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Just a quick post today to clarify three issues highlighted on recent threads - fisheries decline, renewable energy and the use of DDT in malaria prevention.

On my post on regime shifts in the natural world, you mentioned, hrp1000, the transformation that appears to be preventing the once mighty cod stocks of the Grand Banks from rebuilding themselves.

was good enough to send me a reference for exploring the idea that other fish which cod used to eat - in this paper, herring and mackerel, although capelin have also been proposed - are now able to thrive, eating cod larvae and so becoming the new bosses of the situation.

You need a subscription to access the full article, but anyway the full reference is there. Essentially the authors found that cod fell during years when herring and mackerel (which have a taste for cod larvae) were abundant, and vice versa.

As they admit, it's a correlational study but may nevertheless have pinpointed the reason the cod are failing to recover.

In several threads, not least "Predict and survive" - or not, we've got into discussion of renewable energy and in particular the issue of how much back-up is needed.

CuckooToo, you posited the idea that wind turbines need 100% backup because the wind doesn't blow all the time.

It's intuitively logical - but not backed by a number of studies, including , at least when the proportion of wind-generated electricity remains below 20%.

There are a number of reasons:

• if turbines are disbursed around the country, their output won't vary in step
• supply and demand can be balanced by ramping up power stations running at less than full capacity, firing up standby generators or temporarily cutting supply to industrial clients which can cope with it - all things
• by trading internationally - in the UK's case, via the interconnector with France
• using storage systems, such as the pumped water facility at or some other technology

In the UK's case, the report concludes that to keep the supply as reliable as it is now (and no electrical system is 100% reliable) the backup would need to have about one-fifth the capacity of the wind turbine fleet - ie if wind supplied 20% of the electricity, the backup would have to be able to supply about 4%.

This shouldn't really be a surprise because the current system has to cope with large variations in demand between different seasons and between night and day, and with sudden unexpected events such as the emergency shutdown of a large power station - a different kind of intermittency.

The intermittency of the wind supply could be decreased by putting turbines where the wind blows more reliably, and the move to more offshore wind farms should in principle help here.

Generating a higher proportion of electricity from wind would require more backup (or storage or flexibility), but predicting how much isn't easy - largely because there isn't any experience of it. .

A wider mix of renewables would of course be more predictable and could generate more flexibly. Tides can be forecast as far in advance as you like; geothermal, hydro-electric and biomass stations can be operated on demand just like gas turbines.

As to cost... well, that's a different issue. Some aspects, such as the long-term cost of tidal power, are, I would suggest, un-forecastable at the moment.

Thirdly, and finally; my post on Heathrow's third runway took us into discussion about the role of DDT in malaria prevention and the question, ClaphamBusman, of where it's being used now.

The pointed me to last year's , which lists 53 countries deploying insecticides, 13 of which use DDT.

The report's summary notes that "a revival of support for indoor residual spraying of insecticide (IRS) presents a new opportunity for large-scale malaria control".

Some years ago, - a fascinating experience, and a striking demonstration of the fact that in the real world, the decisions that have to be taken are often about balancing risks rather than eliminating them.

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