Acronym GILPAT
Category
Aquaculture
Title Investigations of Increased Mortalities on Marine Salmon Sites due to Gill Pathologies Programme Nat. Programme (supported by ESIF)
Instrument (FP6)
Contact Type (FP7)
Strand (Interreg)
NA
Theme (FP7)
Activity Area (FP6)
Regional Area (Interreg)
Action (COST)
NA
Specific Programme (FP7)
NA
Funding source National-European Coordinator Neil Ruane Coordinator email neil.ruane@marine.ie
Coordinator institution
MI - Marine Institute (Ireland)
Institutions involved
NA
Start year 2008 End year 2012 Funding (€) € 452,474 Website NA Summary The role of jellyfish as causative agents of gill disorders and mortalities in marine-farmed finfish had received very little attention until the advent of the GILPAT project. Without knowledge of the abundance, distribution, and seasonality of the detrimental jellyfish species around aquaculture sites an understanding of such interactions cannot be developed. There are numerous species of jellyfish throughout our coastal waters capable of forming high density blooms, however their potential to cause gill damage remains largely unknown. It was the aim of this project (GILPAT) to begin an investigation into the jellyfish species found around the coast of Ireland and their potential impact on the finfish aquaculture industry. Therefore extended targeted monitoring of the health of marine-farmed fish and jellyfish populations, in combination with the development of controlled experimental studies, were performed to gain a more comprehensive understanding of mechanisms behind jellyfish-induced gill disorders. In addition to this a range of molecular diagnostic methods were developed and used to screen farm samples for potential pathogens which have been linked to the development of gill disease in other countries. This project has resulted in 14 peer-reviewed publications (either published, in press or accepted) and a PhD thesis. Work from GILPAT has been presented at a range of international meetings including those organised by the European Parliament, European Association of Fish Pathologists, and the Fish Veterinary Society. A number of training workshops were also held for industry at the Marine Institute and the National Maritime College of Ireland. Literature reviews and epidemiological investigations A review of the literature highlighted the large knowledge gaps which exist at present. Gill disease is a multi-factorial condition occurring due to the interactions of particular environmental conditions, the presence of phyto/zooplankton blooms, and pathogen (viral, bacterial or parasitic) involvement. The Marine Institute's phytoplankton monitoring programme provides a lot of information on the occurrence and distribution of these species, however there is no current programme available for zooplankton/jellyfish species. There is also little known about the prevalence and role played in gill disease by pathogens such as the bacterium Tenacibaculum maritimum, and the parasite Desmozoon lepeophtherii. Epitheliocystis also appears to be linked to gill disease, but it has yet to be established whether this is a primary or secondary factor. An epidemiological investigation highlighted gill disease as a significant cause of mortality, it resulted in fish welfare challenges and gave rise to loss of growth in the majority of the marine salmon farms during the study period. Results indicated that mortalities due to gill disorders were lower in the salmon S0s compared to the S1s . Results also indicated that sites positive for the presence of epitheliocystis had significantly higher mortalities due to gill disease when compared to negative sites. There were no statistically significant differences in gill disorder losses when net washing (using high pressure underwater cleaners) was compared to net changing although the authors feel that this is an area which needs to be investigated further. Zooplankton monitoring The role of gelatinous zooplankton in the gill disorders of marine-farmed salmon was investigated using both field sampling and controlled challenge experiments. To aid these investigations, a semi-quantitative gill scoring methodology was developed over a longitudinal study to enable gill damage to be rated in terms of extent and severity. Over the longitudinal study, the highest observed gill scores coincided with an intrusion of the scyphozoan jellyfish Pelagia noctiluca into one particular farm. Small hydrozoan jellyfish (the siphonophore Muggiaea atlantica and the hydromedusa Solmaris corona) were linked to clinically significant gill damage at densities in orders of magnitude lower than for previously reported problems. The abundance of small hydrozoan jellyfish was also significantly correlated with the average daily fish mortality (with a lag of three to four days) indicating the potential for such abundances to cause background level mortalities. Experimental challenge trials Through the use of controlled experimental challenge trials, the role of two cnidarian agents in causing gill disorders of marine-farmed salmon was investigated. The potential for gill damage caused by the common jellyfish Aurelia aurita may have been previously underestimated and understudied due to the benign nature of its sting to humans. However, following exposure to macerated A. aurita, it was possible to explicitly show for the first time that Atlantic salmon post-smolts suffered significant and persistent gill damage. Through the use of histological sampling over a time-series from initial exposure, the pathogenesis of the gill damage could be tracked and was shown to increase in severity and extent up to 38 hours after the jellyfish were removed. This lag in the maximum gill damage observed may similarly relate to the lag in mortality that was observed in the monitoring programme, three to four days after the peak in jellyfish abundance. Even three weeks from the start of the challenge, fish had longstanding gill damage. No mortalities were observed, although it should be considered that the fish were in controlled conditions (with UV sterilisation of the water) where the likelihood of secondary bacterial infections was minimal. In the