this stopped at about 26°C and about 39 ppm. Wild fish richness increased when the DO was at a level of 7 mg l−1 and the pH at about 7.9 in Izmir. Between 10 and 20 m, light transmittance showed greater wild fish species richness in Izmir region. In contrast, the wild fish species richness of the Muğla region fluctuated more. In terms of wild fish species richness, these fluctuations increased with salinity and DO, while they decreased with SST, pH and light transmittance. However, the range of variation of the recorded physico-chemical variables is rather narrow. The results of the correlation matrix indicate that the relationship between wild fish species richness and pH and SST was statistically significant in Izmir region (P < 0.05).

Seasonal variations in wild fish aggregation near sea-cage fish farms in the Turkish Aegean Sea

This study sought to determine seasonal variations in wild fish assemblage events associated with fish farms with respect to fish biodiversity, abundance and biomass along the Turkish Aegean Sea. Assemblages of wild fish were determined bimonthly with six rapid visual counts (RVCs) at six fish farms in both the Izmir (Northern Aegean Sea, NAS) and Mugla (Southern Aegean Sea, SAS) regions. Data were pooled based on each season. Thirty-nine total fish species representing 21 families were identified, with three particularly abundant families: Sparidae (seven species), Carangidae (six species) and Mugilidae (four species). The greatest fish diversity was recorded in the summer, followed by spring, autumn and winter. Regionally, the greatest fish diversity was observed in the southern Aegean Sea. Significant differences across seasons and regions were observed. Boops boops was very abundant at all stations, except for the first station in the NAS. Approximately 80.5 % of farm-associated fish were B. boops, followed by Atherina boyeri (4.14 %), Scomber colias (2.82 %), Sardinella aurita (2.78 %) and Oblada melanura (2.73 %). A total of 105,041 individual fish and 3637 kg.11,250 m−3 fish were identified in the NAS; 448,331 individual fish and 47,070 kg.11,250 m−3 fish were identified in the SAS. These results indicate that the diversity of wild fish communities is relatively high, and vary between regions. Boops boops represents the predominant species, although the Sparids, Carangids, Mugilids, Scombrids, Clupeids and Pomatomids are also highly represented throughout the Mediterranean.

Ege Denizi’nde kıyı balıkçılığı ve deniz kuşları arasındaki bazı etkileşimler

In this study, it is aimed to determine the some interactions between various fishery types and seabirds, results of this interaction and sea bird species that have been interacting due to secondary attraction factors. A total of 80 fishermen, working in fish farms, small scale fishery (SSF) and lagoons located in Izmir, Aydın and Muğla were face-to-face interviewed between September 2016 and December 2018. The great cormorant (Phalacrocorax carbo), yellow-legged gull (Larus michahellis), great white egret (Ardea alba), some yelkouan shearwater (Puffinus yelkouan) and great white pelican (Pelecanus onocrotalus) are the bird species that have an interaction with coastal fishermen and sea-cage fish farms. 82% of employees in sea-cage fish farms mentioned that they have an interaction with sea birds in winter, besides %50 of SSF have an interaction with sea birds in summer. The difference between interaction rate according to seasons has been found as statistically significant (p<0.05). 33% of employees in fish farms expressed that they see sea birds during the day. This ratio is 21.7% and 15% for SSF and fishermen in lagoon, respectively. Furthermore, 8.3% of fishermen in lagoon, 11% of employees in fish farms and SSF mentioned that they have an interaction with seabirds especially in the morning time.

Determining potential conflicts between small-scale fisheries and sea-cage fish farms in the Aegean Sea

Background. Fish farms attract both juvenile and adult wild fishes in great numbers and diversity. These wild fishes also become an easy target stock for local fishers, both professional and recreational. There are three groups of interest in this study, the fish farmers, small-scale fishermen (professional), and recreational fishers (mostly anglers but also spear fishers). This study was intended to determine conflicts derived from the adverse social and bioecological interactions from the perspective of both local fishermen and fish farmers and to determine the observed incidence of predators. Materials and methods. In this study, 48 randomly selected fish farms, 28 fishery cooperatives, and 33 fishing ports located close to aquaculture areas were visited for face-to-face interviews with the fishermen of small-scale fisheries (SSF) between July 2015 and July 2017. The main purpose of the survey questionnaire was to ask SSF fishermen for their opinion about offshore aquaculture and vice versa—ask the fish farmers about SSF, emphasizing issues potentially creating problems. Data were collected mainly on sociodemographic profiles, current problems, reciprocal conflict issues, the impact of offshore fish farming on SSF activities, and on bioecological interactions such as predator attacks and problems with farm fish escapes. Results. Approximately 77% of fishermen in Izmir and 71% of fishermen in Muğla declared that they had problems with the sea-cage fish farms. On the other hand, 40% of fish farmers in Izmir and 54% of fish farmers in Muğla reported problems with local SSF. The fishermen believe that in recent years, Mediterranean monk seal (Monachus monachus), sea turtle (Caretta caretta), and invasive fish (pufferfish, etc.) populations have been increasing in Muğla. Additionally, 46% of Muğla fish farmers and 60% of Izmir fish farmers acknowledged cases of fishes escaping from sea-cages. In terms of the predators, 84% of Muğla and 75% of Izmir’s fishermen expressed their concerns regarding predator attacks on the sea-cage farms. Conclusion. Further studies, based on socioeconomic issues, are needed for better understanding the dimensions of incomes and economic losses of SSF and cooperation issues in the area where the interaction is high. We suggest that all stakeholders, small-scale fishermen, in particular, must be integrated with a management plan, and the cooperation among both fishing groups should be encouraged.

