Trout farming, and in particular organic trout farming is critically challenged by relatively high skin/gill infections caused by various pathogens, especially the parasite Costia (Ichthyobodo necator), which is ultimately lethal for fry/smaller fish. In addition, a bacterium Midichloria-like organism (MLO) causes Red Mark Syndrome (RMS) with large red patches in the skin, which makes the fish less attractive, hereby lowering their value.

Treatment by use of antibiotics/auxiliary compounds is only possible to a limited extent in organic trout production. Organic regulations for aquaculture requests also fry to be 100% organic, this in turn compromises supply of organic fry as basic for future production of organic trout.

The project ShelterFish has aimed to develop methods to solve key health problems in organic rainbow trout production related to infectious diseases, thereby enabling a larger and more cost efficient production with high animal welfare and low environmental impact. Especially, solutions to prevent and/or treat Costia and RMS are seen as urgently needed.

Does shelters support welfare of rainbow trout juveniles?

The researchers have evaluated the fish welfare in response to providing artificial shelters for the fish. Rainbow trout were exposed to two types of submerged shelters: Full screens or partial semi screens. The welfare was assessed and compared to a control group (without shelters) by evaluating growth and condition, extent of external lesions, and also the neuroendocrine responses to acute and repeated stress.

A separate study of potential effects of early exposure to shelters on the juvenile performance later in the life cycle showed that shelter-grown fish performed less competitive compared to non-shelter-grown fish in challenging conditions favoring competition (low stocking density plus restricted feeding). However, this effect was clearly mitigated when letting shelter-grown fish to acclimate to non-shelter conditions before the competition tests.

Finally, a field study was performed in an organic rainbow trout farm with simply designed shelters to investigate rainbow trout fingerlings’ preference for shelter or not, and the behavioral response to farm-related stress, as well as the cortisol response and growth performance.

The experiments showed that:
1) Fish have a clear behavioral preference to use/get protected under the shelters, particularly when disturbed, and
2) shelters had no deleterious effect on fish performance, welfare indicators or survival - though, shelter resulted in reduced fin damages.

Based on the results, in particular the field studies, it is suggested that future trials, designed to study the effects of shelter on rainbow trout welfare, would benefit from longer habituation period prior to stress testing and sampling.

Hence, the studies performed showed potential for shelters to be used as a welfare promoting strategy in trout farming, but further research is needed to optimize the shelter type and design and the proper timing for its application.

Sustainable anti-parasitic agents against parasites

The parasitic pathogens Costia (Ichthyobodo necator), Ichthyopthirius multifilis (ICH-white spot disease) and different amoebae (Vanella sp.) have been identified in gill samples collected from trout in organic farms using Q-PCR array and histology. Test for establishment of Costia cultures have been performed at organic farm with positive results.

ShelterFish has tested extracts of oregano, thyme, coriander, garlic and astaxanthin in vitro and in vivo against the ciliate, Ichthyophthirius and the flagellate Costia. These natural compounds showed an effect on the infection in the early stage but not after 8 days.

Garlic exhibited the highest in vitro killing capacity towards theronts (the infective stage of the parasite), followed by oregano, thyme, and astaxanthin, whereas coriander did not have in vitro parasiticidal effect. The results suggest that the investigated additives inhibit early parasite development directly - as well as indirectly (i.e. by elevating the host immune response).

However, more promising results were found with a novel biological control agent, the surfactant from Pseudomonas H6, a kind of biological soap, which opens the cell membrane of the parasite so the content float out, and the parasite is killed. The tests showed that this agent has a strong effect on a series of organisms colonizing gills, skin and fins of organic trout. The compound could kill many of the organisms including Ichthyophthirus, but more importantly, the surfactant seemed to clean off the colonizers (Amoebae, Ichthyobodo, Trichodina and Ambiphrya).

The auxiliary water treatment compounds, the biocides formalin, peracetic acid and hydrogen peroxide induced the mucus secreting cells on fish surfaces (gills, skin, and fins) to empty their content upon stimulation.

Experiments on prophylaxis/treatment of gill pathogens showed that amoebae were killed at the following concentrations: 16.9 mg/ml (raw garlic-extract), 100 mg/L (hydrogen peroxide), 250 microgram/ml (bacteria surfactant), 17.9 mg/ml (oregano extract), 7.5 ppt (NaCl).

No effect of shelters could be detected in gills, skin and fins on colonization of fish surfaces with amoebae and ciliates.

Testing natural immunity against RMS

Red Mark Syndrome (RMS) is a disease affecting rainbow trout under farming conditions. The disease is not lethal but is costly for the fish farmer as the symptoms of the disease are large, red skin lesions, which often appear in fish of market size. There are currently no good treatment options for this disease.

ShelterFish knew that RMS does rarely occur twice in the same fish population, therefore the project investigated whether early exposure to RMS as fingerlings could result in protective immunity throughout the remainder of the production cycle, thus preventing skin lesions in larger fish and thereby severely reducing the problems associated with RMS.

Pathogen free (SPF) rainbow trout fingerlings were exposed to RMS by cohabitation. Most of the cohabitants developed clinical signs of RMS during the following three months. The fish were now maintained until 13 months after initiating the first round of cohabitation, at which time they were once again exposed to RMS via cohabitation. This time, non-exposed control fish were also added. The second cohabitation was terminated after 77 days. By then, all negative controls had developed several RMS-lesions, whereas no skin pathology was observed in the pre-exposed fish. All experimental procedures were carried out at 12°C.

The results showed that under the experimental conditions, protective immunity against RMS can be induced in fingerlings and last for at least 13 months.

Less organic material makes more healthy fish

Use of chemical disinfectants and organic aquaculture may seem contradictory.

There are however no direct contradictions between these two terms, as long as they are developed and improved towards more sustainable solutions. A natural and immediate aversion to words like “sanitizers, chemicals and auxiliary compounds” are frequently met. Similarly, aquaculture production is often criticized and have a tarnished reputation. Organic aquaculture is a way to differentiate from conventional fish production. Some of the challenges still remain to be solved and during that process, ideally important knowledge and experience can also accelerate better management in the conventional aquaculture industry.  

Based on the investigations in ShelterFish, the project recommends the following procedures to achieve optimal water quality in organic trout farms:

• Reduce particulate organic matter, as the microbial water quality becomes improved by further removal of organic matter, i.e. lowered bacterial load, increased oxygen concentration and decreased dissolved CO2.
• Increase water flow and circulation/internal recirculation to improve oxygen conditions and reduce retention time and potentially dampen intra specific aggression to change fish behavior from erratic to more streamlined swimming (welfare).
• Proactively learn to use safe chemicals with caution to become confident with optimized chemical water treatment (dosages and fish behavior).

The project was completed in a cooperation between Technical University of Denmark/DTU Aqua, University of Copenhagen/Department of Veterinary and Animal Sciences, The Danish Aquaculture Organisation, Lihme Dambrug (organic trout farm), Vork Dambrug (organic trout farm) and Ravnstrup Mølle (processing).

The project has been part of the Organic RDD programme, which is coordinated by ICROFS and granted by GUDP (under the Danish Ministry of Food, Agriculture and Fisheries).

Visit the website of ShelterFish