second experimental challenge trial, the significant and rapid growth of biofouling hydroids was quantified and the potential for damage caused by these organisms was investigated for the first time. The results show that the hydroids have a significant impact on the health of the gills after exposure. These studies have implications for management practices, such as in situ net washing, which will require further investigations. Molecular diagnostic methods During this project, sensitive molecular diagnostic methods were developed for five potential pathogens which have been associated with gill disease in other countries: Atlantic salmon paramyxovirus; Neoparamoeba perurans; Tenacibaculum maritimum; Candidatus Piscichlamydia salmonis; and Desmozoon lepeophtherii. The Atlantic salmon paramyxovirus was not found in any samples tested. N. perurans was detected in fish with amoebic gill disease and levels increased in fish with more severe pathology. The bacterium T. maritimum appears to be commonly found in fish gill tissues, however high levels of the bacteria were associated with gill disease. The bacteria were also found in some species of jellyfish, suggesting that they may act as vectors. The occurrence of epitheliocystis, caused by the bacterium Piscichlamydia salmonis, has also been associated with gill disease and increased loads of the bacteria on gill tissue occurred in cases with severe gill pathology. Engineering review This report looked at the threat of harmful jellyfish blooms to salmon aquaculture and a number of possible mitigation measures were described and discussed. Cessation of feeding, oxygenation of cages, and forcing salmon deeper in cages are methods that are being or have been used by salmon farm operators in order to defend against jellyfish blooms. While there is logical reasoning behind the use of each of these methods, no evidence of the effectiveness of these techniques has been found. It is recommended that control studies for each of these techniques be undertaken to assess their effectiveness. In terms of mitigation technologies that have not been used previously, air bubble curtains may be the most promising. Air bubble curtains have long been suggested to prevent jellyfish blooms entering salmon cages but there is little evidence of their effectiveness. Small scale trials using water filled ping pong balls to stimulate jellyfish have indicated that bubble curtains may be effective in pushing jellyfish to the surface where they would accumulate at a float and collected using a suction pump. The energy required to run a bubble curtain may limit its use, especially where there is a strong current as costs may be prohibitive. Closed containment systems are still not cost competitive with conventional salmon cages. The main reason for the costs of closed containment aquaculture being so high are the energy costs associated with pumping water. This needs to be reduced in order for it to become competitive. Concepts where renewable energies are used to supply aquaculture energy requirements should be developed and the economics of combining the two industries studied. Removal of biofouling is a labour and capital intensive task for the aquaculture industry. Current methods of dealing with fouling in Ireland are replacing nets for cleaning and in situ cleaning with power washers. Replacing nets has the effect of causing stress on the salmon while power washing results in bits of fouling drifting in to the cages where they may cause gill problems in the salmon. Research on anti-fouling coatings which are suitable for use on organic salmon farms should be undertaken. Field trials should also be undertaken on the combined water jet and vacuum system which would prevent removed fouling from entering the salmon cages. Recommendations for future areas of work: This project has identified two general areas where more research is required: (1) early warning systems consisting of a combination of zooplankton/jellyfish monitoring and investigations into pathogens; (2) developing mitigation measures. Zooplankton/jellyfish monitoring: as no official monitoring programme exists, the training of people involved in the aquaculture industry to sample and identify zooplankton species will be necessary. Developing models to track and predict the movement of jellyfish is an area which needs to be developed as well as studies identifying which particular species are harmful to fish. Pathogen investigations: the roles of the various suspect pathogens in gill disease need to be fully elucidated. It is also important to determine how these pathogens are transmitted to the fish and what the risk factors for gill disease are. Once the roles have been determined, research into the development of vaccines, therapeutants or special diets should be carried out. Mitigation measures: the effectiveness of various strategies such as the use of bubble curtains, flocculating agents, and the use of anti-foulants needs to be investigated. Alternative cage design is also an area which could prove useful in mitigating the effects of harmful jellyfish blooms.
The aims of the GILPAT project were to take a multidisciplinary approach in order to further understand the underlying causes of gill disease in Irish farmed fish. A specific aim was to establish a pilot zooplankton monitoring programme and use training workshops to enable fish farmers to upskill in areas such as zooplankton sampling and basic identification of the main zooplankton/jellyfish species common to Irish waters. Complimenting this was the development of a number of molecular diagnostic methods for the detection of potential pathogens suspected of being involved in the development of the condition. Together with a comprehensive literature review, epidemiological study, and experimental challenge studies, the project aimed to bring all these elements together with the objective of outlining potential mitigation measures and identifying areas for future research.
Keywords
Parasite;
Monitoring;
Jellyfish;
Open sea aquaculture;
Zooplankton;
Fish;
Cage aquaculture;
Salmon;
Disease;
Fish health;
Marine Region
29
West of Ireland (27.VIIb)
1
Marine Region Map
