PlanetCatfish.com

In lab we have a respirometer for fish. The fish goes in a clear tube (the chamber) with a sponge-covered intake on one side and a sponge-covered outlet on the other. The respirometer pushes water through the chamber, measuring the oxygen levels coming out, and switches between circulating the old water and flushing the chamber with new water. We've put some of the in my lab in the respirometer and after we did some mild adjustments (always face the fish into the flow!) everything has worked fine.

However, we have now had two bad failure with . In the first instance the fish flipped out and jammed itself against the sponge and we just let it out. My research assistant said she thought maybe she had pinched a barbel closing the chamber, and that sounded like the sort of pain that would make the fish react badly. Today we tried again using a large diameter chamber to make sure the fish had plenty of room. My assistant was very careful of the barbels transferring the fish into the chamber, but it still reacted badly and ended up lying on its side on the bottom of the chamber. In the smaller chamber we had thought it had actually become stuck because the sponge in that chamber projects a bit, but in the larger chamber it's impossible for a fish that size to get stuck because the gap is so much larger than its head. We monitored the fish, it was definitely breathing, but it just lay there (which is really strange for a Kryptopterus). Any time I moved the chamber it would recover and move around, but then lay back down. When we released it back into the main tank it moved well for a minute and then lay down. Eventually I moved it to a ten gallon tank so it could (hopefully!) recover without other fish picking at it.

Anyway, I have no idea why this behavior occurred, and especially why the fish remained poorly/disoriented after being moved back into a large tank. Any thoughts?
Note: it didn't happen in the glass catfish, which are the same size, and it's not oxygen levels, because we're actively monitoring that and they barely changed. The respirometer is also using water from the fish's original tank, so there's no change in water parameters.

I'm going to update this since I think the mystery is solved.
The catfish died the next day. Symptoms appeared consistent with a damaged swim bladder. This made me suspect that the fish had been exposed to high pressure in the chamber. Sure enough, my RA and I played around with the larger chamber (empty) and determined that when the chamber is pressed closed it can fairly easily seal before chamber cap "seats", and that the result is that the closing chamber-cap acts as a piston. We opened a few bubble-release valves in the lines and we can see water get expelled as the chamber closed. However, that also means that we have a way to let that pressure equalize in the future.

The analysis does seem to be sound. I think it was quite a nasty way to die.

Now you know what happened, and how to prevent this from happening again, I think the chamber can be used again for your research.

Sorry for the loss. I've seen that type of compression in fish swimming chambers I used to use in the 1990s. We had a built- in vent, but the "pop" of the seal still created surges. Better luck to you going forward.

Cheers, Eric

A timely article appeared today, not so much about injury in a swimming chamber, but includes their design of one:

https://www.mdpi.com/2311-5521/6/3/127/htm

Cheers.

Bas Pels wrote: 26 Mar 2021, 07:36 Now you know what happened, and how to prevent this from happening again, I think the chamber can be used again for your research.

Yes, I think we have a good strategy for preventing the pressure build-up now. I suspect that we didn't have issues with K. vitreolus because we had the bubble release valve open or something. I doubt it has anything to do with species.