Ege Denizi‘nde Balık Çiftliklerinden Balık Kaçışlarının Nedenleri Üzerine

Temmuz 2015 ve Temmuz 2017 arasında, Ege Denizi‘nde İzmir ve Muğla kıyılarından rastgele seçilmiş 48 balık çiftliği ziyaret edilmiş, ayrıca balık çiftlikleri etrafında av yapan toplam 329 kıyı balıkçısıyla yüz yüze anketler yapılmıştır. Anketlerde balık çiftliklerinden balık kaçış nedenleri sorulmuştur. Verilen yanıtlar predatör saldırıları, fırtınalar, kafes ağlarının yetiştirilen balıklar ve deniz kuşları tarafından ısırılması, ağ yırtıkları, balıkların aktarılması esnasındaki düşmeler olarak katılımcılar tarafından sınıflandırılmıştır.

Ege Denizi’nde balık çiftlikleri etrafındaki doğal balık tür çeşitliliği

Structures, floating on the surface of the ocean attract both juvenile and adult fishes in great numbers and diversity. Natural and artificial Fish Aggregation Devices (FADs) in open seas are widely recognised for their capacity to attract pelagic fishes. It can be assumed that floating sea-cages act as a kind of FAD. Even, fish aggregations beneath the sea-cages increase much more by the influence of feeding. In this study, aggregations of wild fish were counted around six sea-cage fish farms in both northern and southern Aegean Sea. Each fish farm cultivated Sparus aurata and Dicentrarchus labrax and the farms deployed between 720 m and 3 km far from the coast. Between July 2015 and July 2017, assemblages of wild fish were counted bimonthly on two separate days at each of these farms. The Rapid Visual Counts (RVC) in five minutes with SCUBA and covering 11250 m3 were performed for six times within each farm. A total of 40 species, belonging to 22 families were recorded at fish farms, with 3 families, Sparidae (7 species), Carangidae (6 species) and Mugilidae (4 species) being particularly abundant. The results of the study and other studies conducted in other parts of the Mediterranean were merged; it was found that a total of 99 fish species were listed around sea-cage fish farms in the Mediterranean Sea.

Haemorrhagic kidney syndrome may not be a variation of infectious salmon anaemia

Haemorrhagic kidney syndrome (HKS) of Atlantic salmon is a condition of unknown aetiology that was responsible in the late 1990's for large scale losses in Eastern Canada in sea-caged fish. Lesions included renal interstitial haemorrhage, plus acute renal tubular necrosis with eosinophilic casts. Affected fish were shown to have erythrocyte inclusion body virus (EIBS) but the significance of this infection was unknown at the time. Following the initial report describing this syndrome, data were published showing that infectious salmon anaemia virus (ISAV) was involved with HKS, that lesions typical of HKS could be reproduced using extracts of kidney from fish with HKS, and that these extracts contained ISAV. Even though no liver lesions were ever seen (the hallmark of ISA) the conclusion was reached that HKS was a pathological variation of ISA.Piscine orthoreovirus (PRV) is an emerging viral pathogen of both Atlantic and Pacific salmon. In the former, it is causally linked to heart and skeletal muscle inflammation (HSMI), while in Pacific salmon, a range of jaundice syndromes and distinctive renal tubular necrosis are reported. The similarity of the renal lesions in Pacific salmon to those seen in HKS prompted a re-evaluation of HKS, using in-situ hybridization to identify and localize both PRV and ISAV, using the archived material from which HKS was originally described. We show here the presence of both ISAV and PRV in affected tissues, concentrated in lesions.These findings show that fish with HKS had a dual viral infection, and that HKS was not, therefore, necessarily due to ISAV alone. Given the similarity between the renal lesions in HKS and those in chinook salmon with PRV, these findings also suggest that PRV was a more plausible aetiological candidate. It is just as likely, however, that both viruses were required, and that they acted in a synergistic fashion. The lesions in renal tubules in HKS, partly driven by haemoglobin and partly by one or both viruses, probably led to release of locally high levels of vasoactive factors that were able to target the peritubular capillaries of the renal portal system, leading to necrosis and interstitial haemorrhage.

