Hej Joacim, good party on Wednesday. Sorry to hear about your fish dying. Logically, one might suspect stress effects from the transfer from one environment to another. However, even if stress effects are energetically costly, the secondary effects, i.e. the effects of the release of stress hormones would logically include more blood being pumper through the gills, not less. So there's no "internal" reason why blood flow should be shut down through one set of gills. So perhaps you're looking at tertiary stress effects, i.e. reduced immune response, and that there are some pathogens in the water which are now succeeding in infecting the gill tissue of some fish. Infecting the gills on one side would not necessarily lead to an infection on the opposite side, but would probably cause such a loss in oxygen uptake capacity that the fish are dieing of hypoxamia as you describe.

But we need a good pathologist in here to suggest what bug could be causing this and what you could do to stop it.

I regularly check the Hatcheries. If the water don't have sufficient Dissolved oxygen most of the fry die after hatched from eggs at this stage they require more D.O levels. The Symptoms that you have mentioned in the fry is the same there .We have improved the D.O levels it got controlled.

I'm with Bjorn on this one. It sounds to me like a 'stress-induced' pathogen infection that was kept at bay in the first aquarium but was induced by the second transfer. Proper pathology will likely be your only real answer. Sorry to hear this has happened to your experiment, it's not fun.

Hi,

This is an interesting observation, and I would support the need for moribund fish presenting with the clinical signs you have reported to be sampled for histopathology. This should be done to determine if the problem you are observing is pathogen or non-pathogen driven. So I would support the previous statements that you need to have some tissue samples processed for pathology prior to any "bug' hunting as this may not be due to a pathogen but may be aggression/stress/behavioural adaptation. You mention that you use anaesthetic but not what kind of anaesthetic and how often this has been used - some have an immunomodulatory effect in certain fish species, which might leave them predisposed to secondary infections.

It could be the lack of surface area for bacteria has allowed nitrite levels to rise, creating a toxic environment for the fish. It might not be a bad idea transfer some of the structure from the old tank in order to proliferate the bacteria colonies in your new tank. Good luck!

Thanks for all the answers! This has been helpful for getting an overview of what is happening to the fish and has helped in making some decisions regarding the experiment.

To answer some questions and comments:

Oxygen levels are good. The system is flow through and we have quite a good current.

Fish had been in the lab for about a week when we made the first transfer.

Fish were captured by electrofishing in the wild - if the fish survives the first day after that, they are usually perfectly fine.

No external disinfectant was used - we never do and have never had this problem before.

Anaesthesia used was 2-phenoxyethanol.

Unfortunately, fish pathologists are rarely found around here.

I belive the tertiary stress effect may actually be the cause. The pattern of outbreak seems to match one that would be due to distress.

The last days we have observed that the gills of some fish dying has also had fungus on them - perhaps this is the main cuase of the mortality, or a secondary effect of another pathogen.

In any case, the problem is now "solved", as we sacrificed all fish. We will try the experiment again with another batch of fry, and this time we will take extra care to minimize stress. Fortunately, we are able to use the fish for validating some experimental techniques - so it is not a complete waste of fish lives and research time.

A mild salt-water bath when the wild fish arrive in your lab might be a good idea.

@Sean: Do you have any advice on what concentration to use?

I think we've used about 5 ppm on the rainbow trout in the past when we suspect an infection

I agree, we have used 5-10 ppm with freshwater wild-caught eels.

Coming in a bit late here. 2-Phenoxyethanol would unlikely be the cause of the problem. What concentration did you use out of interest? You may want to check for nitrogen supersaturation as I have seen similar pathology associated with infarction of the lamellae where emboli have blocked blood flow, and the patholgy presented when you moved the fish into a different sytem. For some reason one side always seems to become affected rather than both. Eyes also become exophthalmic. However, you are clutching at straws here unless you investigate the pathology further with histology as Mags from Stirling suggests. Both BGD and infarct tissue are easily diagnosed with histology.

Regards, David

It is essential to control the temperature, nitrite and dissolved oxygen level.

To revive the fish, after anesthesia and / or electric fishing is very important to provide much oxygen and equal or lower temperature. Ensure that the oxygen is located at the bottom of the tank with a good diffuser, creating very small bubbles. When handling the fish, stress that produced them is extremely high, ideally using dark tanks and reduce the concentration of fish.

