There is increasing concern in the aquaculture industry that biofouling, the unwanted growth of organisms on farm nets, may impact gill health. Among the main biofouling species are hydroids, which are related to jellyfish and bear similar stinging cells. The exposure of salmon to hydroids during biofouling management operations such as net cleaning is suspected to cause gill disorder. Biofouling management on salmon farms involves regular (up to weekly) cleaning of production cage nets. Each year, individual farms can release dozens of tons of biofouling material during net cleaning into the surrounding water. This cleaning waste comprises organisms, fragments, larvae and, potentially, pathogens. The effects this material may have on salmon are not well understood. Similarly, no knowledge exists on the dispersal and deposition of net cleaning waste and its ability to facilitate the spread of pathogens.
The aim of the STING project was to determine the risks of current biofouling management in Norwegian salmon farming to fish health and biosecurity.
To determine the proportion of Norwegian salmon farms where fish health and welfare may be affected by the presence of hydroid biofouling and to learn more about current biofouling management strategies, a survey with 51 Norwegian salmon farms was conducted. Hydroids were present on almost every site, in addition to algae, caprellids and mussels. The main motivation for cleaning the nets was to keep cleaner fish from feeding on the biofouling. Most farmers (81%) reported that fish behaviour is affected when the cage is cleaned (e.g., reduced appetite, general signs of stress i.e. jumping) and many farmers (43%) report similar impacts on fish in the neighbouring cage. As a consequence, feeding is often stopped or reduced for up to 8 hrs during net cleaning. (Published in Norsk Fiskeoppdrett 12/2018)
To assess the direct impacts of cleaning waste on salmon gill health and possible interaction with other gill diseases, a challenge trial was conducted where salmon were exposed to hydroid-dominated cleaning waste for 3 hours. A second group of fish was further exposed to Paramoeba perurans, the causative agent for amoebic gill disease (AGD). To assess the effects on gill health, the fish were monitored for 5 weeks using gill scoring, histological and PCR analyses. The results showed that exposure to hydroids can negatively affect gill health. Gill health was significantly poorer in treated fish than in control groups up to 7 days after exposure. Exposure to hydroids had, however, no impact on the development or severity of Amoebic Gill Disease.
(Published in Plos One, https://doi.org/10.1371/journal.pone.0199842)
To assess the release and distribution of cleaning waste and its associated risks, cages at three salmon farm sites were analysed with regard to biofouling condition of the net and particulate material in the cage before and during net cleaning operation. Biofouling covered up to 72% of the net and was dominated by hydroids and algae. During net cleaning, the abundance of biofouling particles in the water increased up to 19-fold, with the number of hydroid particles increasing up to 73-fold. Of the particulate material collected during net cleaning, biofouling organisms accounted for up to 46%, more than half of it being hydroids. The rest were planktonic organisms from the surrounding water masses. Most particles were smaller than 3 mm; few particles reached larger size, and their number increased during cleaning.
Based on measurements of particle sinking speed conducted with sampled material, particle distribution was modelled. The results show that cleaning waste particles accumulate in the cage during net washing, potentially injuring the fish. By spreading throughout the farm site, also fish in neighbouring cages are at risk for gill injuries and infection with pathogens associated with the waste particles. However, choosing the farm location based on sea bottom geography and current conditions can prevent transport of cleaning waste particles to neighbouring sites. (The data will be published as a scientific article).
Additional data on the interaction between farmed salmon, hydroids, and other cnidarian species growing on cage nets was collected during a research visit with The Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Tasmania, in collaboration with an Australian salmon producer. This knowledge on international salmon farming supplements the local data collected during the postdoctoral work and has been included in an article on 'Environmental considerations in aquaculture health management'. This work will be published as a chapter in the book 'Aquaculture Health Management' in 2019.
Finally, the knowledge gathered during this postdoc project will further be incorporated into a review article on 'The impact and control of biofouling in marine aquaculture', to be published in 'Biofouling' in 2019.
Gill health is one of the current key topics in Norwegian salmon aquaculture. There is growing concern that exposure to cnidarian biofouling such as hydroids (the dominant biofouling organism on Norwegian salmon farms) can cause gill disorder in salmon, but there is a lack of robust experimental evidence. Biofouling management on Norwegian salmon farms involves regular in-situ washing of production cage nets. During this process, individual farms can release dozens of tons of biofouling material into the surrounding water annually, comprising organisms, fragments, larvae and, potentially, pathogens. The effects this material may have on salmon are not well understood. Similarly, no knowledge exists on the dispersal and deposition of net cleaning waste and its ability to facilitate the spread of pathogens.
The main objective of the STING project is to use field and laboratory studies to determine the risks of current biofouling management in Norwegian salmon farming to fish health and biosecurity.
STING will build on data from an experiment conducted in parallel in 2015 that will elucidate the ability of hydroids to cause significant gill pathologies and thereby increase the susceptibility of farmed salmon to infection with Amoebic Gill Disease. The STING project will use field and modelling approaches to: (a) determine whether during net cleaning operations in Norwegian farms salmon are exposed to potentially harmful levels of hydroid material, (b) identify the proportion of Norwegian salmon farms that may be exposed to gill health risks posed by hydroids, and (c) determine the dispersal of cleaning waste released from salmon farms and its ability to facilitate pathogen transport between production cages or adjacent farms. STING's results will directly feed into the development of improved biofouling management practices and technologies to advance fish welfare in Norwegian and international salmon farms.