Food composition of whiting Merlangius merlangus, captured around the sea-cage fish farms in Ordu, south-eastern Black Sea

AbstractThe food composition of whiting, Merlangius merlangus, caught around the sea-cage fish farms off the coast of Perşembe, was analysed and compared with the diet of whiting from a control site off the coast of Fatsa (Ordu Province, south-eastern Black Sea). A total of 815 stomach samples were collected during the study, and of these, 195 (23.9%) were empty. According to the percentage of relative importance index (IRI%), pellet food (47.8%) and Annelida (25%) were the main prey groups of whiting in the sea-cage fish farms area, while unidentified teleost (85.3%) and Engraulis encrasicolus (8.2%) were dominant in the control site. The other prey groups in both areas were Crustaceans (Mysidae, Amphipoda, Upogebia sp.), Sprattus sprattus and Gobius spp. Seasonally, pellet food was the most consumed food in all seasons, but Mysidae was the first preference of whiting around the sea-cage fish farms in spring. In the control site, unidentified teleost was the first preference in all seasons, except winter, where E. encrasicolus was the first choice, followed by crustaceans and S. sprattus in winter. Bray–Curtis analysis shows that seasonally, there is no significant difference in the sea-cage fish farms, while there is significant difference in the control site, and two-dimensional nMDS of IRI% revealed a clear separation between both sites. SIMPER analysis revealed that the most contributing factors to the differences between seasons were pellet food in the sea-cage fish farms, and E. encrasicolus in the control site.

The final countdown: Continuous physiological welfare evaluation of farmed fish during common aquaculture practices before and during harvest

Welfare of farmed fish has become of increasing concern for consumers, producers, interest groups and authorities. To improve fish welfare, it is necessary to find indicators that can identify stressful situations early enough so that an intervention can take place before detrimental effects occur. By using heart rate bio-loggers in freely swimming rainbow trout (Oncorhynchus mykiss), combined with plasma cortisol levels and a range of haematological and blood chemistry parameters, we assessed the severity of stress responses induced by a range of common aquaculture practices before and during harvest. Following surgery, transportation and reintroduction with conspecifics in the sea cage, it took ~4 days for heart rate to stabilize and for a clear circadian rhythm in heart rate to emerge (i.e. average circadian fluctuation in heart rate of ~25 to 27 beats min−1). The presence or absence of this circadian rhythm in heart rate could inform researchers in the aquaculture industry whether or not specific farming routines induce unnecessary and prolonged stress. The elevations in heart rate caused by common farming practises such as crowding and transportation (e.g. heart rate increased by ~8 and 9 beats min−1 above what would normally be expected for that time of day, respectively) corresponded well with increases in plasma cortisol levels. Stressful farming practises or events (indicated by elevated plasma cortisol levels) such as air exposure during brailing and aquatic hypoxia triggered a hypoxic bradycardia until fish were released back into oxygenated water whereupon heart rate significantly increased to repay the accumulated oxygen debt. Repeated stress induced by multiple farming practises (i.e. combined stressors of crowding, brailing and transportation) clearly had a cumulative and long-lasting effect as heart rate peaked at ~25 beats min−1 above what would normally be expected for that time of day. Heart rate also remained significantly elevated by ~9 beats min−1 the following morning, suggesting that if rainbow trout need to be subjected to multiple stressors during consequtive days, it is recommended that sufficient time for recovery is provided between stressors. This study demonstrates that heart rate monitoring can be useful to assess stress levels of freely swimming fish in sea cages. Moreover, the use of implantable bio-loggers opens up a broad range of possible applications that will allow researchers to investigate the effects of environmental and/or anthropogenic stressors on the welfare of fish under conditions more realistic to the aquaculture industry.