Beware the change in diet!

Salt baths or Aqua-iodine, solve the problem with fungi.

Good luck

I concur that you need more information and that the best way to do this is have a someone look at tissues from the affected fish. There are many labs throughout the EU and the USA that given some properly taken and preserved samples could give you a good idea of what is occurring. Do you have a microscope? If so, look at a gill smear from a dying animal (not dead) and compare it with that from a healthy animal. Play with the light to see if you can visualize fungal hyphae or filamentous bacteria. You should also be able to determine if there are other organisms present such as protozoa. This would allow you, in theory, to address the problem, depending upon what you find.

When a disease breaks down it is always due to several and cumulative reasons, not just one. In order to stop the disease, or at least trying to stop it, you should first think on all possible triggering factors.

As already said, STRESS (everything that moves away form normal and healthy living conditions might cause stress) is the main entry door for all kind of diseases.

In your case Stress factors are;

- Wild animals don’t get along with being captured and confined,

- The catching method "electrofishing",

- Higher densities in the barren tank than allowed,

- Wild animals use to carry parasites and microorganisms that in healthy conditions don’t appear or grow. You haven’t mentioned any sort of quarantine process or bath. If you have planned to group fishes from different places and catches I recommend some sort of quarantine, in order to avoid future problems.

- LACK OF OXIGEN (bad circulation of water inside the tank, big bubbles, contaminated inflow air, lack of water movement or aireation, high temperatures),

- TEMPERATURE (normally too high, besides of lowering oxygen levels, could be a triggering factor of bacteria and fungus fast growth) ,

- HIGH LEVELS OF TOXIC SUBSTANCES AS NITRITES (water parameters should be measured first, as well as filtering system checked up, but you pointed that in your first aquarium you kept "quite a lot of structural enrichment" that was not brought at your barren tank, so depending on your filtration system, some bacteria in charge of the nitrification process NH3 => NO2 => NO3 (nitrobacter/nitrosoma/nitro..) ..that could be living in those structures weren’t brought to the barren tank, causing a possible peak of NITRITES.

- Your barren tank was not properly clean and disinfected, and opu

All these factors could trigger an infectious process, moreover if they happen at the same time. So avoiding them or mitigate them, the disease might disappear.

“Swimming at surface and quick beating of gill covers” means that they are lack of oxygen, but this is a symptom, not the cause. Trout fries do not get all the oxygen they need because their gills don’t do their job, O2/CO2 exchange. Since your system seems to have good water conditions I incline to think that something bad is happening in their gills. All clues described seem to point at an infection disease, and should be treated as such.

I am not a ictiopathologist but I have found the following two disease that matches with your described symptons.

FUNGAL DISEASE. Branchiomyces sanguinis / Branchiomyces demigrans

Profilaxis:

Baths of copper sulphate ( 8mg/l for 60 min),

Baths of non-iodized sodium chloride (15gr/lit.).

Baths of malachite green (1mg/ 10 lit.)

BACTERIAL DISEASE: “Gill Disease” Myxobacteria

Profilaxis:

To improve aeration and bottom clean up, and to disinfect the tank

Oxitetracicline or tetracicline (50 mg/ lit).

Acriflavine + methylene blue, Baths of Malachite green (1mg / 10 lit.)

Methylene blue + tripaflavina

Hope this helps.

Good luck.

I would have to agree with the notion that stress is the culprit. Between catching them handling them and the secondary transfer the effects of the corticosteroids released did not have sufficient time to return to acceptable levels. This would not have had such a significant impact on larger fish, but fry are very susceptible to these effects. The bright side is depending on your experiment you have just eliminated the fish with the lowest fitness in this regard, although depending on your experiment this could also not give you an accurate snapshot of the population.

Dear Naslund

In line with what others have said, I too suspect some kind of bacterial infection. Please do histology of the gills and also check if there is any secondary infection because of parasitic protozoa or crustaceans. But before treating please confirm the causative agent.

Good luck!

Dola.

Again, thank you all for comments. As I wrote before, the fish were killed. In the end we could not continue the experiment as 50% of the fish had died after just a couple of days - before we had even started the experimental manipulation. Instead we collected new fry and adjusted the handling of the fish to minimize stress. For this the comments in this thread were of much value! We still do not know exactly what killed the fish, but the new ones are doing very well and have no signs of stress or disease.