On 13th September Aline Yuri Otsuki Matsuo asked for information about how to control the predatory nymphs of Odonata in fish culture facilities.
You don't mention the type of fish you are trying to rear, but here is some information which I have found to be generally useful in aquaculture.
The problem of predation of larval and post-larval fish in hatcheries and rearing ponds is not confined to the activities of Odonata - even mature Daphnia will attack and damage newly emerged fry. The first time I came across this problem was when I was examining the freshwater aquaculture industry in Israel in 1973. The technique used there was to rear fry in very small ponds for several weeks, until they were big enough to escape from or resist attack by crustacean predators. The small ponds were dosed with Dipterex, and OP pesticide with very low toxicity to fish fry, a couple of days before the fry were introduced to the ponds. The dose level was around 0.5 to 1mg/l - this is lethal to most crustacea and insects, but fish fry were apparently unaffected by concentrations of at least 10mg/l. The pesticide breaks down fairly rapidly - in a couple of weeks it is virtually all gone.
The Dipterex kills off the crustacea and insects, but does not affect rotifers, which are the preferred food of the larvae when they first hatch. After a couple of weeks, the decay of the pesticide allows the populations of crustaceans such as Daphnia to recolonise the ponds - at exactly the right time for the young fish to switch over to using them as a food!
The effect of adopting this technique was dramatic in the Israeli commercial ponds - losses of fry were around 90% before the use of Dipterex was adopted; afterwards, the survival rate was over 90% - i.e the technique enabled around 9 to 10 times as many fry to survive this very hazardous period. I adopted the same method on my ornamental carp farm in Britain for years, and found it to be extremely effective.
Odonata generally take at least one year to reach their full size, so a modification of this technique should be effective in the grow-out ponds. The fry should be reared to a reasonable size - say 20mm or so - in small ponds as described above, then transferred to the larger grow-out ponds which have been treated in the same way to remove any resident predators (you could also try drying them out in advance as an alternative, of course). If the fish are fed and grow well, then they should reach a large enough size to be safe from predation by the Odonata nymphs before the latter get large enough to cause problems.
As far as estimating the population of nymphs is concerned, you may well find that different species have different habitat preferences. Especially if there are water plants in thepond you will find a large proportion of the nymphs in them - best bet is to sample in all parts of the pond and find out where most of them are. Then target these areas in future to keep an eye on population growth.
If your fish need natural foods but will also accept artificial, and the Odonata get out of control, you could try giving the fish more supplemental feed for a few days before dosing the ponds with something like Dipterex to kill off the nymphs. Keep the supplementary feed going until natural feed animal populations return - using extra manure after the Dipterex has done its job should encourage the regrowth of the feed organisms. In some cases, you may need to take out some of the food animals first and put them into a separate pond, so that you have a seed stock available to recolonise your treated ponds afyer the pesticide has broken down.
I don't know of any predator/control organism that will manage your Odonata population without at the same time rejoicing in the wonderful supply of fish protein available in your ponds! Maybe someone else can suggest something exotic that has such a benign preference for Odonata but hates fish?
I hope this is useful.
Doug Cross, Environmental Consultant and Forensic Ecologist
Tel/Fax (+44) 1884 277627
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