The short-term effects of farmed fish food consumed by wild fish congregating outside the farms

We simulated in the laboratory the possible effects on fatty acids and immune status of wild fish arriving for the first time in the vicinity of a sea-cage fish farm, shifting their natural diet to commercial feed consumption, rich in fatty acids of vegetable origin. The flesh fatty acid profile of golden mullet specimens was altered after 2weeks of commercial feed consumption, showing an increase in fatty acids of vegetable origin. The serum peroxidase and bactericidal activities, and head-kidney leucocyte phagocytic capacity, increased after eight weeks of the new diet, while the respiratory burst activity decreased. The extent of these changes cannot be considered large enough to regard them as compromising the health status of fish. More research is needed in order to elucidate whether the rapid assimilation of the dietary fatty acids could harm the immune status of fish when feeding for longer periods than two months.

Monitoring a Massive Escape of European Sea Bass (Dicentrarchus Labrax) at an Oceanic Island: Potential Species Establishment

The post-escape behavior of aquaculture escapees is a growing topic of research. We monitored a massive escape event of the European sea bass, Dicentrarchus labrax, which occurred at a sea-cage fish farm off the oceanic island of La Palma, Canary Islands, eastern Atlantic. Stomach contents and gonadal development of escapees were analyzed from two islands (Gran Canaria and La Palma) in order to assess the degree of postescape establishment,. We also tested (at both islands), the suitability of fatty acid profiles as biomarkers of aquaculture escapes, processing recaptured escaped fish at a range of distances away from aquaculture facilities. Escaped European sea bass concentrated within breakwaters and decreased in abundance through time after the massive escape at La Palma. Decapod crustaceans (particularly Percnon gibbesi and Rhynchocinetes sp) were the main diet constituents of escapees, followed by fishes (mainly the parrotfish, Sparisoma cretense). Only one spawner male was found. Crude lipid, oleic acid, linoleic acid, linolenic acid, eicosapentaenoic acid, Σn-9 fatty acids and Σmonounsatured fatty acids showed higher values in cultured or escaped individuals near cages relative to fish far away from farms. Arachidonic acid, docosahexaenoic acid, Σn-3, satured fatty acids, Σn-3/Σn-6 ratio and Palmitic acid showed the opposite pattern. Our data showed that escaped European sea bass is able to exploit natural recourses, altering their fatty acid profiles relative to farmed conspecifics. The usefulness of fatty acids as biomarkers is, however, limited to a short period of time after escape events.

Effects of proximity to offshore fish farms over soft-bottom macrofauna

Sea-cage fish farms impact the seabed within their immediate vicinity, potentially affecting recipient communities. We assessed whether proximity to three sea-cage fish farms at the Canary Islands altered patterns in the abundance, assemblage structure and richness of soft-bottom macrofauna. We related among-farm variability in dissimilarities in macrofaunal assemblage structure between seabeds beneath cages and controls to differences in hydrodynamics, production and seabed topography. Contrasting patterns of species abundances with varying proximity to fish farms were observed: some species decreased while other species increased their abundances with increasing distance at some farms. Although faunal assemblages at 0 m (i.e. beneath the cages) were different, in terms of assemblage structure, from those found at controls, pairwise differences in assemblage structure among distances away varied among the studied fish farms. Species richness showed inconsistent patterns with proximity to cages among fish farms. In summary, inconsistent patterns of macrofaunal assemblages with varying proximity to aquaculture facilities preclude confident predictions on the way offshore aquaculture alters macrofauna in the study region.

Scientific Opinion on the safety and efficacy of furanones and tetrahydrofurfuryl derivatives: 5‐ethyl‐3‐hydroxy‐4‐methylfuran‐2(5H)‐one and 3‐hydroxy‐4, 5‐dimethylfuran‐2(5H)‐one (chemical group 13) when used as flavourings for all animal species

5-Ethyl-3-hydroxy-4-methylfuran-2(5H)-one and 3-hydroxy-4,5-dimethylfuran-2(5H)-one belong to chemical group 13 (furanones and tetrahydrofurfuryl derivatives) and are authorised for use as flavours in food. These additives are safe at concentrations of 0.05 mg/kg feed for poultry and pigs and 0.08 mg/kg feed for cattle, salmonids and non food-producing animals. The concentration of the additives should be appropriately reduced if used in water for drinking. If used simultaneously in feed and water for drinking, the total intake should not exceed the maximum dose resulting from the use of the flavourings in feed alone. At the levels safe for target animal species these products are safe for the consumers of animal products. The FEEDAP Panel considers it prudent to treat the compounds under assessment as irritant to skin, eyes and the respiratory tract and as skin sensitisers. The proposed concentration of 0.08 mg flavouring compound/kg feed is unlikely to have detrimental effects on the environment except when used in feed for fish in sea cages, in which case the safe concentration would be 0.047 mg/kg. Since both compounds are used in food as flavourings, and their function in feed is essentially the same as that in food, no further demonstration of efficacy is necessary. © European Food Safety Authority, 2014

Fluctuating sea-cage environments modify the effects of stocking densities on production and welfare parameters of Atlantic salmon ( Salmo salar L.)

