What Killed These Trout?

[Elton]

Posted on behalf of Fliss. Please add all replies to this thread:

 

Hi im hoping that someone can help, i work for a couple in dorset who have a large pond which untill recently (last week) had some beautiful trout in. suddenly they have dissapeared, we know that there has been a heron around but the fish were too big for it to take. all that remains of the fish are small white fleshy lumps in the water and an oily film on the waters surface, even the heads have been eaten. there are no natural rivers or streams around us so no mink and we are completly puzzeled as to what has the speed and strength to catch and eat about a dozen trout in a week, any ideas would be appriciated. thanks

[Worms]

Posted on behalf of Fliss. Please add all replies to this thread:

Some pictures might be useful. The oily film indicates possible organic decomposition or even planktonic bloom. Both of these can affect dissolved oxygen content in the water and affect aquatic life.

 

The fish remains, are there carcasses or just odd fleshy lumps?

 

Whereabouts in Devon are you? Otters would take the fish but usually any remains would be left on the bank.

[Steve Walker]

Sounds like a water quality problem to me. The oily scum could be fats from the decomposing fish.

 

The size of the pond and the number of fish in it would be a good starting point.

[chaseroyale]

Sounds like a water quality problem to me. The oily scum could be fats from the decomposing fish.

 

The size of the pond and the number of fish in it would be a good starting point.

 

 

Thanks for the response, the pond is roughly 30 ft by 15 foot and 3-3.5 ft deep, it has a pump moving the water up and down a cascading waterfall and plenty of plants to keep the water oxygenated. It contained 14 trout, the fish were living in it quite happily for 3 years perious to this. there is no running waterways for 6 miles around us and no-one else in the village has a pond.

 

Fliss

[chaseroyale]

Some pictures might be useful. The oily film indicates possible organic decomposition or even planktonic bloom. Both of these can affect dissolved oxygen content in the water and affect aquatic life.

 

The fish remains, are there carcasses or just odd fleshy lumps?

 

Whereabouts in Devon are you? Otters would take the fish but usually any remains would be left on the bank.

 

 

thanks for the reply, there is just the odd fleshy lump left in the water, not enough to even make 1 fish up and we have lost a dozen, the pond has been established for three years before this and has pumps to keep the water clean and moving well, and there are many diffent plants to keep the water oxygenated.

 

there are no running waterways for 6 miles around us as we are in chalk hills of Dorset.

 

thanks for your help, Fliss

[Worms]

thanks for the reply, there is just the odd fleshy lump left in the water, not enough to even make 1 fish up and we have lost a dozen, the pond has been established for three years before this and has pumps to keep the water clean and moving well, and there are many diffent plants to keep the water oxygenated.

 

there are no running waterways for 6 miles around us as we are in chalk hills of Dorset.

 

thanks for your help, Fliss

What is the source for the water supply to the pond? Obviously we have had very little rain over the summer and levels will have dropped. Is the source natural or domestic?

[chesters1]

bands of roaming Romanians?

[Steve Walker]

Depending how big the trout are, that could be a very high stocking density. For instance, big reservoirs are stocked at about 12kg of fish per hectare, while a natural brown trout water may support 40-150kg per hectare. If the fish are, say, 8oz each, that would be running at about 760kg per hectare - that's a lot, that's in the same order as heavily stocked commercial carp fisheries. If they were too big for a heron, they must have been a fair bit bigger than 8oz!

 

Obviously, a well filtered and fed ornamental pond can support a higher stocking density than a wild river, but I do wonder if this just pushed it over the edge. What surprises me is that the fish have done so well in a shallow pond for three years, and that when things have gone wrong it has been in the cooler weather - I would have expected water temperatures in a hot summer to be close to the upper lethal limit for trout.

 

If I had to hazard a guess for what happened, I would say that over the last three years the fish have got bigger, and the ability of the system to maintain good enough water quality has become a bit borderline. Something has happened - possibly something external, possibly aggression or territorial behaviour in the fish - and one of more of the fish has died and rotted. That has pushed the water quality over the edge, leading to the other fish snuffing it in a chain reaction. Dead fish will quite quickly rot and be scavenged upon so that they look more like an anchovy fillet than a whole fish - just a lump of gooey flesh held together by the backbone.

 

I would suspect that chain of events rather than a predator - I think that if you had a predator problem, the fish would have just vanished with no trace other than maybe some scraps and scales on the bankside.

[chaseroyale]

Thank you so much to all who replied to the post and for all the advice recieved, unfortunatly all the trout have now been eaten with no deffinate answers.

 

thank you all again, fliss