BEST PRACTICES FOR PREVENTING WINTER ULCERS IN ATLANTIC SALMON Salmo salar L. IN NORTHERN NORWAY

 Introduction

 Winter ulcers in  Atlantic  salmon  generally lead to reduced welfare and downgraded product quality.  Wound development is mainly linked  to  the  winter  ulcer bacteria ,  particularly Moritella viscosa or  a  cocktail of  other bacteria, i.e., M. viscosa , Tenacibaculum spp , Vibrio spp. These pathogens thrive at low temperatures (< 8 °C).  Naturally enough,  the  winter ulcers  outbreak in salmon aquaculture is a significant  fish health challenge in northern Norway, where  temperatures are lower  and production time is longer .

An essential measure  against winter ulcers is avoiding handling  the fish  as much as possible . However, due to high salmon lice infestations, the fish must undergo delousing treatments. These treatments can be both medicinal and non-medicinal.  In the case of non-medicinal  delousing treatment s  (which are more effective) , th ere are una voidable  injuries  to the fish skin,  where the outcome is usu ally ulcer outbreaks at low temperatures . The severity of the outcome depends on many factors, but handling intensity ;  among others,  how hard or gently the fish is handled, is very important. From our field experience, there is a significant knowledge gap in the cause-and-effect relationships and limited knowledge about the factors associated with this variation. Results from  our  pilot studies show that sedating the fish during crowding and delousing operations makes fish calmer , reducing the risk of ulcer development . In addition, there is little data about which indicators can be used to assess the risk of  winter ulcers. This lack of knowledge gives limited room for action to adapt the production plan or implement measures to reduce  the risk.

Furthermore, there is no  systematized knowledge about how crowding affects  the  risk of winter- ulcer outbreaks during delousing operations .  We will fill in this knowledge gap by  documenting  the effects of handling and different crowding intensities  (CI)  on the susceptibility of Atlantic salmon to winter ulcer outbreaks. We  will document this by answering the  following questions -

1. Does the level of CI during cold seasons  significantly affect  ulcer outbreaks?
2.  Is it primarily the physical damage to the skin and mucous layers,  stress, a compromised immune system,  pathogens, or combinations of all these factors that are decisive for wound development after handling?
3.  Can indicators for ulcer risk be established to be used as decision support when preventive measures against ulcers are to be defined?

The main goal of this study is to  identify best practices for crowding and mapping wound risks in connection with handling second autumn and winter at sea to reduce the risk of, or extent of, winter ulcer outbreaks in Atlantic salmon.  Results from this project will help to shed light on what is today’s best practice for crowding and how  it should be adapted to a situation with low temperatures and an increased risk of wound development. The author will  present and  discuss the experimental design at Aquaculture Europe 2023.

Material and Methods

The project is financed by the Norwegian Seafood Research Fund (FHF 901835 ), with a total budget of 1,112,261 Euros

. The project  (named ReduSår; duration: 2023-2025) will be a  close collaboration  between  the  Norwegian industry and research institutes/Universities.

 The  experiment will be performed at the  R&D Sea  facility of LetSea AS at Dønna, Norway, in October 2023 .  We will use mesoscale cages (n=12)  in triplicates  with large pit-tagged salmon (approx. 2.5 kg; n = 200 per cage ). All cages will be treated uniformly, except for  crowding methods during  the  corresponding delousing . We will test different  combinations of CIs, i.e., grades  1 and 3, according to FISHWELL ( referred to as T1 and T3) with or without sedation ( referred to as S+ and S- )  during a 60-minute crowding operation

, giv ing us three test groups T1 S+, T3 S-, T3 S+ and a control group (T1 S-) . Grade 1 is an acceptable CI in the Norwegian aquaculture industry,  whereas Grade 3 is  an undesirable  practice (see FISHWELL for more info). Water and fish mucus samples will be collected  throughout the experiment  to investigate the  presence of wound bacteria; furthermore ,  camera-based monitoring of wounds at  the fish group level will also be done.  The aim is to utilize the detailed monitoring of wound development in the experiment to test indicators for predicting ("early warning") of wound outbreaks.

Results

 Sedation as a measure for more gentle handling must be systematically investigated. In the short term, our results will help  salmon  breeders learn from each other’s experiences and increase the basis for decision-making when planning and carrying out handling operations. In the longer term, knowledge developed in the project will provide a basis for optimizing production planning, methods, or measures for lice prevention or treatment.

The project will also map possible new indicators for increased winter ulcer risk or "early warning" in case of ulcer outbreaks. Such indicators can initially help to improve the decision-making basis for c rowding methods or adjusting handling procedures before and during delousing . Over time,  new mapping tools can be established to develop knowledge about the causes of an increased risk of winter sores. The project will be able to contribute both knowledge and new tools that can improve preventive and mitigating measures to reduce wound problems in the industry, thereby improving fish welfare, profitability, reputation, and sustainability.

References

1.                       FHF. Best practice measures for the prevention of winter ulcers in the second autumn and winter at sea (ReduSår).

2.                       Noble C, Gismervik S, Iversen M, et al. Welfare Indicators for farmed Atlantic Salmon: tools for assessing fish welfare. 2018.