Possibly the fish that die are suffering from "gas bubble disease", since the oxygen solubility decreases with temperature, what is in solution earlier can form bubbles at higher temperature. The death of the fish depends on if the bubble prevents capillary flow in vital organs.

You say that you have seen that happening in the last ouple of years. Has any changes in getting water taken place when it started?

Dear Mikko,

 In the dams it is difficult to follow water, beucause sometimes they run the system to produce electricity.  So the level of water sometimes increase sometimes decreases. To me in such dams loading biomass seem to change water.  If farmers loads too much fish, then  we observe some organic pollution in the dams.  Timing of the event is very interesting.  It starts exactly at 18 and continues above.  But 18-21C is the hottest time of mortalities. 

the symptoms described look like oxygen deficiency. In meso-eutrophic waters oxygen concentration may vary in a daily rhythm with a minimum in early morning/late night. It is therefore suggested to install within the cage a continuous monitoring of the oxygen concentration.

That in the first hit the highest mortality occurs and later less mortality is a further indication of rhythms of oxygen content. With a high fish stock oxygen consumption of the fish stock is highest and may lead to oxygen concentrations in the cage below the tolerance level of the fish in the early morning. With decreasing fish stock  (after first mortality) oxygen consumption  is lower and therefore oxygen concentration may be not as low as with high fish stock.

Thanks Roland,

That is exactly what we see frequently, mortality occur very early morning or if you feed late afternoon.  Is this beucase of oxygen or not, that is not I am sure about it. 

Very early morning and late afternoon just before sunset fish become very active (instrincly).  It should find food quickly before sunrise, or sunset.  It is an behavioral intrinstic survival matter for fish. 

It is not the feeding regime of fish, though we strategically do not feed afternoons in the peak summers, because the contrast between daytime and nighttime is drastic.  Change from sunset to the night is extreme.  Think about it, due to high light phytoplanctonic -positive- effect on oxygen (and interestingly on pH) is extreme during day time, As days changes everthing turns upside down.  The real time adaptation to this phenomenon is cumbersome for fish.   However effect on each fish seems also different depending on the load on the fish, or physiological status of fish.  Yes since it is a drowning, it should be related to the Oxygen in some way.  But why it occurs on very healthy fish?   Our situation is that a big wave of something hits (at 18 and above) the system than it resolves over time ( a  month or so). 

I will post pictures if I can get them.

Dear Hamdi,

reading your statement my suggestion is to reduce feeding intensity at temperatures exceeding 18°C, oxygen problems are independent on the health status of the individual fish, oxygen depletion kills also healthy fish. Again my suggestion to install a permanent oxygen logger.

Dear Roland, thanks again,

to complicate further "reduced feeding" increases mortality.  I know that is awkard, but it is the only way to reduce mortalities.  My theory on this that reduced feeding at the beginning of the summer starts developing external feeding (zooplankton) habit.  There is some sort of zoo that paralyses fish to death.  I always succeeded to reduce mortality by heavy feeding after seeing drowning syndrome.  This could be some sort of bacteria externally as well.  However I could not find any evidence of bacteria affecting operculums. 

Dear Hamdi,

ok, then the problem seems not to derive from low oxygen but from a toxin incorporated by the fish through feeding. I know an example when zooplankton killed Coregonus larvae in early spring in Lake Constance (Eckmann et al, 1986).  You mentioned very healthy fish affected, these are the fish feeding most effectively and thus ingesting more zooplankton than already affected ones. I do not think it is bacteria, it seems to be a toxin ingested via the zooplankton. 

Is there any mortality in fish outside the cages ?

Yes that is my most outstanding theory at this time as well.  Studying zooplancton in the lake could be very rewording.  I tried to encourage some people for that, but failed unfortunately.  Around the cages there is no mortality, but it would be very difficult to observe due to the birds and some morts released by the farmers.  But I do not recall any observance by the workers or fisherman. 

This is a classic lack of oxygen problem. Sudden death in the early morning before the sun rises on still hot nights, oxygen levels normal when measured in the morning when the dead fish are seen. I studied it intensively in aquaculture ponds using  remote sensing in the 1980's.  Basically the algae in the lake (there doesn't need to be a visible bloom) remove oxygen at night. Warm water holds less dissolved oxygen anyway, so the oxygen levels fall below the level to sustain life. Then when the first rays of the sun strike the water, the algae begin producing oxygen which rapidly rises to "normal" levels. The key is the wind level. Oxygen diffuses poorly in water - If there is wind on the lake at night, the surface water turns over and oxygenates, if it is still  there is a fish kill. Fish can survive also if a thermocline develops protecting cold deep oxygenated water in which fish can survive.

An interesting finding was that "paddle wheels" have an effect simply by moving the surface water, exposing water to the surface where it oxygenates by diffusion -, not by the splashing which has minimal effect. A pump can be more energy efficient, especially where there is deep anoxic water.

Dear John,

Thanks much for the input.  I liked your input on the effect of the wind or paddling.  I did not understand use of deep anoxic water with pump.  . 

On paddling, we have two types of cages: one is offshore chages, and the other one is square ones (12x12m) on the floats.  Fish perform better in the square ones.  The surface water exchange is weak in offshore cages in the dams. 

The use of a pump or a bubble plume to move deep anoxic water to the surface has been well proven. See http://www.aslo.org/lo/toc/vol_40/issue_2/0359.pdf

The technique can be improved by using the bubbles in a pipe as an airlift to take subsurface water directly to the surface. Once the bubles form there is very little diffusion of oxygen out into the surrounding water.

Thanks John,

The depth of water is about 65 m below the cages.  It would be very difficult to give air to that depth.  Never tried, but would be tried.  Anoxic water is one thing need to be considered.  This layer looks like slowly is lifted up by slow temperature increase, slow turnover , We feel it when it is close to the 10 m, then around surface 4-0m mixes up.  At that period (estimated around 10 days) fish is disturbed.  Giving air at some point would help mixing way below the cages and prevent anoxia close surfaces. 

To overcome anoxic water below a Japanese scientist used fibed optic cables to give light to stimulate plankton growth under cages to remove anoxia.  It is a good iea to be explored further.