Outbreak of mortality among cage-reared cobia (Rachycentron canadum) associated with parasitism

Abstract

This study reports a disease outbreak among juvenile cobia (Rachycentron canadum) farmed in cages in the state of Rio de Janeiro, Brazil, caused by the dinoflagellate Amyloodinium ocellatum and the monogenean Neobenedenia melleni. Two thousand five hundred fish were stocked at 0.4 kg/m³ in a set of 12 m³ tanks, in autumn (mean weight 15.0 ± 7.3 g) and in winter (mean weight 43.0 ± 5.6 g). Freshwater baths were administered as a routine treatment, as the symptoms were detected followed by two collection samples. Firstly in May 2011 (n = 5) and secondly in September 2011 (n = 10). In the first sample, the prevalence of N. melleni on the body surface was 100% and the mean intensity was 42.0 ± 1.7, while in the second sample the prevalence was 60% with a mean intensity 3.0 ± 0.2 and mean abundance 1.8 ± 0.4. Amyloodinium ocellatum was only found in the second sample, at a prevalence 100% and mean intensity 46.8 ± 3.4. The cause of fish mortality was possibly associated with a decrease in fish resistance after the first contact with monogenean parasites, allied with respiratory difficulty caused by the presence of A. ocellatum in the gills.Keywords: Marine fish, parasitology, disease, culture, Brazil. ResumoEste estudo relata a mortalidade em massa de juvenis de bijupirá (Rachycentron canadum) criados em tanques-rede no estado do Rio de Janeiro, Brasil, causada pelo dinoflagelado Amyloodinium ocellatum e o monogenea Neobenedenia melleni. Dois mil e quinhentos peixes estavam estocados à densidade de 0,4 kg/m³ em tanques-rede de 12 m³, no período do outono (15,0 ± 7,3 g; peso médio ± DP) e inverno (peso médio de 43,0 ± 5,6 g). Banhos de água doce foram realizados rotineiramente à medida que os sintomas eram detectados, quando então, foram efetuadas duas colheitas, sendo a primeira em maio de 2011 (n = 5) e a segunda em setembro de 2011 (n = 10). Na primeira colheita, a prevalência de N. melleni foi de 100% e a intensidade média de 42,0 ± 1,7, e na segunda, a prevalência foi de 60% e intensidade média de 3,0 ± 0,2 e abundância média de 1,8 + 0,4. Na segunda colheita, observou-se a presença de A. ocellatum (prevalência de 100%, intensidade média de 46,8 ± 3,4). Possivelmente, a causa da mortalidade estava relacionada à redução na resistência dos peixes após o primeiro contato com o parasito monogenea, juntamente com a dificuldade respiratória causada pelo dinoflagelado nas brânquias.Palavras-chave: Peixe marinho, parasitologia, doença, cultivo, Brasil.

Protozoan infections in farmed fish from Brazil: diagnosis and pathogenesis

“…The causative agent of velvet disease A. ocellatum is ubiquitous, affects marine farmed fish and may provoke important outbreak mortalities and economical losses in aquaculture systems (LEVY et al, 2007;PEREIRA et al, 2011;MOREIRA et al, 2013). The trophont stage is pear or roundedshaped, has golden to brownish color, presents chloroplasts and is capable of feeding when adhered on the fish surface or gills by its rhizocysts.…”

An overall estimation of losses caused by diseases in the Brazilian fish farms

“…In the Brazilian territory few reports of farmed fish mortality have been recorded, as for example those caused by bacterial (Kubitza 2005;Martins et al 2008;Figueiredo et al 2012;Silva et al 2012;Pádua et al 2014) and parasitic diseases (Martins et al 2000(Martins et al , 2001SantAna et al 2012;Silva et al 2012;Moreira et al 2013;Pádua et al 2013Pádua et al , 2015Valladão et al 2013Valladão et al , 2014Videira et al 2016). Large-scale mortality is rarely registered despite of their non-diagnosed occurrence in the country.…”

Neobenedenia melleni (Monogenea: Capsalidae) in ornamental reef fish imported to Brazil

“…The earlier adoption of diagnostic and therapeutic measures with freshwater bath, followed by treatment with praziquantel, did improve the prognostic of the diseased fish of great economic value in this study. Freshwater bathing was previously performed successfully in the cage-farmed cobia, Rachycentron canadum, parasitized by N. melleni (MOREIRA et al, 2013). The success of prognosis is even greater when the fish are maintained under optimal water quality conditions and fed with balanced diet (CARDOSO et al, 2017).…”