Stocking densities are commonly used to set limits for the production of fish in sea-cages, yet limited information exists to assess how environmental fluctuations modify the effects of stocking densities on the production and welfare of fish. Atlantic salmon ( Salmo salar L.) of average size 1.3 kg were held at high (15.7–32.1 kg m −3) or normal (5.6–14.5 kg m −3) stocking densities in triplicate 2000 m 3 sea-cages from August to December. Intense crowding within both the high (189 kg m − 3 , 10 × stocking density) and normal (147 kg m − 3 , 17 × stocking density) density cages occurred when sub-optimal temperatures limited the amount of vertical space available. In addition, when stocking density in the high treatment exceeded 26.5 kg m − 3 , feed intake, growth rate and feed utilisation declined and a greater number of cataracts, fin erosions and skin lesions developed. Fish with cataracts on both eyes were smaller than fish with only one or no cataracts. High stocking densities have significant detrimental effects on production and welfare, particularly when they are exacerbated by environments that drive crowding. Stocking densities should therefore be based on the characteristics of each location, to account for the influence of environmental variability.

Mauve Stingers (Pelagia noctiluca) as carriers of the bacterial fish pathogen Tenacibaculum maritimum

Aggregations or blooms of jellyfish are increasingly problematic for the aquaculture industry. Jellyfish-associated mass mortalities of sea-caged fish are most often caused by swarms of oceanic species like Pelagia noctiluca. These relatively large jellyfish get carried by tides and currents onto fish cages, causing them to break up into pathogenic nematocyst-containing pieces that are capable of passing through the mesh of the cages. The main effect on fish is gill damage leading to respiratory distress, but the lesions may also be compounded by bacterial infection, Tenacibaculum maritimum being one of the pathogens involved. In our previous study, we highlighted the ability of the jellyfish Phialella quadrata to carry this important pathogen. However, since these small jellyfish were collected around sea-cages of infected salmon, it was not possible to determine if the jellyfish or the fish themselves were the original source of the bacteria. Results of the current study demonstrate that these filamentous bacteria are present on the mouth of P. noctiluca that had no previous contact with farmed fish. These new results highlight the fact that some Cnidarian species harbour T. maritimum and suggest that jellyfish might be a natural host for these bacteria whose environmental reservoir has not yet been determined.

Two mortality events in sea‐caged yellowtail kingfish Seriola lalandi Valenciennes, 1833 (Nannopercidae) from Western Australia

In November 2008 a commercial sea-cage operator farming yellowtail kingfish, Seriola lalandi, in Western Australia reported more than 70% mortality of the sea-caged fish. Several parasites and potentially pathogenic bacteria were isolated from the fish, but despite a detailed laboratory investigation the cause of the mortality event was never ascertained. That episode of deaths was followed in January 2009 by a smaller mortality event in fish in a sea cage that had been stocked several weeks earlier. Pathogens similar to those seen in the first mortality event were isolated, but again a single causative agent could not be identified. Multiple stress factors resulting in immunosuppression may have precipitated the mortality events.

In vitro toxicity of bithionol and bithionol sulphoxide to Neoparamoeba spp., the causative agent of amoebic gill disease (AGD)

The objective of the present study was to evaluate the in vitro toxicity of bithionol and bithionol sulphoxide to Neoparamoeba spp., the causative agent of amoebic gill disease (AGD). The current treatment for AGD-affected Atlantic salmon involves bathing sea-caged fish in freshwater for a minimum of 3 h, a labour-intensive and costly exercise. Previous attempts to identify alternative treatments have suggested bithionol as an alternate therapeutic, but extensive in vitro efficacy testing has not yet been done. In vitro toxicity to Neoparamoeba spp. was examined using amoebae isolated from the gill of AGD-affected Atlantic salmon and exposing the parasites to freshwater, alumina (10 mg l(-1)), seawater, bithionol or bithionol sulphoxide at nominal concentrations of 0.1, 0.5, 1, 5 and 10 mg l(-1) in seawater. The numbers of viable amoebae were counted using the trypan blue exclusion method at 0, 24, 48 and 72 h. Both bithionol and bithionol sulphoxide demonstrated in vitro toxicity to Neoparamoeba spp. at all concentrations examined (0.1 to 10 mg l(-1) over 72 h), with a comparable toxicity to freshwater observed for both chemicals at concentrations > 5 mg l(-1) following a 72 h treatment. Freshwater remained the most effective treatment, with only 6% viable amoebae seen after 24 h and no viable amoebae observed after 48 h.