The Fish-kill on No5
Myland Monery, Thames Water ranger,
has given us the following feedback:
We have discussed this amongst the team and we believe there was a probable algal bloom on Reservoir 5 after a sudden spell of raised temperature and hours of sunlight, as a result the dissolved oxygen levels crashed overnight. Cliff was on duty the following morning (when oxygen levels are at their lowest) and received calls from worried anglers describing fish in distress on the No.5 Reservoir. He immediately went to investigate and on discovering approximately 100 rainbow trout around the curtilage of No.5 reservoir, alive but in distress and no issue on the No.4, he called me to inform me of the situation. I then came to assist Cliff in resolving the problem.
As we at this point had very few fatalities we decided the best course of action was to get the water in the No.5 moving by discharging the poorer quality water and replacing from another source. That halted the problem although realistically the better quality water did take some time to filter its way through the reservoir chain.
We monitored the situation over the following days and as an estimate we lost in the region of 100-200 rainbow trout. We have managed to clear up approximately 60 rainbow trout carcasses from the curtilage of No.5.
We do not believe that all the trout in the No.5 have died as no other species of fish have been affected. This also seems to confirm that there was indeed a drop in oxygen brought on by the algal bloom, as rainbow trout are a North American species and require a higher level of oxygen than that of the native species. There have been no brown trout fatalities to our knowledge (a native species) which backs up this assumption. It also indicates that some fish have survived in the No.5. If all the fish in the No.5 had died we would have discovered far in excess of the dead fish than we have, even taking into consideration the fish lying on the bottom and the fish eaten by wildlife. To put the 100-200 dead fish estimate into perspective, we stock the No.5 reservoir with 5000 rainbow trout per year and there is always fish from the 5000 that do not get caught leaving a positive balance each year.
Although we are still disappointed that we have lost any fish at all, we can be confident we did everything we could to reduce the impact of the crash in oxygen brought on by an algal bloom.
We are looking in to purchasing a portable multiparameter probe for testing dissolved oxygen levels. We will also have a meeting with Operations to discuss and implement a future event plan, to reduce the likelihood of this happening again.
On Wednesday of last week (8th May) we had a stocking of 1000 rainbow trout which we were very cautious with and only stocked 300 of these fish into the No.5. The remainder were stocked into the No.4 which has been fishing really well with three to four anglers catching 30 fish a day. We have had no further trout mortalities since the original event and subsequent restocking.
I hope this gives you a better understanding of the circumstances, if you have any more questions please contact me.
Kind Regards,
Myland Monery
Walthamstow Fishery,
Thames Water Utilities Ltd,
2 Forest Road,
London
N17 9NH
Tel: 020 8808 1527
Mob: 07747 641179 (41179)
